auth-yes/public/utils/bip39.js

import { WORDLIST } from "./bip39_wordlist.js";

export async function entropyToMnemonic(entropy) {
  if (entropy.length < 16 || entropy.length > 32 || entropy.length % 4 !== 0) {
    throw new Error("Invalid entropy length");
  }

  const entropyBits = Array.from(entropy)
    .map((b) => b.toString(2).padStart(8, "0"))
    .join("");

  const entropyBuffer = new Uint8Array(entropy.length);
  entropyBuffer.set(entropy);
  const hashBuffer = await crypto.subtle.digest("SHA-256", entropyBuffer);
  const hashBits = Array.from(new Uint8Array(hashBuffer))
    .map((b) => b.toString(2).padStart(8, "0"))
    .join("");

  const checksumLength = entropy.length / 4;
  const checksum = hashBits.slice(0, checksumLength);

  const bits = entropyBits + checksum;
  const chunks = bits.match(/(.{1,11})/g) || [];

  const mnemonic = chunks.map((binaryStr) => {
    const index = parseInt(binaryStr, 2);
    return WORDLIST[index];
  });

  return mnemonic.join(" ");
}

export async function mnemonicToEntropy(mnemonic) {
  const words = mnemonic.normalize("NFKD").trim().split(/\s+/);
  if (words.length % 3 !== 0) {
    throw new Error("Invalid mnemonic length");
  }

  const bits = words
    .map((word) => {
      const index = WORDLIST.indexOf(word);
      if (index === -1) {
        throw new Error(`Invalid word in mnemonic: ${word}`);
      }
      return index.toString(2).padStart(11, "0");
    })
    .join("");

  const dividerIndex = Math.floor(bits.length / 33) * 32;
  const entropyBits = bits.slice(0, dividerIndex);
  const checksumBits = bits.slice(dividerIndex);

  const entropy = new Uint8Array(entropyBits.length / 8);
  for (let i = 0; i < entropy.length; i++) {
    entropy[i] = parseInt(entropyBits.slice(i * 8, (i + 1) * 8), 2);
  }

  const hashBuffer = await crypto.subtle.digest("SHA-256", entropy);
  const hashBits = Array.from(new Uint8Array(hashBuffer))
    .map((b) => b.toString(2).padStart(8, "0"))
    .join("");
  const expectedChecksum = hashBits.slice(0, checksumBits.length);

  if (expectedChecksum !== checksumBits) {
    // Explicitly zeroize on failure
    entropy.fill(0);
    throw new Error("Invalid mnemonic checksum");
  }

  return entropy;
}