.

Reviewed-on: https://git.reya.su/reya/coop-mobile/pulls/52

composeApp/src/androidMain/composeResources/drawable/ic_communities.xml

@@ -0,0 +1,18 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="960"
+    android:viewportHeight="960">
+    <path
+        android:fillColor="#00000000"
+        android:strokeColor="#000000"
+        android:strokeWidth="56"
+        android:strokeLineCap="round"
+        android:pathData="M120,220 Q120,120 220,120 L740,120 Q840,120 840,220 L840,740 Q840,840 740,840 L220,840 Q120,840 120,740 Z" />
+    <path
+        android:fillColor="#00000000"
+        android:strokeColor="#000000"
+        android:strokeWidth="72"
+        android:strokeLineCap="round"
+        android:pathData="M400,320 L400,640 M560,320 L560,640 M320,400 L640,400 M320,560 L640,560" />
+</vector>

composeApp/src/androidMain/composeResources/drawable/ic_lock.xml

@@ -0,0 +1,15 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="960"
+    android:viewportHeight="960">
+    <path
+        android:fillColor="#000000"
+        android:pathData="M240,500 Q240,440 300,440 L660,440 Q720,440 720,500 L720,780 Q720,840 660,840 L300,840 Q240,840 240,780 Z" />
+    <path
+        android:fillColor="#00000000"
+        android:strokeColor="#000000"
+        android:strokeWidth="64"
+        android:strokeLineCap="round"
+        android:pathData="M340,500 L340,440 A140,140 0 0 1 620,440 L620,500" />
+</vector>

composeApp/src/androidMain/kotlin/su/reya/coop/App.kt

@@ -39,6 +39,7 @@ import androidx.navigation3.ui.NavDisplay
 import kotlinx.coroutines.launch
 import su.reya.coop.repository.AccountRepository
 import su.reya.coop.repository.ChatRepository
+import su.reya.coop.repository.ConcordRepository
 import su.reya.coop.repository.SettingsRepository
 import su.reya.coop.screens.ContactListScreen
 import su.reya.coop.screens.HomeScreen
@@ -54,9 +55,15 @@ import su.reya.coop.screens.ScanScreen
 import su.reya.coop.screens.SettingsScreen
 import su.reya.coop.screens.UpdateProfileScreen
 import su.reya.coop.screens.chat.ChatScreen
+import su.reya.coop.screens.communities.ChannelScreen
+import su.reya.coop.screens.communities.CommunitiesScreen
+import su.reya.coop.screens.communities.CommunityScreen
+import su.reya.coop.screens.communities.JoinCommunityScreen
 import su.reya.coop.viewmodel.AccountViewModel
+import su.reya.coop.viewmodel.ChannelScreenViewModel
 import su.reya.coop.viewmodel.ChatScreenViewModel
 import su.reya.coop.viewmodel.ChatViewModel
+import su.reya.coop.viewmodel.ConcordViewModel
 import su.reya.coop.viewmodel.ProfileCache
 import su.reya.coop.viewmodel.SettingsViewModel
 
@@ -90,6 +97,7 @@ fun App(
     profileCache: ProfileCache,
     accountRepository: AccountRepository,
     chatRepository: ChatRepository,
+    concordRepository: ConcordRepository,
     settingsRepository: SettingsRepository,
     connectivityMonitor: ConnectivityMonitor,
 ) {
@@ -109,6 +117,10 @@ fun App(
                         settingsRepository
                     )
 
+                    modelClass.isAssignableFrom(ConcordViewModel::class.java) -> ConcordViewModel(
+                        concordRepository
+                    )
+
                     else -> throw IllegalArgumentException("Unknown ViewModel class")
                 }
                 @Suppress("UNCHECKED_CAST")
@@ -120,6 +132,7 @@ fun App(
     val accountViewModel: AccountViewModel = viewModel(factory = viewModelFactory)
     val chatViewModel: ChatViewModel = viewModel(factory = viewModelFactory)
     val settingsViewModel: SettingsViewModel = viewModel(factory = viewModelFactory)
+    val concordViewModel: ConcordViewModel = viewModel(factory = viewModelFactory)
 
     val context = LocalContext.current
     val activity = context as? ComponentActivity
@@ -170,6 +183,11 @@ fun App(
                 snackbarHostState.showSnackbar(message)
             }
         }
+        launch {
+            concordViewModel.errorEvents.collect { message ->
+                snackbarHostState.showSnackbar(message)
+            }
+        }
         launch {
             profileCache.errorEvents.collect { message ->
                 snackbarHostState.showSnackbar(message)
@@ -237,7 +255,7 @@ fun App(
                 ),
                 entryProvider = entryProvider {
                     entry<Screen.Home> {
-                        HomeScreen(accountViewModel, chatViewModel)
+                        HomeScreen(accountViewModel, chatViewModel, concordViewModel)
                     }
                     entry<Screen.RequestList> {
                         RequestListScreen(chatViewModel)
@@ -297,6 +315,36 @@ fun App(
                     entry<Screen.Settings> {
                         SettingsScreen(settingsViewModel)
                     }
+                    entry<Screen.Communities> {
+                        CommunitiesScreen(concordViewModel)
+                    }
+                    entry<Screen.JoinCommunity> { key ->
+                        JoinCommunityScreen(concordViewModel, key.link)
+                    }
+                    entry<Screen.Community> { key ->
+                        CommunityScreen(key.communityId, concordViewModel)
+                    }
+                    entry<Screen.Channel> { key ->
+                        val factory = remember(key) {
+                            object : ViewModelProvider.Factory {
+                                override fun <T : ViewModel> create(modelClass: Class<T>): T {
+                                    @Suppress("UNCHECKED_CAST")
+                                    return ChannelScreenViewModel(
+                                        key.communityId,
+                                        key.channelId,
+                                        accountRepository,
+                                        concordRepository
+                                    ) as T
+                                }
+                            }
+                        }
+                        ChannelScreen(
+                            viewModel<ChannelScreenViewModel>(
+                                key = "${key.communityId}:${key.channelId}",
+                                factory = factory
+                            )
+                        )
+                    }
                 }
             )
         }

composeApp/src/androidMain/kotlin/su/reya/coop/MainActivity.kt

@@ -13,6 +13,7 @@ import kotlinx.coroutines.MainScope
 import su.reya.coop.nostr.NostrManager
 import su.reya.coop.repository.AccountRepository
 import su.reya.coop.repository.ChatRepository
+import su.reya.coop.repository.ConcordRepository
 import su.reya.coop.repository.MediaRepository
 import su.reya.coop.repository.SettingsRepository
 import su.reya.coop.viewmodel.ProfileCache
@@ -52,6 +53,10 @@ class MainActivity : ComponentActivity() {
         ChatRepository(NostrManager.instance, mediaRepository, settingsRepository, scope)
     }
 
+    private val concordRepository by lazy {
+        ConcordRepository(NostrManager.instance, scope)
+    }
+
     override fun onCreate(savedInstanceState: Bundle?) {
         Thread.setDefaultUncaughtExceptionHandler { thread, throwable ->
             throwable.printStackTrace()
@@ -97,6 +102,7 @@ class MainActivity : ComponentActivity() {
                 profileCache = profileCache,
                 accountRepository = accountRepository,
                 chatRepository = chatRepository,
+                concordRepository = concordRepository,
                 settingsRepository = settingsRepository,
                 connectivityMonitor = connectivityMonitor,
             )

composeApp/src/androidMain/kotlin/su/reya/coop/Navigation.kt

@@ -61,4 +61,16 @@ sealed interface Screen : NavKey {
 
     @Serializable
     data object Settings : Screen
+
+    @Serializable
+    data object Communities : Screen
+
+    @Serializable
+    data class Community(val communityId: String) : Screen
+
+    @Serializable
+    data class Channel(val communityId: String, val channelId: String) : Screen
+
+    @Serializable
+    data class JoinCommunity(val link: String? = null) : Screen
 }

composeApp/src/androidMain/kotlin/su/reya/coop/NostrForegroundService.kt

@@ -63,7 +63,7 @@ class NostrForegroundService : Service() {
 
                 // Initialize Nostr client
                 try {
-                    nostr.init(dbDir.absolutePath)
+                    nostr.init(dbDir.absolutePath, AppStore(this@NostrForegroundService))
                 } catch (e: Exception) {
                     throw IllegalStateException("Failed to initialize Nostr Client", e)
                 }

composeApp/src/androidMain/kotlin/su/reya/coop/screens/HomeScreen.kt

@@ -102,17 +102,20 @@ import su.reya.coop.RoomKind
 import su.reya.coop.RoomUiState
 import su.reya.coop.Screen
 import su.reya.coop.ago
+import su.reya.coop.concord.parseInviteLink
 import su.reya.coop.shared.Avatar
 import su.reya.coop.shared.getExpressiveFontFamily
 import su.reya.coop.uiStateFlow
 import su.reya.coop.viewmodel.AccountViewModel
 import su.reya.coop.viewmodel.ChatViewModel
+import su.reya.coop.viewmodel.ConcordViewModel
 
 @OptIn(ExperimentalMaterial3ExpressiveApi::class, ExperimentalMaterial3Api::class)
 @Composable
 fun HomeScreen(
     accountViewModel: AccountViewModel,
-    chatViewModel: ChatViewModel
+    chatViewModel: ChatViewModel,
+    concordViewModel: ConcordViewModel,
 ) {
     val context = LocalContext.current
     val navigator = LocalNavigator.current
@@ -166,16 +169,22 @@ fun HomeScreen(
 
     LaunchedEffect(qrScanResult.content) {
         qrScanResult.content?.let { result ->
-            runCatching { PublicKey.parse(result) }
+            // A Concord invite is a link, not a npub, so it is routed to the Join screen instead of
-                .onSuccess { pubkey ->
+            // being force-fed to PublicKey.parse.
-                    try {
+            if (parseInviteLink(result) != null) {
-                        val roomId = chatViewModel.createChatRoom(listOf(pubkey))
+                navigator.navigate(Screen.JoinCommunity(result))
-                        navigator.navigate(Screen.Chat(roomId))
+            } else {
-                    } catch (e: Exception) {
+                runCatching { PublicKey.parse(result) }
-                        e.message?.let { snackbarHostState.showSnackbar(it) }
+                    .onSuccess { pubkey ->
+                        try {
+                            val roomId = chatViewModel.createChatRoom(listOf(pubkey))
+                            navigator.navigate(Screen.Chat(roomId))
+                        } catch (e: Exception) {
+                            e.message?.let { snackbarHostState.showSnackbar(it) }
+                        }
                     }
-                }
+                    .onFailure { e -> println("Failed to parse QR: ${e.message}") }
-                .onFailure { e -> println("Failed to parse QR: ${e.message}") }
+            }
             // Clear the nav state
             qrScanResult.clear()
         }
@@ -473,7 +482,8 @@ fun HomeScreen(
                 BottomMenuList(
                     onDismiss = dismissAndRun,
                     accountViewModel = accountViewModel,
-                    chatViewModel = chatViewModel
+                    chatViewModel = chatViewModel,
+                    concordViewModel = concordViewModel
                 )
             }
         }
@@ -788,12 +798,14 @@ fun BottomMenuList(
     onDismiss: (suspend () -> Unit) -> Unit,
     accountViewModel: AccountViewModel,
     chatViewModel: ChatViewModel,
+    concordViewModel: ConcordViewModel,
 ) {
     val navigator = LocalNavigator.current
 
     val defaultMenuList = listOf(
         "Update Profile" to { navigator.navigate(Screen.UpdateProfile) },
         "Contact List" to { navigator.navigate(Screen.ContactList) },
+        "Communities (beta)" to { navigator.navigate(Screen.Communities) },
         "Relay Management" to { navigator.navigate(Screen.Relay) },
         "Settings" to { navigator.navigate(Screen.Settings) }
     )
@@ -824,6 +836,8 @@ fun BottomMenuList(
                     accountViewModel.logout(onLogout = {
                         accountViewModel.resetInternalState()
                         chatViewModel.resetInternalState()
+                        // Concord's keys are identity-scoped, so they leave with the identity.
+                        concordViewModel.resetInternalState()
                     })
                 }
             },

composeApp/src/androidMain/kotlin/su/reya/coop/screens/chat/ChatInput.kt

@@ -38,8 +38,8 @@ fun ChatInput(
     value: String,
     onValueChange: (String) -> Unit,
     onSend: () -> Unit,
-    onUpload: () -> Unit,
+    onUpload: (() -> Unit)? = null,
-    onMicClick: () -> Unit
+    onMicClick: (() -> Unit)? = null
 ) {
     Row(
         modifier = Modifier.padding(horizontal = 16.dp, vertical = 8.dp),
@@ -60,12 +60,16 @@ fun ChatInput(
                 capitalization = KeyboardCapitalization.Sentences,
                 imeAction = ImeAction.Default
             ),
-            leadingIcon = {
+            leadingIcon = if (onUpload == null) {
-                IconButton(onClick = onUpload) {
+                null
-                    Icon(
+            } else {
-                        painter = painterResource(Res.drawable.ic_add_circle),
+                {
-                        contentDescription = "Upload",
+                    IconButton(onClick = onUpload) {
-                    )
+                        Icon(
+                            painter = painterResource(Res.drawable.ic_add_circle),
+                            contentDescription = "Upload",
+                        )
+                    }
                 }
             },
         )
@@ -75,9 +79,21 @@ fun ChatInput(
             transitionSpec = { (scaleIn() + fadeIn()) togetherWith (scaleOut() + fadeOut()) },
             label = "send_mic_transition"
         ) { isNotEmpty ->
-            if (isNotEmpty) {
+            // Nothing to send and nowhere to dictate to: the send button stays, greyed out.
+            if (!isNotEmpty && onMicClick != null) {
+                FilledTonalIconButton(
+                    onClick = onMicClick,
+                    modifier = Modifier.size(56.dp),
+                ) {
+                    Icon(
+                        painter = painterResource(Res.drawable.ic_audio),
+                        contentDescription = "Speech to Text"
+                    )
+                }
+            } else {
                 IconButton(
                     onClick = onSend,
+                    enabled = isNotEmpty,
                     modifier = Modifier.size(56.dp),
                     colors = IconButtonDefaults.iconButtonColors(
                         containerColor = MaterialTheme.colorScheme.surfaceVariant,
@@ -89,16 +105,6 @@ fun ChatInput(
                         contentDescription = "Send"
                     )
                 }
-            } else {
-                FilledTonalIconButton(
-                    onClick = onMicClick,
-                    modifier = Modifier.size(56.dp),
-                ) {
-                    Icon(
-                        painter = painterResource(Res.drawable.ic_audio),
-                        contentDescription = "Speech to Text"
-                    )
-                }
             }
         }
     }

composeApp/src/androidMain/kotlin/su/reya/coop/screens/chat/ChatMessage.kt

@@ -5,9 +5,13 @@ import androidx.compose.foundation.interaction.MutableInteractionSource
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.offset
 import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.layout.widthIn
+import androidx.compose.foundation.shape.CircleShape
 import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Surface
@@ -34,6 +38,7 @@ import androidx.compose.ui.text.buildAnnotatedString
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.text.style.TextDecoration
 import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
 import coil3.compose.AsyncImage
 import rust.nostr.sdk.EventId
 import rust.nostr.sdk.PublicKey
@@ -46,6 +51,13 @@ import su.reya.coop.formatAsTime
 import su.reya.coop.isImageUrl
 import su.reya.coop.removeImageUrls
 
+@Immutable
+data class ReactionGroup(
+    val emoji: String,
+    val authors: List<PublicKey>,
+    val containsMe: Boolean
+)
+
 @Immutable
 data class MessageModel(
     val id: EventId,
@@ -54,15 +66,20 @@ data class MessageModel(
     val images: List<String>,
     val timestamp: String,
     val isMine: Boolean,
-    val replyEventIds: List<EventId>
+    val replyEventIds: List<EventId>,
+    val reactions: List<ReactionGroup> = emptyList()
 )
 
 @Composable
-fun rememberMessageModel(event: UnsignedEvent, currentUser: PublicKey? = null): MessageModel {
+fun rememberMessageModel(
+    event: UnsignedEvent,
+    reactions: List<UnsignedEvent> = emptyList(),
+    currentUser: PublicKey? = null
+): MessageModel {
     val settings = LocalSettings.current
     val isMobileData = LocalConnectivity.current
 
-    return remember(event, currentUser, settings, isMobileData) {
+    return remember(event, reactions, currentUser, settings, isMobileData) {
         val id = event.ensureId().id()!!
         val isMine = currentUser == event.author()
         val content = event.content()
@@ -104,6 +121,16 @@ fun rememberMessageModel(event: UnsignedEvent, currentUser: PublicKey? = null):
             append(cleanedContent.substring(lastIndex))
         }
 
+        val groupedReactions = reactions.groupBy { it.content() }
+            .map { (emoji, events) ->
+                val authors = events.map { it.author() }
+                ReactionGroup(
+                    emoji = emoji,
+                    authors = authors,
+                    containsMe = authors.any { it == currentUser }
+                )
+            }
+
         MessageModel(
             id = id,
             author = event.author(),
@@ -111,7 +138,8 @@ fun rememberMessageModel(event: UnsignedEvent, currentUser: PublicKey? = null):
             images = images,
             timestamp = event.createdAt().formatAsTime(),
             isMine = isMine,
-            replyEventIds = replyEventIds
+            replyEventIds = replyEventIds,
+            reactions = groupedReactions
         )
     }
 }
@@ -158,46 +186,110 @@ fun ChatMessage(
             horizontalAlignment = if (model.isMine) Alignment.End else Alignment.Start,
             verticalArrangement = Arrangement.spacedBy(4.dp)
         ) {
-            if (model.annotatedContent.isNotBlank()) {
+            Box(contentAlignment = Alignment.BottomEnd) {
-                Surface(
+                Column(
-                    modifier = Modifier.widthIn(max = 280.dp),
+                    modifier = Modifier.padding(
-                    color = containerColor,
+                        bottom = if (model.reactions.isNotEmpty()) 4.dp else 0.dp,
-                    contentColor = contentColor,
+                        end = if (model.reactions.isNotEmpty() && !model.isMine) 8.dp else 0.dp
-                    shape = bubbleShape,
+                    ),
+                    horizontalAlignment = if (model.isMine) Alignment.End else Alignment.Start,
+                    verticalArrangement = Arrangement.spacedBy(4.dp)
                 ) {
-                    Text(
+                    if (model.annotatedContent.isNotBlank()) {
-                        text = model.annotatedContent,
+                        Surface(
-                        modifier = Modifier.padding(horizontal = 16.dp, vertical = 8.dp),
+                            modifier = Modifier.widthIn(max = 280.dp),
-                        style = MaterialTheme.typography.bodyLarge
+                            color = containerColor,
-                    )
+                            contentColor = contentColor,
+                            shape = bubbleShape,
+                        ) {
+                            Text(
+                                text = model.annotatedContent,
+                                modifier = Modifier.padding(horizontal = 16.dp, vertical = 8.dp),
+                                style = MaterialTheme.typography.bodyLarge
+                            )
+                        }
+                    }
+                    model.images.forEach { imageUrl ->
+                        Surface(
+                            shape = RoundedCornerShape(16.dp),
+                            color = MaterialTheme.colorScheme.surfaceVariant,
+                            modifier = Modifier.widthIn(max = 280.dp)
+                        ) {
+                            AsyncImage(
+                                model = imageUrl,
+                                contentDescription = "Image from chat",
+                                modifier = Modifier
+                                    .fillMaxWidth()
+                                    .clip(RoundedCornerShape(16.dp)),
+                                contentScale = ContentScale.FillWidth
+                            )
+                        }
+                    }
                 }
-            }
+
-            model.images.forEach { imageUrl ->
+                if (model.reactions.isNotEmpty()) {
-                Surface(
+                    MessageReactions(
-                    shape = RoundedCornerShape(16.dp),
+                        emojis = model.reactions.map { it.emoji },
-                    color = MaterialTheme.colorScheme.surfaceVariant,
+                        total = model.reactions.sumOf { it.authors.size },
-                    modifier = Modifier.widthIn(max = 280.dp)
+                        modifier = Modifier.offset(y = 12.dp)
-                ) {
-                    AsyncImage(
-                        model = imageUrl,
-                        contentDescription = "Image from chat",
-                        modifier = Modifier
-                            .fillMaxWidth()
-                            .clip(RoundedCornerShape(16.dp)),
-                        contentScale = ContentScale.FillWidth
                     )
                 }
             }
+
             if (isMessageClicked) {
                 Text(
                     text = model.timestamp,
                     style = MaterialTheme.typography.labelSmall,
                     color = MaterialTheme.colorScheme.outline,
-                    modifier = Modifier.align(
+                    modifier = Modifier.padding(top = if (model.reactions.isNotEmpty()) 8.dp else 0.dp)
-                        if (model.isMine) Alignment.End else Alignment.Start
-                    )
                 )
             }
         }
     }
 }
+
+/** The reaction chips under a bubble, shared by DMs and Channels. */
+@Composable
+fun MessageReactions(
+    emojis: List<String>,
+    total: Int,
+    modifier: Modifier = Modifier
+) {
+    val displayEmojis = emojis.take(3)
+
+    Row(
+        modifier = modifier,
+        verticalAlignment = Alignment.CenterVertically,
+        horizontalArrangement = Arrangement.spacedBy(4.dp)
+    ) {
+        displayEmojis.forEach { emoji ->
+            Surface(
+                modifier = Modifier.size(24.dp),
+                color = MaterialTheme.colorScheme.surface,
+                shape = CircleShape,
+            ) {
+                Box(contentAlignment = Alignment.Center) {
+                    Text(
+                        text = emoji,
+                        fontSize = 12.sp,
+                    )
+                }
+            }
+        }
+        if (total > 2) {
+            Surface(
+                modifier = Modifier.size(24.dp),
+                color = MaterialTheme.colorScheme.surface,
+                shape = CircleShape,
+            ) {
+                Box(contentAlignment = Alignment.Center) {
+                    Text(
+                        text = total.toString(),
+                        style = MaterialTheme.typography.labelSmall,
+                        fontSize = 10.sp,
+                    )
+                }
+            }
+        }
+    }
+}

composeApp/src/androidMain/kotlin/su/reya/coop/screens/chat/ChatScreen.kt

@@ -80,6 +80,7 @@ import androidx.compose.ui.platform.LocalWindowInfo
 import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.unit.IntOffset
 import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import coop.composeapp.generated.resources.Res
 import coop.composeapp.generated.resources.ic_arrow_back
@@ -90,6 +91,8 @@ import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
 import org.jetbrains.compose.resources.painterResource
+import rust.nostr.sdk.EventId
+import rust.nostr.sdk.KindStandard
 import rust.nostr.sdk.UnsignedEvent
 import su.reya.coop.LocalNavigator
 import su.reya.coop.LocalProfileCache
@@ -143,10 +146,21 @@ fun ChatScreen(
     val loading = viewModel.loading
     val newOtherMessages = viewModel.newOtherMessages
     val requireScreening = viewModel.requireScreening
-    val messages = viewModel.messages
+    val allEvents = viewModel.messages
+
+    val displayMessages by remember {
+        derivedStateOf { allEvents.filter { it.kind().asStd() != KindStandard.REACTION } }
+    }
+
+    val reactionsByMessage by remember {
+        derivedStateOf {
+            allEvents.filter { it.kind().asStd() == KindStandard.REACTION }
+                .groupBy { it.tags().eventIds().firstOrNull() }
+        }
+    }
 
     val groupedMessages =
-        remember { derivedStateOf { messages.groupBy { it.createdAt().formatAsGroup() } } }
+        remember { derivedStateOf { displayMessages.groupBy { it.createdAt().formatAsGroup() } } }
 
     val roomState by remember(id, currentUser?.publicKey) {
         (room as Room).uiStateFlow(profileCache, currentUser?.publicKey)
@@ -161,6 +175,29 @@ fun ChatScreen(
         label = "blurAnimation"
     )
 
