doc: how to sign the transaction

[coolaj86]

Forgive the slop.

In short:

• signatures do NOT have salt/nonce
• hashes are DOUBLE
• hashes are NOT reversed when signed (the "serialized" form is a lie)
• signatures are NOT reversed

The transition has 2 forms:

• null sigs (signed by each identity key, and the asset lock key)
• full sigs (includes all signature data)

let textEncoder = new TextEncoder();
let Secp256k1 = require("@dashincubator/secp256k1");

async function sha256(data) {
  let hashBuffer = await crypto.subtle.digest("SHA-256", data);
  let hashBytes = new Uint8Array(hashBuffer);
  return hashBytes;
}

function toHex(bytes) {
  return Array.from(bytes, (byte) => byte.toString(16).padStart(2, "0")).join(
    "",
  );
}

let Bytes = {};
/** @typedef {String} Hex */
/**
 * @param {Hex} hex
 */
Bytes.hexToBytes = function (hex) {
  let bufLen = hex.length / 2;
  let bytes = new Uint8Array(bufLen);

  let i = 0;
  let index = 0;
  let lastIndex = hex.length - 2;
  for (;;) {
    if (i > lastIndex) {
      break;
    }

    let h = hex.substr(i, 2);
    let b = parseInt(h, 16);
    let nan = isNaN(b);
    if (nan) {
      throw new Error(`'${h}' could not be parsed as hex`);
    }
    bytes[index] = b;

    i += 2;
    index += 1;
  }

  return bytes;
};

let KeyUtils = {};

/**
 * @param {Uint8Array} privKeyBytes
 * @param {Uint8Array} doubleHashBytes
 * @returns {Promise<Uint8Array>} - magic signature
 */
KeyUtils.magicSign = async function (privKeyBytes, doubleHashBytes) {
  let MAGIC_OFFSET = 27 + 4; // 27 because bitcoin, 4 because "compressed" key
  let testing = true;
  let sigOpts = { canonical: true, der: false, recovered: true };
  if (!testing) {
    Object.assign({ extraEntropy: true });
  }
  let recoverySig = await Secp256k1.sign(
    doubleHashBytes,
    privKeyBytes,
    sigOpts,
  );
  let magicSig = new Uint8Array(65);
  let recovery = MAGIC_OFFSET + recoverySig[1];
  // the magic byte is prepended (the signature is NOT reversed)
  magicSig[0] = recovery;
  magicSig.set(recoverySig[0], 1);
  return magicSig;
};

async function runTest() {
  let keyHex =
    "561349d51e3682f542adf609a95e579caf2974e823c7cf283d9bab001b2e4f7e";
  let privKeyBytes = Bytes.hexToBytes(keyHex);

  // let input = "";
  // let data = textEncoder.encode(input);

  let input =
    "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";
  let data = Bytes.hexToBytes(input);

  let expectedOutputStr =
    //"56944c5d3f98413ef45cf54545538103cc9f298e0575820ad3591376e2e0f65d";
    "5cd6e6496d81d285df139834c05c87e438ce9b73874d3f074aebbc249e032638";

  let firstHash = await sha256(data);
  let secondHash = await sha256(firstHash);

  // Reverse the result
  // let reversedHash = secondHash.reverse();

  // Convert to hex string
  let resultHex = toHex(secondHash);
  // let resultHex = toHex(reversedHash);

  console.log("Computed Hex:", resultHex);
  console.log("Matches Expected:", resultHex === expectedOutputStr);

  let sig = await KeyUtils.magicSign(privKeyBytes, secondHash);
  let sigHex = toHex(sig);
  console.log(sigHex);
}

runTest();