Relay transport

The relay layer handles everything below the application protocol: WebSocket connectivity, message encryption, reconnection, and message queuing. Application code (wallet, dapp) should never touch NDK or Nostr directly.

Stack

Application messages (hdwalletv1 JSON)
    ↓ JSON.stringify
NDK PrivateDirectMessage (kind 14, rumor)
    ↓ NIP-17 gift wrap (kind 1059, encrypted to recipient)
NDK → Nostr relay (WebSocket)
    ↓ stored as kind 1059 event, tagged with recipient pubkey
NDK ← Nostr relay subscription (filter: kind=1059, #p=<our_pubkey>)
    ↓ giftUnwrap → PrivateDirectMessage → JSON.parse
Application message handler

RelayClient

RelayClient is the low-level building block. One instance per peer connection.

Responsibilities

• Owns an NDK instance configured with explicit relay URLs.
• Subscribes to kind:1059 (GiftWrap) events tagged with its own pubkey.
• Decrypts and unwraps incoming events using NIP-17 giftUnwrap.
• Publishes outbound messages using giftWrap addressed to the paired peer.
• Filters incoming messages by peer pubkey (rejects messages from unknown senders).
• Discards messages older than lastProcessedTimestamp (replay protection).
• Queues outbound messages until at least one relay is ready.

Construction

new RelayClient({
  explicitRelayUrls: string[];  // WebSocket URLs, e.g. ["wss://relay.riften.net:443"]
  signerPrivateKey: Uint8Array;  // 32-byte secp256k1 private key (this client's identity)
  pairedPublicKey?: Uint8Array;  // 32-byte x-only pubkey of the peer (set after key exchange)
  logNetworkActivity?: boolean;  // default true
})

Key methods

connect(): Promise<void>
  // NDK connect, subscribe to GiftWrap events, start waiting for relays.

disconnect(): Promise<void>
  // Stop subscription, mark queue not-ready, update lastProcessedTimestamp.

relay(message: ProtocolMessage): Promise<void>
  // Send a message. Enqueues if relays not ready. Throws if paired key not set.

setPairedPublicKey(key: Uint8Array): void
  // Called after key exchange. Enables outbound messages and incoming peer filtering.

isKeyExchangeComplete(): boolean
  // true once pairedPublicKey is set and non-zero.

nextSequence(): number
  // Returns a unique sequence number (starts at random offset, increments by 2).

getLastProcessedTimestamp() / setLastProcessedTimestamp(ts): void
  // Persist/restore across reconnects to avoid re-delivering buffered messages.

Message queue

Before any relay is confirmed as connected, outbound relay() calls are enqueued in a MessageQueue. The client polls every 100 ms for up to 5 seconds for at least one relay with status 1 (connected). After that timeout it assumes the relay is ready and flushes the queue regardless (avoiding silent message loss on slow connections).

On disconnect(), the queue is marked not-ready so messages sent during a reconnect gap are held rather than dropped.

Replay protection

lastProcessedTimestamp is set to now - 2 on the first connection. On reconnect it is updated to now in disconnect(). Any incoming message with time < lastProcessedTimestamp is silently dropped. This prevents the relay from re-delivering messages that were already handled before a disconnect.

Keepalive

RelayClient creates its SimplePool with enablePing: true. This enables nostr-tools' built-in heartbeat: every 29 seconds the pool pings each connected relay and expects a response within 20 seconds. In Node.js this uses native WebSocket ping/pong frames; in browsers (where the WebSocket API doesn't expose ping) it falls back to sending a dummy subscription request and waiting for EOSE.

If a relay fails to respond, nostr-tools closes the WebSocket, which fires the subscription onclose callback, which emits "disconnect" on RelayClient, which triggers the connection manager's existing reconnect loop. This detects "zombie" TCP connections where the socket appears open but the relay is unreachable.

Additionally, if a publishMessage() call fails (all relays reject the event), RelayClient emits "disconnect" alongside the thrown error. This ensures the reconnect loop starts immediately rather than waiting for the next ping cycle.

Sequence numbers

nextSequence() starts at a random offset in the safe integer range and increments by 2. This means two instances are unlikely to collide (they start at different offsets), and the step of 2 leaves room for error responses at odd offsets if needed in the future.

initiateRelay

initiateRelay() (in relay-handler.ts) wraps RelayClient with a reconnect loop:

initiateRelay(
  statusCallback: RelayStatusCallback,
  privateKey: Uint8Array,
  initialPairedKey: Uint8Array,
  options: { explicitRelayUrls, reconnectInterval?, maxReconnectAttempts? }
): () => void  // returns cleanup function

It calls statusCallback with a RelayUpdatePayload on every state change:

interface RelayUpdatePayload {
  client: RelayClient;
  status: RelayStatus;  // { status: "connected" | "reconnecting" | "disconnected" }
}

