Javascript CSPRNG for Integers

crypto-rand.js

function secureRand(min, max) {
  let range = max - min;
  if (range < 2) {
    return min;
  }

  let bits_needed = Math.ceil(Math.log2(range));
  if (bits_needed > 31) {
    throw new Error("cannot generate numbers larger than 31 bits.");
  }

  let mask = (1 << bits_needed) - 1;

  let start = window.performance.now();
  const maxTime = 100; // 100ms

  // Create byte array
  let byteArray = new Uint32Array(1);
  while (true) {
    // fill it with random numbers
    window.crypto.getRandomValues(byteArray);

    let rval = bigEndian(byteArray);

    // Use & to apply the mask and reduce the number of lookups
    rval = rval & mask;

    if (rval < range) {
      // Return an integer that falls within the range
      return min + rval;
    }
    // Integer out of acceptable range
    if (window.performance.now() - start > maxTime) {
      throw new Error("took too long generating random number");
    }
  }
}

function bigEndian(byteArray) {
  return new DataView(byteArray.buffer).getUint32();
}