ApiCore/ApiCore-Backend/src/auth/mfa/mfa-crypto.util.ts

import {
  createCipheriv,
  createDecipheriv,
  createHash,
  randomBytes,
} from 'node:crypto';

const ALGORITHM = 'aes-256-gcm';
const IV_BYTES = 12;

function getKey(): Buffer {
  const secret = process.env.JWT_ACCESS_SECRET;
  if (!secret) {
    throw new Error('JWT_ACCESS_SECRET is not set');
  }
  return createHash('sha256').update(secret).digest();
}

/**
 * TOTP secrets must be reversible (unlike passwords/refresh tokens, which
 * are one-way hashed) so the server can compute the expected code on every
 * login. AES-256-GCM at rest, keyed off JWT_ACCESS_SECRET, keeps the secret
 * out of the DB in plaintext without introducing a new secret to manage.
 */
export function encryptMfaSecret(plaintext: string): string {
  const iv = randomBytes(IV_BYTES);
  const cipher = createCipheriv(ALGORITHM, getKey(), iv);
  const encrypted = Buffer.concat([
    cipher.update(plaintext, 'utf8'),
    cipher.final(),
  ]);
  const authTag = cipher.getAuthTag();
  return [iv, authTag, encrypted]
    .map((buffer) => buffer.toString('base64'))
    .join('.');
}

export function decryptMfaSecret(ciphertext: string): string {
  const [ivB64, authTagB64, dataB64] = ciphertext.split('.');
  if (!ivB64 || !authTagB64 || !dataB64) {
    throw new Error('Invalid MFA secret ciphertext');
  }
  const decipher = createDecipheriv(
    ALGORITHM,
    getKey(),
    Buffer.from(ivB64, 'base64'),
  );
  decipher.setAuthTag(Buffer.from(authTagB64, 'base64'));
  return Buffer.concat([
    decipher.update(Buffer.from(dataB64, 'base64')),
    decipher.final(),
  ]).toString('utf8');
}