feat: add community ui (#52)

Reviewed-on: reya/coop#52

crates/concord/src/utils/base64url.rs

@@ -0,0 +1,34 @@
+use anyhow::{Result, anyhow};
+use base64::engine::{DecodePaddingMode, GeneralPurpose, GeneralPurposeConfig};
+use base64::{Engine as _, alphabet};
+
+/// Unpadded base64url (RFC 4648 §5), 43 characters for 32 bytes: §8's value
+/// encoding at any depth.
+///
+/// The reader tolerates non-zero trailing bits; the writer never emits them.
+/// The spec's own worked example (`examples.md` §6.2) contains five such
+/// values, and a reader cannot tell a mis-encoded named field from a correctly
+/// encoded one, so the boundary is the writer's alone.
+const BASE64URL: GeneralPurpose = GeneralPurpose::new(
+    &alphabet::URL_SAFE,
+    GeneralPurposeConfig::new()
+        .with_encode_padding(false)
+        .with_decode_padding_mode(DecodePaddingMode::RequireNone)
+        .with_decode_allow_trailing_bits(true),
+);
+
+pub(crate) fn encode(bytes: &[u8]) -> String {
+    BASE64URL.encode(bytes)
+}
+
+/// Decodes one 32-byte value, the width every §8 field has.
+pub(crate) fn decode_32(value: &str) -> Result<[u8; 32]> {
+    let bytes = BASE64URL
+        .decode(value.trim())
+        .map_err(|error| anyhow!("invalid base64url: {error}"))?;
+
+    bytes
+        .as_slice()
+        .try_into()
+        .map_err(|_| anyhow!("expected 32 bytes, got {}", bytes.len()))
+}