diff --git a/Cargo.lock b/Cargo.lock
index 0ebd346..6284232 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2159,6 +2159,7 @@ dependencies = [
  "rand 0.9.5",
  "rusqlite",
  "serde",
+ "serde_json",
  "tempfile",
  "thiserror 2.0.19",
  "tokio",
@@ -3299,6 +3300,19 @@ dependencies = [
  "syn 3.0.2",
 ]
 
+[[package]]
+name = "serde_json"
+version = "1.0.151"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c841b55ecdae098c80dcae9cf767f6f8a0c2cdb3416bbef72181df4d0fe73f14"
+dependencies = [
+ "itoa",
+ "memchr",
+ "serde",
+ "serde_core",
+ "zmij",
+]
+
 [[package]]
 name = "serdect"
 version = "0.4.3"
@@ -4681,3 +4695,9 @@ dependencies = [
  "quote",
  "syn 2.0.119",
 ]
+
+[[package]]
+name = "zmij"
+version = "1.0.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "29666d0abbfad1e3dc4dcf6144730dd3a3ab225bbbdac83319345b1b44ccfc1b"
diff --git a/Cargo.toml b/Cargo.toml
index 4b27dca..f249e9e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -21,6 +21,7 @@ iroh-tickets = "1"
 mainline = "7"
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "sync", "time", "io-util", "io-std", "signal"] }
 serde = { version = "1", features = ["derive"] }
+serde_json = "1"
 postcard = { version = "1", features = ["alloc"] }
 blake3 = "1"
 futures = "0.3"
diff --git a/crates/music-dht/Cargo.toml b/crates/music-dht/Cargo.toml
index c9d8111..166a4d3 100644
--- a/crates/music-dht/Cargo.toml
+++ b/crates/music-dht/Cargo.toml
@@ -10,6 +10,7 @@ rust-version.workspace = true
 federation-net = { workspace = true }
 tokio = { workspace = true }
 serde = { workspace = true }
+serde_json = { workspace = true }
 postcard = { workspace = true }
 blake3 = { workspace = true }
 futures = { workspace = true }
diff --git a/crates/music-dht/src/device_sync.rs b/crates/music-dht/src/device_sync.rs
new file mode 100644
index 0000000..d60beeb
--- /dev/null
+++ b/crates/music-dht/src/device_sync.rs
@@ -0,0 +1,733 @@
+//! Wire protocol shared by furumi personal-device synchronization clients.
+//!
+//! The storage and UI policy live in applications. This module only defines
+//! the stable JSON-lines messages spoken over the auxiliary iroh stream ALPN,
+//! plus small helpers for opaque `frid://i/...` invites.
+
+use std::collections::BTreeMap;
+use std::str::FromStr;
+
+use federation_net::{ByteStream, NetworkId, PeerTicket, SecretKey};
+use serde::{Deserialize, Serialize};
+use tokio::io::{AsyncRead, AsyncReadExt};
+
+use crate::error::{MusicDhtError, Result};
+
+/// Auxiliary ALPN used for personal-device sync streams.
+pub const SYNC_ALPN: &[u8] = b"furumi/sync/1";
+
+/// Version of the personal-device sync wire protocol.
+pub const DEVICE_SYNC_PROTOCOL_VERSION: u16 = 1;
+
+/// Default invite lifetime used by applications unless they need a custom TTL.
+pub const DEFAULT_INVITE_TTL_MS: i64 = 10 * 60 * 1000;
+
+/// Maximum JSON line accepted by the protocol.
+pub const MAX_SYNC_LINE: usize = 8 * 1024 * 1024;
+
+/// Opaque invite payload encoded into `frid://i/<base64url-json>`.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct InviteWire {
+    /// Invite schema version.
+    pub v: u16,
+    /// Transport ticket for the inviting endpoint.
+    #[serde(rename = "t")]
+    pub ticket: String,
+    /// Device id of the inviter.
+    #[serde(rename = "d")]
+    pub device_id: String,
+    /// Short invite id stored by the inviter.
