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Protocol

The application-level protocol has two layers:

  • Base protocol — the handshake messages (dapp_ready, wallet_ready, disconnect) that are shared across all application protocols and live in @wizardconnect/core/protocols/base.ts.
  • hdwalletv1 — the BCH HD-wallet protocol, carrying session data (xpubs) and sign request round-trips. Defined in @wizardconnect/core/protocols/hdwalletv1.ts.

Both layers use the same encrypted relay channel (see transport.md).

Protocol selection happens during the handshake via supported_protocols lists, not via a hard-coded field. This allows forward-compatible negotiation when future protocol versions are added.

Message envelope

Every message shares a base shape:

interface ProtocolMessage {
  action: string;  // one of the RelayMsgAction values below
  time: number;    // Unix timestamp (seconds). Used for replay filtering.
}

The time field is checked by the relay client: messages older than the last-processed timestamp are silently dropped. This prevents stale messages buffered at the relay from being re-delivered after a reconnect.

Actions

dapp_ready                — dapp → wallet, signals dapp is alive + lists supported protocols
wallet_ready              — wallet → dapp, signals wallet is alive + delivers session data + key exchange
sign_transaction_request  — dapp → wallet, asks wallet to sign a transaction
sign_transaction_response — wallet → dapp, returns signed tx or error
sign_cancel               — dapp → wallet only, cancels an in-flight sign_transaction_request
disconnect                — either → either, courtesy notification before tearing down

Base protocol

dapp_ready

interface DappReadyMessage {
  action: "dapp_ready";
  supported_protocols: string[];  // protocols this dapp supports, in preference order
  selected_protocol?: string;     // set only on the reactive dapp_ready (after seeing wallet_ready)
  wallet_discovered: boolean;     // true if dapp already saw this wallet this session
  dapp_name?: string;             // optional, sent on first message for wallet UI
  dapp_icon?: string;             // optional icon URL or data-URI
  time: number;
}

dapp_name and dapp_icon are captured by the wallet on the first dapp_ready that includes them. The wallet shows these in its connections list.

selected_protocol is absent on the proactive dapp_ready (sent before the dapp has seen the wallet). Once the dapp receives wallet_ready and picks a protocol, the reactive dapp_ready carries selected_protocol so the wallet can confirm the agreed protocol.

wallet_ready

interface WalletReadyMessage {
  action: "wallet_ready";
  supported_protocols: string[];       // protocols this wallet supports
  wallet_name: string;
  wallet_icon: string;
  dapp_discovered: boolean;            // true if wallet already saw this dapp this session
  session: Record<string, unknown>;    // keyed by protocol name; each value is protocol-specific
  public_key: string;                  // wallet's Nostr x-only pubkey (hex, 32 bytes) — key exchange
  secret: string;                      // echo of the shared secret from the URI — MITM prevention
  time: number;
}

wallet_ready is the most important message in the protocol. It serves two purposes:

  1. Key exchange — public_key is the wallet's Nostr pubkey; secret is echoed from the connection URI for MITM prevention. The dapp verifies the secret and calls setPairedPublicKey(public_key) before processing the rest of the message. This is why wallet_ready bypasses the relay-client peer filter.

  2. Application handshake — the wallet populates session for every protocol it supports. The dapp picks the first protocol from its own supported_protocols list that also appears in the wallet's list, then reads session[selectedProtocol] for the protocol-specific data.

Protocol negotiation

  1. The dapp sends its supported_protocols list in the proactive dapp_ready.
  2. The wallet replies with its own supported_protocols and the session map.
  3. The dapp selects agreed = dapp.supported_protocols.find(p => wallet.supported_protocols.includes(p)).
  4. If no overlap: the dapp sends disconnect(reason: "protocol_mismatch") and emits a disconnect event. No further communication happens.
  5. If agreed: the dapp reads session[agreed] and sends a reactive dapp_ready with selected_protocol = agreed.

Handshake

The handshake uses a mutual-discovery pattern. The goal is for both sides to converge to a live session regardless of who reconnects first. "Discovered" means "I have received and processed a ready message from the other side in this runtime session."

