Message Signing

BSV Message Signing

Three approaches for message signing on BSV, from simple to full-featured.

Quick Reference

1. Direct SDK Signing

For simple message authentication without transaction context.

BSM (Bitcoin Signed Message)

import { BSM, PrivateKey, Signature, BigNumber } from "@bsv/sdk";

const privateKey = PrivateKey.fromWif("L1...");
const message = [/* byte array */];

// Sign
const signature = BSM.sign(message, privateKey, "raw") as Signature;
const h = new BigNumber(BSM.magicHash(message));
const recovery = signature.CalculateRecoveryFactor(privateKey.toPublicKey(), h);
const compactSig = signature.toCompact(recovery, true, "base64");

// Verify - recover public key and check address
const sig = Signature.fromCompact(compactSig, "base64");
for (let r = 0; r < 4; r++) {
  try {
    const pubKey = sig.RecoverPublicKey(r, new BigNumber(BSM.magicHash(message)));
    if (BSM.verify(message, sig, pubKey)) {
      console.log("Signer:", pubKey.toAddress());
      break;
    }
  } catch { /* try next */ }
}

BRC-77 (SignedMessage)

import { SignedMessage, PrivateKey, PublicKey } from "@bsv/sdk";

const signer = PrivateKey.fromWif("L1...");
const message = [/* byte array */];

// Public signature (anyone can verify)
const sig = SignedMessage.sign(message, signer);
const valid = SignedMessage.verify(message, sig);

// Private signature (only recipient can verify)
const recipientPubKey = PublicKey.fromString("02...");
const privateSig = SignedMessage.sign(message, signer, recipientPubKey);

const recipientKey = PrivateKey.fromWif("K1...");
const privateValid = SignedMessage.verify(message, privateSig, recipientKey);

See: examples/bsm-sign-verify.ts, examples/brc77-private-sig.ts

2. Template-Based Signing

For working with BitCom protocols and script parsing. Uses @bopen-io/templates.

Sigma Template

import { Sigma, SigmaAlgorithm, BitCom } from "@bopen-io/templates";
import { PrivateKey, Hash } from "@bsv/sdk";

// Decode from existing script
const bitcom = BitCom.decode(script);
const sigmas = Sigma.decode(bitcom);

// Create signature (given pre-computed hashes)
const inputHash = Array.from(Hash.sha256(outpointBytes));
const dataHash = Array.from(Hash.sha256(scriptDataBytes));

const sigma = Sigma.sign(inputHash, dataHash, privateKey, {
  algorithm: SigmaAlgorithm.BRC77,  // or SigmaAlgorithm.BSM
  vin: 0
});

// Verify
const valid = sigma.verifyWithHashes(inputHash, dataHash);

// Generate locking script
const lockingScript = sigma.lock();

AIP Template

import { AIP, BitCom } from "@bopen-io/templates";

// Decode AIP from script
const bitcom = BitCom.decode(script);
const aips = AIP.decode(bitcom);

// Create AIP signature
const aip = await AIP.sign(dataBytes, privateKey, {
  algorithm: "BITCOIN_ECDSA",
  fieldIndexes: [0, 1, 2]  // optional: specific fields to sign
});

const valid = aip.verify();

See: examples/sigma-template.ts

Note: Sigma template requires sigma-protocol as peer dependency for Algorithm enum.

3. Sigma Protocol (Full Transaction Signing)

For complete transaction-bound signatures. Use when:

• Signing OP_RETURN data in transactions
• Need multiple signatures on same output
• Want automatic hash calculation from transaction context

bun add sigma-protocol

import { Sigma, Algorithm } from "sigma-protocol";
import { Transaction, PrivateKey } from "@bsv/sdk";

// Create Sigma instance
const sigma = new Sigma(tx, targetVout, sigmaInstance, refVin);

// Sign with BSM (default)
const { signedTx } = sigma.sign(privateKey);

// Sign with BRC-77
const { signedTx: tx2 } = sigma.sign(privateKey, Algorithm.BRC77);

// Sign with BRC-77 for specific verifier (private)
const { signedTx: tx3 } = sigma.sign(privateKey, Algorithm.BRC77, verifierPubKey);

// Verify
const valid = sigma.verify();
const privateValid = sigma.verify(recipientPrivateKey);  // for private BRC-77

Multiple Signatures

// First signature (user with BSM)
const sigma1 = new Sigma(tx, 0, 0);
const { signedTx } = sigma1.sign(userKey, Algorithm.BSM);

// Second signature (platform with BRC-77)
const sigma2 = new Sigma(signedTx, 0, 1);
sigma2.sign(platformKey, Algorithm.BRC77);

// Verify each
sigma2.setSigmaInstance(0);
sigma2.verify();  // User

sigma2.setSigmaInstance(1);
sigma2.verify();  // Platform

See: examples/sigma-multi-sig.ts

Algorithm Comparison

When to Use What

BAP Identity Signing (BSM)

BAP identities use BSM for signing. The bsv-bap library handles this:

import { BAP } from "bsv-bap";
import { Utils } from "@bsv/sdk";
const { toArray } = Utils;

const bap = new BAP({ rootPk: wif });
const identity = bap.getId(bap.listIds()[0]);

// Sign with BSM (uses derived identity signing key)
const message = toArray("Hello World", "utf8");
const { address, signature } = identity.signMessage(message);

// Verify BSM signature
const isValid = bap.verifySignature("Hello World", address, signature);

A CLI is also available: npm install -g bsv-bap (see create-bap-identity skill).

Additional Resources

Reference Files

• references/brc-77-spec.md - Full BRC-77 specification
• references/sigma-advanced.md - Remote signing, hash details, advanced patterns

Examples

• examples/bsm-sign-verify.ts - Direct BSM signing
• examples/brc77-private-sig.ts - BRC-77 private signatures
• examples/sigma-template.ts - Template-based Sigma operations
• examples/sigma-multi-sig.ts - Full sigma-protocol with multi-sig