+    #[serde(rename = "i")]
+    pub invite_id: String,
+    /// Runtime pairing secret.
+    #[serde(rename = "s")]
+    pub secret: String,
+    /// Expiration timestamp in unix milliseconds.
+    #[serde(rename = "e")]
+    pub expires_at_ms: i64,
+}
+
+/// Public device profile exchanged inside a trusted sync group.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct DeviceProfileWire {
+    /// Stable device id.
+    pub device_id: String,
+    /// User-visible device name.
+    pub name: String,
+    /// Application version.
+    pub client_version: String,
+    /// Wire protocol version.
+    pub protocol_version: u16,
+    /// Transport endpoint id, as string.
+    pub endpoint_id: String,
+    /// Transport ticket used for direct reconnects.
+    pub endpoint_ticket: String,
+    /// Whether the device has been revoked.
+    #[serde(default)]
+    pub revoked: bool,
+    /// Last accepted seq from this device after revocation.
+    #[serde(default)]
+    pub revoke_cutoff_seq: Option<i64>,
+    /// Profile update timestamp in unix milliseconds.
+    #[serde(default)]
+    pub updated_at_ms: i64,
+}
+
+/// Portable track reference embedded into sync payloads for unresolved tracks.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct SyncedFedTrack {
+    /// DHT item id, when known.
+    pub item_id: String,
+    /// DHT owner endpoint id, when known.
+    pub owner: String,
+    /// Track title.
+    pub title: String,
+    /// Main artist names.
+    #[serde(default)]
+    pub artist_names: Vec<String>,
+    /// Featured artist names.
+    #[serde(default)]
+    pub featured_artist_names: Vec<String>,
+    /// Release or track year.
+    pub year: Option<i32>,
+    /// Duration in seconds.
+    pub duration_seconds: Option<i64>,
+    /// Stable content id (`b3:<hex>`).
+    pub content_id: String,
+    /// Release title, when known.
+    pub release_title: Option<String>,
+    /// Track number.
+    pub track_number: Option<i32>,
+    /// Disc number.
+    pub disc_number: Option<i32>,
+}
+
+/// Portable playback queue track.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct PlaybackTrack {
+    /// Local or placeholder id on the sending device.
+    pub id: i64,
+    /// Track title.
+    pub title: String,
+    /// Track number.
+    pub track_number: Option<i32>,
+    /// Disc number.
+    pub disc_number: Option<i32>,
+    /// Duration in seconds.
+    pub duration_seconds: f64,
+    /// Main artist names.
+    #[serde(default)]
+    pub artist_names: Vec<String>,
+    /// Featured artist names.
+    #[serde(default)]
+    pub featured_artist_names: Vec<String>,
+    /// Local or placeholder release id.
+    pub release_id: i64,
+    /// Release title.
+    pub release_title: String,
+    /// Release year.
+    pub release_year: Option<i32>,
+    /// Legacy compatibility only. Paths are device-local and should be empty.
+    #[serde(default)]
+    pub file_path: String,
+    /// Stable content id.
+    pub content_id: Option<String>,
+    /// Audio format label.
+    pub audio_format: Option<String>,
+    /// Audio bitrate.
+    pub audio_bitrate: Option<i32>,
+    /// Audio sample rate.
+    pub audio_sample_rate: Option<i32>,
+    /// Audio bit depth.
+    pub audio_bit_depth: Option<i32>,
+    /// File size in bytes.
+    pub file_size_bytes: Option<i64>,
+    /// Sender-side play count.
+    #[serde(default)]
+    pub play_count: i64,
+    /// Federation metadata for unresolved tracks.
+    #[serde(default)]
+    pub fed: Option<SyncedFedTrack>,
+}
+
+/// Playback repeat mode.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
+#[serde(rename_all = "snake_case")]
+pub enum PlaybackRepeat {
+    /// Repeat disabled.
+    #[default]
+    Off,
+    /// Repeat current track.