Rules

  1. On every connect/reconnect, each side sends its own "ready" message proactively. The wallet sends wallet_ready immediately (it already knows the dapp's pubkey from the URI). The dapp sends dapp_ready once the relay is connected and key exchange resolves.
  2. Each "ready" message carries a boolean indicating whether the sender has already seen the other party (wallet_discovered in dapp_ready, dapp_discovered in wallet_ready).
  3. On receiving a "ready" with the discovery flag false, the receiver must send back its own "ready" — even if it already sent one — because the other side has lost state and needs a fresh delivery.
  4. The wallet guards against duplicate wallet_ready messages within a single connection cycle via walletReadySentThisCycle. This resets to false on each new connect/reconnect. Receiving dapp_ready(wallet_discovered=false) also resets this flag.

Scenarios

Initial connect (neither has seen the other):

Dapp ──dapp_ready(supported=["hdwalletv1"], wallet_discovered=false)──▶ Wallet   (proactive)
Dapp ◀──wallet_ready(supported=["hdwalletv1"], session={...}, dapp_discovered=false)── Wallet
Dapp ──dapp_ready(supported=["hdwalletv1"], selected="hdwalletv1", wallet_discovered=true)──▶ Wallet

After step 3 the wallet sets dappDiscovered = true. No more ready messages unless a reconnect.

Wallet reconnects (dapp still running, walletDiscovered=true):

Dapp ──dapp_ready(wallet_discovered=true)──────────────────────────────▶ Wallet   (proactive)
Dapp ◀──wallet_ready(dapp_discovered=false, session={...})────────────── Wallet   (proactive)
Dapp ──dapp_ready(selected="hdwalletv1", wallet_discovered=true)────────▶ Wallet  (reactive)

Dapp reconnects (browser refresh, wallet still running):

Dapp ──dapp_ready(wallet_discovered=false)──────────────────────────────▶ Wallet  (proactive)
Dapp ◀──wallet_ready(dapp_discovered=true, session={...})─────────────── Wallet   (reactive)
(No third message: dapp_discovered=true means the dapp does not need to send a reactive reply.)

Design decision: why mutual discovery?

An alternative is a fixed initiator/responder role (only the dapp initiates). That breaks when the wallet reconnects while the dapp is still alive — the wallet would wait for a dapp message that never comes because the dapp thinks the session is live. Mutual discovery means each side sends a "hello" on reconnect without depending on the other side's state.

hdwalletv1 protocol

The hdwalletv1 session data is carried in wallet_ready.session["hdwalletv1"]. It delivers everything the dapp needs to derive an unlimited number of addresses without further contact with the wallet.

Hdwalletv1Session

interface Hdwalletv1Session {
  paths: PathXpub[];  // BIP32 xpubs for receive/change/defi
}

Carried as wallet_ready.session["hdwalletv1"]. The dapp validates it with isHdwalletv1Session().

See pubkey-derivation.md for the full xpub story.

PathXpub

interface PathXpub {
  name: PathName;  // "receive" | "change" | "defi"
  xpub: string;   // BIP32 base58-encoded extended public key
}

name is the protocol-level identifier. The dapp uses the name to know what kind of addresses to derive from the xpub; it does not need to know (or care) where the wallet derived the xpub from.

Highly recommended derivation paths. To ensure addresses are recognised by other wallets and blockchain explorers, wallets should derive xpubs from the standard BIP44 paths for BCH:

Name Recommended derivation path Purpose
receive m/44'/145'/0'/0 External receive addresses
change m/44'/145'/0'/1 Internal change addresses
defi m/44'/145'/0'/7 DeFi / Cauldron addresses

Using these paths means the same addresses will appear in any BIP44-compatible wallet that holds the same seed, making fund recovery straightforward.

Privacy-first alternative: any path per session. The protocol does not enforce the recommended paths. A wallet that prioritises privacy may derive xpubs from non-standard or randomly-chosen paths, and may even rotate them each session. The dapp derives addresses correctly regardless — it never sees the path, only the xpub. The trade-off is that funds sent to session-specific paths will not be found by standard wallet recovery tools without additional metadata.

Design decision: names instead of child indices. The protocol uses human-readable names rather than numeric child indices because the derivation path is a wallet-internal detail. A name like "receive" is stable and meaningful; the corresponding BIP44 index is an implementation concern that only the wallet (and internal dapp state) need to know.

