WizardConnect/docs/protocol.md

# 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](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:

```typescript
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
chunk                     — either → either, transport-level. Carries one slice of a larger message
                            that exceeds NIP-44's 65,535-byte plaintext ceiling. Not tied to
                            hdwalletv1 semantics. See transport.md.
```

The action names above are the well-known set. Extensions may define additional action strings
(e.g. `decrypt_request`, `decrypt_response`). Both sides should ignore unknown actions gracefully.
See [extensions.md](extensions.md) for conventions on defining extension messages.

---

## Base protocol

### dapp_ready

```typescript
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

```typescript
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`. Both `walletReadySentThisCycle` and `dappDiscovered` reset to
   `false` on each new connect/reconnect. This ensures the wallet always sends
   `wallet_ready(dapp_discovered=false)` at the start of a new connection cycle, matching the
   "Wallet reconnects" scenario. Receiving `dapp_ready(wallet_discovered=false)` also resets
   `walletReadySentThisCycle`.

#### 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

```typescript
interface Hdwalletv1Session {
  paths: PathXpub[];                        // BIP32 xpubs for each named path
  extensions?: Record<string, unknown>;     // optional extension capabilities and data
}
```

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

The `extensions` field is optional. When present, each key is an extension name and its presence
indicates the wallet supports that extension. The value carries extension-specific handshake data,
or `{}` if no data is needed. See [extensions.md](extensions.md) for the full extension system.

See [pubkey-derivation.md](pubkey-derivation.md) for the full xpub story.

### PathXpub

```typescript
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

```typescript
type PathName = string;

// Well-known path names:
const PATH_RECEIVE = "receive";
const PATH_CHANGE  = "change";
const PATH_DEFI    = "defi";
```

`PathName` is an open string. The well-known values are:

| 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 |

Wallets may include additional paths via extensions (e.g. `stealth_scan`, `stealth_spend`, `rpa`).
Dapps should ignore path names they do not recognize. See [extensions.md](extensions.md) for
conventions on defining new path names.

### sign_transaction_request

```typescript
interface SignTransactionRequest {
  action: "sign_transaction_request";
  transaction: WcSignTransactionRequest;  // from @bch-wc2/interfaces
  sequence: number;
  inputPaths: [number, PathName, number][];  // [inputIndex, pathName, addressIndex]
  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.

`inputPaths` is a sparse array of `[inputIndex, PathName, addressIndex]` tuples. Each entry identifies
the HD derivation path name and address index the dapp used to derive the locking script for the input
at position `inputIndex`. Only inputs that require wallet signing need an entry — contract inputs with
pre-set unlocking bytecode can be omitted. This allows the wallet to sign each input without scanning
or guessing which key was used.

#### SIGHASH requirement (security-critical)

Wallets **MUST** sign every input with `SIGHASH_ALL | SIGHASH_FORKID | SIGHASH_UTXOS` and **MUST**
reject any request that would require different flags.

`SIGHASH_ALL` ensures the signature commits to the entire transaction (all inputs and all outputs).
Without it, an attacker could collect a valid signature and graft it onto a different transaction —
for example, using `SIGHASH_NONE` an attacker could replace every output to redirect funds.

Because `inputPaths` lets the dapp specify which key signs each input, the wallet no longer
independently verifies that the key matches the UTXO's locking bytecode. This is safe **only** when
`SIGHASH_ALL` is enforced: if the dapp provides a wrong path, the resulting signature is invalid
(public key hash mismatch) and the transaction cannot broadcast. Without `SIGHASH_ALL`, a
wrong-key signature could still be repurposed in a different transaction context.

Summary of the flags:

| Flag | Purpose |
|------|---------|
| `SIGHASH_ALL` | Commits to all inputs and outputs — prevents output substitution |
| `SIGHASH_FORKID` | Prevents cross-fork replay (BCH ↔ BTC) |
| `SIGHASH_UTXOS` | Commits to all input UTXOs — prevents input substitution after signing |

### sign_transaction_response

```typescript
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.

### Re-delivery on reconnect

If the wallet is not connected (or reconnects) while a `sign_transaction_request` is in flight,
the dapp automatically re-sends all pending requests when it receives `wallet_ready`. This
handles the common case where the user triggers a transaction in the dapp and then opens the
wallet app several seconds later.

The wallet deduplicates incoming requests by `sequence` number — if it has already emitted
`pendingSignRequest` for a given sequence and has not yet responded, the duplicate is silently
dropped. The dedup guard is cleared when the wallet sends a response (`sign_transaction_response`)
or receives a `sign_cancel`.

### sign_cancel

```typescript
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).

No acknowledgement does not mean fire-and-forget on the sender's side. `relay()` resolves only
after the publish has been settled against every configured relay, so the sender must keep the
relay connection open until then — closing it first kills the publish in flight and the peer
never learns of the disconnect. Sending a courtesy `disconnect` and immediately tearing the
transport down is the same as not sending one.

```typescript
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`, then tears the connection down once the publish
  settles — bounded, so an unreachable relay cannot hold the socket open. The connection leaves
  the registry synchronously. See [wallet.md § Sending disconnect](wallet.md#sending-disconnect).
- 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:

```typescript
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

```typescript
function childIndexOfPathName(name: PathName): number | undefined
// "receive" → 0, "change" → 1, "defi" → 7, unknown → undefined
```

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 for the well-known path names. It returns `undefined` for extension path names
(e.g. `"stealth_scan"`). Callers must handle the `undefined` case — typically by skipping the path.
It is not a protocol concern — the numeric indices never appear on the wire.