+    val goToMessage = { eventId: EventId? ->
+        if (eventId != null) {
+            scope.launch {
+                var targetIndex = -1
+                var currentIndex = 0
+
+                for (group in groupedMessages.value) {
+                    val msgInGroup = group.value
+                    val idx = msgInGroup.indexOfFirst { it.ensureId().id() == eventId }
+                    if (idx != -1) {
+                        targetIndex = currentIndex + idx
+                        break
+                    }
+                    currentIndex += msgInGroup.size + 1
+                }
+
+                if (targetIndex != -1) {
+                    listState.animateScrollToItem(targetIndex)
+                }
+            }
+        }
+    }
+
     val sendFile = { uri: Uri ->
         scope.launch {
             // Read file on IO dispatcher
@@ -190,15 +227,13 @@ fun ChatScreen(
             }
         }
 
-    LaunchedEffect(messages.size) {
+    LaunchedEffect(allEvents.size) {
-        if (messages.isNotEmpty()) {
+        if (displayMessages.isNotEmpty()) {
             listState.animateScrollToItem(0)
         }
     }
 
-    Box(
+    Box(modifier = Modifier.fillMaxSize()) {
-        modifier = Modifier.fillMaxSize()
-    ) {
         Scaffold(
             modifier = Modifier.blur(blurAmount),
             contentWindowInsets = ScaffoldDefaults.contentWindowInsets.union(WindowInsets.ime),
@@ -271,7 +306,7 @@ fun ChatScreen(
                             room?.let { ScreenerCard(accountViewModel, it) }
                         }
 
-                        when (messages.isNotEmpty()) {
+                        when (displayMessages.isNotEmpty()) {
                             true -> {
                                 LazyColumn(
                                     modifier = Modifier
@@ -286,14 +321,15 @@ fun ChatScreen(
                                             items = messagesInGroup,
                                             key = { it.ensureId().id()?.toHex()!! }
                                         ) { event ->
+                                            val msgReactions = reactionsByMessage[event.id()] ?: emptyList()
                                             val model =
-                                                rememberMessageModel(event, currentUser?.publicKey)
+                                                rememberMessageModel(event, msgReactions, currentUser?.publicKey)
 
                                             val replyPreview =
-                                                remember(model.replyEventIds, messages.size) {
+                                                remember(model.replyEventIds, displayMessages.size) {
                                                     model.replyEventIds.firstOrNull()
                                                         ?.let { replyId ->
-                                                            messages.find { it.id() == replyId }
+                                                            displayMessages.find { it.ensureId().id() == replyId }
                                                         }
                                                 }
 
@@ -303,7 +339,13 @@ fun ChatScreen(
                                                     .animateItem(),
                                                 verticalArrangement = Arrangement.spacedBy(2.dp)
                                             ) {
-                                                replyPreview?.let { ReplyPreview(it, model.isMine) }
+                                                replyPreview?.let { previewEvent ->
+                                                    ReplyPreview(
+                                                        event = previewEvent,
+                                                        isMine = model.isMine,
+                                                        onClick = { goToMessage(previewEvent.id()) }
+                                                    )
+                                                }
                                                 ChatMessage(
                                                     model = model,
                                                     modifier = Modifier.graphicsLayer {
@@ -394,8 +436,9 @@ fun ChatScreen(
                                     value = text,
                                     onValueChange = { text = it },
                                     onSend = {
-                                        viewModel.sendMessage(text)
+                                        viewModel.sendMessage(text, replyingTo?.id)
                                         text = ""
+                                        replyingTo = null
                                     },
                                     onUpload = {
                                         fileLauncher.launch("image/*")
@@ -435,14 +478,11 @@ fun ChatScreen(
             val (model, bounds) = selectedMessage ?: return@AnimatedVisibility
 
             val density = LocalDensity.current
-            val windowInfo = LocalWindowInfo.current
-            val windowHeight = windowInfo.containerSize.height
             val scrollState = rememberScrollState()
 
             var menuHeight by remember { mutableFloatStateOf(0f) }
-            val spacing = with(density) { 12.dp.toPx() }
+            var toolbarHeight by remember { mutableFloatStateOf(0f) }
-            val showAbove =
+            val spacing = with(density) { 6.dp.toPx() }
-                (windowHeight - bounds.bottom) < (menuHeight + spacing) && bounds.top > (menuHeight + spacing)
 
             Box(
                 modifier = Modifier
@@ -451,11 +491,30 @@ fun ChatScreen(
                     .clickable { selectedMessage = null }
                     .verticalScroll(scrollState),
             ) {
-                val totalExtraHeight = if (menuHeight > 0) menuHeight + spacing else 300f
+                val totalExtraHeight = (if (menuHeight > 0) menuHeight + spacing else 300f) +
+                        (if (toolbarHeight > 0) toolbarHeight + spacing else 100f)
                 val contentBottom = with(density) { (bounds.bottom + totalExtraHeight).toDp() }
 
                 Spacer(modifier = Modifier.height(contentBottom + 200.dp))
 
+                // Reaction Toolbar (Above)
+                Box(
+                    modifier = Modifier
+                        .offset { IntOffset(0, (bounds.top - toolbarHeight - spacing).toInt().coerceAtLeast(0)) }
+                        .onGloballyPositioned { toolbarHeight = it.size.height.toFloat() }
+                        .fillMaxWidth()
+                        .padding(horizontal = 16.dp),
+                    contentAlignment = if (model.isMine) Alignment.CenterEnd else Alignment.CenterStart
+                ) {
+                    ReactionToolbar(
+                        onReaction = { reaction ->
+                            viewModel.sendReaction(model.id, reaction)
+                            selectedMessage = null
+                        }
+                    )
+                }
+
+                // Message Preview
                 ChatMessage(
                     model = model,
                     modifier = Modifier
@@ -463,38 +522,35 @@ fun ChatScreen(
                         .padding(horizontal = 16.dp)
                 )
 
-                val menuOffset = if (showAbove) {
+                // Action Menu (Below)
-                    bounds.top - menuHeight - spacing
-                } else {
-                    bounds.bottom + spacing
-                }
-
                 Box(
                     modifier = Modifier
-                        .offset { IntOffset(0, menuOffset.toInt().coerceAtLeast(0)) }
+                        .offset { IntOffset(0, (bounds.bottom + spacing).toInt()) }
                         .onGloballyPositioned { menuHeight = it.size.height.toFloat() }
                         .fillMaxWidth()
                         .padding(horizontal = 16.dp),
                     contentAlignment = if (model.isMine) Alignment.CenterEnd else Alignment.CenterStart
                 ) {
-                    ContextMenu { action ->
+                    ContextMenu(
-                        when (action) {
+                        onAction = { action ->
-                            "Copy" -> {
+                            when (action) {
-                                scope.launch {
+                                "Copy" -> {
-                                    val content = model.annotatedContent
+                                    scope.launch {
-                                    val data = ClipData.newPlainText(content, content)
+                                        val content = model.annotatedContent
-                                    clipboardManager.setClipEntry(ClipEntry(data))
+                                        val data = ClipData.newPlainText(content, content)
+                                        clipboardManager.setClipEntry(ClipEntry(data))
+                                    }
                                 }
-                            }
 
-                            "Reply" -> {
+                                "Reply" -> {
-                                replyingTo = model
+                                    replyingTo = model
-                            }
+                                }
 
-                            else -> {}
+                                else -> {}
+                            }
+                            selectedMessage = null
                         }
-                        selectedMessage = null
+                    )
-                    }
                 }
             }
         }
@@ -544,7 +600,11 @@ private fun ReplyBox(model: MessageModel, onDismiss: () -> Unit) {
 }
 
 @Composable
-private fun ReplyPreview(event: UnsignedEvent, isMine: Boolean = false) {
+private fun ReplyPreview(
+    event: UnsignedEvent,
+    isMine: Boolean = false,
+    onClick: () -> Unit
+) {
     val profileCache = LocalProfileCache.current
     val profileFlow = remember(event) { profileCache.getMetadata(event.author()) }
     val profile by profileFlow.collectAsStateWithLifecycle()
@@ -560,7 +620,9 @@ private fun ReplyPreview(event: UnsignedEvent, isMine: Boolean = false) {
         contentAlignment = if (isMine) Alignment.CenterEnd else Alignment.CenterStart
     ) {
         Surface(
-            modifier = Modifier.widthIn(max = 280.dp),
+            modifier = Modifier
+                .widthIn(max = 280.dp)
+                .clickable(onClick = onClick),
             color = MaterialTheme.colorScheme.tertiaryContainer,
             shape = bubbleShape,
         ) {
@@ -568,13 +630,13 @@ private fun ReplyPreview(event: UnsignedEvent, isMine: Boolean = false) {
                 modifier = Modifier
                     .fillMaxWidth()
                     .padding(horizontal = 16.dp, vertical = 8.dp),
+                verticalArrangement = Arrangement.spacedBy(2.dp)
             ) {
                 Text(
                     text = profile?.name ?: "Unknown",
-                    style = MaterialTheme.typography.labelSmall,
+                    style = MaterialTheme.typography.bodyMedium,
-                    color = MaterialTheme.colorScheme.onTertiaryContainer.copy(
+                    fontWeight = FontWeight.SemiBold,
-                        alpha = 0.6f
+                    color = MaterialTheme.colorScheme.onTertiaryContainer,
-                    ),
                 )
                 Text(
                     text = event.content(),
@@ -587,9 +649,41 @@ private fun ReplyPreview(event: UnsignedEvent, isMine: Boolean = false) {
     }
 }
 
+@Composable
+fun ReactionToolbar(
+    onReaction: (String) -> Unit
+) {
+    val reactionEmojis = listOf("👍", "❤️", "😂", "😮", "😢", "😡", "🎉")
+
+    Surface(
+        color = MaterialTheme.colorScheme.surfaceContainerHigh,
+        shape = RoundedCornerShape(24.dp),
+        shadowElevation = 1.dp
+    ) {
+        Row(
+            modifier = Modifier
+                .padding(horizontal = 12.dp, vertical = 8.dp),
+            horizontalArrangement = Arrangement.spacedBy(8.dp),
+            verticalAlignment = Alignment.CenterVertically
+        ) {
+            reactionEmojis.forEach { emoji ->
+                Text(
+                    text = emoji,
+                    modifier = Modifier
+                        .clickable { onReaction(emoji) }
+                        .padding(4.dp),
+                    fontSize = 24.sp
+                )
+            }
+        }
+    }
+}
+
 @OptIn(ExperimentalMaterial3ExpressiveApi::class)
 @Composable
-private fun ContextMenu(onAction: (String) -> Unit) {
+private fun ContextMenu(
+    onAction: (String) -> Unit
+) {
     val menuItems = listOf(
         "Copy" to Res.drawable.ic_copy,
         "Reply" to Res.drawable.ic_reply
@@ -598,6 +692,8 @@ private fun ContextMenu(onAction: (String) -> Unit) {
     DropdownMenuGroup(
         shapes = MenuDefaults.groupShape(1, 1),
         containerColor = MenuDefaults.groupVibrantContainerColor,
+        tonalElevation = 1.dp,
+        shadowElevation = 1.dp,
         modifier = Modifier.width(220.dp)
     ) {
         val itemCount = menuItems.size

composeApp/src/androidMain/kotlin/su/reya/coop/screens/communities/ChannelScreen.kt

@@ -0,0 +1,365 @@
+package su.reya.coop.screens.communities
+
+import androidx.compose.foundation.combinedClickable
+import androidx.compose.foundation.interaction.MutableInteractionSource
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.PaddingValues
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.WindowInsets
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.ime
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.layout.union
+import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.lazy.items
+import androidx.compose.foundation.lazy.rememberLazyListState
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.Icon
+import androidx.compose.material3.IconButton
+import androidx.compose.material3.LoadingIndicator
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Scaffold
+import androidx.compose.material3.ScaffoldDefaults
+import androidx.compose.material3.SnackbarHost
+import androidx.compose.material3.Surface
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.material3.TopAppBar
+import androidx.compose.material3.TopAppBarDefaults
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.derivedStateOf
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.style.TextOverflow
+import androidx.compose.ui.unit.dp
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import coop.composeapp.generated.resources.Res
+import coop.composeapp.generated.resources.ic_arrow_back
+import kotlinx.coroutines.flow.flowOf
+import org.jetbrains.compose.resources.painterResource
+import rust.nostr.sdk.PublicKey
+import su.reya.coop.LocalNavigator
+import su.reya.coop.LocalProfileCache
+import su.reya.coop.LocalSnackbarHostState
+import su.reya.coop.Profile
+import su.reya.coop.concord.ConcordMessage
+import su.reya.coop.dayLabel
+import su.reya.coop.sanitizeName
+import su.reya.coop.screens.chat.ChatInput
+import su.reya.coop.screens.chat.DateSeparator
+import su.reya.coop.screens.chat.MessageReactions
+import su.reya.coop.screens.chat.ReactionToolbar
+import su.reya.coop.shared.Avatar
+import su.reya.coop.short
+import su.reya.coop.timeLabel
+import su.reya.coop.viewmodel.ChannelScreenViewModel
+
+/**
+ * One Channel's history, laid out Discord-style: every message carries its author, because a Channel
+ * has many speakers rather than the two a DM has.
+ *
+ * The list is reversed, exactly as `ChatScreen` is, so the newest message sits at the bottom without
+ * any scroll bookkeeping — the item order below is bottom-to-top, and the day separator is placed
+ * after its group for that reason.
+ */
+@OptIn(ExperimentalMaterial3Api::class, ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun ChannelScreen(viewModel: ChannelScreenViewModel) {
+    val navigator = LocalNavigator.current
+    val snackbarHostState = LocalSnackbarHostState.current
+    val listState = rememberLazyListState()
+
+    val channel by viewModel.channel.collectAsStateWithLifecycle()
+    val currentUser by viewModel.currentUser.collectAsStateWithLifecycle()
+
+    var text by remember { mutableStateOf("") }
+
+    /** The message whose action row is open, if any. */
+    var activeMessageId by remember { mutableStateOf<String?>(null) }
+
+    /** The message an Edit is being written for, if any. Its text lives in [text]. */
+    var editing by remember { mutableStateOf<ConcordMessage?>(null) }
+
+    val grouped by remember {
+        derivedStateOf {
+            viewModel.messages
+                .groupBy { it.dayLabel() }
+                .toList()
+                .reversed()
+                .map { (day, dayMessages) -> day to dayMessages.asReversed() }
+        }
+    }
+
+    Scaffold(
+        contentWindowInsets = ScaffoldDefaults.contentWindowInsets.union(WindowInsets.ime),
+        containerColor = MaterialTheme.colorScheme.surfaceContainer,
+        snackbarHost = { SnackbarHost(snackbarHostState) },
+        topBar = {
+            TopAppBar(
+                title = {
+                    Row(verticalAlignment = Alignment.CenterVertically) {
+                        Text(
+                            text = "#${channel?.name ?: "channel"}",
+                            style = MaterialTheme.typography.titleMediumEmphasized,
+                            maxLines = 1,
+                            overflow = TextOverflow.Ellipsis,
+                        )
+                        if (channel?.hasKey == false) {
+                            Spacer(modifier = Modifier.size(8.dp))
+                            Text(
+                                text = "No key",
+                                style = MaterialTheme.typography.labelSmall,
+                                color = MaterialTheme.colorScheme.outline,
+                            )
+                        }
+                    }
+                },
+                colors = TopAppBarDefaults.topAppBarColors(
+                    containerColor = MaterialTheme.colorScheme.surfaceContainer,
+                ),
+                navigationIcon = {
+                    IconButton(onClick = { navigator.goBack() }) {
+                        Icon(
+                            painter = painterResource(Res.drawable.ic_arrow_back),
+                            contentDescription = "Back"
+                        )
+                    }
+                },
+            )
+        },
+        content = { innerPadding ->
+            Surface(
+                modifier = Modifier
+                    .fillMaxSize()
+                    .padding(top = innerPadding.calculateTopPadding()),
+                color = MaterialTheme.colorScheme.surface,
+                shape = RoundedCornerShape(topStart = 24.dp, topEnd = 24.dp),
+            ) {
+                Column(
+                    modifier = Modifier
+                        .fillMaxSize()
+                        .padding(bottom = innerPadding.calculateBottomPadding())
+                ) {
+                    if (viewModel.loading) {
+                        Box(
+                            modifier = Modifier
+                                .weight(1f)
+                                .fillMaxWidth(),
+                            contentAlignment = Alignment.Center
+                        ) {
+                            LoadingIndicator()
+                        }
+                    } else if (viewModel.messages.isEmpty()) {
+                        Box(
+                            modifier = Modifier
+                                .weight(1f)
+                                .fillMaxWidth(),
+                            contentAlignment = Alignment.Center
+                        ) {
+                            Column(
+                                horizontalAlignment = Alignment.CenterHorizontally,
+                                verticalArrangement = Arrangement.spacedBy(4.dp),
+                            ) {
+                                Text(
+                                    text = "No messages yet",
+                                    style = MaterialTheme.typography.titleLargeEmphasized.copy(
+                                        fontWeight = FontWeight.SemiBold
+                                    ),
+                                    color = MaterialTheme.colorScheme.onSurface
+                                )
+                                // Nothing to add for a Channel we hold no key for: the bar at the
+                                // bottom of this screen is already saying why it is empty.
+                                if (channel?.hasKey != false) {
+                                    Text(
+                                        text = "Say something to start it off.",
+                                        style = MaterialTheme.typography.bodyMedium,
+                                        color = MaterialTheme.colorScheme.outline,
+                                    )
+                                }
+                            }
+                        }
+                    } else {
+                        LazyColumn(
+                            modifier = Modifier
+                                .weight(1f)
+                                .fillMaxWidth(),
+                            state = listState,
+                            reverseLayout = true,
+                            contentPadding = PaddingValues(vertical = 8.dp),
+                        ) {
+                            grouped.forEach { (day, dayMessages) ->
+                                items(dayMessages, key = { it.idHex }) { message ->
+                                    ChannelMessageRow(
+                                        message = message,
+                                        isMine = message.author == currentUser?.publicKey?.toHex(),
+                                        active = activeMessageId == message.idHex,
+                                        onDismissActions = { activeMessageId = null },
+                                        onLongPress = { activeMessageId = message.idHex },
+                                        onReaction = { emoji ->
+                                            viewModel.sendReaction(message, emoji)
+                                            activeMessageId = null
+                                        },
+                                        onEdit = {
+                                            editing = message
+                                            text = message.content
+                                            activeMessageId = null
+                                        },
+                                    )
+                                }
+                                item(key = "day:$day") { DateSeparator(day) }
+                            }
+                        }
+                    }
+
+                    // A Private Channel we hold no key for is listable but not writable, so the input
+                    // is replaced by the reason rather than left to fail on send.
+                    if (channel?.hasKey == false) {
+                        Text(
+                            text = "This channel was not part of the invite, so this device has no key " +
+                                "for it.",
+                            modifier = Modifier.padding(16.dp),
+                            style = MaterialTheme.typography.bodySmall,
+                            color = MaterialTheme.colorScheme.outline,
+                        )
+                    } else {
+                        if (editing != null) {
+                            Row(
+                                modifier = Modifier
+                                    .fillMaxWidth()
+                                    .padding(horizontal = 16.dp, vertical = 4.dp),
+                                verticalAlignment = Alignment.CenterVertically,
+                            ) {
+                                Text(
+                                    text = "Editing a message",
+                                    modifier = Modifier.weight(1f),
+                                    style = MaterialTheme.typography.labelMedium,
+                                    color = MaterialTheme.colorScheme.primary,
+                                )
+                                TextButton(onClick = { editing = null; text = "" }) { Text("Cancel") }
+                            }
+                        }
+
+                        // No attachments and no dictation in v1, so the input shows neither: a button
+                        // that does nothing would be worse than its absence.
+                        ChatInput(
+                            value = text,
+                            onValueChange = { text = it },
+                            onSend = {
+                                val target = editing
+                                if (target == null) viewModel.sendMessage(text)
+                                else viewModel.editMessage(target.idHex, text)
+                                editing = null
+                                text = ""
+                            },
+                        )
+                    }
+                }
+            }
+        },
+    )
+}
+
+@Composable
+private fun ChannelMessageRow(
+    message: ConcordMessage,
+    isMine: Boolean,
+    active: Boolean,
+    onDismissActions: () -> Unit,
+    onLongPress: () -> Unit,
+    onReaction: (String) -> Unit,
+    onEdit: () -> Unit,
+) {
+    val (pubkey, profile) = rememberAuthor(message.author)
+    val name = profile?.name?.sanitizeName()?.takeIf { it.isNotBlank() }
+        ?: pubkey?.short()
+        ?: message.author.take(8)
+
+    Row(
+        modifier = Modifier
+            .fillMaxWidth()
+            .padding(horizontal = 16.dp, vertical = 4.dp)
+    ) {
+        Avatar(picture = profile?.picture, description = name, size = 36.dp)
+        Spacer(modifier = Modifier.size(10.dp))
+        Column(
+            modifier = Modifier
+                .weight(1f)
+                .combinedClickable(
+                    interactionSource = remember { MutableInteractionSource() },
+                    indication = null,
+                    onClick = onDismissActions,
+                    onLongClick = onLongPress,
+                )
+        ) {
+            Row(verticalAlignment = Alignment.CenterVertically) {
+                Text(
+                    text = if (isMine) "$name (you)" else name,
+                    style = MaterialTheme.typography.labelLarge,
+                    color = MaterialTheme.colorScheme.onSurface,
+                    maxLines = 1,
+                    overflow = TextOverflow.Ellipsis,
+                    modifier = Modifier.weight(1f, fill = false),
+                )
+                Spacer(modifier = Modifier.size(6.dp))
+                Text(
+                    text = message.timeLabel(),
+                    style = MaterialTheme.typography.labelSmall,
+                    color = MaterialTheme.colorScheme.outline,
+                )
+                if (message.edited) {
+                    Spacer(modifier = Modifier.size(4.dp))
+                    Text(
+                        text = "(edited)",
+                        style = MaterialTheme.typography.labelSmall,
+                        color = MaterialTheme.colorScheme.outline,
+                    )
+                }
+            }
+            Text(
+                text = message.content,
+                style = MaterialTheme.typography.bodyMedium,
+                color = MaterialTheme.colorScheme.onSurface,
+            )
+            if (message.reactions.isNotEmpty()) {
+                MessageReactions(
+                    emojis = message.reactions.map { it.emoji },
+                    total = message.reactions.sumOf { it.authors.size },
+                    modifier = Modifier.padding(top = 4.dp),
+                )
+            }
+            if (active) {
+                Column(modifier = Modifier.padding(top = 8.dp)) {
+                    ReactionToolbar(onReaction = onReaction)
+                    // Only the author may edit, so only they are offered it.
+                    if (isMine) {
+                        TextButton(onClick = onEdit) { Text("Edit") }
+                    }
+                }
+            }
+        }
+    }
+}
+
+/** The profile behind a message's author, fetched the ordinary Nostr way — see `Avatar`. */
+@Composable
+private fun rememberAuthor(authorHex: String): Pair<PublicKey?, Profile?> {
+    val profileCache = LocalProfileCache.current
+    val pubkey = remember(authorHex) { runCatching { PublicKey.parse(authorHex) }.getOrNull() }
+    val profile by remember(pubkey) {
+        pubkey?.let { profileCache.getMetadata(it) } ?: flowOf<Profile?>(null)
+    }.collectAsStateWithLifecycle(null)
+
+    return pubkey to profile
+}

composeApp/src/androidMain/kotlin/su/reya/coop/screens/communities/CommunitiesScreen.kt