PathName

type PathName = "receive" | "change" | "defi";
Name Recommended BIP44 path Purpose
receive m/44'/145'/0'/0 External receive addresses
change m/44'/145'/0'/1 Internal change addresses
defi m/44'/145'/0'/7 DeFi / Cauldron addresses

sign_transaction_request

interface SignTransactionRequest {
  action: "sign_transaction_request";
  transaction: WcSignTransactionRequest;  // from @bch-wc2/interfaces
  sequence: number;
  time: number;
}

sequence is a unique number generated by RelayClient.nextSequence(). It starts at a random offset (to avoid collisions across sessions) and increments by 2 per call. The dapp uses sequence to match responses to requests.

WcSignTransactionRequest describes a Bitcoin Cash transaction: inputs, outputs, source outputs (for signing), version, locktime, and an optional userPrompt string shown to the user in the wallet UI.

sign_transaction_response

interface SignTransactionResponse {
  action: "sign_transaction_response";
  sequence: number;
  signedTransaction: string;  // hex-encoded fully signed transaction
  error?: string;             // if present, signing failed; signedTransaction is ""
  time: number;
}

The wallet either returns the signed transaction hex or an error string. The dapp rejects the pending Promise associated with the sequence in the error case.

sign_cancel

interface SignCancelMessage {
  action: "sign_cancel";
  sequence: number;   // must match the sequence of the sign_transaction_request being cancelled
  reason?: string;    // optional human-readable explanation
  time: number;
}

Sent by the dapp only to cancel an in-flight sign_transaction_request. The wallet should dismiss the corresponding sign dialog immediately upon receipt.

Use cases: - User presses cancel on the dapp side while waiting for the wallet to sign. - Dapp replaces a stale request with a new one (e.g., trade price has changed).

Dapp side (DappConnectionManager): - sendSignCancel(sequence, reason?) — immediately rejects the pending Promise for that sequence, then sends sign_cancel to the wallet.

Wallet side (WalletConnectionManager): - Incoming sign_cancel emits a signCancelled event (connectionId, sequence, reason). The host app is responsible for dismissing the sign dialog.

disconnect

Either side may send a disconnect message before tearing down the relay connection. This is a courtesy notification — the remote side treats the connection as closed immediately upon receipt (no acknowledgement).

enum DisconnectReason {
  ProtocolMismatch = "protocol_mismatch",  // no common protocol found during handshake
  UserDisconnect   = "user_disconnect",    // explicit user or application action
}

interface DisconnectMessage {
  action:   "disconnect";
  reason:   DisconnectReason;
  message?: string;  // optional human-readable detail
  time:     number;
}

Wallet side (WalletConnectionManager): - disconnect(id) sends UserDisconnect before cleaning up. - Incoming disconnect emits a remoteDisconnect event (connectionId, reason, message) and removes the connection.

Dapp side (DappConnectionManager): - sendDisconnect(message?) sends UserDisconnect. Caller then calls dappRelay.cleanup(). - Protocol mismatch during handleWalletReady sends ProtocolMismatch and emits a disconnect event (reason, message). - Incoming disconnect emits a disconnect event.

Type guards

@wizardconnect/core exports runtime type guards for all protocol messages:

isProtocolMessage(obj)        → ProtocolMessage
isDappReadyMessage(obj)       → DappReadyMessage
isWalletReadyMessage(obj)     → WalletReadyMessage
isDisconnectMessage(obj)      → DisconnectMessage
isHdwalletv1Session(obj)      → Hdwalletv1Session
isPathXpub(obj)               → PathXpub
isErrorMessage(obj)           → ErrorMessage
isSignTransactionRequest(obj) → SignTransactionRequest
isSignCancelMessage(obj)      → SignCancelMessage

These are used internally to validate incoming messages before dispatch.

Helper: childIndexOfPathName

function childIndexOfPathName(name: PathName): number
// "receive" → 0, "change" → 1, "defi" → 7

The protocol uses string names for paths, but code that manages key state internally (such as DappPubkeyStateManager) keys its maps by numeric child index. This helper converts between the two representations. It is not a protocol concern — the numeric indices never appear on the wire.