The reconnect loop is simple: on a disconnect event from the client it waits reconnectInterval (default 5000 ms) and calls client.connect() again. No exponential backoff currently — connections are expected to be stable (relay and mobile network) and fast to re-establish.

initiateDappRelay

Higher-level helper that bundles credential generation, URI encoding, key exchange handling, and reconnection into one call:

initiateDappRelay(
  statusCallback: RelayStatusCallback,
  options?: {
    explicitRelayUrls?: string[];
    reconnectInterval?: number;
    maxReconnectAttempts?: number;
    existingCredentials?: { privateKey: string; secret: string };
  }
): DappRelayResult

interface DappRelayResult {
  client: RelayClient;
  uri: string;                   // wiz:// URI to encode into QR
  credentials: KeyExchangeCredentials;
  events: EventEmitter<{ keyexchangecomplete: [walletPublicKey: Uint8Array] }>;
  cleanup: () => void;
}

Internally it wraps the caller's statusCallback to intercept wallet_ready messages, verify the secret, and call client.setPairedPublicKey() before the message is processed by DappConnectionManager. On reconnect, if walletPublicKeyNostr is already known it immediately restores the paired key so outbound messages can be sent before the next wallet_ready arrives.

initiateWalletRelay

initiateWalletRelay(
  statusCallback: RelayStatusCallback,
  options: {
    uri: string;              // wiz:// URI from QR scan
    walletPrivateKey: Uint8Array;  // wallet's relay identity key (may be ephemeral)
  }
): { cleanup: () => void }

Decodes the URI, sets the dapp's public key as the initial paired key, and starts the relay. On each connect/reconnect, the wallet sends wallet_ready which carries the key exchange fields (public_key, secret) alongside the session data. This gives the dapp the wallet's pubkey atomically with the application handshake data.

Encryption: NIP-17 gift wrap

NIP-17 gift wrap works like a sealed envelope:

1. Rumor (kind 14, PrivateDirectMessage): the plaintext message, signed by the sender. Content is the JSON payload. Tags include ["p", recipient_pubkey_hex].
2. Seal (kind 13): the rumor encrypted with a shared ECDH secret derived from sender's private key and recipient's public key. No p tag — unlinkable to sender/recipient.
3. Wrap (kind 1059, GiftWrap): the seal encrypted to the recipient's public key using a freshly generated throwaway key. Has a ["p", recipient_pubkey_hex] tag so the relay can route it to the right subscription.

The relay sees only kind 1059 with a recipient tag. It cannot read content or link messages to senders. The throwaway outer key means even the relay cannot correlate multiple messages from the same sender.

NDK handles all three layers in giftWrap() / giftUnwrap().

Default relays

wss://relay.riften.net:443     (primary)
wss://relay.cauldron.quest:443 (secondary)

Both relays are used by default on both dapp and wallet sides for redundancy. Since Nostr relays do not federate (they don't forward events to each other), connecting to multiple relays ensures messages are delivered even if one relay is temporarily unavailable.

The connection URI encodes only the primary relay; the secondary is added programmatically by the library. When a custom relay is specified (via URI or explicitRelayUrls), only that relay is used — default relays are not auto-added.

Nothing in the protocol prevents using any other standard Nostr relay.

Duplicate message handling

When subscribed to multiple relays, the same event may arrive from more than one relay. nostr-tools SimplePool.subscribeMany() deduplicates events by ID — it tracks seen event IDs in a per-subscription _knownIds set and only fires onevent once per unique ID. Since pool.publish(urls, event) sends the identical event (same ID) to all relays, the receiving side's pool delivers it exactly once.

Transport-level extensions

Transport-level extensions are capabilities of the relay/gift-wrap transport itself, independent of any application protocol. They're distinct from the protocol-level extensions documented in extensions.md, which extend hdwalletv1 specifically.

Negotiation

Both sides advertise transport-level extensions in a base extensions field on their handshake message (dapp_ready / wallet_ready). The shape is identical on both sides:

interface DappReadyMessage {
  // ...
  extensions?: Record<string, unknown>;
}

interface WalletReadyMessage {
  // ...
  extensions?: Record<string, unknown>;
}

A capability is considered enabled only when both sides advertise it. RelayClient exposes setPeerCapabilities({...}) so the connection managers can inform it once they've parsed the peer's _ready message.

Extension values are per-extension; today chunk uses { version: 1 }. Unknown extension keys are ignored by implementations that don't recognise them, so adding new capabilities is always backward-compatible — an old peer simply doesn't advertise them and the new side falls back to the non-extended behaviour.

Known transport extensions

Extension	Purpose
chunk	Split messages larger than NIP-44's 65,535-byte plaintext ceiling across multiple gift-wrapped events. Symmetric — enabled when both sides advertise. See below.

