Get Random Bytes from Webcrypto #webcrypto #js

random.ts

/**
 * Webcrypto random bytes is limited to 65,536 in one call.
 * The below functions will make repeated calls to acquire random bytes.
 */

const webcrypto = globalThis.crypto?.webcrypto || globalThis.window?.crypto;

async function sleep(ms: number): Promise<void> {
  return await new Promise<void>((r) => setTimeout(r, ms));
}

/**
 * Get random bytes asynchronously.
 * This yields the event loop each 65,536 bytes.
 */
async function getRandomBytes(size: number): Promise<Buffer> {
  const randomBytes = Buffer.allocUnsafe(size);
  let i = 0;
  while (size > 0) {
    // Webcrypto limits a max 65,536 random bytes at a time
    const chunkSize = Math.min(size, 65536);
    const chunk = randomBytes.slice(i, chunkSize);
    webcrypto.getRandomValues(chunk);
    i += chunkSize;
    size -= chunkSize;
    if (size > 0) {
      await sleep(0);
    }
  }
  return randomBytes;
}

/**
 * Get random bytes synchronously.
 * This loops each 65,536 bytes until the buffer is filled.
 */
function getRandomBytesSync(size: number): Buffer {
  const randomBytes = Buffer.allocUnsafe(size);
  let i = 0;
  while (size > 0) {
    const chunkSize = Math.min(size, 65536);
    const chunk = randomBytes.slice(i, chunkSize);
    webcrypto.getRandomValues(chunk);
    i += chunkSize;
    size -= chunkSize;
  }
  return randomBytes;
}

export {
  getRandomBytes,
  getRandomBytesSync
};

I'm using libsodium now which doesn't have this limitation, so we don't really need an async and sync functions and we can produce as much data as we want.