archon

Archon Satoshi Wallet — Service Specification

Language-agnostic contract for the Satoshi wallet — the HD Bitcoin wallet service used by the satoshi-mediator to broadcast OP_RETURN anchor transactions. The canonical implementation is services/mediators/satoshi-wallet/.

This is the only Archon component that builds and signs Bitcoin transactions. It does not hold its own mnemonic on disk: at signing time it fetches the wallet’s BIP39 mnemonic from the local Keymaster (via the admin-keyed GET /api/v1/wallet/mnemonic) and discards it after the signing pass.

Related specs. Paired with the satoshi-mediator (which calls its HTTP routes to anchor batches and manage fees) and the Keymaster (source of the mnemonic). The UTXO / balance / history side of the wallet is delegated either to a locally-reachable Bitcoin Core wallet via RPC (core backend) or to a hosted Bitcoin JSON-RPC + UTXO provider (alchemy backend).

1. Service responsibilities

Two halves, both thin wrappers:

  1. Watch-only wallet — derives xpubs from the Keymaster mnemonic, imports wpkh descriptors into Bitcoin Core as a descriptor watch- only wallet, and forwards balance/address/UTXO/history queries to Core. The descriptor wallet name is ${ARCHON_WALLET_NAME} (default archon-watch-<nodeID>).

  2. Hosted UTXO wallet — when ARCHON_WALLET_BACKEND=alchemy, derives BIP84 external/internal addresses locally, scans a gap window through the configured UTXO API, persists scan state on disk, builds funded PSBTs locally, signs from the Keymaster mnemonic, and broadcasts through Bitcoin JSON-RPC sendrawtransaction.

  3. Signing authority — for send, anchor, and bump-fee operations, re-derives the private keys on demand from the mnemonic, signs locally with bitcoinjs-lib + ECPair, and broadcasts via sendrawtransaction. RBF bumping is currently supported only by the core backend.

The mnemonic never lives in this service’s memory between requests. Every signing call fetches it afresh and scope-bounds its use to a single request.

2. HTTP API contract

Binds to ${ARCHON_WALLET_PORT} (default 4240). Routes under /api/v1.

2.1 Routes

Method Path Admin? Notes
GET /api/v1/wallet/version no { version, commit }
POST /api/v1/wallet/setup yes Creates the watch-only wallet in Core and imports wpkh descriptors. Returns { ok: true, network, walletName, descriptors: [external, internal] }. Idempotent — safe to re-run.
GET /api/v1/wallet/balance yes { balance, unconfirmed_balance, network } in BTC.
GET /api/v1/wallet/address yes { address, network } — next unused external bech32 address.
GET /api/v1/wallet/transactions?count=N&skip=N yes { transactions: ListTransactionsEntry[], network }. Pagination: count (default 10), skip (default 0).
GET /api/v1/wallet/utxos?minconf=N yes { utxos: UnspentOutput[], network }. minconf default 1.
GET /api/v1/wallet/fee-estimate?blocks=N yes { feerate, blocks, network } — BTC/kB from Bitcoin JSON-RPC estimatesmartfee, with a conservative fallback in hosted mode.
GET /api/v1/wallet/info yes Wallet status block (balance, tx count, network, etc.).
POST /api/v1/wallet/send yes { to, amount, feeRate?, subtractFee? } → { txid, ... }. amount in BTC. feeRate in sat/vB.
POST /api/v1/wallet/anchor yes { data, feeRate? } → { txid, ... }. data is the UTF-8 string to put in OP_RETURN (≤ 80 bytes). Called by satoshi-mediator.
GET /api/v1/wallet/transaction/:txid yes { txid, confirmations, blockhash, fee, network } from Core’s gettransaction. HTTP 404 if the tx isn’t in this wallet.
POST /api/v1/wallet/bump-fee yes { txid, feeRate? } → { txid, ... }. RBF the given tx; txid MUST be a wallet tx currently in the mempool. Called by satoshi-mediator when a pending anchor stays unconfirmed.
GET /metrics no Prometheus. Binds to ${ARCHON_WALLET_METRICS_PORT} (default 4241), NOT the main port.

