message-signing

name	message-signing
description	This skill should be used when the user asks to "sign a message", "verify a signature", "use BSM", "use BRC-77", "implement Sigma signing", "create signed messages", "authenticate with Bitcoin", or mentions message signing, signature verification, or authentication protocols on BSV.

BSV Message Signing

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

Quick Reference

Approach	Package	Use Case
Direct SDK	@bsv/sdk	Simple message signing (BSM, BRC-77)
Templates	@bopen-io/templates	Script parsing/creation for protocols
Sigma Protocol	sigma-protocol	Transaction-bound signatures

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

Feature	BSM	BRC-77
Key derivation	Direct	Child key per message
Recovery	From signature	Embedded in signature
Private verify	No	Yes (with recipient key)
SDK module	BSM	SignedMessage

When to Use What

Scenario	Approach
Simple message auth	Direct SDK (BSM or BRC-77)
BAP identity signing	bsv-bap library
Parse existing Sigma/AIP scripts	Templates
Build BitCom transaction outputs	Templates
Sign transaction OP_RETURN data	sigma-protocol
Multiple signatures per output	sigma-protocol
Platform + user dual signing	sigma-protocol

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