+    One,
+    /// Repeat the whole queue.
+    All,
+}
+
+/// Portable playback state.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct PlaybackStateWire {
+    /// Queue tracks.
+    #[serde(default)]
+    pub queue: Vec<PlaybackTrack>,
+    /// Current queue index.
+    #[serde(default)]
+    pub queue_pos: usize,
+    /// Whether playback is active.
+    pub playing: bool,
+    /// Whether playback is paused.
+    pub paused: bool,
+    /// Active device idle timestamp, when paused/stopped.
+    #[serde(default)]
+    pub idle_since_ms: Option<i64>,
+    /// Current position in seconds.
+    pub position_secs: f64,
+    /// Volume 0..100.
+    #[serde(default)]
+    pub volume: u8,
+    /// Shuffle mode.
+    pub shuffle: bool,
+    /// Repeat mode.
+    pub repeat: PlaybackRepeat,
+}
+
+/// Playback status broadcast by a device during sync.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct PlaybackSnapshot {
+    /// Device id.
+    pub device_id: String,
+    /// Device name.
+    pub device_name: String,
+    /// Whether this device considers itself active.
+    pub active: bool,
+    /// Update timestamp in unix milliseconds.
+    pub updated_at_ms: i64,
+    /// Playback state.
+    pub state: PlaybackStateWire,
+}
+
+/// Playback command delivered through the sync op log.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(tag = "kind", rename_all = "snake_case")]
+pub enum PlaybackCommand {
+    /// Replace target state. `seek` means the target should also seek audio.
+    SetState {
+        /// New state.
+        state: PlaybackStateWire,
+        /// Whether to seek the audio sink to `state.position_secs`.
+        #[serde(default)]
+        seek: bool,
+    },
+    /// Another device became active.
+    ActiveChanged {
+        /// New active device id.
+        active_device_id: String,
+        /// New active device name.
+        active_device_name: String,
+        /// Transferred playback state.
+        state: PlaybackStateWire,
+    },
+}
+
+/// One operation in the device-sync log.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct SyncOpWire {
+    /// Unique op id, usually `<origin_device_id>:<seq>`.
+    pub op_id: String,
+    /// Origin device id.
+    pub origin_device_id: String,
+    /// Monotonic sequence inside the origin device log.
+    pub seq: i64,
+    /// Hybrid/logical timestamp in unix milliseconds.
+    pub hlc_ms: i64,
+    /// Operation payload.
+    pub payload: SyncOpPayload,
+}
+
+/// Personal-device sync operation payload.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(tag = "kind", rename_all = "snake_case")]
+pub enum SyncOpPayload {
+    /// Set like state for a content id.
+    TrackLikeSet {
+        /// Content id.
+        content_id: String,
+        /// Desired like state.
+        liked: bool,
+        /// Optional unresolved federated metadata.
+        #[serde(default)]
+        fed: Option<SyncedFedTrack>,
+    },
+    /// Create playlist.
+    PlaylistCreated {
+        /// Stable playlist sync id.
+        playlist_id: String,
+        /// Playlist title.
+        title: String,
+    },
+    /// Rename playlist.
+    PlaylistRenamed {
+        /// Stable playlist sync id.
+        playlist_id: String,
+        /// Playlist title.
+        title: String,
+    },
+    /// Delete playlist.
+    PlaylistDeleted {
+        /// Stable playlist sync id.
+        playlist_id: String,
+    },
+    /// Add track to playlist.
+    PlaylistTrackAdded {
+        /// Stable playlist sync id.
+        playlist_id: String,
+        /// Content id.
+        content_id: String,
+        /// Playlist position.
+        position: i64,
+        /// Optional unresolved federated metadata.
+        #[serde(default)]
+        fed: Option<SyncedFedTrack>,
+    },
+    /// Remove track from playlist.
+    PlaylistTrackRemoved {
+        /// Stable playlist sync id.
+        playlist_id: String,
+        /// Content id.
