Implement OP_CHECKSIG

In Bitcoin script, OP_CHECKSIG verifies a digital signature against a public key and a message hash. It's a crucial operation for validating transactions.

Task

Complete the OP_CHECKSIG function in challenges/OpcodeFunctions.js:

  1. Implement OP_CHECKSIG to verify a signature against a public key and a message hash.
  2. The function should pop the top two items from the stack: the public key and the signature.
  3. It should then prompt the user for a message hash.
  4. Finally, it should push true (0x01) or false (0x00) onto the stack based on the signature verification result.

Implementation

P2PK ScriptPubKey Decoder Exercise
import CryptoJS from 'crypto-js';
import { ec as EC } from 'elliptic';
import * as bitcoin from 'bitcoinjs-lib';

const ec = new EC('secp256k1');

export function OP_CHECKSIG(stack, getMessageHash) {
    if (stack.length < 2) {
        throw new Error("Insufficient items for OP_CHECKSIG")
    }

    const pubKeyBuffer = stack.pop()
    const signatureBuffer = stack.pop()

    try {
        const pubKey = ec.keyFromPublic(pubKeyBuffer)
        const signature = signatureBuffer.slice(0, -1) // Remove the hash type
        const messageHash = getMessageHash()
        const isValid = pubKey.verify(messageHash, signature)

        stack.push(isValid ? Buffer.from([0x01]) : Buffer.from([0x00]))
    } catch (error) {
        console.error("Error in OP_CHECKSIG:", error)
        stack.push(Buffer.from([0x00]))
    }

}

export const OP_CODE_FUNCTIONS = {
    OP_CHECKSIG
    // Add more opcodes here
}
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Hints

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  • Use the elliptic library's EC class for signature verification
  • Remember to handle the DER-encoded signature format
  • The public key should be in compressed or uncompressed format
  • Ensure proper error handling for invalid inputs or verification failures

Solution

Solution Code

Test your implementation using the Bitcoin OP_CHECKSIG Simulator above. Try various inputs to ensure your opcode handles different scenarios correctly.

Next
Implement Script Parser