@@ -0,0 +1,185 @@
+package su.reya.coop.screens.communities
+
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.PaddingValues
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.lazy.items
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.ExtendedFloatingActionButton
+import androidx.compose.material3.Icon
+import androidx.compose.material3.IconButton
+import androidx.compose.material3.ListItemDefaults
+import androidx.compose.material3.LoadingIndicator
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.PlainTooltip
+import androidx.compose.material3.Scaffold
+import androidx.compose.material3.SnackbarHost
+import androidx.compose.material3.Text
+import androidx.compose.material3.TooltipAnchorPosition
+import androidx.compose.material3.TooltipBox
+import androidx.compose.material3.TooltipDefaults
+import androidx.compose.material3.TopAppBar
+import androidx.compose.material3.TopAppBarDefaults
+import androidx.compose.material3.rememberTooltipState
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import coop.composeapp.generated.resources.Res
+import coop.composeapp.generated.resources.ic_arrow_back
+import coop.composeapp.generated.resources.ic_plus
+import kotlinx.coroutines.launch
+import org.jetbrains.compose.resources.painterResource
+import su.reya.coop.LocalNavigator
+import su.reya.coop.LocalSnackbarHostState
+import su.reya.coop.Screen
+import su.reya.coop.displayName
+import su.reya.coop.viewmodel.ConcordViewModel
+
+/**
+ * Every Community this device has joined, plus any Direct Invite still waiting.
+ *
+ * There is no file upload, no message search and no per-Community settings page — v1 is the read
+ * path and sending, and the screen says so rather than pretending otherwise.
+ */
+@OptIn(ExperimentalMaterial3Api::class, ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun CommunitiesScreen(concordViewModel: ConcordViewModel) {
+    val navigator = LocalNavigator.current
+    val snackbarHostState = LocalSnackbarHostState.current
+    val scope = rememberCoroutineScope()
+
+    val communities by concordViewModel.communities.collectAsStateWithLifecycle()
+    val directInvites by concordViewModel.directInvites.collectAsStateWithLifecycle()
+    val isReady by concordViewModel.isReady.collectAsStateWithLifecycle()
+
+    val sorted = remember(communities) { communities.sortedBy { it.displayName().lowercase() } }
+
+    Scaffold(
+        containerColor = MaterialTheme.colorScheme.surfaceContainer,
+        snackbarHost = { SnackbarHost(snackbarHostState) },
+        topBar = {
+            TopAppBar(
+                title = {
+                    Text(
+                        text = "Communities",
+                        style = MaterialTheme.typography.titleMediumEmphasized
+                    )
+                },
+                colors = TopAppBarDefaults.topAppBarColors(
+                    containerColor = MaterialTheme.colorScheme.surfaceContainer,
+                ),
+                navigationIcon = {
+                    IconButton(onClick = { navigator.goBack() }) {
+                        Icon(
+                            painter = painterResource(Res.drawable.ic_arrow_back),
+                            contentDescription = "Back"
+                        )
+                    }
+                },
+            )
+        },
+        floatingActionButton = {
+            TooltipBox(
+                positionProvider = TooltipDefaults.rememberTooltipPositionProvider(
+                    TooltipAnchorPosition.Above,
+                    spacingBetweenTooltipAndAnchor = 8.dp,
+                ),
+                tooltip = {
+                    PlainTooltip { Text("Join a community") }
+                },
+                state = rememberTooltipState(),
+            ) {
+                ExtendedFloatingActionButton(
+                    onClick = { navigator.navigate(Screen.JoinCommunity()) },
+                    expanded = false,
+                    icon = {
+                        Icon(
+                            painter = painterResource(Res.drawable.ic_plus),
+                            contentDescription = "Join a community"
+                        )
+                    },
+                    text = { Text("Join") },
+                )
+            }
+        },
+        content = { innerPadding ->
+            when {
+                !isReady && sorted.isEmpty() -> {
+                    Box(
+                        modifier = Modifier
+                            .fillMaxSize()
+                            .padding(innerPadding),
+                        contentAlignment = Alignment.Center
+                    ) {
+                        LoadingIndicator()
+                    }
+                }
+
+                sorted.isEmpty() && directInvites.isEmpty() -> {
+                    Box(
+                        modifier = Modifier
+                            .fillMaxSize()
+                            .padding(innerPadding)
+                    ) {
+                        CommunityEmptyState(
+                            title = "No communities yet",
+                            subtitle = "Join one with an invite link.",
+                        )
+                    }
+                }
+
+                else -> {
+                    LazyColumn(
+                        modifier = Modifier
+                            .fillMaxSize()
+                            .padding(top = innerPadding.calculateTopPadding()),
+                        contentPadding = PaddingValues(16.dp),
+                        verticalArrangement = Arrangement.spacedBy(ListItemDefaults.SegmentedGap),
+                    ) {
+                        items(directInvites, key = { it.communityId }) { invite ->
+                            DirectInviteCard(
+                                invite = invite,
+                                onJoin = {
+                                    scope.launch {
+                                        // A Direct Invite already carries its bundle, so there is
+                                        // nothing to fetch — only to validate and accept.
+                                        val preview = concordViewModel.previewDirectInvite(invite)
+                                        val joined = concordViewModel.join(preview)
+                                        if (joined != null) {
+                                            navigator.navigate(
+                                                Screen.Community(joined.membership.communityId)
+                                            )
+                                        }
+                                    }
+                                },
+                                onDismiss = {
+                                    concordViewModel.dismissDirectInvite(invite.communityId)
+                                },
+                            )
+                        }
+
+                        items(sorted, key = { it.membership.communityId }) { state ->
+                            CommunityRow(
+                                state = state,
+                                onClick = {
+                                    navigator.navigate(
+                                        Screen.Community(state.membership.communityId)
+                                    )
+                                },
+                            )
+                        }
+                    }
+                }
+            }
+        },
+    )
+}

composeApp/src/androidMain/kotlin/su/reya/coop/screens/communities/CommunityComponents.kt

@@ -0,0 +1,254 @@
+package su.reya.coop.screens.communities
+
+import androidx.compose.foundation.clickable
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material3.Badge
+import androidx.compose.material3.BadgedBox
+import androidx.compose.material3.Button
+import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.Icon
+import androidx.compose.material3.ListItem
+import androidx.compose.material3.ListItemDefaults
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.SegmentedListItem
+import androidx.compose.material3.Surface
+import androidx.compose.material3.Text
+import androidx.compose.material3.TextButton
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.style.TextOverflow
+import androidx.compose.ui.unit.dp
+import coop.composeapp.generated.resources.Res
+import coop.composeapp.generated.resources.ic_communities
+import coop.composeapp.generated.resources.ic_lock
+import org.jetbrains.compose.resources.painterResource
+import su.reya.coop.concord.CommunityInvite
+import su.reya.coop.concord.CommunityState
+import su.reya.coop.concord.ConcordChannel
+import su.reya.coop.displayName
+import su.reya.coop.shared.Avatar
+import su.reya.coop.unreadTotal
+
+/**
+ * Rows and small blocks shared by the Communities screens, mirroring `HomeScreen`'s `ChatRoom` and
+ * `ContactListItem`: `ListItem` for the top level, `SegmentedListItem` inside a group.
+ */
+
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun CommunityRow(state: CommunityState, onClick: () -> Unit) {
+    val unread = state.unreadTotal()
+
+    ListItem(
+        modifier = Modifier.clickable(onClick = onClick),
+        leadingContent = {
+            BadgedBox(
+                badge = {
+                    if (unread > 0) {
+                        Badge { Text(unread.toString()) }
+                    }
+                }
+            ) {
+                // Concord icons are encrypted blobs v1 never fetches, so this is always the placeholder.
+                Avatar(picture = null, description = state.displayName())
+            }
+        },
+        headlineContent = {
+            Text(
+                text = state.displayName(),
+                style = MaterialTheme.typography.titleMediumEmphasized.copy(
+                    fontWeight = if (unread > 0) FontWeight.SemiBold else FontWeight.Normal
+                ),
+                maxLines = 1,
+                overflow = TextOverflow.Ellipsis,
+            )
+        },
+        supportingContent = {
+            Text(
+                text = channelCountLabel(state.channels.size),
+                style = MaterialTheme.typography.bodyMedium,
+                color = MaterialTheme.colorScheme.outline,
+                maxLines = 1,
+                overflow = TextOverflow.Ellipsis,
+            )
+        },
+        colors = ListItemDefaults.colors(containerColor = MaterialTheme.colorScheme.surface)
+    )
+}
+
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun ChannelRow(channel: ConcordChannel, index: Int, total: Int, onClick: () -> Unit) {
+    val unread = channel.unreadCount
+
+    SegmentedListItem(
+        onClick = onClick,
+        shapes = ListItemDefaults.segmentedShapes(index = index, count = total),
+        leadingContent = {
+            BadgedBox(
+                badge = {
+                    if (unread > 0) {
+                        Badge { Text(unread.toString()) }
+                    }
+                }
+            ) {
+                Box(modifier = Modifier.size(24.dp), contentAlignment = Alignment.Center) {
+                    if (channel.private) {
+                        Icon(
+                            painter = painterResource(Res.drawable.ic_lock),
+                            contentDescription = "Private",
+                            modifier = Modifier.size(20.dp),
+                            tint = MaterialTheme.colorScheme.outline,
+                        )
+                    } else {
+                        Text(
+                            text = "#",
+                            style = MaterialTheme.typography.titleMedium,
+                            color = MaterialTheme.colorScheme.outline,
+                        )
+                    }
+                }
+            }
+        },
+        content = {
+            Row(verticalAlignment = Alignment.CenterVertically) {
+                Text(
+                    text = channel.name,
+                    modifier = Modifier.weight(1f),
+                    style = MaterialTheme.typography.bodyLarge.copy(
+                        fontWeight = if (unread > 0) FontWeight.SemiBold else FontWeight.Normal
+                    ),
+                    maxLines = 1,
+                    overflow = TextOverflow.Ellipsis,
+                )
+                // A Private Channel with no key is listable but unreadable: the row says so rather
+                // than opening an empty room and looking broken.
+                if (!channel.hasKey) {
+                    Text(
+                        text = "No key",
+                        style = MaterialTheme.typography.labelSmall,
+                        color = MaterialTheme.colorScheme.outline,
+                    )
+                }
+            }
+        }
+    )
+}
+
+/**
+ * A Direct Invite (CORD-05 §6) waiting to be accepted. It arrived giftwrapped to this npub, so it is
+ * shown on the Communities list rather than in any chat.
+ */
+@Composable
+fun DirectInviteCard(invite: CommunityInvite, onJoin: () -> Unit, onDismiss: () -> Unit) {
+    Surface(
+        modifier = Modifier.fillMaxWidth(),
+        shape = RoundedCornerShape(16.dp),
+        color = MaterialTheme.colorScheme.secondaryContainer,
+        contentColor = MaterialTheme.colorScheme.onSecondaryContainer,
+    ) {
+        Column(modifier = Modifier.padding(16.dp)) {
+            Text(
+                text = "Invited to a community",
+                style = MaterialTheme.typography.labelMedium,
+            )
+            Spacer(modifier = Modifier.size(4.dp))
+            Text(
+                text = invite.name ?: "Untitled community",
+                style = MaterialTheme.typography.titleMediumEmphasized,
+                maxLines = 1,
+                overflow = TextOverflow.Ellipsis,
+            )
+            Spacer(modifier = Modifier.size(2.dp))
+            Text(
+                text = shortCommunityId(invite.communityId),
+                style = MaterialTheme.typography.bodySmall,
+            )
+            Spacer(modifier = Modifier.size(8.dp))
+            Row(horizontalArrangement = Arrangement.spacedBy(8.dp)) {
+                TextButton(onClick = onDismiss) { Text("Dismiss") }
+                Button(onClick = onJoin) { Text("Join") }
+            }
+        }
+    }
+}
+
+/** The app's empty-state convention: two centred lines, with the feature's mark above them. */
+@Composable
+fun CommunityEmptyState(title: String, subtitle: String) {
+    Box(modifier = Modifier.fillMaxSize(), contentAlignment = Alignment.Center) {
+        Column(
+            horizontalAlignment = Alignment.CenterHorizontally,
+            verticalArrangement = Arrangement.spacedBy(4.dp),
+        ) {
+            Icon(
+                painter = painterResource(Res.drawable.ic_communities),
+                contentDescription = null,
+                modifier = Modifier.size(56.dp),
+                tint = MaterialTheme.colorScheme.outlineVariant,
+            )
+            Spacer(modifier = Modifier.size(8.dp))
+            Text(
+                text = title,
+                style = MaterialTheme.typography.titleLargeEmphasized.copy(
+                    fontWeight = FontWeight.SemiBold
+                ),
+                color = MaterialTheme.colorScheme.onSurface,
+            )
+            Text(
+                text = subtitle,
+                style = MaterialTheme.typography.bodyMedium,
+                color = MaterialTheme.colorScheme.outline,
+            )
+        }
+    }
+}
+
+/**
+ * Says out loud what v1 does not do yet. Concord's authority is a signed roster every client folds
+ * (CORD-04), and this build folds none of it — a stale claim of moderation would be worse than none.
+ */
+@Composable
+fun BetaNotice() {
+    Surface(
+        modifier = Modifier.fillMaxWidth(),
+        shape = RoundedCornerShape(16.dp),
+        color = MaterialTheme.colorScheme.surfaceVariant,
+        contentColor = MaterialTheme.colorScheme.onSurfaceVariant,
+    ) {
+        Row(
+            modifier = Modifier.padding(16.dp),
+            verticalAlignment = Alignment.CenterVertically,
+        ) {
+            Icon(
+                painter = painterResource(Res.drawable.ic_lock),
+                contentDescription = null,
+                modifier = Modifier.size(20.dp),
+            )
+            Spacer(modifier = Modifier.size(12.dp))
+            Text(
+                text = "Beta. Messages, channels and invites work. Roles, kicks and bans are not " +
+                    "enforced by this build yet — only by the other clients in the community.",
+                style = MaterialTheme.typography.bodySmall,
+            )
+        }
+    }
+}
+
+private fun channelCountLabel(count: Int): String =
+    if (count == 1) "1 channel" else "$count channels"
+
+/** A Community's identity is a hash, so show enough of it to tell two apart. */
+internal fun shortCommunityId(communityId: String): String =
+    if (communityId.length <= 16) communityId else communityId.take(16) + "..."

composeApp/src/androidMain/kotlin/su/reya/coop/screens/communities/CommunityScreen.kt

@@ -0,0 +1,171 @@
+package su.reya.coop.screens.communities
+
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.PaddingValues
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.lazy.itemsIndexed
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.Icon
+import androidx.compose.material3.IconButton
+import androidx.compose.material3.ListItemDefaults
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Scaffold
+import androidx.compose.material3.SnackbarHost
+import androidx.compose.material3.Text
+import androidx.compose.material3.TopAppBar
+import androidx.compose.material3.TopAppBarDefaults
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.remember
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.style.TextOverflow
+import androidx.compose.ui.unit.dp
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import coop.composeapp.generated.resources.Res
+import coop.composeapp.generated.resources.ic_arrow_back
+import org.jetbrains.compose.resources.painterResource
+import su.reya.coop.LocalNavigator
+import su.reya.coop.LocalSnackbarHostState
+import su.reya.coop.Screen
+import su.reya.coop.displayName
+import su.reya.coop.viewmodel.ConcordViewModel
+
+/**
+ * One Community's Channels, split public and private the way the protocol splits them: a Public
+ * Channel's key derives from `community_root`, so anyone in the Community can read it, while a
+ * Private one is only listable to those an invite or a rekey handed a key to (CORD-03 §1).
+ */
+@OptIn(ExperimentalMaterial3Api::class, ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun CommunityScreen(communityId: String, concordViewModel: ConcordViewModel) {
+    val navigator = LocalNavigator.current
+    val snackbarHostState = LocalSnackbarHostState.current
+
+    val communities by concordViewModel.communities.collectAsStateWithLifecycle()
+    val state = remember(communities, communityId) {
+        communities.firstOrNull { it.membership.communityId == communityId }
+    }
+
+    if (state == null) {
+        // Reachable if the Community was dropped underneath this screen (a logout while it was open),
+        // so it keeps a way back rather than dead-ending here.
+        Scaffold(
+            containerColor = MaterialTheme.colorScheme.surfaceContainer,
+            topBar = {
+                TopAppBar(
+                    title = {
+                        Text(
+                            text = "Community",
+                            style = MaterialTheme.typography.titleMediumEmphasized
+                        )
+                    },
+                    colors = TopAppBarDefaults.topAppBarColors(
+                        containerColor = MaterialTheme.colorScheme.surfaceContainer,
+                    ),
+                    navigationIcon = {
+                        IconButton(onClick = { navigator.goBack() }) {
+                            Icon(
+                                painter = painterResource(Res.drawable.ic_arrow_back),
+                                contentDescription = "Back"
+                            )
+                        }
+                    },
+                )
+            },
+        ) { innerPadding ->
+            Box(
+                modifier = Modifier
+                    .fillMaxSize()
+                    .padding(innerPadding),
+                contentAlignment = Alignment.Center
+            ) {
+                Text(
+                    text = "This community is not on this device.",
+                    style = MaterialTheme.typography.titleMediumEmphasized,
+                    color = MaterialTheme.colorScheme.onSurface,
+                )
+            }
+        }
+        return
+    }
+
+    val (privateChannels, publicChannels) = remember(state.channels) {
+        state.channels.partition { it.private }
+    }
+
+    Scaffold(
+        containerColor = MaterialTheme.colorScheme.surfaceContainer,
+        snackbarHost = { SnackbarHost(snackbarHostState) },
+        topBar = {
+            TopAppBar(
+                title = {
+                    Text(
+                        text = state.displayName(),
+                        style = MaterialTheme.typography.titleMediumEmphasized,
+                        maxLines = 1,
+                        overflow = TextOverflow.Ellipsis,
+                    )
+                },
+                colors = TopAppBarDefaults.topAppBarColors(
+                    containerColor = MaterialTheme.colorScheme.surfaceContainer,
+                ),
+                navigationIcon = {
+                    IconButton(onClick = { navigator.goBack() }) {
+                        Icon(
+                            painter = painterResource(Res.drawable.ic_arrow_back),
+                            contentDescription = "Back"
+                        )
+                    }
+                },
+            )
+        },
+        content = { innerPadding ->
+            LazyColumn(
+                modifier = Modifier
+                    .fillMaxSize()
+                    .padding(top = innerPadding.calculateTopPadding()),
+                contentPadding = PaddingValues(16.dp),
+                verticalArrangement = Arrangement.spacedBy(ListItemDefaults.SegmentedGap),
+            ) {
+                item { BetaNotice() }
+
+                itemsIndexed(publicChannels) { index, channel ->
+                    ChannelRow(
+                        channel = channel,
+                        index = index,
+                        total = publicChannels.size,
+                        onClick = {
+                            navigator.navigate(Screen.Channel(communityId, channel.idHex))
+                        },
+                    )
+                }
+
+                itemsIndexed(privateChannels) { index, channel ->
+                    ChannelRow(
+                        channel = channel,
+                        index = index,
+                        total = privateChannels.size,
+                        onClick = {
+                            navigator.navigate(Screen.Channel(communityId, channel.idHex))
+                        },
+                    )
+                }
+
+                if (state.channels.isEmpty()) {
+                    item {
+                        Text(
+                            text = "No channels yet. They arrive with the community's metadata.",
+                            style = MaterialTheme.typography.bodyMedium,
+                            color = MaterialTheme.colorScheme.outline,
+                        )
+                    }
+                }
+            }
+        },
+    )
+}

composeApp/src/androidMain/kotlin/su/reya/coop/screens/communities/JoinCommunityScreen.kt

