generateChallenge.ts

import { isoCrypto } from './iso/index.ts';
import type { Uint8Array_ } from '../types/index.ts';

/**
 * Generate a suitably random value to be used as an attestation or assertion challenge
 */
export async function generateChallenge(): Promise<Uint8Array_> {
  /**
   * WebAuthn spec says that 16 bytes is a good minimum:
   *
   * "In order to prevent replay attacks, the challenges MUST contain enough entropy to make
   * guessing them infeasible. Challenges SHOULD therefore be at least 16 bytes long."
   *
   * Just in case, let's double it
   */
  const challenge = new Uint8Array(32);

  await isoCrypto.getRandomValues(challenge);

  return _generateChallengeInternals.stubThis(challenge);
}

/**
 * Make it possible to stub the return value during testing
 * @ignore Don't include this in docs output
 */
export const _generateChallengeInternals = {
  stubThis: (value: Uint8Array_) => value,
};