SilentPayments (BIP-352)

Send & receive bitcoins via SilentPayment (aka static payment codes), in pure typescript.

Installation

• npm i "github:BlueWallet/SilentPayments" --save

Library is implemented in pure typescript without js-compiled version committed - you might need to configure javascript build on your side. For example, to use it in jest tests:

package.json:

  "jest": {
    "transform": {
      "^.+\\.(ts|tsx)$": "ts-jest"
    },
    "transformIgnorePatterns": [
      "node_modules/(?!((jest-)?react-native(-.*)?|@react-native(-community)?)|silent-payments/)"
    ],

If youre using webpack you might need to add a loader in webpack.config.js, something like this:

...
    {
        test: /node_modules\/silent-payments\/.*\.ts$/,
        use: [
          {
            loader: require.resolve('ts-loader'),
            options: {
              allowTsInNodeModules: true,
              getCustomTransformers: () => ({
                before: [isDevelopment && ReactRefreshTypeScript()].filter(
                  Boolean
                ),
              }),
              transpileOnly: isDevelopment,
            },
          },
        ],
      },
...

Usage

Send

You must provide UTXOs and targets (which might or might not include SilentPayment codes):

createTransaction(utxos: UTXO[], targets: Target[]): Target[]

Finally:

const sp = new SilentPayment();

const targets = sp.createTransaction(
  [
    {
      txid: "f4184fc596403b9d638783cf57adfe4c75c605f6356fbc91338530e9831e9e16",
      vout: 0,
      wif: ECPair.fromPrivateKey(Buffer.from("1cd5e8f6b3f29505ed1da7a5806291ebab6491c6a172467e44debe255428a192", "hex")).toWIF(),
      utxoType: "p2wpkh",
    },
  ],
  [
    {
      address: "3FiYaHYHQTmD8n2SJxVYobDeN1uQKvzkLe",
      value: 11_111,
    },
    {
      address: "sp1qqgste7k9hx0qftg6qmwlkqtwuy6cycyavzmzj85c6qdfhjdpdjtdgqjuexzk6murw56suy3e0rd2cgqvycxttddwsvgxe2usfpxumr70xc9pkqwv",
      value: 22_222,
    },
    {
      // no address, which should be interpreted as change
      value: 33_333,
    },
  ]
);

Library will unwrap sp1... codes into correct receivers address. You must provide correct UTXO types to the library, and you must use the same UTXOs in an actual transaction you create. Library will not do coin selection for you.

Receive

Scanning is a two-step job. First derive the per-transaction tweak from the spending transaction and the scripts it spent (prevoutScripts[i] is the script of tx.ins[i]). Returns null when the transaction should be skipped (no eligible inputs, or the pubkey sum is the identity).

const tweak = SilentPayment.computeTweakForTx(tx, prevoutScripts);
if (!tweak) {
  // skip this transaction
}
const tweakHex = Buffer.from(tweak).toString("hex");

The tweak can also come from a tweak-indexing backend (same 33-byte compressed pubkey, hex-encoded). Then detect outputs that belong to you.

With a BIP-39 seed (optional BIP-352 account index and passphrase; both default to account 0 and ""):

const utxos = SilentPayment.detectOurUtxos(tx, mnemonic, tweakHex);
// or: SilentPayment.detectOurUtxos(tx, mnemonic, tweakHex, accountNum, passphrase)

Each result is a taproot UTXO with a WIF spend key (b_spend + t_k). Detection walks unlabeled k = 0, 1, … until a gap or K_MAX.

If you already have bscan and Bspend (scan private key and spend public key as hex):

const utxos = SilentPayment.detectOurUtxosUsingTweakbscanBspend(tx, tweakHex, bscanHex, BspendHex);

To check a single output script in isolation (only k = 0; later k need the full transaction via detectOurUtxos*):

SilentPayment.isOurUtxoUsingTweakbscanBspendAndOutputScript(outputScriptHex, tweakHex, bscanHex, BspendHex);
SilentPayment.isOurUtxoUsingTweakbscanBspendAndOutputScriptUint8array(outputScript, tweak, bscan, Bspend);

Labeled addresses (m ≠ unlabeled B_spend) are not scanned. seedToCode produces the unlabeled sp1… payment code plus the scan/spend keys used above.

Development

• npm i
• npm t

License

MIT

References

• https://github.com/josibake/silent-payments-workshop/blob/main/silent-payments-workshop.ipynb
• bitcoin/bitcoin#27827
• https://medium.com/@ottosch/how-bip47-works-ee641cc14bf3
• https://medium.com/@ottosch/how-silent-payments-work-41bea907d6b0
• https://github.com/bitcoin/bips/blob/master/bip-0352.mediawiki
• https://github.com/bitcoin/bips/blob/97012a82064c7247df502a170c03b053825cdd15/bip-0352/reference.py