@@ -0,0 +1,237 @@
+package su.reya.coop.screens.communities
+
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.foundation.verticalScroll
+import androidx.compose.material3.Button
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.Icon
+import androidx.compose.material3.IconButton
+import androidx.compose.material3.LoadingIndicator
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.OutlinedTextField
+import androidx.compose.material3.Scaffold
+import androidx.compose.material3.SnackbarHost
+import androidx.compose.material3.Surface
+import androidx.compose.material3.Text
+import androidx.compose.material3.TopAppBar
+import androidx.compose.material3.TopAppBarDefaults
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import coop.composeapp.generated.resources.Res
+import coop.composeapp.generated.resources.ic_arrow_back
+import coop.composeapp.generated.resources.ic_scanner
+import kotlinx.coroutines.launch
+import org.jetbrains.compose.resources.painterResource
+import su.reya.coop.LocalNavigator
+import su.reya.coop.LocalScanResult
+import su.reya.coop.LocalSnackbarHostState
+import su.reya.coop.Screen
+import su.reya.coop.concord.InvitePreview
+import su.reya.coop.viewmodel.ConcordViewModel
+
+/**
+ * Paste or scan an invite link, see what joining would mean, then join.
+ *
+ * The preview is deliberately a separate step: a link is fetched and decrypted, but nothing is
+ * subscribed and no presence is announced until the user accepts (CORD-05 §1), so backing out here
+ * leaves no trace on any relay.
+ */
+@OptIn(ExperimentalMaterial3Api::class, ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun JoinCommunityScreen(concordViewModel: ConcordViewModel, initialLink: String? = null) {
+    val navigator = LocalNavigator.current
+    val qrScanResult = LocalScanResult.current
+    val snackbarHostState = LocalSnackbarHostState.current
+    val scope = rememberCoroutineScope()
+
+    var link by remember(initialLink) { mutableStateOf(initialLink.orEmpty()) }
+    var preview by remember { mutableStateOf<InvitePreview?>(null) }
+    var isBusy by remember { mutableStateOf(false) }
+
+    LaunchedEffect(qrScanResult.content) {
+        qrScanResult.content?.let { scanned ->
+            link = scanned
+            preview = null
+            qrScanResult.clear()
+        }
+    }
+
+    Scaffold(
+        containerColor = MaterialTheme.colorScheme.surfaceContainer,
+        snackbarHost = { SnackbarHost(snackbarHostState) },
+        topBar = {
+            TopAppBar(
+                title = {
+                    Text(
+                        text = "Join a community",
+                        style = MaterialTheme.typography.titleMediumEmphasized
+                    )
+                },
+                colors = TopAppBarDefaults.topAppBarColors(
+                    containerColor = MaterialTheme.colorScheme.surfaceContainer,
+                ),
+                navigationIcon = {
+                    IconButton(onClick = { navigator.goBack() }) {
+                        Icon(
+                            painter = painterResource(Res.drawable.ic_arrow_back),
+                            contentDescription = "Back"
+                        )
+                    }
+                },
+                actions = {
+                    IconButton(onClick = { navigator.navigate(Screen.Scan) }) {
+                        Icon(
+                            painter = painterResource(Res.drawable.ic_scanner),
+                            contentDescription = "Scanner"
+                        )
+                    }
+                },
+            )
+        },
+        content = { innerPadding ->
+            Column(
+                modifier = Modifier
+                    .fillMaxSize()
+                    .padding(innerPadding)
+                    .verticalScroll(rememberScrollState())
+                    .padding(16.dp),
+                verticalArrangement = Arrangement.spacedBy(16.dp),
+            ) {
+                OutlinedTextField(
+                    value = link,
+                    onValueChange = {
+                        link = it
+                        // Any edit invalidates what was fetched for the old text.
+                        preview = null
+                    },
+                    modifier = Modifier.fillMaxWidth(),
+                    label = { Text("Invite link") },
+                    placeholder = { Text("https://.../invite/...") },
+                    maxLines = 4,
+                )
+
+                Button(
+                    onClick = {
+                        scope.launch {
+                            isBusy = true
+                            preview = concordViewModel.previewInvite(link)
+                            isBusy = false
+                        }
+                    },
+                    enabled = link.isNotBlank() && !isBusy,
+                    modifier = Modifier.fillMaxWidth(),
+                ) {
+                    Text("Look it up")
+                }
+
+                preview?.let { shown ->
+                    InvitePreviewCard(
+                        preview = shown,
+                        isBusy = isBusy,
+                        onJoin = {
+                            scope.launch {
+                                isBusy = true
+                                val joined = concordViewModel.join(shown)
+                                isBusy = false
+                                if (joined != null) {
+                                    // Replace this screen with the Community it just joined.
+                                    navigator.goBack()
+                                    navigator.navigate(
+                                        Screen.Community(joined.membership.communityId)
+                                    )
+                                }
+                            }
+                        },
+                    )
+                }
+            }
+        },
+    )
+}
+
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+private fun InvitePreviewCard(preview: InvitePreview, isBusy: Boolean, onJoin: () -> Unit) {
+    val invite = preview.invite
+
+    Surface(
+        modifier = Modifier.fillMaxWidth(),
+        shape = RoundedCornerShape(16.dp),
+        color = MaterialTheme.colorScheme.surfaceVariant,
+        contentColor = MaterialTheme.colorScheme.onSurfaceVariant,
+    ) {
+        Column(
+            modifier = Modifier.padding(16.dp),
+            verticalArrangement = Arrangement.spacedBy(4.dp),
+        ) {
+            Text(
+                text = invite.name ?: "Untitled community",
+                style = MaterialTheme.typography.titleMediumEmphasized,
+            )
+            Text(
+                text = shortCommunityId(invite.communityId),
+                style = MaterialTheme.typography.bodySmall,
+            )
+            Spacer(modifier = Modifier.size(4.dp))
+            Text(
+                text = plural(invite.channels.size, "channel"),
+                style = MaterialTheme.typography.bodyMedium,
+            )
+            Text(
+                text = plural(preview.relays.size, "relay"),
+                style = MaterialTheme.typography.bodyMedium,
+            )
+
+            // Everything that would stop this invite from being accepted, in plain English.
+            preview.problems.forEach { problem -> Refusal(problem) }
+            if (preview.expired) Refusal("This invite has expired.")
+            if (preview.alreadyJoined) {
+                Text(
+                    text = "You are already in this community.",
+                    style = MaterialTheme.typography.bodySmall,
+                )
+            }
+
+            Spacer(modifier = Modifier.size(8.dp))
+            Button(
+                onClick = onJoin,
+                enabled = preview.joinable && !isBusy,
+                modifier = Modifier.fillMaxWidth(),
+            ) {
+                if (isBusy) {
+                    LoadingIndicator(modifier = Modifier.size(20.dp))
+                } else {
+                    Text(if (preview.alreadyJoined) "Rejoin" else "Join")
+                }
+            }
+        }
+    }
+}
+
+@Composable
+private fun Refusal(message: String) {
+    Text(
+        text = message,
+        style = MaterialTheme.typography.bodySmall,
+        color = MaterialTheme.colorScheme.error,
+    )
+}
+
+private fun plural(count: Int, noun: String): String =
+    if (count == 1) "1 $noun" else "$count ${noun}s"

shared/src/commonMain/kotlin/su/reya/coop/Community.kt

@@ -0,0 +1,46 @@
+package su.reya.coop
+
+import kotlinx.datetime.DateTimeUnit
+import kotlinx.datetime.TimeZone
+import kotlinx.datetime.minus
+import kotlinx.datetime.number
+import kotlinx.datetime.toLocalDateTime
+import su.reya.coop.concord.CommunityState
+import su.reya.coop.concord.ConcordMessage
+import kotlin.time.Clock
+
+/** Control metadata first, then the snapshot the invite carried, then a last-resort label. */
+fun CommunityState.displayName(): String {
+    val name = meta?.name?.sanitizeName()?.takeIf { it.isNotBlank() }
+        ?: membership.name?.sanitizeName()?.takeIf { it.isNotBlank() }
+    return name ?: "Untitled community"
+}
+
+/** Every Channel's badge added up, for the Community row. */
+fun CommunityState.unreadTotal(): Int = channels.sumOf { it.unreadCount }
+
+/** `HH:mm`, local time. */
+fun ConcordMessage.timeLabel(): String {
+    val time = createdAt.toLocalDateTime(TimeZone.currentSystemDefault())
+    val hour = time.hour.toString().padStart(2, '0')
+    val minute = time.minute.toString().padStart(2, '0')
+    return "$hour:$minute"
+}
+
+/** `Today` / `Yesterday` / `DD/MM/YY`, matching [formatAsGroup] for DMs. */
+fun ConcordMessage.dayLabel(): String {
+    val zone = TimeZone.currentSystemDefault()
+    val date = createdAt.toLocalDateTime(zone).date
+    val today = Clock.System.now().toLocalDateTime(zone).date
+
+    return when (date) {
+        today -> "Today"
+        today.minus(1, DateTimeUnit.DAY) -> "Yesterday"
+        else -> {
+            val day = date.day.toString().padStart(2, '0')
+            val month = date.month.number.toString().padStart(2, '0')
+            val year = date.year.toString().takeLast(2)
+            "$day/$month/$year"
+        }
+    }
+}

shared/src/commonMain/kotlin/su/reya/coop/concord/ConcordCommunityList.kt

@@ -0,0 +1,129 @@
+package su.reya.coop.concord
+
+import kotlin.io.encoding.Base64
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * §8's 32-byte values are unpadded base64url rather than hex — the Community List is the one
+ * encrypted document CORD-01 lets choose its own encoding. Padding is tolerated on read.
+ */
+private val listBase64 = Base64.UrlSafe.withPadding(Base64.PaddingOption.ABSENT_OPTIONAL)
+
+/** One fragment's plaintext. `frags` and `seed` are read by writers, not here. */
+@Serializable
+internal data class CommunityListFragment(
+    val entries: List<CommunityListEntry> = emptyList(),
+    val tombstones: List<CommunityListTombstone> = emptyList(),
+)
+
+@Serializable
+internal data class CommunityListEntry(
+    /** base64url, and case-significant: decoded, never case-folded. */
+    @SerialName("community_id") val communityId: String = "",
+    /** The freshest snapshot. `seed`, the backfill anchor, is not read here. */
+    val current: CommunityListSnapshot? = null,
+    /** Unix ms. Tiebreaks against the tombstone and between two entries. */
+    @SerialName("added_at") val addedAt: Long = 0L,
+)
+
+@Serializable
+internal data class CommunityListTombstone(
+    @SerialName("community_id") val communityId: String = "",
+    @SerialName("removed_at") val removedAt: Long = 0L,
+)
+
+/**
+ * The invite bundle's *membership* subset (CORD-05), in §8's base64url: never the icon, never the
+ * link fields.
+ */
+@Serializable
+internal data class CommunityListSnapshot(
+    val owner: String = "",
+    @SerialName("owner_salt") val ownerSalt: String = "",
+    @SerialName("community_root") val communityRoot: String = "",
+    @SerialName("root_epoch") val rootEpoch: ULong = 0u,
+    @SerialName("control_pk") val controlPk: String? = null,
+    val channels: List<CommunityListChannel> = emptyList(),
+    val relays: List<String> = emptyList(),
+    val name: String? = null,
+)
+
+@Serializable
+internal data class CommunityListChannel(
+    val id: String = "",
+    val key: String = "",
+    val epoch: ULong = 0u,
+    val name: String? = null,
+)
+
+/**
+ * Unions fragments into one membership per Community (CORD-02 §8).
+ *
+ * Only the merges a *reader* needs: the newest entry per Community wins, and a Community whose
+ * tombstone is at least as new as its entry reads as left. `seed`, the canonical-bytes tiebreak and
+ * repacks exist to keep two writers byte-identical — a client that never republishes cannot flap.
+ */
+internal fun List<CommunityListFragment>.listedMemberships(): List<Membership> {
+    val newest = mutableMapOf<String, Pair<Long, Membership>>()
+
+    for (entry in flatMap { it.entries }) {
+        val membership = entry.toMembership() ?: continue
+        val held = newest[membership.communityId]
+        if (held == null || entry.addedAt > held.first) {
+            newest[membership.communityId] = entry.addedAt to membership
+        }
+    }
+
+    val removed = mutableMapOf<String, Long>()
+    for (tombstone in flatMap { it.tombstones }) {
+        val id = tombstone.communityId.listId() ?: continue
+        removed[id] = maxOf(removed[id] ?: 0L, tombstone.removedAt)
+    }
+
+    return newest
+        .filter { (id, entry) -> (removed[id] ?: 0L) < entry.first }
+        .values.map { it.second }
+}
+
+/** One entry as a [Membership], reusing the invite bundle's own self-certification. */
+private fun CommunityListEntry.toMembership(): Membership? {
+    val id = communityId.listId() ?: return null
+    val invite = (current ?: return null).toInvite(id) ?: return null
+    if (invite.problems().isNotEmpty()) return null
+    return invite.toMembership(invite.relaySet())
+}
+
+/** A snapshot normalised to the hex [CommunityInvite] the invite path already validates. */
+private fun CommunityListSnapshot.toInvite(communityIdHex: String): CommunityInvite? {
+    val ownerHex = owner.listId() ?: return null
+    val saltHex = ownerSalt.listId() ?: return null
+    val rootHex = communityRoot.listId() ?: return null
+
+    // Strict: a malformed control key is a malformed entry, not a key to quietly drop null. Dropping
+    // it to null would leave the Control Plane unaddressable with nothing to show for it.
+    val controlHex = controlPk?.let { it.listId() ?: return null }
+
+    // Lenient by contrast: one unreadable Channel key costs that Channel, not the membership.
+    val granted = channels.mapNotNull { channel ->
+        val id = channel.id.listId() ?: return@mapNotNull null
+        val key = channel.key.listId() ?: return@mapNotNull null
+        InviteChannel(id = id, key = key, epoch = channel.epoch, name = channel.name)
+    }
+
+    return CommunityInvite(
+        communityId = communityIdHex,
+        owner = ownerHex,
+        ownerSalt = saltHex,
+        communityRoot = rootHex,
+        rootEpoch = rootEpoch,
+        controlPk = controlHex,
+        channels = granted,
+        relays = relays,
+        name = name,
+    )
+}
+
+/** §8's unpadded base64url for a 32-byte value, or null when it is not one. */
+private fun String.listId(): String? =
+    runCatching { listBase64.decode(this) }.getOrNull()?.takeIf { it.size == 32 }?.toHex()

shared/src/commonMain/kotlin/su/reya/coop/concord/ConcordControl.kt

@@ -0,0 +1,92 @@
+package su.reya.coop.concord
+
+import kotlinx.serialization.decodeFromString
+import rust.nostr.sdk.UnsignedEvent
+import kotlin.time.Instant
+
+object ConcordControl {
+    fun editionOf(rumor: UnsignedEvent): ControlEdition? {
+        if (rumor.kind().asU16() != ConcordKind.CONTROL_EDITION.toUShort()) return null
+
+        val tags = rumor.tags().toVec()
+        val vsk = tags.firstOrNull { it.kind() == ConcordTag.VSK }?.content()?.toIntOrNull() ?: return null
+        if (vsk == ConcordVsk.DISSOLVED) return null
+
+        val eidHex = tags.firstOrNull { it.kind() == ConcordTag.EID }?.content() ?: return null
+        if (eidHex.hex32() == null) return null
+
+        val version = tags.firstOrNull { it.kind() == ConcordTag.EV }?.content()?.toULongOrNull() ?: return null
+        if (version == 0uL) return null
+
+        val prev = tags.firstOrNull { it.kind() == ConcordTag.EP }?.content()
+        if (prev != null && prev.hex32() == null) return null
+
+        return ControlEdition(
+            vsk = vsk,
+            eidHex = eidHex.lowercase(),
+            version = version,
+            prevHashHex = prev?.lowercase(),
+            content = rumor.content(),
+            actorHex = rumor.author().toHex(),
+            rumorIdHex = rumor.id()?.toHex() ?: return null,
+            createdAt = Instant.fromEpochSeconds(rumor.createdAt().asSecs().toLong()),
+        )
+    }
+
+    fun fold(editions: List<ControlEdition>, communityIdHex: String): ControlFold {
+        var meta: CommunityMeta? = null
+        val channels = mutableMapOf<String, ChannelMeta>()
+
+        editions.groupBy { it.vsk to it.eidHex }.forEach { (entity, group) ->
+            val (vsk, eidHex) = entity
+            if (vsk != ConcordVsk.METADATA && vsk != ConcordVsk.CHANNEL) return@forEach
+            // The metadata entity *is* the Community, so its coordinate must be the community_id;
+            // an edition naming anything else is not this Community's metadata.
+            if (vsk == ConcordVsk.METADATA && eidHex != communityIdHex) return@forEach
+
+            val winner = winner(group) ?: return@forEach
+            when (vsk) {
+                ConcordVsk.METADATA -> meta = decode(winner.content)
+                else -> decode<ChannelMeta>(winner.content)?.let { channels[eidHex] = it }
+            }
+        }
+
+        return ControlFold(meta = meta, channels = channels)
+    }
+
+    private fun winner(group: List<ControlEdition>): ControlEdition? {
+        val byVersion = group
+            .groupBy { it.version }
+            .mapValues { (_, candidates) -> candidates.minByOrNull { it.rumorIdHex } ?: return null }
+
+        val eid = byVersion.values.firstOrNull()?.eidHex?.hex32() ?: return null
+
+        for (top in byVersion.keys.sortedDescending()) {
+            var prev: ByteArray? = null
+            var candidate: ControlEdition? = null
+            var intact = true
+
+            for (version in 1uL..top) {
+                val edition = byVersion[version]
+                if (edition == null) {
+                    intact = false
+                    break
+                }
+                // The first edition has no predecessor; every later one must chain to the last.
+                val expected = if (version == 1uL) null else prev?.toHex()
+                if (edition.prevHashHex != expected) {
+                    intact = false
+                    break
+                }
+                prev = editionHash(eid, version, prev, edition.content.encodeToByteArray())
+                candidate = edition
+            }
+
+            if (intact) return candidate
+        }
+        return null
+    }
+
+    private inline fun <reified T> decode(content: String): T? =
+        runCatching { concordJson.decodeFromString<T>(content) }.getOrNull()
+}

shared/src/commonMain/kotlin/su/reya/coop/concord/ConcordCrypto.kt

@@ -0,0 +1,157 @@
+package su.reya.coop.concord
+
+import okio.Buffer
+import okio.ByteString
+import okio.ByteString.Companion.decodeHex
+import okio.ByteString.Companion.toByteString
+import rust.nostr.sdk.Keys
+import rust.nostr.sdk.PublicKey
+import rust.nostr.sdk.SecretKey
+
+fun hkdfSha256(
+    ikm: ByteArray,
+    info: ByteArray,
+    salt: ByteArray = ByteArray(0),
+    length: Int = 32,
+): ByteArray {
+    require(length in 1..255 * 32) { "HKDF output length must be 1..8160, was $length" }
+
+    val saltKey = if (salt.isEmpty()) ByteArray(32).toByteString() else salt.toByteString()
+    val prk = ikm.toByteString().hmacSha256(saltKey)
+
+    val out = Buffer()
+    var t = ByteString.EMPTY
+    var counter = 1
+
+    while (out.size < length) {
+        t = Buffer()
+            .write(t)
+            .write(info)
+            .writeByte(counter)
+            .readByteString()
+            .hmacSha256(prk)
+        out.write(t)
+        counter++
+    }
+
+    return out.readByteArray(length.toLong())
+}
+
+fun hkdfInfo(label: String, id: ByteArray, epoch: ULong? = null): ByteArray {
+    require(id.size == 32) { "Concord HKDF id must be 32 bytes, was ${id.size}" }
+    return Buffer().apply {
+        writeUtf8(label)
+        writeByte(0)
+        write(id)
+        if (epoch != null) writeLong(epoch.toLong())
+    }.readByteArray()
+}
+
+/** A plane's derived keypair: `(sk, xonly(sk))` from CORD-02 A.2 `group_key`. */
+data class GroupKey(val secretKey: SecretKey, val publicKey: PublicKey)
+
+fun groupSeed(
+    label: String,
+    secret: ByteArray,
+    id: ByteArray,
+    epoch: ULong? = null,
+    isValid: (ByteArray) -> Boolean = ::isValidScalar,
+): ByteArray {
+    val base = hkdfInfo(label, id, epoch)
+    var counter = -1
+
+    while (counter <= 255) {
+        val info = if (counter < 0) base else base + byteArrayOf(counter.toByte())
+        val seed = hkdfSha256(secret, info)
+        if (isValid(seed)) return seed
+        counter++
+    }
+
+    error("Concord group_key: scalar_normalize exhausted for label $label")
+}
+
+fun isValidScalar(seed: ByteArray): Boolean {
+    if (seed.size != 32) return false
+    var anyNonZero = false
+
+    for (byte in seed) {
+        if (byte != 0.toByte()) {
+            anyNonZero = true
+            break
+        }
+    }
+
+    if (!anyNonZero) return false
+    for (i in 0 until 32) {
+        val candidate = seed[i].toInt() and 0xff
+        val order = SECP256K1_ORDER[i].toInt() and 0xff
+        if (candidate != order) return candidate < order
+    }
+
+    return false
+}
+
+private val SECP256K1_ORDER =
+    "fffffffffffffffffffffffffffffffebaaedce6af48a03bbfd25e8cd0364141".hexToBytes()
+
+fun groupKey(label: String, secret: ByteArray, id: ByteArray, epoch: ULong? = null): GroupKey {
+    val secretKey = SecretKey.fromBytes(groupSeed(label, secret, id, epoch))
+    return GroupKey(secretKey, Keys(secretKey).publicKey())
+}
+
+fun communityId(ownerXonly: ByteArray, ownerSalt: ByteArray): ByteArray {
+    require(ownerXonly.size == 32) { "owner_xonly must be 32 bytes, was ${ownerXonly.size}" }
+    require(ownerSalt.size == 32) { "owner_salt must be 32 bytes, was ${ownerSalt.size}" }
+    return Buffer().apply {
+        writeUtf8(ConcordLabel.COMMUNITY)
+        write(ownerXonly)
+        write(ownerSalt)
+    }.readByteString().sha256().toByteArray()
+}
+
+fun prevCommit(prevEpoch: ULong, prevKey: ByteArray): ByteArray {
+    require(prevKey.size == 32) { "prev_key must be 32 bytes, was ${prevKey.size}" }
+    return Buffer().apply {
+        writeUtf8(ConcordLabel.EPOCH_KEY_COMMITMENT)
+        writeLong(prevEpoch.toLong())
+        write(prevKey)
+    }.readByteString().sha256().toByteArray()
+}
+
+fun editionHash(entityId: ByteArray, version: ULong, prev: ByteArray?, content: ByteArray): ByteArray {
+    require(entityId.size == 32) { "entity_id must be 32 bytes, was ${entityId.size}" }
+    require(prev == null || prev.size == 32) { "prev must be 32 bytes" }
+    val label = ConcordLabel.EDITION_HASH.encodeToByteArray()
+    return Buffer().apply {
+        writeLong(label.size.toLong())
+        write(label)
+        write(entityId)
+        writeLong(version.toLong())
+        if (prev != null) {
+            writeByte(1)
+            write(prev)
+        } else {
+            writeByte(0)
+            write(ByteArray(32))
+        }
+        writeLong(content.size.toLong())
+        write(content)
+    }.readByteString().sha256().toByteArray()
+}
+
+/** 64 lowercase hex chars, the encoding CORD-01 mandates for every 32-byte value on the wire. */
+internal fun ByteArray.toHex(): String = toByteString().hex()
+
+/** Inverse of [toHex]. Throws on odd length or a non-hex character. */
+internal fun String.hexToBytes(): ByteArray = decodeHex().toByteArray()
+
+/** Inverse of [toHex], returning null instead of throwing — for reading untrusted input. */
+internal fun String.hexToBytesOrNull(): ByteArray? = runCatching { hexToBytes() }.getOrNull()
+
+/**
+ * A 32-byte value read from hex, or null when it is not exactly 32 bytes.
+ *
+ * The length check is the point: most of Concord's ids and keys are 32 bytes, and a short or long
+ * value must be rejected before it reaches a derivation that assumes a fixed width.
+ */
+internal fun String.hex32(): ByteArray? = if (length == 64) hexToBytesOrNull() else null

