axic/ethereum-bsm

Bitcoin Signed Message in Ethereum

ethereum-bsm

Bitcoin Signed Message x Ethereum

A tutorial on how to validate in Ethereum, that a message has been signed with a valid Bitcoin key. This doesn't work with vanilla Bitcoin addresses, but a slightly transformed Bitcoin public key is needed.

Steps

The steps in short:

1) Encode a Bitcoin public key as an Ethereum address.

Take an uncompressed key, hash it with Keccak-256 and use the lower 160-bits of this hash. The result can be zero padded to 256-bits if needed. Call this Be.

2) Sign a message (call it m) with the Bitcoin key and keep the raw signature parameters.

The Bitcoin message signature is usually encoded as Base64, with the following underlying structure:

• v: 8 bits
• r: 256 bits
• s: 256 bits

3) Give Be, m, v, r and s to the contract to verify.

Important: this will only work with uncompressed Bitcoin keys! The tools provided below will keep keys in an uncompressed format.

Tools

We supply two tools:

1. A Javascript library to generate the above needed parameters. See bsm.js.

2. A Solidity source code for Ethereum to validate these parameters in a smart contract. See bsm.sol.

Example

In the below example we encode the message Hello World with a compressed Bitcoin key (KzoNBaVaUqxRQo7P7kqvY5XP7DPzvauhikzWBLQFq4c76gi7Bzx4).

var bitcoin = require('bitcoinjs-lib')
var bsm = require('./bsm.js')

var keyPair = bitcoin.ECPair.fromWIF('KzoNBaVaUqxRQo7P7kqvY5XP7DPzvauhikzWBLQFq4c76gi7Bzx4')

console.log(bsm.pretty(bsm.address(keyPair))) // 0x20a6a4cf2be3b58136c5a54e0614fd50342a1204

var sig = bsm.sign(keyPair, 'Hello World')
console.log(sig.v) // 27
console.log(bsm.pretty(sig.r)) // 0x9e04c5756c7ae43976057db9af52bd7f343dd06e3f881ca2e12849c4bf73f567
console.log(bsm.pretty(sig.s)) // 0x482a007dcd848b88ba133f786db0e2205ae35ce452f557c6b04d1fcc0497b87a

var sig = bsm.sign(keyPair, 'Hello World Too')
console.log(sig.v) // 28
console.log(bsm.pretty(sig.r)) // 0x3e6de3b0544103489dd8d3c5be20d540353a17f1830b3646b5decbae4b8fbe6e
console.log(bsm.pretty(sig.s)) // 0x67f9236d0af4e9fc929f2105d05de5f2b1d737de5e6fbf67b1862f7fa18020f1

These parameters can be supplied to the validateBSM method in Solidity, which should return true if the signature belongs to that key:

return validateBSM(
  'Hello World',
  0x20a6a4cf2be3b58136c5a54e0614fd50342a1204,
  27,
  0x9e04c5756c7ae43976057db9af52bd7f343dd06e3f881ca2e12849c4bf73f567,
  0x482a007dcd848b88ba133f786db0e2205ae35ce452f557c6b04d1fcc0497b87a
);

License

MIT License

Copyright (C) 2016 Alex Beregszaszi