+        content_id: String,
+    },
+    /// Update origin device profile.
+    DeviceProfileSet {
+        /// Display name.
+        name: String,
+        /// Client version.
+        client_version: String,
+        /// Endpoint ticket.
+        endpoint_ticket: String,
+        /// Endpoint id.
+        endpoint_id: String,
+    },
+    /// Trust a device.
+    DeviceTrusted {
+        /// Trusted device id.
+        target_device_id: String,
+    },
+    /// Revoke a device.
+    DeviceRevoked {
+        /// Revoked device id.
+        target_device_id: String,
+        /// Max seq seen from the target at revoke time.
+        target_max_seq_seen: i64,
+    },
+    /// Playback command for a target device.
+    PlaybackCommand {
+        /// Target device id.
+        target_device_id: String,
+        /// Command.
+        command: PlaybackCommand,
+    },
+}
+
+impl SyncOpPayload {
+    /// Returns true for operations that remove state and may eventually compact.
+    pub fn is_tombstone(&self) -> bool {
+        matches!(
+            self,
+            SyncOpPayload::TrackLikeSet { liked: false, .. }
+                | SyncOpPayload::PlaylistDeleted { .. }
+                | SyncOpPayload::PlaylistTrackRemoved { .. }
+                | SyncOpPayload::DeviceRevoked { .. }
+        )
+    }
+}
+
+/// Materialized sync snapshot sent with every handshake.
+#[derive(Debug, Clone, Default, PartialEq, Serialize, Deserialize)]
+pub struct SyncSnapshot {
+    /// Current likes.
+    #[serde(default)]
+    pub likes: Vec<SnapshotLike>,
+    /// Like tombstones.
+    #[serde(default)]
+    pub unlikes: Vec<SnapshotLikeTombstone>,
+    /// Current playlists.
+    #[serde(default)]
+    pub playlists: Vec<SnapshotPlaylist>,
+    /// Deleted playlists.
+    #[serde(default)]
+    pub deleted_playlists: Vec<SnapshotPlaylistTombstone>,
+    /// Removed playlist items.
+    #[serde(default)]
+    pub removed_playlist_items: Vec<SnapshotPlaylistItemTombstone>,
+}
+
+/// Snapshot like row.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct SnapshotLike {
+    /// Content id.
+    pub content_id: String,
+    /// Last write timestamp.
+    pub hlc_ms: i64,
+    /// Last write op id.
+    pub op_id: String,
+    /// Optional unresolved federated metadata.
+    #[serde(default)]
+    pub fed: Option<SyncedFedTrack>,
+}
+
+/// Snapshot unlike tombstone.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct SnapshotLikeTombstone {
+    /// Content id.
+    pub content_id: String,
+    /// Last write timestamp.
+    pub hlc_ms: i64,
+    /// Last write op id.
+    pub op_id: String,
+}
+
+/// Snapshot playlist.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct SnapshotPlaylist {
+    /// Stable playlist sync id.
+    pub playlist_id: String,
+    /// Title.
+    pub title: String,
+    /// Last write timestamp.
+    pub hlc_ms: i64,
+    /// Last write op id.
+    pub op_id: String,
+    /// Current items.
+    #[serde(default)]
+    pub items: Vec<SnapshotPlaylistItem>,
+}
+
+/// Snapshot deleted playlist tombstone.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct SnapshotPlaylistTombstone {
+    /// Stable playlist sync id.
+    pub playlist_id: String,
+    /// Last write timestamp.
+    pub hlc_ms: i64,
+    /// Last write op id.
+    pub op_id: String,
+}
+
+/// Snapshot playlist item.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct SnapshotPlaylistItem {
+    /// Content id.
+    pub content_id: String,
+    /// Playlist position.
+    pub position: i64,
+    /// Last write timestamp.
+    pub hlc_ms: i64,
+    /// Last write op id.
+    pub op_id: String,
+    /// Optional unresolved federated metadata.