shared/src/commonMain/kotlin/su/reya/coop/concord/ConcordInvite.kt

@@ -0,0 +1,193 @@
+package su.reya.coop.concord
+
+import kotlin.io.encoding.Base64
+import rust.nostr.sdk.Nip19Coordinate
+import rust.nostr.sdk.RelayUrl
+import rust.nostr.sdk.nip44Decrypt
+
+/** The stock relay dictionary (CORD-05 §3). Referenced by one byte so links stay short. */
+internal object ConcordRelayDictionary {
+    val STOCK = listOf(
+        "wss://jskitty.com/nostr",
+        "wss://asia.vectorapp.io/nostr",
+        "wss://relay.ditto.pub",
+        "wss://relay.dreamith.to",
+    )
+}
+
+/** The link's secret half: `[version][flags][relays?][token:16]`, base64url without padding. */
+data class InviteFragment(
+    val version: Int,
+    val relays: List<String>,
+    /** The 16-byte unlock token, hex. Derives exactly one thing: the bundle key. */
+    val tokenHex: String,
+)
+
+/** A parsed invite link: the public locator plus the secret fragment. */
+data class ParsedInvite(
+    val signerHex: String,
+    val identifier: String,
+    /** Relays the naddr itself names. Usually empty; the compact form travels in the fragment. */
+    val naddrRelays: List<RelayUrl>,
+    val fragment: InviteFragment,
+)
+
+private const val FRAGMENT_VERSION = 4
+
+/** The one flag v1 knows: the stock set is in use, so zero relay bytes follow. */
+private const val FLAG_STOCK_RELAYS = 1
+private const val TOKEN_BYTES = 16
+
+/** CORD-05 §3: the fragment only needs to *find* the bundle; the bundle carries the real set. */
+private const val MAX_BOOTSTRAP_RELAYS = 3
+
+/** CORD-05 §1: the spec's reference ceiling on a bundle's Channel list. */
+private const val MAX_INVITE_CHANNELS = 256
+
+/** CORD-02 §6: five stable relays is the recommendation, not a rule; a client may truncate. */
+private const val MAX_COMMUNITY_RELAYS = 5
+
+/** The fragment is unpadded base64url; ABSENT_OPTIONAL also tolerates a padded paste. */
+private val fragmentBase64 = Base64.UrlSafe.withPadding(Base64.PaddingOption.ABSENT_OPTIONAL)
+
+fun parseInviteLink(text: String): ParsedInvite? {
+    val trimmed = text.trim()
+    val hash = trimmed.indexOf('#')
+    if (hash <= 0 || hash == trimmed.lastIndex) return null
+
+    val naddr = trimmed.substring(0, hash).substringAfterLast('/')
+    val fragment = decodeInviteFragment(trimmed.substring(hash + 1)) ?: return null
+
+    val coordinate = runCatching { Nip19Coordinate.fromBech32(naddr) }.getOrNull() ?: return null
+    if (coordinate.coordinate().kind().asU16() != ConcordKind.INVITE_BUNDLE.toUShort()) return null
+
+    return ParsedInvite(
+        signerHex = coordinate.coordinate().publicKey().toHex(),
+        identifier = coordinate.coordinate().identifier(),
+        naddrRelays = coordinate.relays(),
+        fragment = fragment,
+    )
+}
+
+fun decodeInviteFragment(fragment: String): InviteFragment? {
+    val bytes = runCatching { fragmentBase64.decode(fragment) }.getOrNull() ?: return null
+    // Two header bytes plus the token at minimum.
+    if (bytes.size < 2 + TOKEN_BYTES) return null
+
+    val reader = FragmentReader(bytes)
+    if (reader.byte() != FRAGMENT_VERSION) return null
+
+    val flags = reader.byte() ?: return null
+
+    val relays = if (flags and FLAG_STOCK_RELAYS != 0) {
+        ConcordRelayDictionary.STOCK
+    } else {
+        val count = reader.byte() ?: return null
+        buildList {
+            repeat(count) {
+                val lead = reader.byte() ?: return null
+                val relay = when (lead) {
+                    0 -> reader.slice(reader.byte() ?: return null)?.let { "wss://$it" }
+                    255 -> reader.slice(reader.byte() ?: return null)
+                    else -> ConcordRelayDictionary.STOCK.getOrNull(lead - 1)
+                } ?: return null
+                add(relay)
+            }
+        }.take(MAX_BOOTSTRAP_RELAYS)
+    }
+
+    // Whatever is left must be exactly the token: a short one silently weakens the link.
+    val token = bytes.copyOfRange(reader.index, bytes.size)
+    if (token.size != TOKEN_BYTES) return null
+
+    return InviteFragment(
+        version = FRAGMENT_VERSION,
+        relays = relays,
+        tokenHex = token.toHex(),
+    )
+}
+
+fun inviteBundleKey(tokenHex: String): GroupKey? =
+    tokenHex.hexToBytesOrNull()?.let { groupKey(ConcordLabel.INVITE_KEY, it, ByteArray(32)) }
+
+fun decryptInviteBundle(content: String, key: GroupKey): CommunityInvite? {
+    val plaintext = runCatching { nip44Decrypt(key.secretKey, key.publicKey, content) }.getOrNull()
+        ?: return null
+    return runCatching { concordJson.decodeFromString<CommunityInvite>(plaintext) }.getOrNull()
+}
+
+fun isInviteTombstone(vsk: String?): Boolean = vsk == ConcordVsk.INVITE_TOMBSTONE.toString()
+
+fun CommunityInvite.problems(): List<String> {
+    val problems = mutableListOf<String>()
+
+    val ownerBytes = owner.hex32()
+    val saltBytes = ownerSalt.hex32()
+    val rootBytes = communityRoot.hex32()
+
+    if (ownerBytes == null) problems += "The invite's owner key is malformed"
+    if (saltBytes == null) problems += "The invite's owner salt is malformed"
+    if (rootBytes == null) problems += "The invite's community key is malformed"
+
+    if (ownerBytes != null && saltBytes != null) {
+        if (communityId(ownerBytes, saltBytes).toHex() != communityId.lowercase()) {
+            problems += "The invite does not prove its community id"
+        }
+    }
+
+    if (communityId.hex32() == null) problems += "The invite's community id is malformed"
+    if (controlPk != null && controlPk.hex32() == null) problems += "The invite's control key is malformed"
+
+    if (channels.size > MAX_INVITE_CHANNELS) {
+        problems += "The invite carries ${channels.size} channels (max $MAX_INVITE_CHANNELS)"
+    }
+
+    channels.forEachIndexed { index, channel ->
+        if (channel.id.hex32() == null) problems += "Channel $index has a malformed id"
+        if (channel.key.hex32() == null) problems += "Channel $index has a malformed key"
+    }
+
+    return problems
+}
+
+fun CommunityInvite.isExpired(nowMs: Long): Boolean = expiresAt != null && expiresAt <= nowMs
+
+fun CommunityInvite.relaySet(fallback: List<String> = emptyList()): List<String> {
+    val source = relays.ifEmpty { fallback }
+    return source.map { it.trim() }.filter { it.isNotEmpty() }.distinct().take(MAX_COMMUNITY_RELAYS)
+}
+
+fun CommunityInvite.toMembership(relays: List<String>): Membership = Membership(
+    communityId = communityId.lowercase(),
+    owner = owner.lowercase(),
+    ownerSalt = ownerSalt.lowercase(),
+    communityRoot = communityRoot.lowercase(),
+    rootEpoch = rootEpoch,
+    controlPk = controlPk?.lowercase(),
+    controlRoot = null,
+    relays = relays,
+    name = name,
+    channels = channels.map { channel ->
+        StoredChannelKey(
+            id = channel.id.lowercase(),
+            key = channel.key.lowercase(),
+            epoch = channel.epoch,
+            name = channel.name,
+            private = !channel.key.equals(communityRoot, ignoreCase = true),
+        )
+    },
+)
+
+private class FragmentReader(private val bytes: ByteArray) {
+    var index = 0
+        private set
+
+    fun byte(): Int? = if (index < bytes.size) bytes[index++].toInt() and 0xff else null
+
+    fun slice(length: Int): String? {
+        if (length < 0 || index + length > bytes.size) return null
+        val value = bytes.decodeToString(index, index + length)
+        index += length
+        return value
+    }
+}

shared/src/commonMain/kotlin/su/reya/coop/concord/ConcordKind.kt

@@ -0,0 +1,263 @@
+package su.reya.coop.concord
+
+object ConcordKind {
+    /** NIP-59 gift wrap. Concord reuses it but reverses the roles: fixed author, ephemeral `p`. */
+    const val WRAP = 1059
+
+    /** Ephemeral wrap for kinds that must never be stored (typing, voice presence). */
+    const val EPHEMERAL_WRAP = 21059
+
+    /**
+     * Sealed content for everything that must stay secret: Chat, Guestbook, rekey blobs
+     * and Direct Invites. CORD-02 §5 makes this choice normative, never stylistic.
+     */
+    const val SEAL = 20013
+
+    /**
+     * Plaintext seal. **Control plane only** (CORD-02 §5) — the roster is public by design
+     * so that membership, and therefore authority, is verifiable by anyone.
+     */
+    const val PLAINTEXT_SEAL = 20014
+
+    /** Channel message (NIP-C7 shape). Chat plane. */
+    const val MESSAGE = 9
+
+    /** Threaded reply (NIP-22 shape). Chat plane. */
+    const val REPLY = 1111
+
+    /** Reaction (NIP-25 shape). Chat plane. */
+    const val REACTION = 7
+
+    /** Delete (NIP-09 shape). Chat plane. */
+    const val DELETE = 5
+
+    /** Disappearing-message timer notice (CORD-08 §4). Chat plane. */
+    const val TIMER_NOTICE = 1740
+
+    /** Edit. Chat plane. */
+    const val EDIT = 3302
+
+    /** Rekey blobs (CORD-06). Rekey addresses. */
+    const val REKEY = 3303
+
+    /** Join / Leave. Guestbook plane. */
+    const val JOIN_LEAVE = 3306
+
+    /** Control edition — sub-kinded by [ConcordVsk]. Control plane. */
+    const val CONTROL_EDITION = 3308
+
+    /** Kick. Guestbook plane. */
+    const val KICK = 3309
+
+    /** WebXDC peer signal. Chat plane. */
+    const val WEBXDC_SIGNAL = 3310
+
+    /** Guestbook snapshot, chunked, refounder-signed. Guestbook plane. */
+    const val GUESTBOOK_SNAPSHOT = 3312
+
+    /** Typing indicator. Chat plane, ephemeral. */
+    const val TYPING = 23311
+
+    /** Voice presence (CORD-07). Chat plane, ephemeral. */
+    const val VOICE_PRESENCE = 23313
+
+    /** Direct Invite — giftwrapped straight to an npub. Rides a standard wrap, not a stream. */
+    const val DIRECT_INVITE = 3313
+
+    /** Public invite bundle. Addressable, signed by its per-link keypair at an empty `d`. */
+    const val INVITE_BUNDLE = 33301
+
+    /** Community List — one addressable event per fragment, NIP-44 to self. */
+    const val COMMUNITY_LIST = 33302
+
+    /**
+     * The single-event Community List [COMMUNITY_LIST] superseded. Retired and never written;
+     * queried only so "no List exists" can be told apart from "a client still speaks the old shape".
+     */
+    const val RETIRED_COMMUNITY_LIST = 13302
+
+    /** Invite List — replaceable, NIP-44 to self. */
+    const val INVITE_LIST = 13303
+}
+
+/** Control edition sub-kinds, the `vsk` tag on [ConcordKind.CONTROL_EDITION] (CORD-02 Appendix A). */
+object ConcordVsk {
+    /** Community metadata. */
+    const val METADATA = 0
+
+    /** Role. */
+    const val ROLE = 1
+
+    /** Channel metadata — the only other edition v1 folds. */
+    const val CHANNEL = 2
+
+    /** Grant. */
+    const val GRANT = 3
+
+    /** Banlist. */
+    const val BANLIST = 4
+
+    /** Reserved for role ordering. */
+    const val RESERVED_ROLE_ORDER = 5
+
+    /** Claimed by the invite bundle's live marker. */
+    const val INVITE_LIVE = 6
+
+    /** Retired (was the v1 owner attestation). */
+    const val RETIRED_OWNER_ATTESTATION = 7
+
+    /** Invite-link registry. */
+    const val INVITE_REGISTRY = 8
+
+    /** Claimed by the invite bundle's revocation tombstone. */
+    const val INVITE_TOMBSTONE = 9
+
+    /** Dissolved tombstone — chainless, exempt from version discipline. */
+    const val DISSOLVED = 10
+
+    /** Pin List. */
+    const val PIN_LIST = 11
+}
+
+object ConcordPermission {
+    const val MANAGE_ROLES = 1 shl 0
+    const val MANAGE_CHANNELS = 1 shl 1
+    const val MANAGE_METADATA = 1 shl 2
+    const val KICK = 1 shl 3
+    const val BAN = 1 shl 4
+    const val MANAGE_MESSAGES = 1 shl 5
+    const val CREATE_INVITE = 1 shl 6
+
+    /** Retired (was `MANAGE_INVITES`). Never grant it. */
+    const val RETIRED_MANAGE_INVITES = 1 shl 7
+
+    const val VIEW_AUDIT_LOG = 1 shl 8
+    const val MENTION_EVERYONE = 1 shl 9
+
+    const val PIN_MESSAGES = 1 shl 11
+
+    /**
+     * Staff = anyone holding a staff bit, plus the owner. Staff are the ones who hold
+     * `control_root` and can therefore write to the Control plane.
+     */
+    const val STAFF = MANAGE_ROLES or MANAGE_CHANNELS or MANAGE_METADATA or
+        BAN or CREATE_INVITE or PIN_MESSAGES
+}
+
+object ConcordLabel {
+    /** A Channel's group key. `secret` = channel key, or `community_root` for a public channel. */
+    const val CHANNEL = "concord/channel"
+
+    /** Control Plane **read** key. `secret` = `community_root`. */
+    const val CONTROL = "concord/control"
+
+    /** Control Plane signer, held by staff only. `secret` = `control_root`. */
+    const val CONTROL_SIGNER = "concord/control-signer"
+
+    /** Guestbook Plane group key. `secret` = `community_root`. */
+    const val GUESTBOOK = "concord/guestbook"
+
+    /** Dissolution tombstone address. `secret` = `community_id`, id = 32 zero bytes, no epoch. */
+    const val DISSOLVED = "concord/dissolved"
+
+    /** A channel rekey address. `secret` = the prior `community_root`. */
+    const val REKEY_PSEUDONYM = "concord/rekey-pseudonym"
+
+    /** A base (community-wide) rekey address. `secret` = the prior `community_root`. */
+    const val BASE_REKEY_PSEUDONYM = "concord/base-rekey-pseudonym"
+
+    /** A rekey blob locator. `secret` = `rotator_xonly ‖ recipient_xonly`. */
+    const val RECIPIENT_PSEUDONYM = "concord/recipient-pseudonym"
+
+    /** SFU room keypair (CORD-07). */
+    const val VOICE_SIGNER = "concord/voice-signer"
+
+    /** 32-byte call media key (CORD-07). */
+    const val VOICE_MEDIA = "concord/voice-media"
+
+    /** Per-sender frame key (CORD-07). id = `sha256(identity)`, no epoch. */
+    const val VOICE_SENDER = "concord/voice-sender"
+
+    /** A member's Grant coordinate. no epoch. */
+    const val GRANT = "concord/grant"
+
+    /** Banlist coordinate. no epoch. */
+    const val BANLIST = "concord/banlist"
+
+    /** A Channel's Pin List coordinate. no epoch. */
+    const val PINS = "concord/pins"
+
+    /** A creator's invite-link registry coordinate. no epoch. */
+    const val INVITE_LINKS = "concord/invite-links"
+
+    /** Public-invite decrypt key. `secret` = the link's unlock token, no epoch. */
+    const val INVITE_KEY = "concord/invite-key"
+
+    /**
+     * Prefix for the `community_id` commitment (CORD-02 A.4). Not an HKDF label — it is
+     * hashed directly, with no `0x00` separator and no length prefix.
+     */
+    const val COMMUNITY = "concord/community"
+
+    /** Prefix for `prevcommit` (CORD-02 A.8). Also hashed directly, not through HKDF. */
+    const val EPOCH_KEY_COMMITMENT = "concord/epoch-key-commitment"
+
+    /**
+     * Label for `edition_hash` (CORD-02 A.8).
+     *
+     * Deliberately **not** under the `concord/` prefix: the spec pins the reference
+     * implementation's own `vector` label here. Reproduce it exactly.
+     */
+    const val EDITION_HASH = "vector-community/v1/edition"
+}
+
+/** Tag names (CORD-01, CORD-02 §5 and the per-kind sections). */
+object ConcordTag {
+    /** `["ms", "<0..999>"]` — sub-second ordering. True time = `created_at * 1000 + ms`. */
+    const val MS = "ms"
+
+    /** `["p", "<ephemeral pubkey>"]` — the one tag a stream wrap carries (NIP-59 reversed). */
+    const val P = "p"
+
+    /** `["channel", "<channel_id>"]` — MUST also be committed inside the author-signed rumor. */
+    const val CHANNEL = "channel"
+
+    /** `["epoch", "<n>"]` — MUST also be committed inside the author-signed rumor. */
+    const val EPOCH = "epoch"
+
+    /** `["q", "<rumor id>", "", "<author>"]` — NIP-C7 inline quote. */
+    const val QUOTE = "q"
+
+    /** `["e", "<rumor id>"]` — the message a reaction, edit or delete targets (CORD-03 §2.3–2.5). */
+    const val E = "e"
+
+    /** `["vsk", "<n>"]` — Control edition entity type. */
+    const val VSK = "vsk"
+
+    /** `["eid", "<hex32>"]` — stable coordinate of a Control edition's entity. */
+    const val EID = "eid"
+
+    /** `["ev", "<n>"]` — this edition's version, climbing from 1. */
+    const val EV = "ev"
+
+    /** `["ep", "<hash hex>"]` — previous edition hash. Absent on the first edition. */
+    const val EP = "ep"
+
+    /** `["vac", "<grant eid>", "<version>", "<hash>"]` — authority citation. Absent when the owner acts. */
+    const val VAC = "vac"
+
+    /** `["expiration", "<secs>"]` — NIP-40. MUST be on both wrap and rumor, same value. */
+    const val EXPIRATION = "expiration"
+
+    /** `["d", ""]` / `["d", "<index>"]` — addressable coordinate. */
+    const val D = "d"
+
+    /** `["snap", "<id>", "<i>", "<n>"]` — Guestbook snapshot chunk. */
+    const val SNAP = "snap"
+
+    /** `["invite", "<creator hex>", "<label>"]` — optional invite attribution on a join. */
+    const val INVITE = "invite"
+
+    /** `["k", "<kind>"]` — the target's kind on a reaction, the invite hint on a `3313` wrap. */
+    const val K = "k"
+}

