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SatMask

Proof of Concept code for signing Bitcoin transactions via MetaMask.

DO NOT USE THIS CODE IN PRODUCTION.

How it works

MetaMask's eth_sign api allows the user to sign arbitrary digests. Unlike geth, it does not modify or hash the argument. This means we can calculate the sighash of a Bitcoin transaction and pass it to eth_sign. After that, we need to translate the signature from Ethereum's hex-encoded RSV standard to Bitcoin's DER-Encoded RS format, and add the signature to the transaction.

Calculating the sighash requires knowledge of the public key that will sign, so an extra personal_sign call is made. This generates a signature from which we can recover the users' pubkey to use in the sighash algorithm.

The current version signs only transactions with 1 witness pubkeyhash input and one witness pubkeyhash output. The resulting transaction is logged to console.

A brief tour of the codebase

  • src/tx.js handles transaction datastructure manipulation. This includes construction, sighash serialization, and adding the signature to the transaction (inside a witness).

  • src/sigs.js handles interfacing with MetaMask, retrieving the signature, and translating from RSV to DER.

  • src/App.js is a simple react app that provides a convenient interface for specifying a transaction. All credit to @tynes for this part :)

How to run it

Don't. Like really don't. Read the code, but don't run it.