+    #[serde(default)]
+    pub fed: Option<SyncedFedTrack>,
+}
+
+/// Snapshot removed playlist item tombstone.
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub struct SnapshotPlaylistItemTombstone {
+    /// Stable playlist sync id.
+    pub playlist_id: String,
+    /// Content id.
+    pub content_id: String,
+    /// Last write timestamp.
+    pub hlc_ms: i64,
+    /// Last write op id.
+    pub op_id: String,
+}
+
+/// Top-level message spoken on [`SYNC_ALPN`].
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub enum WireMessage {
+    /// Pairing request sent by the invite consumer.
+    PairRequest {
+        /// Invite id.
+        invite_id: String,
+        /// Invite secret.
+        secret: String,
+        /// Requester device profile.
+        profile: DeviceProfileWire,
+        /// Requester sync group id.
+        #[serde(default)]
+        group_id: Option<String>,
+        /// Number of active devices in requester group.
+        #[serde(default)]
+        group_active_devices: usize,
+        /// Requester trusted devices.
+        #[serde(default)]
+        devices: Vec<DeviceProfileWire>,
+        /// Requester vector clock.
+        vector: BTreeMap<String, i64>,
+        /// Requester pending operations.
+        ops: Vec<SyncOpWire>,
+        /// Requester materialized snapshot.
+        snapshot: SyncSnapshot,
+        /// Requester playback snapshot.
+        #[serde(default)]
+        playback: Option<PlaybackSnapshot>,
+    },
+    /// Pairing response sent by the inviter.
+    PairResponse {
+        /// Whether pairing was accepted.
+        accepted: bool,
+        /// Whether the request is waiting for user confirmation.
+        #[serde(default)]
+        pending: bool,
+        /// Optional rejection error.
+        #[serde(default)]
+        error: Option<String>,
+        /// Accepted group id.
+        #[serde(default)]
+        group_id: Option<String>,
+        /// Inviter profile.
+        #[serde(default)]
+        profile: Option<DeviceProfileWire>,
+        /// Inviter trusted devices.
+        #[serde(default)]
+        devices: Vec<DeviceProfileWire>,
+        /// Inviter vector clock.
+        #[serde(default)]
+        vector: BTreeMap<String, i64>,
+        /// Inviter pending operations.
+        #[serde(default)]
+        ops: Vec<SyncOpWire>,
+        /// Inviter materialized snapshot.
+        #[serde(default)]
+        snapshot: SyncSnapshot,
+        /// Inviter playback snapshot.
+        #[serde(default)]
+        playback: Option<PlaybackSnapshot>,
+    },
+    /// Regular sync hello.
+    Hello {
+        /// Sync group id.
+        group_id: String,
+        /// Sender profile.
+        profile: DeviceProfileWire,
+        /// Sender trusted devices.
+        devices: Vec<DeviceProfileWire>,
+        /// Sender vector clock.
+        vector: BTreeMap<String, i64>,
+        /// Sender pending operations.
+        ops: Vec<SyncOpWire>,
+        /// Sender materialized snapshot.
+        snapshot: SyncSnapshot,
+        /// Sender playback snapshot.
+        #[serde(default)]
+        playback: Option<PlaybackSnapshot>,
+    },
+    /// Regular sync response.
+    SyncResponse {
+        /// Whether sync was accepted.
+        accepted: bool,
+        /// Optional rejection error.
+        #[serde(default)]
+        error: Option<String>,
+        /// Receiver trusted devices.
+        #[serde(default)]
+        devices: Vec<DeviceProfileWire>,
+        /// Receiver vector clock.
+        #[serde(default)]
+        vector: BTreeMap<String, i64>,
+        /// Receiver pending operations.
+        #[serde(default)]
+        ops: Vec<SyncOpWire>,
+        /// Receiver materialized snapshot.
+        #[serde(default)]
+        snapshot: SyncSnapshot,
+        /// Receiver playback snapshot.
+        #[serde(default)]
+        playback: Option<PlaybackSnapshot>,
+    },
+}
+
+/// Encodes an invite as a compact `frid://i/...` link.