shared/src/commonMain/kotlin/su/reya/coop/concord/ConcordManager.kt

@@ -0,0 +1,742 @@
+package su.reya.coop.concord
+
+import kotlinx.coroutines.CancellationException
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.flow.update
+import rust.nostr.sdk.AckPolicy
+import rust.nostr.sdk.Event
+import rust.nostr.sdk.EventId
+import rust.nostr.sdk.Filter
+import rust.nostr.sdk.Kind
+import rust.nostr.sdk.PublicKey
+import rust.nostr.sdk.RelayUrl
+import rust.nostr.sdk.ReqTarget
+import rust.nostr.sdk.SendEventTarget
+import rust.nostr.sdk.Tag
+import rust.nostr.sdk.UnsignedEvent
+import su.reya.coop.AppStorage
+import su.reya.coop.nostr.Nostr
+import kotlin.concurrent.Volatile
+import kotlin.time.Clock
+import kotlin.time.Duration.Companion.seconds
+
+class ConcordManager(private val nostr: Nostr) {
+    @Volatile
+    private var planes: Map<String, Plane> = emptyMap()
+
+    /** Latest Control fold per community id. Replaced wholesale; see [planes]. */
+    @Volatile
+    private var folds: Map<String, ControlFold> = emptyMap()
+
+    /**
+     * Unread badges by Channel id. Replaced wholesale; see [planes]. In memory only, mirroring
+     * `Room.unreadCount` in the chat layer, so a restart begins with every badge cleared.
+     */
+    @Volatile
+    private var unread: Map<String, Int> = emptyMap()
+
+    /**
+     * Bumped whenever a plane rumor is cached or a message is sent, so a screen showing a Channel
+     * has something to re-query on. A counter rather than a set of changed scopes, because two
+     * messages to the same Channel must both be observable.
+     */
+    private val _revision = MutableStateFlow(0L)
+    val revision: StateFlow<Long> = _revision.asStateFlow()
+
+    private val _memberships = MutableStateFlow<List<Membership>>(emptyList())
+    val memberships: StateFlow<List<Membership>> = _memberships.asStateFlow()
+
+    private val _communities = MutableStateFlow<List<CommunityState>>(emptyList())
+    val communities: StateFlow<List<CommunityState>> = _communities.asStateFlow()
+
+    /** Direct Invites that arrived giftwrapped to us and have not been accepted or dismissed. */
+    private val _directInvites = MutableStateFlow<List<CommunityInvite>>(emptyList())
+    val directInvites: StateFlow<List<CommunityInvite>> = _directInvites.asStateFlow()
+
+    /**
+     * True once [restore] has run. An empty [memberships] means the same thing before and after it,
+     * so a screen needs this to tell "joined nothing" from "not loaded yet".
+     */
+    private val _restored = MutableStateFlow(false)
+    val restored: StateFlow<Boolean> = _restored.asStateFlow()
+
+    private var store: ConcordStore? = null
+
+    /**
+     * Hands over the encrypted storage [Nostr] cannot reach on its own.
+     *
+     * [Nostr] is a Context-free singleton, so the Android layer passes [AppStorage] in through
+     * [Nostr.init]. Until this is called nothing can be persisted or restored, and the manager
+     * simply routes nothing.
+     */
+    internal fun attach(storage: AppStorage) {
+        store = ConcordStore(storage, nostr)
+    }
+
+    /** True when a wrap's author is one of our planes' stream keys. */
+    fun isPlaneAddress(authorHex: String): Boolean = planes.containsKey(authorHex)
+
+    /**
+     * Handles one stream wrap from a plane we hold a key for.
+     *
+     * [PlaneKey.unwrap] runs every check a reader must make and returns null on any failure, so a
+     * forged, unverifiable, mis-bound or undecryptable wrap is dropped here without ever being
+     * rendered. What survives is stored under its logical scope and, for Control, re-folded.
+     */
+    suspend fun handlePlaneEvent(event: Event) {
+        val plane = planes[event.author().toHex()] ?: return
+        val rumor = plane.key.unwrap(event) ?: return
+        val store = store ?: return
+
+        store.cacheRumor(plane.scopeIdHex, event.id().toHex(), rumor)
+        _revision.update { it + 1 }
+
+        // A message from someone else is the only thing a badge counts. Our own wraps come back
+        // through the same subscription and so does a re-fetch, so the check is on the rumor's
+        // author rather than on the arrival.
+        if (plane.role == PlaneRole.Channel) countUnread(plane.scopeIdHex, rumor)
+
+        // Control carries consensus state, so every edition changes what we can read: a new
+        // Channel means a new plane address, and therefore a new subscription.
+        if (plane.role == PlaneRole.Control) refreshControl(plane.communityIdHex)
+    }
+
+    /**
+     * Intercepts rumors that arrive through the ordinary NIP-17 giftwrap pipeline and belong to
+     * Concord instead.
+     *
+     * Returns true when Concord consumed the rumor. A Direct Invite (kind `3313`) rides a *standard*
+     * NIP-59 wrap, so it unwraps fine — but it is a key handoff, not a message, and letting it
+     * through would have the chat layer build a room out of it. Returning true here keeps all
+     * Concord kind knowledge inside this package and leaves the chat code untouched.
+     */
+    fun onInboxRumor(rumor: UnsignedEvent): Boolean {
+        if (rumor.kind().asU16() != ConcordKind.DIRECT_INVITE.toUShort()) return false
+
+        val invite =
+            runCatching { concordJson.decodeFromString<CommunityInvite>(rumor.content()) }.getOrNull()
+        if (invite == null) {
+            println("Concord: a direct invite arrived but its bundle could not be read")
+            return true
+        }
+        if (invite.problems().isNotEmpty()) {
+            println("Concord: refusing a direct invite that does not prove its community id")
+            return true
+        }
+
+        _directInvites.update { current ->
+            if (current.any {
+                    it.communityId.equals(
+                        invite.communityId,
+                        ignoreCase = true
+                    )
+                }) current
+            else current + invite
+        }
+        return true
+    }
+
+    /** Local half of startup: load memberships, fold the cached Control plane, index the planes. */
+    suspend fun restore() {
+        val store = store ?: return
+        val memberships = store.loadMemberships()
+        _memberships.value = memberships
+
+        // Fold what is already on disk so Channels and metadata render immediately. Live editions
+        // arrive later through handlePlaneEvent and re-fold.
+        folds = memberships.associate { membership ->
+            membership.communityId to ConcordControl.fold(
+                editions = store.cachedRumors(membership.communityId)
+                    .mapNotNull { ConcordControl.editionOf(it) },
+                communityIdHex = membership.communityId,
+            )
+        }
+        refreshPlanes()
+        _restored.value = true
+    }
+
+    /**
+     * Drops every membership and everything derived from it.
+     *
+     * Membership keys are identity-scoped — holding them *is* being in the Community (CORD-02 §2) —
+     * so signing out has to take them with it, or the next identity on this device holds a seat it
+     * never took. The cached rumors go with `Nostr.prune()`; the subscriptions are dropped here,
+     * since nothing else would.
+     */
+    suspend fun reset() {
+        _memberships.value.forEach { membership ->
+            nostr.client?.unsubscribe(subscriptionId(membership.communityId))
+        }
+
+        store?.clearMemberships()
+
+        _memberships.value = emptyList()
+        _directInvites.value = emptyList()
+
+        folds = emptyMap()
+        unread = emptyMap()
+
+        refreshPlanes()
+    }
+
+    /** Network half of startup: adopt what other clients hold, then connect and subscribe. */
+    suspend fun sync() {
+        adoptCommunityList()
+
+        for (membership in _memberships.value) {
+            try {
+                subscribeCommunity(membership)
+            } catch (e: CancellationException) {
+                throw e
+            } catch (e: Exception) {
+                println("Concord: could not subscribe to ${membership.communityId}: ${e.message}")
+            }
+        }
+    }
+
+    /**
+     * Adopts the memberships this identity holds on other clients (CORD-02 §8).
+     *
+     * The Community List is the only way a Community joined elsewhere can appear here: an invite
+     * hands keys to one device, while the List is the member's own self-encrypted vault on their
+     * own relays. Add-only — a tombstone for a Community we do not hold is honoured, but nothing
+     * this device joined is dropped, because Coop never writes the List and so cannot tell which
+     * of the two states is newer.
+     */
+    private suspend fun adoptCommunityList() {
+        val client = nostr.client
+        if (client == null) {
+            println("Concord: Community List skipped — no client yet")
+            return
+        }
+        val store = store
+        if (store == null) {
+            println("Concord: Community List skipped — storage was never attached")
+            return
+        }
+
+        val me = try {
+            nostr.signer.getPublicKeyAsync()
+        } catch (e: CancellationException) {
+            throw e
+        } catch (e: Exception) {
+            println("Concord: Community List skipped — could not read the signer: ${e.message}")
+            return
+        }
+        if (me == null) {
+            println("Concord: Community List skipped — the signer has no public key")
+            return
+        }
+
+        val filter = Filter()
+            .kind(Kind(ConcordKind.COMMUNITY_LIST.toUShort()))
+            .author(me)
+
+        val events = try {
+            client.fetchEvents(ReqTarget.auto(listOf(filter)), timeout = 8.seconds).toVec()
+        } catch (e: CancellationException) {
+            throw e
+        } catch (e: Exception) {
+            println("Concord: Community List fetch failed: ${e.message}")
+            return
+        }
+
+        println(
+            "Concord: Community List for ${me.toHex().take(8)}… came back with " +
+                "${events.size} event(s) from ${client.relays().size} relay(s)"
+        )
+
+        val fragments = events.mapNotNull { event ->
+            try {
+                val plaintext = nostr.signer.nip44DecryptAsync(me, event.content())
+                concordJson.decodeFromString<CommunityListFragment>(plaintext)
+            } catch (e: CancellationException) {
+                throw e
+            } catch (e: Exception) {
+                println(
+                    "Concord: Community List fragment ${event.id().toHex().take(8)}… " +
+                        "(d=${event.tagValue(ConcordTag.D)}) is unusable: ${e.message}"
+                )
+                null
+            }
+        }
+
+        val known = _memberships.value.mapTo(mutableSetOf()) { it.communityId }
+        val listed = fragments.listedMemberships()
+        val adopted = listed.filterNot { it.communityId in known }
+
+        println(
+            "Concord: Community List gave ${fragments.size} fragment(s), " +
+                "${listed.size} membership(s), ${adopted.size} new"
+        )
+
+        if (adopted.isEmpty()) return
+
+        val updated = _memberships.value + adopted
+        try {
+            store.saveMemberships(updated)
+        } catch (e: CancellationException) {
+            throw e
+        } catch (e: Exception) {
+            println("Concord: could not store the adopted memberships: ${e.message}")
+            return
+        }
+
+        _memberships.value = updated
+        refreshPlanes()
+    }
+
+    /**
+     * Fetches and decrypts the bundle behind a public invite link, so the UI can show what joining
+     * would mean. Nothing is joined, nothing is subscribed and no presence is announced here
+     * (CORD-05 §1: a bundle is passive until the user accepts).
+     */
+    suspend fun previewInvite(link: String): InvitePreview {
+        val client = nostr.client ?: throw IllegalStateException("Nostr client is not ready")
+
+        val parsed = parseInviteLink(link)
+            ?: throw IllegalArgumentException("That is not a Concord invite link")
+
+        val bundleKey = inviteBundleKey(parsed.fragment.tokenHex)
+            ?: throw IllegalArgumentException("The invite link's token is malformed")
+
+
+        val relays = (parsed.naddrRelays.map { it.toString() } + parsed.fragment.relays)
+            .mapNotNull { runCatching { RelayUrl.parse(it) }.getOrNull() }
+            .distinct()
+
+        if (relays.isEmpty()) throw IllegalStateException("The invite link names no relay to fetch from")
+
+        relays.forEach { relay ->
+            client.addRelay(relay)
+            client.connectRelay(relay)
+        }
+
+        val filter = Filter()
+            .kind(Kind(ConcordKind.INVITE_BUNDLE.toUShort()))
+            .author(PublicKey.fromBytes(parsed.signerHex.hexToBytes()))
+            .identifier(parsed.identifier)
+
+        val bundle = client
+            .fetchEvents(
+                ReqTarget.manual(relays.associateWith { listOf(filter) }),
+                timeout = 8.seconds
+            )
+            .toVec()
+            .firstOrNull()
+            ?: throw IllegalStateException("No invite bundle was found at that link")
+
+        if (isInviteTombstone(bundle.tagValue(ConcordTag.VSK))) {
+            throw IllegalStateException("This invite link has been revoked")
+        }
+
+        val invite = decryptInviteBundle(bundle.content(), bundleKey)
+            ?: throw IllegalStateException("The invite bundle could not be opened")
+
+        return preview(invite, link, parsed.fragment.relays)
+    }
+
+    /** The same preview for a Direct Invite, which arrived with no link to fetch. */
+    fun previewDirectInvite(invite: CommunityInvite): InvitePreview =
+        preview(invite, link = null, fallbackRelays = emptyList())
+
+    /**
+     * Forgets a Direct Invite the user declined. It was never persisted, so this only clears the
+     * in-memory list — there is nothing on a relay to take back.
+     */
+    fun dismissDirectInvite(communityIdHex: String) {
+        _directInvites.update { invites ->
+            invites.filterNot { it.communityId.equals(communityIdHex, ignoreCase = true) }
+        }
+    }
+
+    /** Accepts an invite: persist the keys, connect, subscribe, and announce the join. */
+    suspend fun join(preview: InvitePreview): CommunityState {
+        preview.problems.firstOrNull()?.let { throw IllegalArgumentException(it) }
+        if (preview.expired) throw IllegalArgumentException("That invite has expired")
+
+        val store = store ?: throw IllegalStateException("Concord storage is not ready")
+        val membership = preview.invite.toMembership(preview.relays)
+
+        val updated =
+            _memberships.value.filterNot { it.communityId == membership.communityId } + membership
+        store.saveMemberships(updated)
+        _memberships.value = updated
+
+        refreshPlanes()
+        subscribeCommunity(membership)
+
+        if (!preview.alreadyJoined) publishJoin(membership, preview.invite)
+
+        _directInvites.update { invites ->
+            invites.filterNot { it.communityId.equals(membership.communityId, ignoreCase = true) }
+        }
+
+        return _communities.value.firstOrNull { it.membership.communityId == membership.communityId }
+            ?: CommunityState(membership, null, emptyList())
+    }
+
+    /** A Channel's messages, oldest first, with reactions and edits folded on top. */
+    suspend fun channelMessages(channelIdHex: String): List<ConcordMessage> =
+        store?.cachedRumors(channelIdHex).orEmpty().toChannelMessages()
+
+    /** A Channel's unread badge. See [ConcordChannel.unreadCount] for what it does and does not survive. */
+    fun unreadCount(channelIdHex: String): Int = unread[channelIdHex] ?: 0
+
+    /** Clears a Channel's badge — the Channel screen calls this once its messages are on display. */
+    fun markChannelRead(channelIdHex: String) {
+        if ((unread[channelIdHex] ?: 0) == 0) return
+        unread = unread - channelIdHex
+        publishUnread(channelIdHex, 0)
+    }
+
+    /**
+     * Sends a message to a Channel (CORD-03 §3), and returns it as a reader will see it.
+     *
+     * Throws when the Channel is hidden behind a key we were never granted — a Private Channel is
+     * listable without being writable.
+     */
+    suspend fun sendChannelMessage(channelIdHex: String, content: String): ConcordMessage =
+        publish(channelPlane(channelIdHex), ConcordKind.MESSAGE, content)
+            .toConcordMessage()
+            ?: throw IllegalStateException("Concord: could not read back the message just sent")
+
+    /**
+     * Reacts to a message (CORD-03 §2.3). The `e` tag names the target's *rumor* id, and `p`/`k`
+     * carry its author and kind, which is what lets a reader group it without a lookup.
+     */
+    suspend fun sendChannelReaction(
+        channelIdHex: String,
+        targetIdHex: String,
+        targetAuthorHex: String,
+        reaction: String,
+    ) {
+        publish(
+            plane = channelPlane(channelIdHex),
+            kind = ConcordKind.REACTION,
+            content = reaction,
+            extraTags = listOf(
+                Tag.event(EventId.parse(targetIdHex)),
+                Tag.publicKey(PublicKey.parse(targetAuthorHex)),
+                Tag.custom(ConcordTag.K, listOf(ConcordKind.MESSAGE.toString())),
+            ),
+        )
+    }
+
+    /**
+     * Replaces one of our own messages (CORD-03 §2.5). The `e` tag names the message being edited;
+     * the edit rides the same Channel plane, so it reaches exactly the readers the original did.
+     */
+    suspend fun sendChannelEdit(channelIdHex: String, targetIdHex: String, content: String) {
+        publish(
+            plane = channelPlane(channelIdHex),
+            kind = ConcordKind.EDIT,
+            content = content,
+            extraTags = listOf(Tag.event(EventId.parse(targetIdHex))),
+        )
+    }
+
+    private fun channelPlane(channelIdHex: String): Plane = planes.values.firstOrNull {
+        it.role == PlaneRole.Channel && it.scopeIdHex.equals(channelIdHex, ignoreCase = true)
+    } ?: throw IllegalArgumentException("That Channel is not one we hold a key for")
+
+    /**
+     * The whole write path: build the rumor from the Channel's own key, wrap it under that key's
+     * stream half, cache it locally, then publish to the Community's own relays.
+     *
+     * The rumor is cached *before* it is published, so it is visible even if every relay is
+     * unreachable and so the subscription's echo of the wrap lands on the same `d` slot instead of
+     * duplicating it. The relays are always the Community's, never the app's defaults.
+     */
+    private suspend fun publish(
+        plane: Plane,
+        kind: Int,
+        content: String,
+        extraTags: List<Tag> = emptyList(),
+    ): UnsignedEvent {
+        val client = nostr.client ?: throw IllegalStateException("Nostr client is not ready")
+        val author =
+            nostr.signer.getPublicKeyAsync() ?: throw IllegalStateException("User not signed in")
+
+        val rumor = plane.key.rumor(
+            author = author,
+            kind = kind.toUShort(),
+            content = content,
+            createdAt = Clock.System.now(),
+            extraTags = extraTags,
+        )
+        val wrap = plane.key.wrap(rumor, nostr.signer)
+
+        store?.cacheRumor(plane.scopeIdHex, wrap.id().toHex(), rumor)
+        _revision.update { it + 1 }
+
+        // Always the Community's own relays, never the app's defaults (see [subscribeCommunity]).
+        val relays = _memberships.value
+            .firstOrNull { it.communityId == plane.communityIdHex }
+            ?.relays
+            .orEmpty()
+            .mapNotNull { runCatching { RelayUrl.parse(it) }.getOrNull() }
+        if (relays.isEmpty()) throw IllegalStateException("That Community names no relay to publish to")
+
+        client.sendEvent(
+            event = wrap,
+            target = SendEventTarget.to(relays),
+            ackPolicy = AckPolicy.none()
+        ).failed.forEach { (relay, reason) -> println("Concord: $relay refused a write: $reason") }
+
+        return rumor
+    }
+
+    /** Records a badge change and re-publishes [communities] so a badge drawn from it moves. */
+    private fun publishUnread(channelIdHex: String, count: Int) {
+        _communities.update { states ->
+            if (states.none { state -> state.channels.any { it.idHex == channelIdHex } }) return@update states
+            states.map { state ->
+                state.copy(
+                    channels = state.channels.map {
+                        if (it.idHex == channelIdHex) it.copy(unreadCount = count) else it
+                    }
+                )
+            }
+        }
+    }
+
+    /** Counts one incoming message against its Channel, ignoring our own and anything but a message. */
+    private fun countUnread(channelIdHex: String, rumor: UnsignedEvent) {
+        if (rumor.kind().asU16() != ConcordKind.MESSAGE.toUShort()) return
+        if (rumor.author() == nostr.signer.publicKeyFlow.value) return
+
+        val count = (unread[channelIdHex] ?: 0) + 1
+        unread = unread + (channelIdHex to count)
+        publishUnread(channelIdHex, count)
+    }
+
+    /**
+     * Rebuilds the plane index and the Community read model from the memberships and folds we hold.
+     *
+     * Returns the communities whose set of plane addresses changed, because those are exactly the
+     * ones whose subscription is now stale — a Control edition that adds a Channel adds an address.
+     */
+    private fun refreshPlanes(): Set<String> {
+        val previous = planes
+        val next = mutableMapOf<String, Plane>()
+        val states = mutableListOf<CommunityState>()
+
+        for (membership in _memberships.value) {
+            val communityId = membership.communityId.hex32()
+            val root = membership.communityRoot.hex32()
+            val fold = folds[membership.communityId]
+            val granted = membership.channels.associateBy { it.id }
+
+            if (communityId != null && root != null) {
+                membership.controlPk?.let { controlPkHex ->
+                    if (controlPkHex.hex32() != null) {
+                        val key =
+                            controlPlaneKey(root, communityId, membership.rootEpoch, controlPkHex)
+                        next[key.streamPublicKeyHex] = Plane(
+                            communityIdHex = membership.communityId,
+                            role = PlaneRole.Control,
+                            key = key,
+                            scopeIdHex = membership.communityId,
+                        )
+                    }
+                }
+
+                val guestbook = guestbookPlaneKey(root, communityId, membership.rootEpoch)
+                next[guestbook.streamPublicKeyHex] = Plane(
+                    communityIdHex = membership.communityId,
+                    role = PlaneRole.Guestbook,
+                    key = guestbook,
+                    scopeIdHex = membership.communityId,
+                )
+            }
+
+            val channelIds = (granted.keys + fold?.channels?.keys.orEmpty()).distinct()
+            val channels = channelIds.mapNotNull { channelIdHex ->
+                val meta = fold?.channels?.get(channelIdHex)
+                if (meta?.deleted == true) return@mapNotNull null
+
+                val stored = granted[channelIdHex]
+                val channelId = channelIdHex.hex32()
+                val secret = stored?.key?.hex32()
+                    ?: if (meta?.isPrivate != true) root else null
+                val epoch = stored?.epoch ?: membership.rootEpoch
+
+                var plane: PlaneKey? = null
+                if (channelId != null && secret != null) {
+                    plane = channelPlaneKey(secret, channelId, epoch)
+                    next[plane.streamPublicKeyHex] = Plane(
+                        communityIdHex = membership.communityId,
+                        role = PlaneRole.Channel,
+                        key = plane,
+                        scopeIdHex = channelIdHex,
+                    )
+                }
+
+                ConcordChannel(
+                    idHex = channelIdHex,
+                    name = meta?.name ?: stored?.name ?: channelIdHex.take(8),
+                    private = meta?.isPrivate ?: stored?.private ?: false,
+                    epoch = epoch,
+                    hasKey = plane != null,
+                    // Carried across the re-index rather than recomputed: a Control edition must not
+                    // silently clear every badge, the same way ChatRepository preserves its counters.
+                    unreadCount = unread[channelIdHex] ?: 0,
+                )
+            }
+
+            states += CommunityState(
+                membership = membership,
+                meta = fold?.meta,
+                channels = channels.sortedBy { it.name.lowercase() },
+            )
+        }
+
+        planes = next
+        _communities.value = states
+
+        return _memberships.value
+            .filter { membership ->
+                previous.planeKeys(membership.communityId) != next.planeKeys(membership.communityId)
+            }
+            .map { it.communityId }
+            .toSet()
+    }
+
+    /** Re-folds one Community's Control plane from the cache, then re-indexes if planes shifted. */
+    private suspend fun refreshControl(communityIdHex: String) {
+        val store = store ?: return
+        val editions =
+            store.cachedRumors(communityIdHex).mapNotNull { ConcordControl.editionOf(it) }
+        folds = folds + (communityIdHex to ConcordControl.fold(editions, communityIdHex))
+
+        if (communityIdHex in refreshPlanes()) {
+            _memberships.value.firstOrNull { it.communityId == communityIdHex }
+                ?.let { subscribeCommunity(it) }
+        }
+    }
+
+    /**
+     * Subscribes to one Community: connect its relays, then ask for every plane's address.
+     *
+     * `channel` and `epoch` are multi-letter tags, and Nostr filters only accept single-letter ones,
+     * so plane addresses are the finest granularity available. That is the right level anyway — one
+     * key per plane — and the `channel`/`epoch` binding is a strict check after decryption.
+     */
+    private suspend fun subscribeCommunity(membership: Membership) {
+        val client = nostr.client ?: return
+        val id = subscriptionId(membership.communityId)
+        client.unsubscribe(id)
+
+        val relays = membership.relays
+            .mapNotNull { runCatching { RelayUrl.parse(it) }.getOrNull() }
+            .distinct()
+
+        if (relays.isEmpty()) return
+
+        relays.forEach { relay ->
+            client.addRelay(relay)
+            client.connectRelay(relay)
+        }
+
+        val filters = planes.values
+            .filter { it.communityIdHex == membership.communityId }
+            .mapNotNull { plane -> plane.key.streamPublicKeyHex.hex32() }
+            .distinct()
+            .map {
+                Filter().kind(Kind(ConcordKind.WRAP.toUShort())).author(PublicKey.fromBytes(it))
+            }
+
+        if (filters.isEmpty()) return
+
+        client.subscribe(target = ReqTarget.manual(relays.associateWith { filters }), id = id)
+    }
+
+    /** Publishes a still-fetchable Guestbook Join (CORD-02 §5) — each member's own word. */
+    private suspend fun publishJoin(membership: Membership, invite: CommunityInvite?) {
+        val client = nostr.client ?: return
+        val communityId = membership.communityId.hex32() ?: return
+        val root = membership.communityRoot.hex32() ?: return
+        val author =
+            nostr.signer.getPublicKeyAsync() ?: throw IllegalStateException("User not signed in")
+
+        val plane = guestbookPlaneKey(root, communityId, membership.rootEpoch)
+
+        val extraTags = invite?.creatorNpub
+            ?.let { creator ->
+                listOf(Tag.custom(ConcordTag.INVITE, listOf(creator, invite.label.orEmpty())))
+            }.orEmpty()
+
+        val rumor = plane.rumor(
+            author = author,
+            kind = ConcordKind.JOIN_LEAVE.toUShort(),
+            content = "join",
+            createdAt = Clock.System.now(),
+            extraTags = extraTags,
+        )
+        val wrap = plane.wrap(rumor, nostr.signer)
+
+        val targets =
+            membership.relays.mapNotNull { runCatching { RelayUrl.parse(it) }.getOrNull() }
+
+        client.sendEvent(
+            event = wrap,
+            target = SendEventTarget.to(targets),
+            ackPolicy = AckPolicy.none()
+        )
+    }
+
+    private fun subscriptionId(communityIdHex: String): String =
+        "$SUBSCRIPTION_PREFIX$communityIdHex"
+
+    private fun preview(
+        invite: CommunityInvite,
+        link: String?,
+        fallbackRelays: List<String>,
+    ): InvitePreview {
+        val relays = invite.relaySet(fallbackRelays)
+        val problems =
+            invite.problems() + if (relays.isEmpty()) listOf("The invite names no relays") else emptyList()
+
+        return InvitePreview(
+            link = link,
+            invite = invite,
+            relays = relays,
+            problems = problems,
+            expired = invite.isExpired(Clock.System.now().toEpochMilliseconds()),
+            alreadyJoined = _memberships.value.any {
+                it.communityId.equals(
+                    invite.communityId,
+                    ignoreCase = true
+                )
+            },
+        )
+    }
+
+    private companion object {
+        const val SUBSCRIPTION_PREFIX = "concord:"
+    }
+}
+
+/** What a plane is for — the Community-wide Control and Guestbook planes, or one Channel. */
+private enum class PlaneRole { Control, Guestbook, Channel }
+
+/**
+ * One plane address we hold: the key to decrypt it, the role that says how to fold it, and the
+ * [scopeIdHex] it caches under.
+ */
+private class Plane(
+    val communityIdHex: String,
+    val role: PlaneRole,
+    val key: PlaneKey,
+    val scopeIdHex: String,
+)
+
+private fun Map<String, Plane>.planeKeys(communityIdHex: String): Set<String> =
+    filterValues { it.communityIdHex == communityIdHex }.keys
+
+/** The first content value of a tag, or null. Used for tags read without their companion fields. */
+private fun Event.tagValue(kind: String): String? =
+    tags().toVec().firstOrNull { it.kind() == kind }?.content()