Every admin route requires X-Archon-Admin-Key matching ARCHON_ADMIN_API_KEY. With the key set, missing/wrong header → HTTP

  1. When ARCHON_ADMIN_API_KEY is empty, admin routes return HTTP 403 “Admin API key not configured” — the mediator doesn’t probe for this, so you must configure it matching on both sides.

2.2 Response envelope

Unlike the Keymaster, the satoshi-wallet returns raw-shaped JSON (no top-level key wrapping). Callers read fields directly:

// GET /api/v1/wallet/balance
{ "balance": 0.5, "unconfirmed_balance": 0.0, "network": "signet" }

// POST /api/v1/wallet/anchor
{ "txid": "abc...", "network": "signet" }

2.3 Error shape

application/json { "error": "<message>" } with an appropriate status:

2.4 Body limits

Express default (~100 KB). All bodies are tiny; no custom limit needed.

2.5 CORS

None by default. The wallet is meant to be reached only from the satoshi-mediator on the same private network.

3. Key derivation

The wallet uses BIP84 (native SegWit P2WPKH) derivation throughout:

m / 84' / <coin_type>' / 0' / <chain> / <index>
Field Value
purpose 84' (BIP84)
coin_type 0' for mainnet, 1' for signet/testnet4
account 0' (single account per wallet)
chain 0 (external) / 1 (internal/change)
index non-hardened, grows as needed

Address type: bech32 wpkh(...). Master/account xpub uses xprv/xpub version bytes for mainnet and tprv/tpub for signet/testnet4.

3.1 Descriptors

On POST /wallet/setup, the service:

  1. createwallet with disable_private_keys=true, blank=true, descriptors=true — creates a pure-descriptor watch-only wallet in Core. Loads it if it already exists.
  2. Builds two descriptors with origin info: 
    external: wpkh([<fingerprint>/84h/<coin>h/0h]<xpub>/0/*)
internal: wpkh([<fingerprint>/84h/<coin>h/0h]<xpub>/1/*)
  3. Calls importdescriptors with both descriptors, marking them active and assigning ranges (default 1000 addresses each; the service refreshes the range on setup if Core has consumed too many).

Because Core holds the xpubs only, it can generate addresses and track UTXOs but cannot sign. Signing is done locally in this service.

3.2 Signing flow

For /send, /anchor, /bump-fee:

  1. fetchMnemonic() — GET {keymasterURL}/api/v1/wallet/mnemonic with the admin header.
  2. walletcreatefundedpsbt(inputs=[], outputs=[...], feeRate?) — asks Core to build a funded PSBT from the watch-only wallet’s UTXOs.
  3. For each input in the PSBT, rederive the signing keypair by following the key origin info in the PSBT’s input records (BIP174 — Core populates bip32Derivation fields).
  4. Sign each input with bitcoinjs-lib.
  5. Finalize the PSBT and extract the fully-signed transaction.
  6. sendrawtransaction(<hex>) and return the txid.

For bump-fee, the same flow plus psbtbumpfee from Core’s RPC which returns a new PSBT; same signing pass follows.

OP_RETURN anchors add a single zero-value scriptPubKey: OP_RETURN <data-push> output to the PSBT. data is encoded as UTF-8 bytes. Size validation enforces bytes <= 80.

4. Bitcoin Core interaction

4.1 Connection

JSON-RPC over HTTP, basic auth:

http://<btcUser>:<btcPass>@<btcHost>:<btcPort>/wallet/<walletName>

Wallet name is a URL segment on the RPC path (Core’s multi-wallet addressing convention). The mediator relies on Core being configured with -txindex=0 (default) — gettransaction works against wallet txs, which is all the mediator touches.

4.2 RPC methods used

bitcoind method Used by
createwallet /wallet/setup
loadwallet /wallet/setup (fallback when wallet already exists)
listdescriptors /wallet/setup (idempotency check)
importdescriptors /wallet/setup
getwalletinfo /wallet/info
getbalances /wallet/balance
getnewaddress (with bech32 type) /wallet/address
getreceivedbyaddress /wallet/address (address-rotation check)
listtransactions /wallet/transactions
listunspent /wallet/utxos
estimatesmartfee /wallet/fee-estimate and inside /send//anchor when no feeRate is supplied
walletcreatefundedpsbt /send, /anchor
gettransaction /wallet/transaction/:txid
psbtbumpfee /wallet/bump-fee
sendrawtransaction /send, /anchor, /bump-fee
getblockcount metrics

4.3 Network handling

ARCHON_WALLET_NETWORK must match the Core node’s network. Accepted values and their Core equivalents:

WalletNetwork Core chain Default port
mainnet main 8332
signet signet 38332
testnet4 testnet4 48332

Only these three are supported — regtest is intentionally omitted.