+pub fn encode_invite(invite: &InviteWire) -> Result<String> {
+    let bytes = serde_json::to_vec(invite).map_err(protocol_err)?;
+    Ok(format!("frid://i/{}", base64url_encode(&bytes)))
+}
+
+/// Parses a `frid://i/...` invite.
+pub fn parse_invite(value: &str) -> Result<InviteWire> {
+    let Some(token) = value.trim().strip_prefix("frid://i/") else {
+        return Err(MusicDhtError::Protocol(
+            "usage: frid://i/<invite>".to_string(),
+        ));
+    };
+    let bytes = base64url_decode(token)?;
+    let invite: InviteWire = serde_json::from_slice(&bytes).map_err(protocol_err)?;
+    if invite.v != 1 {
+        return Err(MusicDhtError::Protocol(
+            "unsupported invite version".to_string(),
+        ));
+    }
+    Ok(invite)
+}
+
+/// Extracts the network id carried by an invite's endpoint ticket.
+pub fn invite_network_id(value: &str) -> Result<NetworkId> {
+    let invite = parse_invite(value)?;
+    let ticket = PeerTicket::from_str(&invite.ticket).map_err(protocol_err)?;
+    Ok(ticket.network_id)
+}
+
+/// Hashes an invite secret for durable storage.
+pub fn hash_secret(secret: &str) -> String {
+    blake3::hash(secret.as_bytes()).to_hex().to_string()
+}
+
+/// Generates a random lowercase hex token.
+pub fn random_hex(bytes: usize) -> String {
+    let key = SecretKey::generate();
+    let mut seed = key.to_bytes().to_vec();
+    while seed.len() < bytes {
+        seed.extend_from_slice(blake3::hash(&seed).as_bytes());
+    }
+    hex_encode(&seed[..bytes])
+}
+
+/// Extracts endpoint id from a ticket string.
+pub fn ticket_endpoint_id(ticket: &str) -> Option<String> {
+    let ticket = PeerTicket::from_str(ticket).ok()?;
+    Some(ticket.endpoint_id().to_string())
+}
+
+/// Writes a JSON-lines sync message.
+pub async fn write_msg(stream: &mut ByteStream, message: &WireMessage) -> Result<()> {
+    let mut payload = serde_json::to_vec(message).map_err(protocol_err)?;
+    payload.push(b'\n');
+    stream.send.write_all(&payload).await.map_err(network_err)?;
+    Ok(())
+}
+
+/// Finishes the sending side.
+pub async fn finish_send(stream: &mut ByteStream) -> Result<()> {
+    stream.send.finish().map_err(network_err)?;
+    Ok(())
+}
+
+/// Finishes the response side and waits briefly for peer acknowledgement.
+pub async fn finish_response(
+    stream: &mut ByteStream,
+    drain_timeout: std::time::Duration,
+) -> Result<()> {
+    stream.send.finish().map_err(network_err)?;
+    let _ = tokio::time::timeout(drain_timeout, stream.send.stopped()).await;
+    Ok(())
+}
+
+/// Reads a JSON-lines sync message.
+pub async fn read_msg(stream: &mut ByteStream) -> Result<WireMessage> {
+    let line = read_line(&mut stream.recv).await?;
+    serde_json::from_slice(&line).map_err(protocol_err)
+}
+
+/// Reads one newline-terminated protocol line.
+pub async fn read_line<R: AsyncRead + Unpin>(reader: &mut R) -> Result<Vec<u8>> {
+    read_line_limited(reader, MAX_SYNC_LINE).await
+}
+
+/// Reads one newline-terminated protocol line with a custom size limit.