shared/src/commonMain/kotlin/su/reya/coop/concord/ConcordModels.kt

@@ -0,0 +1,246 @@
+package su.reya.coop.concord
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.Json
+import rust.nostr.sdk.UnsignedEvent
+import kotlin.time.Instant
+
+internal val concordJson = Json {
+    ignoreUnknownKeys = true
+    encodeDefaults = true
+}
+
+@Serializable
+data class Membership(
+    /** Hex `community_id`. Never appears on the wire; every coordinate derives from it one-way. */
+    val communityId: String,
+    /** Owner x-only pubkey hex, the other half of the `community_id` commitment. */
+    val owner: String,
+    /** 32-byte hex salt; not secret, travels in invites so anyone can recompute the commitment. */
+    val ownerSalt: String,
+    /** 32-byte hex `community_root`. The base access key — never leaves the encrypted store. */
+    val communityRoot: String,
+    /** Epoch the root was issued at (CORD-02 §3). */
+    val rootEpoch: ULong,
+    /** Control Plane signer pubkey (CORD-02 §5), used to *address* the plane. Trusted on trust. */
+    val controlPk: String? = null,
+    /** Staff-only write key. v1 never writes to the Control Plane, so this stays null. */
+    val controlRoot: String? = null,
+    /** The Community's relays — always from the invite, never the app's defaults (see PLAN.md 13.2). */
+    val relays: List<String> = emptyList(),
+    /** Last known name, cached so a community renders before its Control fold lands. */
+    val name: String? = null,
+    /** Channels this invite actually granted a key for. */
+    val channels: List<StoredChannelKey> = emptyList(),
+)
+
+@Serializable
+data class StoredChannelKey(
+    val id: String,
+    val key: String,
+    val epoch: ULong,
+    val name: String? = null,
+    val private: Boolean = false,
+)
+
+@Serializable
+data class CommunityInvite(
+    @SerialName("community_id") val communityId: String = "",
+    val owner: String = "",
+    @SerialName("owner_salt") val ownerSalt: String = "",
+    @SerialName("community_root") val communityRoot: String = "",
+    @SerialName("root_epoch") val rootEpoch: ULong = 0u,
+    @SerialName("control_pk") val controlPk: String? = null,
+    val channels: List<InviteChannel> = emptyList(),
+    val relays: List<String> = emptyList(),
+    val name: String? = null,
+    val icon: ConcordIcon? = null,
+    /** Unix **milliseconds** — see [expiryToEpochSeconds]. Optional. */
+    @SerialName("expires_at") val expiresAt: Long? = null,
+    @SerialName("creator_npub") val creatorNpub: String? = null,
+    val label: String? = null,
+)
+
+/** One granted Channel inside a bundle. `key` is `community_root` for a Public one (CORD-03 §1). */
+@Serializable
+data class InviteChannel(
+    val id: String = "",
+    val key: String = "",
+    val epoch: ULong = 0u,
+    val name: String? = null,
+)
+
+@Serializable
+data class ConcordIcon(
+    val url: String? = null,
+    val key: String? = null,
+    val nonce: String? = null,
+    val hash: String? = null,
+)
+
+/** Community metadata — the `vsk 0` entity's content (CORD-02 §6). */
+@Serializable
+data class CommunityMeta(
+    val name: String? = null,
+    val description: String? = null,
+    /** The Community's own relay set. This is the authority; the invite's copy is a snapshot. */
+    val relays: List<String> = emptyList(),
+    val icon: ConcordIcon? = null,
+    val banner: ConcordIcon? = null,
+    /** Disappearing-messages timer in seconds (CORD-08). Read only; v1 does not enforce it. */
+    @SerialName("message_expiration") val messageExpiration: Long? = null,
+)
+
+/** Channel metadata — the `vsk 2` entity's content (CORD-03 §2). */
+@Serializable
+data class ChannelMeta(
+    val name: String? = null,
+    @SerialName("private") val isPrivate: Boolean = false,
+    /** Terminal: the id is never reused and clients drop the Channel (CORD-03 §2). */
+    val deleted: Boolean = false,
+)
+
+data class ControlEdition(
+    val vsk: Int,
+    val eidHex: String,
+    val version: ULong,
+    val prevHashHex: String?,
+    val content: String,
+    val actorHex: String,
+    val rumorIdHex: String,
+    val createdAt: Instant,
+)
+
+/** The projected Control state v1 uses: the Community's metadata and its Channel list. */
+data class ControlFold(
+    val meta: CommunityMeta?,
+    val channels: Map<String, ChannelMeta>,
+)
+
+/** A Channel as the UI sees it: its identity, and whether we actually hold a key to read it. */
+data class ConcordChannel(
+    val idHex: String,
+    val name: String,
+    val private: Boolean,
+    val epoch: ULong,
+    /** False for a Private Channel we were never granted — listable, but not readable. */
+    val hasKey: Boolean,
+    /**
+     * Messages from others since this Channel was last opened. In memory only, exactly like
+     * `Room.unreadCount`: it survives a Control re-fold, but not a restart.
+     */
+    val unreadCount: Int = 0,
+)
+
+/** A Community with its Control fold applied. */
+data class CommunityState(
+    val membership: Membership,
+    /** Null until a `vsk 0` edition has been folded; the invite's name fills in before that. */
+    val meta: CommunityMeta?,
+    val channels: List<ConcordChannel>,
+)
+
+/** One emoji and everyone who used it on a message (CORD-03 §2.3), grouped for display. */
+data class ConcordReaction(
+    val emoji: String,
+    val authors: List<String>,
+)
+
+/** A Chat-plane message, already unwrapped and signature-checked. */
+data class ConcordMessage(
+    val idHex: String,
+    val author: String,
+    val content: String,
+    val createdAt: Instant,
+    /** CORD-02 §4: `created_at * 1000 + ms`. The only ordering basis the protocol uses. */
+    val timestampMs: Long,
+    val reactions: List<ConcordReaction> = emptyList(),
+    /** True once the author's own Edit (kind `3302`) has replaced [content]. */
+    val edited: Boolean = false,
+)
+
+/** A redeemed-in-part invite, ready for the UI to preview before anything joins (CORD-05 §1). */
+data class InvitePreview(
+    /** Null for a Direct Invite, which arrives with no link. */
+    val link: String?,
+    val invite: CommunityInvite,
+    /** Resolved relay set: the bundle's, falling back to the link's bootstrap relays. */
+    val relays: List<String>,
+    /** Non-empty means the bundle is refused — these are the reasons, in plain English. */
+    val problems: List<String>,
+    val expired: Boolean,
+    val alreadyJoined: Boolean,
+) {
+    val joinable: Boolean get() = problems.isEmpty() && !expired
+}
+
+fun CommunityInvite.expiryToEpochSeconds(): Long? = expiresAt?.let { it / 1000 }
+
+internal fun UnsignedEvent.concordTimestampMs(): Long {
+    val ms = tags().toVec()
+        .firstOrNull { it.kind() == ConcordTag.MS }
+        ?.content()
+        ?.toIntOrNull()
+        ?.takeIf { it in 0..999 }
+    return createdAt().asSecs().toLong() * 1000 + (ms ?: 0)
+}
+
+internal fun UnsignedEvent.toConcordMessage(): ConcordMessage? {
+    if (kind().asU16() != ConcordKind.MESSAGE.toUShort()) return null
+    return ConcordMessage(
+        idHex = id()?.toHex().orEmpty(),
+        author = author().toHex(),
+        content = content(),
+        createdAt = Instant.fromEpochSeconds(createdAt().asSecs().toLong()),
+        timestampMs = concordTimestampMs(),
+    )
+}
+
+/**
+ * Projects one Channel's cached rumors into the timeline: its messages, with reactions and edits
+ * folded on top.
+ *
+ * Every one of these kinds names its target by the target's *rumor* id, never the outer wrap's —
+ * that id differs on every re-wrap (CORD-03 §2). A reaction for a message we no longer hold is
+ * dropped along with it. An Edit counts only from the message's own author: the `e` tag is a claim,
+ * and authorship is what the seal actually proves.
+ */
+internal fun List<UnsignedEvent>.toChannelMessages(): List<ConcordMessage> {
+    val messages = filter { it.isKind(ConcordKind.MESSAGE) }
+        .mapNotNull { it.toConcordMessage() }
+        .sortedBy { it.timestampMs }
+
+    val reactions = filter { it.isKind(ConcordKind.REACTION) }
+        .mapNotNull { rumor -> rumor.targetHex()?.let { it to rumor } }
+        .groupBy({ it.first }, { it.second })
+        .mapValues { (_, rumors) ->
+            rumors.groupBy { it.content() }
+                .map { (emoji, reactors) ->
+                    ConcordReaction(
+                        emoji = emoji.ifEmpty { "+" },
+                        authors = reactors.map { it.author().toHex() }.distinct(),
+                    )
+                }
+        }
+
+    val edits = filter { it.isKind(ConcordKind.EDIT) }
+        .mapNotNull { rumor -> rumor.targetHex()?.let { it to rumor } }
+        .groupBy({ it.first }, { it.second })
+        .mapValues { (_, candidates) -> candidates.maxBy { it.concordTimestampMs() } }
+
+    return messages.map { message ->
+        val edit = edits[message.idHex]?.takeIf { it.author().toHex() == message.author }
+        message.copy(
+            content = edit?.content() ?: message.content,
+            edited = edit != null,
+            reactions = reactions[message.idHex].orEmpty(),
+        )
+    }
+}
+
+private fun UnsignedEvent.isKind(expected: Int): Boolean = kind().asU16() == expected.toUShort()
+
+/** The target of a reaction, edit or delete: the first `e` tag (CORD-03 §2.3–2.5). */
+private fun UnsignedEvent.targetHex(): String? =
+    tags().toVec().firstOrNull { it.kind() == ConcordTag.E }?.content()

shared/src/commonMain/kotlin/su/reya/coop/concord/ConcordPlane.kt

@@ -0,0 +1,179 @@
+package su.reya.coop.concord
+
+import kotlinx.coroutines.CancellationException
+import rust.nostr.sdk.AsyncNostrSigner
+import rust.nostr.sdk.Event
+import rust.nostr.sdk.EventBuilder
+import rust.nostr.sdk.Keys
+import rust.nostr.sdk.Kind
+import rust.nostr.sdk.Nip44Version
+import rust.nostr.sdk.PublicKey
+import rust.nostr.sdk.Tag
+import rust.nostr.sdk.Timestamp
+import rust.nostr.sdk.UnsignedEvent
+import rust.nostr.sdk.nip44Decrypt
+import rust.nostr.sdk.nip44Encrypt
+import kotlin.time.Instant
+
+enum class SealForm(private val kindValue: Int) {
+    Encrypted(ConcordKind.SEAL),
+    Plaintext(ConcordKind.PLAINTEXT_SEAL);
+
+    val kind: UShort get() = kindValue.toUShort()
+}
+
+data class ChatBinding(val channelIdHex: String, val epoch: ULong)
+
+class PlaneKey(
+    val read: GroupKey,
+    val stream: GroupKey?,
+    val streamPublicKeyHex: String,
+    val form: SealForm,
+    val chat: ChatBinding?,
+)
+
+fun channelPlaneKey(channelSecret: ByteArray, channelId: ByteArray, epoch: ULong): PlaneKey {
+    val key = groupKey(ConcordLabel.CHANNEL, channelSecret, channelId, epoch)
+    return PlaneKey(
+        read = key,
+        stream = key,
+        streamPublicKeyHex = key.publicKey.toHex(),
+        form = SealForm.Encrypted,
+        chat = ChatBinding(channelId.toHex(), epoch),
+    )
+}
+
+fun guestbookPlaneKey(communityRoot: ByteArray, communityId: ByteArray, epoch: ULong): PlaneKey {
+    val key = groupKey(ConcordLabel.GUESTBOOK, communityRoot, communityId, epoch)
+    return PlaneKey(
+        read = key,
+        stream = key,
+        streamPublicKeyHex = key.publicKey.toHex(),
+        form = SealForm.Encrypted,
+        chat = null,
+    )
+}
+
+fun controlPlaneKey(
+    communityRoot: ByteArray,
+    communityId: ByteArray,
+    epoch: ULong,
+    streamPublicKeyHex: String,
+): PlaneKey {
+    val key = groupKey(ConcordLabel.CONTROL, communityRoot, communityId, epoch)
+    return PlaneKey(
+        read = key,
+        stream = null,
+        streamPublicKeyHex = streamPublicKeyHex,
+        form = SealForm.Plaintext,
+        chat = null,
+    )
+}
+
+fun PlaneKey.rumor(
+    author: PublicKey,
+    kind: UShort,
+    content: String,
+    createdAt: Instant,
+    extraTags: List<Tag> = emptyList(),
+): UnsignedEvent {
+    val tags = buildList {
+        chat?.let {
+            add(Tag.custom(ConcordTag.CHANNEL, listOf(it.channelIdHex)))
+            add(Tag.custom(ConcordTag.EPOCH, listOf(it.epoch.toString())))
+        }
+        add(Tag.custom(ConcordTag.MS, listOf(msOf(createdAt).toString())))
+        addAll(extraTags)
+    }
+    return EventBuilder(Kind(kind), content)
+        .tags(tags)
+        .customCreatedAt(Timestamp.fromSecs(createdAt.epochSeconds.toULong()))
+        .finalizeUnsigned(author)
+        .ensureId()
+}
+
+suspend fun PlaneKey.wrap(rumor: UnsignedEvent, signer: AsyncNostrSigner): Event {
+    val stream = stream ?: error("Concord: this plane is read-only and cannot wrap")
+    val createdAt = rumor.createdAt()
+
+    val rumorJson = rumor.asJson()
+    val content = if (form == SealForm.Encrypted) nip44Seal(read, rumorJson) else rumorJson
+
+    val seal = try {
+        EventBuilder(Kind(form.kind), content).customCreatedAt(createdAt).finalizeAsync(signer)
+    } catch (e: CancellationException) {
+        throw e
+    } catch (e: Exception) {
+        throw IllegalStateException("Concord: failed to seal rumor: ${e.message}", e)
+    }
+
+    check(seal.author() == rumor.author()) {
+        "Concord: seal author ${seal.author().toHex()} does not match rumor author ${
+            rumor.author().toHex()
+        }"
+    }
+
+    return try {
+        val ephemeral = Keys.generate().use { Tag.publicKey(it.publicKey()) }
+        Keys(stream.secretKey).use { keys ->
+            EventBuilder(Kind(concordKind(ConcordKind.WRAP)), nip44Seal(read, seal.asJson()))
+                .tags(listOf(ephemeral))
+                .customCreatedAt(createdAt)
+                .finalize(keys)
+        }
+    } catch (e: CancellationException) {
+        throw e
+    } catch (e: Exception) {
+        throw IllegalStateException("Concord: failed to wrap seal: ${e.message}", e)
+    }
+}
+
+fun PlaneKey.unwrap(event: Event): UnsignedEvent? {
+    if (event.kind().asU16() != ConcordKind.WRAP.toUShort()) return null
+    if (event.author().toHex() != streamPublicKeyHex) return null
+    if (!event.verify()) return null
+
+    val seal = runCatching {
+        Event.fromJson(nip44Decrypt(read.secretKey, read.publicKey, event.content()))
+    }.getOrNull() ?: return null
+    if (!seal.verify()) return null
+
+    val sealKind = seal.kind().asU16()
+    val rumorJson = when {
+        sealKind == SealForm.Encrypted.kind && form == SealForm.Encrypted -> runCatching {
+            nip44Decrypt(read.secretKey, read.publicKey, seal.content())
+        }.getOrNull() ?: return null
+        // Plaintext seal: the rumor's bytes are already the content, never re-parsed as an event.
+        sealKind == SealForm.Plaintext.kind && form == SealForm.Plaintext -> seal.content()
+        else -> return null
+    }
+
+    val rumor =
+        runCatching { UnsignedEvent.fromJson(rumorJson).ensureId() }.getOrNull() ?: return null
+    // NIP-59's impersonation check: the seal proves who wrote the rumor inside it.
+    if (rumor.author() != seal.author()) return null
+    if (!bindsToThisPlane(rumor)) return null
+    return rumor
+}
+
+private fun PlaneKey.bindsToThisPlane(rumor: UnsignedEvent): Boolean {
+    val expected = chat ?: return true
+    val tags = rumor.tags().toVec()
+    val channel = tags.firstOrNull { it.kind() == ConcordTag.CHANNEL }?.content()
+    val epoch = tags.firstOrNull { it.kind() == ConcordTag.EPOCH }?.content()
+    return channel == expected.channelIdHex && epoch == expected.epoch.toString()
+}
+
+private fun msOf(createdAt: Instant): Int = createdAt.toEpochMilliseconds().mod(1000L).toInt()
+
+private fun concordKind(kind: Int): UShort = kind.toUShort()
+
+private fun nip44Seal(key: GroupKey, plaintext: String): String {
+    val size = plaintext.encodeToByteArray().size
+    require(size <= MAX_PLAINTEXT_BYTES) {
+        "Concord: NIP-44 plaintext is $size bytes, over the $MAX_PLAINTEXT_BYTES cap"
+    }
+    return nip44Encrypt(key.secretKey, key.publicKey, plaintext, Nip44Version.V2)
+}
+
+private const val MAX_PLAINTEXT_BYTES = 65_535

shared/src/commonMain/kotlin/su/reya/coop/concord/ConcordStore.kt

@@ -0,0 +1,102 @@
+package su.reya.coop.concord
+
+import kotlinx.coroutines.CancellationException
+import rust.nostr.sdk.EventBuilder
+import rust.nostr.sdk.Filter
+import rust.nostr.sdk.Keys
+import rust.nostr.sdk.Kind
+import rust.nostr.sdk.KindStandard
+import rust.nostr.sdk.Tag
+import rust.nostr.sdk.UnsignedEvent
+import su.reya.coop.AppStorage
+import su.reya.coop.nostr.Nostr
+
+class ConcordStore(private val storage: AppStorage, private val nostr: Nostr) {
+    suspend fun loadMemberships(): List<Membership> {
+        val raw = try {
+            storage.getSecret(MEMBERSHIPS_KEY)
+        } catch (e: CancellationException) {
+            throw e
+        } catch (e: Exception) {
+            println("Concord: could not read memberships: ${e.message}")
+            return emptyList()
+        } ?: return emptyList()
+
+        return try {
+            concordJson.decodeFromString<List<Membership>>(raw)
+        } catch (e: Exception) {
+            println("Concord: stored memberships are unreadable: ${e.message}")
+            emptyList()
+        }
+    }
+
+    suspend fun saveMemberships(memberships: List<Membership>) {
+        try {
+            storage.setSecret(MEMBERSHIPS_KEY, concordJson.encodeToString(memberships))
+        } catch (e: CancellationException) {
+            throw e
+        } catch (e: Exception) {
+            throw IllegalStateException("Concord: could not store memberships: ${e.message}", e)
+        }
+    }
+
+    suspend fun clearMemberships() {
+        try {
+            storage.clear(MEMBERSHIPS_KEY)
+        } catch (e: CancellationException) {
+            throw e
+        } catch (e: Exception) {
+            println("Concord: could not clear memberships: ${e.message}")
+        }
+    }
+
+    suspend fun cacheRumor(scopeIdHex: String, wrapIdHex: String, rumor: UnsignedEvent) {
+        try {
+            val event =
+                EventBuilder(Kind.fromStd(KindStandard.APPLICATION_SPECIFIC_DATA), rumor.asJson())
+                    .tags(
+                        listOf(
+                            Tag.identifier(wrapIdHex),
+                            Tag.custom(INDEX_SCOPE_TAG, listOf(scopeIdHex)),
+                            Tag.custom(INDEX_KIND_TAG, listOf(rumor.kind().asU16().toString())),
+                        )
+                    )
+                    .finalizeAsync(Keys.generate())
+
+            nostr.client?.database()?.saveEvent(event)
+        } catch (e: CancellationException) {
+            throw e
+        } catch (e: Exception) {
+            println("Concord: failed to cache rumor: ${e.message}")
+        }
+    }
+
+    /** Every cached rumor for one scope, oldest first. */
+    suspend fun cachedRumors(scopeIdHex: String): List<UnsignedEvent> {
+        val events = try {
+            val filter = Filter()
+                .kind(Kind.fromStd(KindStandard.APPLICATION_SPECIFIC_DATA))
+                .reference(scopeIdHex)
+            nostr.client?.database()?.query(filter)?.toVec() ?: return emptyList()
+        } catch (e: CancellationException) {
+            throw e
+        } catch (e: Exception) {
+            println("Concord: failed to read cached rumors: ${e.message}")
+            return emptyList()
+        }
+
+        return events
+            .mapNotNull {
+                runCatching {
+                    UnsignedEvent.fromJson(it.content()).ensureId()
+                }.getOrNull()
+            }
+            .sortedBy { it.createdAt().asSecs() }
+    }
+
+    private companion object {
+        const val MEMBERSHIPS_KEY = "concord_memberships"
+        const val INDEX_SCOPE_TAG = "r"
+        const val INDEX_KIND_TAG = "k"
+    }
+}

shared/src/commonMain/kotlin/su/reya/coop/nostr/Messaging.kt

@@ -144,12 +144,15 @@ class MessageManager(private val nostr: Nostr) {
 
     private suspend fun setCachedRumor(giftId: EventId, rumor: UnsignedEvent) {
         try {
+            val isReaction = rumor.kind().asStd() == KindStandard.REACTION
+            val kValue = if (isReaction) "reaction" else "dm"
+
             // Construct reference tags
             val tags = listOf(
                 Tag.identifier(giftId.toHex()),
                 Tag.publicKey(rumor.author()),
                 Tag.custom("r", listOf(rumor.roomId().toString())),
-                Tag.custom("k", listOf("14"))
+                Tag.custom("k", listOf("14", kValue))
             )
 
