generateAccountSalt.ts

// Function to generate AccountSalt
import { buildPoseidon } from 'circomlibjs';

/**
 * Attempt at reproducing the account salt generation that zkemail does on the backend at ->
 * https://github.com/zkemail/relayer-utils/blob/aefb00fdcfbfde7fced50e5a0b086e5cc2ff64cd/src/cryptos.rs#L198
 * @param email
 * @param accountCode
 */
export async function generateAccountSalt(email: string, accountCode: bigint): Promise<string> {
  const maxEmailBytes = 256;

  // 1. Pad the email address
  const paddedEmailBytes = padEmailAddr(email, maxEmailBytes);

  // 2. Convert padded email to Poseidon field elements
  const emailFields = bytesToFields(paddedEmailBytes);

  // 3. Poseidon hash function
  const poseidon = await buildPoseidon();

  // 4. Prepare inputs for Poseidon hashing
  const inputs = [...emailFields, BigInt(accountCode), BigInt(0)];

  // 5. Compute Poseidon hash to get the AccountSalt
  const accountSalt = poseidon(inputs);

  return uint8ArrayToHex(accountSalt);
}

function bytesToFields(bytes: Uint8Array): bigint[] {
  const chunkSize = 31;
  const fieldElements: bigint[] = [];

  // Process the bytes in chunks of 31 bytes
  for (let i = 0; i < bytes.length; i += chunkSize) {
    // Get the current chunk (31 bytes)
    const chunk = bytes.slice(i, i + chunkSize);

    // Extend the chunk to 32 bytes by padding with zeros
    const extended = new Uint8Array(32);
    extended.set(chunk); // Copy chunk into the first part of the 32-byte array

    // Convert the 32-byte array to a BigInt or field element
    const fieldElement = uint8ArrayToBigInt(extended); // Assuming we can treat it as a number here

    fieldElements.push(fieldElement);
  }

  return fieldElements;
}

function uint8ArrayToBigInt(uint8Array: Uint8Array): bigint {
  let result = 0n; // Start with a bigint value of 0
  for (const byte of uint8Array) {
    // eslint-disable-next-line no-bitwise
    result = (result << 8n) + BigInt(byte); // Shift the current value by 8 bits (1 byte) and add the next byte
  }
  return result;
}

function uint8ArrayToHex(uint8Array: Uint8Array): string {
  const noPrepend = Array.from(uint8Array)
    .map((byte) => byte.toString(16).padStart(2, '0')) // Convert each byte to hex and pad with '0' if necessary
    .join(''); // Join all hex strings into one
  return `0x${noPrepend}`;
}

function padEmailAddr(email: string, maxBytes: number): Uint8Array {
  const emailBytes = new TextEncoder().encode(email);
  const paddedBytes = new Uint8Array(maxBytes);
  paddedBytes.set(emailBytes);
  return paddedBytes;
}