5. Lifecycle and configuration

5.1 Startup

  1. Read env; validate network.
  2. Wait for Core by polling until walletcreatefundedpsbt or getblockchaininfo succeeds (the TS reference waits implicitly during /wallet/setup).
  3. Start the metrics HTTP server (separate port).
  4. Start the main HTTP server.

The service eagerly runs setupWatchOnlyWallet at startup with a 12-attempt retry loop before serving requests, so the watch-only wallet is ready by the time the satoshi-mediator hits it.

5.2 Environment variables

Variable Default Meaning
ARCHON_WALLET_PORT 4240 Main HTTP port.
ARCHON_WALLET_METRICS_PORT 4241 Prometheus port (separate listener).
ARCHON_KEYMASTER_URL http://localhost:4226  
ARCHON_ADMIN_API_KEY empty Shared admin key between Keymaster / wallet / mediator.
ARCHON_WALLET_BACKEND core core uses Bitcoin Core descriptor wallet RPCs. alchemy uses local BIP84 signing plus hosted JSON-RPC / UTXO APIs.
ARCHON_WALLET_BTC_RPC_URL unset Full Bitcoin JSON-RPC URL. When set, takes precedence over host / port / user / password. Required for hosted alchemy mode.
ARCHON_WALLET_UTXO_URL derived from ARCHON_WALLET_BTC_RPC_URL Base UTXO REST URL for alchemy mode. Defaults to <rpc-url>/api/v2. The wallet supports Alchemy /utxo/{address} and Esplora/mempool.space /address/{address}/utxo shapes.
ARCHON_WALLET_STATE_PATH ./data/satoshi-wallet-state.json Local scan-state cache used by alchemy mode.
ARCHON_WALLET_REFRESH_TTL_MS 30000 In-process hosted wallet scan cache TTL. Balance/address/UTXO calls inside this window reuse the same gap-window scan.
ARCHON_WALLET_BTC_HOST localhost bitcoind RPC host.
ARCHON_WALLET_BTC_PORT 38332 (signet) bitcoind RPC port.
ARCHON_WALLET_BTC_USER / ARCHON_WALLET_BTC_PASS empty bitcoind RPC auth.
ARCHON_WALLET_NAME archon-watch-<nodeID> Core wallet name.
ARCHON_WALLET_NETWORK signet mainnet / signet / testnet4.
ARCHON_WALLET_GAP_LIMIT 20 Descriptor gap limit (address lookahead).
ARCHON_WALLET_FEE_TARGET 6 Confirmation target for estimatesmartfee.
ARCHON_NODE_ID unset Used only to form the default wallet name.
GIT_COMMIT unknown  

5.3 Shutdown

No explicit handler; the HTTP server closes on SIGTERM/SIGINT by default. The watch-only wallet stays loaded in Core across restarts.

6. Prometheus metrics contract

Binds to the metrics port (separate from the API port). Refreshed every 60 seconds in the background:

Metric Type Labels
wallet_setup_status gauge (none) — 1 if the watch-only wallet is ready, 0 otherwise
wallet_balance_confirmed_btc gauge (none) — BTC
wallet_balance_unconfirmed_btc gauge (none) — BTC
wallet_utxo_count gauge (none)
wallet_fee_estimate_sat_per_vb gauge (none) — sat/vB
wallet_bitcoind_block_height gauge (none)
wallet_sends_total counter status (success / failed)
wallet_http_requests_total counter method, route, status

Plus wallet_version_info{version,commit} and standard Prometheus process metrics.

Route normalization on the route label:

/wallet/transaction/<txid>  -> /wallet/transaction/:txid

Everything else is static.

7. Logging conventions

pino (production) or morgan('dev') (development). Errors are structured via pino with { err }. Nothing in the log output is contractual for dashboards or log aggregators today.

8. Reference implementation and tests

Verified end-to-end against:

No isolated unit tests; conformance is observed via the mediator.

A conformant third implementation MUST:

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