+pub async fn read_line_limited<R: AsyncRead + Unpin>(
+    reader: &mut R,
+    max_line: usize,
+) -> Result<Vec<u8>> {
+    let mut out = Vec::new();
+    loop {
+        let mut byte = [0u8; 1];
+        let read = reader.read(&mut byte).await.map_err(network_err)?;
+        if read == 0 {
+            break;
+        }
+        if byte[0] == b'\n' {
+            break;
+        }
+        out.push(byte[0]);
+        if out.len() > max_line {
+            return Err(MusicDhtError::Protocol(
+                "protocol line is too large".to_string(),
+            ));
+        }
+    }
+    Ok(out)
+}
+
+/// Reads one sync message from any async reader.
+pub async fn read_msg_from<R: AsyncRead + Unpin>(reader: &mut R) -> Result<WireMessage> {
+    let line = read_line(reader).await?;
+    serde_json::from_slice(&line).map_err(protocol_err)
+}
+
+fn base64url_encode(bytes: &[u8]) -> String {
+    const TABLE: &[u8; 64] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
+    let mut out = String::new();
+    let mut i = 0;
+    while i < bytes.len() {
+        let b0 = bytes[i];
+        let b1 = bytes.get(i + 1).copied().unwrap_or(0);
+        let b2 = bytes.get(i + 2).copied().unwrap_or(0);
+        out.push(TABLE[(b0 >> 2) as usize] as char);
+        out.push(TABLE[(((b0 & 0b0000_0011) << 4) | (b1 >> 4)) as usize] as char);
+        if i + 1 < bytes.len() {
+            out.push(TABLE[(((b1 & 0b0000_1111) << 2) | (b2 >> 6)) as usize] as char);
+        }
+        if i + 2 < bytes.len() {
+            out.push(TABLE[(b2 & 0b0011_1111) as usize] as char);
+        }
+        i += 3;
+    }
+    out
+}
+
+fn base64url_decode(value: &str) -> Result<Vec<u8>> {
+    fn val(byte: u8) -> Option<u8> {
+        match byte {
+            b'A'..=b'Z' => Some(byte - b'A'),
+            b'a'..=b'z' => Some(byte - b'a' + 26),
+            b'0'..=b'9' => Some(byte - b'0' + 52),
+            b'-' => Some(62),
+            b'_' => Some(63),
+            _ => None,
+        }
+    }
+    let bytes = value.as_bytes();
+    let mut out = Vec::with_capacity(bytes.len() * 3 / 4);
+    let mut i = 0;
+    while i < bytes.len() {
+        let a = val(bytes[i])
+            .ok_or_else(|| MusicDhtError::Protocol("invalid base64url invite".to_string()))?;
+        let b = val(*bytes
+            .get(i + 1)
+            .ok_or_else(|| MusicDhtError::Protocol("truncated base64url invite".to_string()))?)
+        .ok_or_else(|| MusicDhtError::Protocol("invalid base64url invite".to_string()))?;
+        let c = bytes.get(i + 2).and_then(|byte| val(*byte));
+        let d = bytes.get(i + 3).and_then(|byte| val(*byte));
+        out.push((a << 2) | (b >> 4));
+        if let Some(c) = c {
+            out.push((b << 4) | (c >> 2));
+            if let Some(d) = d {
+                out.push((c << 6) | d);
+            }
+        }
+        i += 4;
+    }
+    Ok(out)
+}
+
+fn hex_encode(bytes: &[u8]) -> String {
+    bytes.iter().map(|byte| format!("{byte:02x}")).collect()
+}
+
+fn protocol_err(err: impl std::fmt::Display) -> MusicDhtError {
+    MusicDhtError::Protocol(err.to_string())
+}
+
+fn network_err(err: impl std::fmt::Display) -> MusicDhtError {
+    MusicDhtError::Network(err.to_string())
+}
diff --git a/crates/music-dht/src/lib.rs b/crates/music-dht/src/lib.rs
index c72beee..bcc61ac 100644
--- a/crates/music-dht/src/lib.rs
+++ b/crates/music-dht/src/lib.rs
@@ -79,6 +79,7 @@
 
 mod config;
 mod database;
+pub mod device_sync;
 mod dht;
 mod error;
 mod message;