Chunking (chunk extension)

Why it exists

NIP-44 caps plaintext at 65,535 bytes — the length is encoded as a U16BE prefix in the wire format, so the limit is structural, not a guardrail that can be raised. NIP-17 gift-wrap additionally encrypts twice (rumor inside seal inside wrap), so a single 50+ KB ProtocolMessage will blow the outer wrap's plaintext budget even when the inner payload itself looks small enough.

Real scenarios that exceed the cap:

• Aggregated swap sign_transaction_request with many pool UTXOs (each with hex lockingBytecode and unlockingBytecode in the per-input sourceOutputs).
• sign_transaction_response carrying a signed transaction hex string. Policy-max BCH transactions are 100 KB (→ 200 KB hex); consensus-max is 1 MB (→ 2 MB hex).

Wire format

interface ChunkMessage extends ProtocolMessage {
  action: "chunk";
  time: number;     // shared across all chunks of one logical message
  msgId: string;    // random identifier, shared across all chunks
  index: number;    // 0-based chunk index within [0, total)
  total: number;    // total number of chunks for this msgId
  data: string;     // base64 slice of the UTF-8 bytes of JSON.stringify(originalMessage)
}

To reconstruct the original message: concatenate the data strings in index order, base64-decode to bytes, UTF-8-decode to a string, JSON.parse.

Sender

RelayClient.publishMessage measures the UTF-8 byte length of the serialized ProtocolMessage. If it exceeds the per-message threshold:

• If the peer's chunk capability is enabled, the message is split into ChunkMessages and each is published individually via the same wrapEvent path as any other message. No ACKs — see "Failure modes" below.
• If the peer has not advertised chunk, publishMessage throws a structured error ("Cannot send <action>: message is larger than NIP-44's 65,535-byte ceiling and the peer does not advertise the 'chunk' transport extension. Please update the connected wallet/dapp..."). This replaces the cryptic invalid plaintext size error from nostr-tools.

The per-chunk budget is sized conservatively. Each chunk's raw data is ≤ 30,000 bytes, which after base64 expansion (~4/3×), JSON envelope overhead, and the two-layer NIP-17 gift-wrap encryption stays well under NIP-44's 65,535-byte ceiling for the outer wrap's plaintext. See packages/core/src/transforms/chunk.ts for the derivation.

Receiver

RelayClient owns a ChunkReassembler instance. Chunks pass the same peer filter and lastProcessedTimestamp dedup as any other message, then are routed to the reassembler. When all chunks for a msgId have arrived (in any order), the bytes are concatenated, decoded, and the resulting ProtocolMessage is handed to the application-level handler — indistinguishable to the application from an unchunked message of equivalent size.

The reassembler holds two maps with TTL eviction:

• in-flight buffers keyed by msgId — collects chunks until complete; default TTL 120 s, sized to comfortably fit a ~35-chunk 2 MB response under real relay latency.
• completed — tracks msgIds we've already delivered, for the same TTL, so late-arriving duplicates (e.g. cross-subscription replay after reconnect) don't spawn a second reassembly and double-deliver.

A background sweeper runs every 10 s while connected and evicts expired entries. Reassembly state is not persisted — reconnects rely on the relay replaying events to complete any in-flight transfer (see below).

Failure modes

Failure	Behaviour
Dapp reloads mid-send	Dapp on reload has no in-flight state. User retries → fresh msgId, all chunks re-sent. Wallet's partial buffer for the old msgId expires via TTL.
Wallet reloads mid-receive	Subscription filter has no since clause, so on reconnect the relay re-delivers all events addressed to the wallet. Chunks reassemble fresh. Works as long as the chunks are still within the relay's retention window.
Network blip / all relays reject a chunk	Promise.allSettled on publish treats each chunk identically to any other single message. If all relays reject, publishMessage throws and emitDisconnect fires — same posture as today's sign-request failure.
Relay prunes mid-reassembly	Receiver's partial times out via TTL. Application sees no response; user retries — same failure mode the protocol already has for any lost sign request.

No persistence layer, no per-chunk ACKs, no new round trips. The assembled ProtocolMessage's own response (e.g. sign_transaction_response) is the effective end-to-end ACK.

Backward compatibility

Dapp	Wallet	Outcome
Old	Old	Unchanged. Oversized message fails at nostr-tools with the raw NIP-44 error.
New	Old	Dapp detects absent chunk capability and throws a clear upgrade-guidance error instead of the cryptic NIP-44 error.
Old	New	Wallet detects absent chunk capability and throws a clear error symmetrically.
New	New	Both advertise, both enable, oversized messages chunk-and-reassemble transparently. Application code is unchanged.

Upgrading the library on both sides is sufficient — no adapter-interface changes, no new configuration, no capability opt-in. The extension is always-on when supported.