             // Set event kind
@@ -176,12 +179,13 @@ class MessageManager(private val nostr: Nostr) {
             val kind = Kind.fromStd(KindStandard.APPLICATION_SPECIFIC_DATA)
             val kTag = SingleLetterTag.lowercase(Alphabet.K)
 
-            // Get all DM events
+            // Get all rumors (DMs and Reactions)
-            val filter = Filter().kind(kind).customTags(kTag, listOf("14", "dm"))
+            val filter = Filter().kind(kind).customTags(kTag, listOf("14", "dm", "reaction"))
             val events = client?.database()?.query(filter)?.toVec() ?: return null
 
             // Collect rooms
             val roomsMap: MutableMap<Long, Room> = mutableMapOf()
+            val lastDmTimestampMap: MutableMap<Long, ULong> = mutableMapOf()
 
             events
                 .map { UnsignedEvent.fromJson(it.content()) }
@@ -189,18 +193,41 @@ class MessageManager(private val nostr: Nostr) {
                 .forEach { rumor ->
                     val id = rumor.roomId()
                     val isFromMe = rumor.author() == userPubkey
+                    val isReaction = rumor.kind().asStd() == KindStandard.REACTION
                     val existing = roomsMap[id]
                     val createdAt = rumor.createdAt()
 
-                    // If the room is new or the current rumor is newer than the existing one
+                    if (existing == null) {
-                    if (existing == null || createdAt.asSecs() > existing.createdAt.asSecs()) {
-                        // A room is "Ongoing" if it was already marked as such or if the current rumor is from the user
-                        val isOngoing = (existing?.kind == RoomKind.Ongoing) || isFromMe
                         val room = Room.new(rumor = rumor, userPubkey = userPubkey, id = id)
-                        roomsMap[id] = if (isOngoing) room.copy(kind = RoomKind.Ongoing) else room
+                        // If the first event we see is a reaction, don't use it as lastMessage
-                    } else if (isFromMe && existing.kind != RoomKind.Ongoing) {
+                        roomsMap[id] = if (isReaction) {
-                        // If it's an older rumor but sent by the user, mark the room as Ongoing
+                            room.copy(lastMessage = null)
-                        roomsMap[id] = existing.copy(kind = RoomKind.Ongoing)
+                        } else {
+                            lastDmTimestampMap[id] = createdAt.asSecs()
+                            room
+                        }
+                        if (isFromMe) {
+                            roomsMap[id] = roomsMap[id]!!.copy(kind = RoomKind.Ongoing)
+                        }
+                    } else {
+                        // Update the overall room timestamp (for sorting) if this event is newer
+                        if (createdAt.asSecs() > existing.createdAt.asSecs()) {
+                            roomsMap[id] = roomsMap[id]!!.copy(createdAt = createdAt)
+                        }
+
+                        // Update the last message content if this is a DM and it's newer than the last DM we've seen
+                        if (!isReaction) {
+                            val lastDmTs = lastDmTimestampMap[id] ?: 0uL
+                            if (createdAt.asSecs() >= lastDmTs) {
+                                lastDmTimestampMap[id] = createdAt.asSecs()
+                                roomsMap[id] = roomsMap[id]!!.copy(lastMessage = rumor.content())
+                            }
+                        }
+
+                        // If any event is from the user, mark the room as Ongoing
+                        if (isFromMe && roomsMap[id]?.kind != RoomKind.Ongoing) {
+                            roomsMap[id] = roomsMap[id]!!.copy(kind = RoomKind.Ongoing)
+                        }
                     }
                 }
 
@@ -348,4 +375,64 @@ class MessageManager(private val nostr: Nostr) {
             throw IllegalStateException("Failed to send message: ${e.message}", e)
         }
     }
+
+    suspend fun sendReaction(
+        to: Set<PublicKey>,
+        targetEventId: EventId,
+        reaction: String,
+        onRumorCreated: ((UnsignedEvent) -> Unit)? = null,
+    ) {
+        try {
+            val currentUser =
+                signer.getPublicKeyAsync() ?: throw IllegalStateException("User not signed in")
+
+            val tags = mutableListOf<Tag>()
+            tags.add(Tag.event(targetEventId))
+            // Add public key tags for each recipient (including me) to ensure roomId consistency
+            to.forEach { pubkey ->
+                tags.add(Tag.publicKey(pubkey))
+            }
+
+            for (receiver in setOf(currentUser) + to) {
+                // Construct the rumor event
+                val rumor = EventBuilder(Kind.fromStd(KindStandard.REACTION), reaction)
+                    .tags(tags)
+                    .finalizeUnsigned(currentUser)
+                    .ensureId()
+
+                // Emit the rumor to the chat screen
+                if (receiver == currentUser) {
+                    onRumorCreated?.invoke(rumor)
+                }
+
+                // Construct the gift wrap event
+                val gift = nip59MakeGiftWrapAsync(
+                    signer = signer,
+                    receiverPubkey = receiver,
+                    rumor = rumor,
+                    extraTags = listOf(
+                        Tag.custom("k", listOf("14"))
+                    )
+                )
+
+                // Send the event to receiver's NIP-17 relays
+                val output = client?.sendEvent(
+                    event = gift,
+                    target = SendEventTarget.toNip17(),
+                    ackPolicy = AckPolicy.none(),
+                    authenticationTimeout = Duration.parse("2s")
+                )
+
+                if (output != null) {
+                    // Keep track of rumor IDs
+                    val id = rumor.id() ?: throw IllegalStateException("Rumor ID is null")
+                    rumorMap[id] = output.id
+                }
+            }
+        } catch (e: CancellationException) {
+            throw e
+        } catch (e: Exception) {
+            throw IllegalStateException("Failed to send reaction: ${e.message}", e)
+        }
+    }
 }

shared/src/commonMain/kotlin/su/reya/coop/nostr/Nostr.kt

@@ -32,6 +32,8 @@ import rust.nostr.sdk.SleepWhenIdle
 import rust.nostr.sdk.Timestamp
 import rust.nostr.sdk.UnsignedEvent
 import rust.nostr.sdk.initLogger
+import su.reya.coop.AppStorage
+import su.reya.coop.concord.ConcordManager
 import kotlin.time.Duration
 import kotlin.time.Duration.Companion.seconds
 
@@ -50,6 +52,7 @@ class Nostr(
     val messages = MessageManager(this)
     val profiles = ProfileManager(this)
     val relays = RelayManager(this)
+    val concord = ConcordManager(this)
 
     private val isInitialized = MutableStateFlow(false)
 
@@ -64,10 +67,18 @@ class Nostr(
         _newEvents.tryEmit(event)
     }
 
-    suspend fun init(dbPath: String, logLevel: LogLevel = LogLevel.WARN) {
+    suspend fun init(
+        dbPath: String,
+        storage: AppStorage,
+        logLevel: LogLevel = LogLevel.WARN,
+    ) {
         try {
             if (isInitialized.value) return
 
+            // Concord's keys live in the encrypted store, which only the Android layer can build.
+            // Handing it over here keeps it out of the Context-free singleton's constructor.
+            concord.attach(storage)
+
             // Initialize the logger for nostr client
             initLogger(logLevel)
 
@@ -144,16 +155,35 @@ class Nostr(
         val notifications = client?.notifications() ?: return@supervisorScope
 
         val giftWrapQueue = Channel<Event>(1024)
+        val concordQueue = Channel<Event>(1024)
+
         var processedCount = 0
         var eoseReceived = false
 
+        try {
+            concord.restore()
+        } catch (e: Exception) {
+            println("Failed to restore Concord state: ${e.message}")
+        }
+
+        launch(defaultDispatcher) { concord.sync() }
+
+        launch(defaultDispatcher) {
+            for (event in concordQueue) {
+                try {
+                    concord.handlePlaneEvent(event)
+                } catch (e: Exception) {
+                    println("Failed to handle Concord plane event: ${e.message}")
+                }
+            }
+        }
+
         launch(defaultDispatcher) {
             for (event in giftWrapQueue) {
                 val rumor = messages.extractRumor(event)
                 processedCount++
 
-                // Trigger new message notification
+                if (rumor != null && !concord.onInboxRumor(rumor)) {
-                if (rumor != null) {
                     val isSelfMessage = rumor.author() == signer.publicKeyFlow.value
                     val isNew = rumor.createdAt().asSecs() >= now.asSecs()
 
@@ -214,7 +244,11 @@ class Nostr(
                                 }
 
                                 KindStandard.GIFT_WRAP -> {
-                                    giftWrapQueue.send(event)
+                                    if (concord.isPlaneAddress(event.author().toHex())) {
+                                        concordQueue.send(event)
+                                    } else {
+                                        giftWrapQueue.send(event)
+                                    }
                                 }
 
                                 else -> {}

shared/src/commonMain/kotlin/su/reya/coop/nostr/RelayManager.kt

@@ -22,13 +22,15 @@ import kotlin.time.Duration
 class RelayManager(private val nostr: Nostr) {
     companion object {
         val BOOTSTRAP_RELAYS = listOf(
-            "wss://relay.primal.net",
             "wss://relay.ditto.pub",
-            "wss://user.kindpag.es",
+            "wss://relay.primal.net",
+            "wss://relay.nostr.net",
+            "wss://profiles.nostr1.com",
         )
 
         val INDEXER_RELAY = listOf(
             "wss://indexer.coracle.social",
+            "wss://user.kindpag.es",
         )
 
         val ALL_RELAYS = BOOTSTRAP_RELAYS + INDEXER_RELAY

shared/src/commonMain/kotlin/su/reya/coop/repository/ChatRepository.kt

@@ -241,8 +241,29 @@ class ChatRepository(
         }
     }
 
+    fun sendReaction(roomId: Long, targetEventId: EventId, reaction: String) {
+        scope.launch(defaultDispatcher) {
+            try {
+                val room = getChatRoom(roomId) ?: throw IllegalArgumentException("Room not found")
+                nostr.messages.sendReaction(
+                    to = room.members,
+                    targetEventId = targetEventId,
+                    reaction = reaction,
+                    onRumorCreated = {
+                        scope.launch(defaultDispatcher) {
+                            updateRoomState(it, roomId)
+                        }
+                    },
+                )
+            } catch (e: Exception) {
+                showError("Error: ${e.message}")
+            }
+        }
+    }
+
     private suspend fun updateRoomState(event: UnsignedEvent, roomId: Long = event.roomId()) {
         val currentUser = nostr.signer.getPublicKeyAsync() ?: return
+        val isReaction = event.kind().asStd() == KindStandard.REACTION
 
         _state.update { currentState ->
             val rooms = currentState.rooms.toMutableMap()
@@ -256,22 +277,24 @@ class ChatRepository(
                 // New room discovery
                 val newRoom = Room.new(event, currentUser, roomId).copy(
                     kind = newKind,
-                    unreadCount = if (isFromMe) 0 else 1
+                    unreadCount = if (isFromMe || isReaction) 0 else 1,
+                    lastMessage = if (isReaction) null else event.content()
                 )
                 rooms[newRoom.id] = newRoom
             } else if (event.createdAt().asSecs() >= existingRoom.createdAt.asSecs()) {
-                // Only update preview if message is newer (handles sync/late arrivals)
+                // Update timestamp for any newer event (DM or Reaction)
+                // But only update preview if it's a DM
                 rooms[roomId] = existingRoom.copy(
-                    lastMessage = event.content(),
+                    lastMessage = if (isReaction) existingRoom.lastMessage else event.content(),
                     createdAt = event.createdAt(),
                     kind = newKind,
-                    unreadCount = if (isFromMe) existingRoom.unreadCount else existingRoom.unreadCount + 1
+                    unreadCount = if (isFromMe || isReaction) existingRoom.unreadCount else existingRoom.unreadCount + 1
                 )
             } else if (isFromMe && existingRoom.kind != RoomKind.Ongoing) {
-                // Even if it's an older message, if it's from me, the room is ongoing
+                // Even if it's an older message or reaction, if it's from me, the room is ongoing
                 rooms[roomId] = existingRoom.copy(kind = RoomKind.Ongoing)
             } else {
-                // Don't update the room list state for older messages
+                // Don't update the room list state for older messages or reactions that don't change preview
                 return@update currentState
             }
             currentState.copy(rooms = rooms)

shared/src/commonMain/kotlin/su/reya/coop/repository/ConcordRepository.kt

@@ -0,0 +1,88 @@
+package su.reya.coop.repository
+
+import kotlinx.coroutines.CancellationException
+import kotlinx.coroutines.CoroutineDispatcher
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.launch
+import kotlinx.coroutines.withContext
+import su.reya.coop.concord.CommunityInvite
+import su.reya.coop.concord.CommunityState
+import su.reya.coop.concord.ConcordMessage
+import su.reya.coop.concord.InvitePreview
+import su.reya.coop.nostr.Nostr
+import su.reya.coop.viewmodel.ErrorHost
+import su.reya.coop.viewmodel.createErrorHost
+
+class ConcordRepository(
+    private val nostr: Nostr,
+    private val scope: CoroutineScope,
+    private val defaultDispatcher: CoroutineDispatcher = Dispatchers.Default,
+) : ErrorHost by createErrorHost() {
+    private val concord = nostr.concord
+
+    val communities: StateFlow<List<CommunityState>> = concord.communities
+
+    /** Direct Invites that arrived giftwrapped to us and have not been accepted or dismissed. */
+    val directInvites: StateFlow<List<CommunityInvite>> = concord.directInvites
+
+    /** True once memberships have been loaded, so an empty list means "joined nothing". */
+    val isReady: StateFlow<Boolean> = concord.restored
+
+    /** Bumped whenever a plane rumor lands, so an open Channel knows to re-read its history. */
+    val revision: StateFlow<Long> = concord.revision
+
+    /** Fetches and decrypts the bundle behind a public invite link. */
+    suspend fun previewInvite(link: String): InvitePreview? = attempt { concord.previewInvite(link) }
+
+    /** The same preview for a Direct Invite, which already arrived with its bundle in hand. */
+    fun previewDirectInvite(invite: CommunityInvite): InvitePreview =
+        concord.previewDirectInvite(invite)
+
+    /** Accepts an invite: persist the keys, connect, subscribe, announce the join. */
+    suspend fun join(preview: InvitePreview): CommunityState? = attempt { concord.join(preview) }
+
+    /** A Channel's history, oldest first, from the local cache the live subscription keeps current. */
+    suspend fun channelMessages(channelIdHex: String): List<ConcordMessage> =
+        attempt { concord.channelMessages(channelIdHex) }.orEmpty()
+
+    suspend fun sendMessage(channelIdHex: String, content: String): ConcordMessage? =
+        attempt { concord.sendChannelMessage(channelIdHex, content) }
+
+    /** Reacts to one message; `targetAuthorHex` is the message's author, which rides the `p` tag. */
+    suspend fun sendReaction(
+        channelIdHex: String,
+        targetIdHex: String,
+        targetAuthorHex: String,
+        reaction: String,
+    ) {
+        attempt { concord.sendChannelReaction(channelIdHex, targetIdHex, targetAuthorHex, reaction) }
+    }
+
+    /** Replaces one of our own messages with [content]. */
+    suspend fun editMessage(channelIdHex: String, targetIdHex: String, content: String) {
+        attempt { concord.sendChannelEdit(channelIdHex, targetIdHex, content) }
+    }
+
+    /** Clears a Channel's badge — the Channel screen calls this once its messages are on display. */
+    fun markChannelRead(channelIdHex: String) = concord.markChannelRead(channelIdHex)
+
+    fun dismissDirectInvite(communityIdHex: String) = concord.dismissDirectInvite(communityIdHex)
+
+    fun resetInternalState() {
+        scope.launch(defaultDispatcher) { attempt { concord.reset() } }
+    }
+
+    /** Runs one Concord call off the main thread, reporting any failure instead of throwing. */
+    private suspend fun <T> attempt(block: suspend () -> T): T? = withContext(defaultDispatcher) {
+        try {
+            block()
+        } catch (e: CancellationException) {
+            throw e
+        } catch (e: Exception) {
+            showError("Error: ${e.message}")
+            null
+        }
+    }
+}

shared/src/commonMain/kotlin/su/reya/coop/viewmodel/ChannelScreenViewModel.kt

@@ -0,0 +1,83 @@
+package su.reya.coop.viewmodel
+
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateListOf
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.setValue
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.flow.SharingStarted
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.drop
+import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.stateIn
+import kotlinx.coroutines.launch
+import su.reya.coop.Profile
+import su.reya.coop.concord.ConcordChannel
+import su.reya.coop.concord.ConcordMessage
+import su.reya.coop.repository.AccountRepository
+import su.reya.coop.repository.ConcordRepository
+
+class ChannelScreenViewModel(
+    val communityId: String,
+    val channelId: String,
+    accountRepository: AccountRepository,
+    private val concordRepository: ConcordRepository,
+) : ViewModel(), ErrorHost by concordRepository {
+    val currentUser: StateFlow<Profile?> = accountRepository.currentUserProfile
+
+    val channel: StateFlow<ConcordChannel?> = concordRepository.communities
+        .map { communities ->
+            communities
+                .firstOrNull { it.membership.communityId == communityId }
+                ?.channels
+                ?.firstOrNull { it.idHex == channelId }
+        }
+        .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5000), null)
+
+    var loading by mutableStateOf(true)
+    val messages = mutableStateListOf<ConcordMessage>()
+
+    private var reloadJob: Job? = null
+
+    init {
+        reload()
+
+        viewModelScope.launch {
+            concordRepository.revision.drop(1).collect { reload() }
+        }
+    }
+
+    private fun reload() {
+        reloadJob?.cancel()
+        reloadJob = viewModelScope.launch {
+            val loaded = concordRepository.channelMessages(channelId)
+            // The whole list, not just its ids: a reaction or an edit changes a message without
+            // changing which messages there are.
+            if (messages != loaded) {
+                messages.clear()
+                messages.addAll(loaded)
+            }
+            loading = false
+            concordRepository.markChannelRead(channelId)
+        }
+    }
+
+    fun sendMessage(text: String) {
+        if (text.isBlank()) return
+        viewModelScope.launch { concordRepository.sendMessage(channelId, text) }
+    }
+
+    fun sendReaction(message: ConcordMessage, reaction: String) {
+        viewModelScope.launch {
+            concordRepository.sendReaction(channelId, message.idHex, message.author, reaction)
+        }
+    }
+
+    /** Replaces one of our own messages; [targetIdHex] is the message being edited. */
+    fun editMessage(targetIdHex: String, text: String) {
+        if (text.isBlank()) return
+        viewModelScope.launch { concordRepository.editMessage(channelId, targetIdHex, text) }
+    }
+}

shared/src/commonMain/kotlin/su/reya/coop/viewmodel/ChatScreenViewModel.kt

@@ -74,4 +74,8 @@ class ChatScreenViewModel(
     fun sendFileMessage(file: ByteArray?, type: String?) {
         chatRepository.sendFileMessage(id, file, type)
     }
+
+    fun sendReaction(targetEventId: EventId, reaction: String) {
+        chatRepository.sendReaction(id, targetEventId, reaction)
+    }
 }

shared/src/commonMain/kotlin/su/reya/coop/viewmodel/ConcordViewModel.kt

@@ -0,0 +1,27 @@
+package su.reya.coop.viewmodel
+
+import androidx.lifecycle.ViewModel
+import kotlinx.coroutines.flow.StateFlow
+import su.reya.coop.concord.CommunityInvite
+import su.reya.coop.concord.CommunityState
+import su.reya.coop.concord.InvitePreview
+import su.reya.coop.repository.ConcordRepository
+
+class ConcordViewModel(
+    private val repository: ConcordRepository,
+) : ViewModel(), ErrorHost by repository {
+    val communities: StateFlow<List<CommunityState>> = repository.communities
+    val directInvites: StateFlow<List<CommunityInvite>> = repository.directInvites
+    val isReady: StateFlow<Boolean> = repository.isReady
+
+    fun previewDirectInvite(invite: CommunityInvite): InvitePreview =
+        repository.previewDirectInvite(invite)
+
+    suspend fun previewInvite(link: String): InvitePreview? = repository.previewInvite(link)
+
+    suspend fun join(preview: InvitePreview): CommunityState? = repository.join(preview)
+
+    fun dismissDirectInvite(communityIdHex: String) = repository.dismissDirectInvite(communityIdHex)
+
+    fun resetInternalState() = repository.resetInternalState()
+}