Javascript CSPRNG for Integers

gistfile1.js

/**
 * License: WTFPL, CC0, ZAP (Zero For 0wned Anti-copyright Pledge), etc
 */
function secure_rand(min, max) {
  var i = rval = bits = bytes = 0;
  var range = max - min;
  if (range < 1) {
    return min;
  }
  if (window.crypto && window.crypto.getRandomValues) {
    // Calculate Math.ceil(Math.log(range, 2)) using binary operators
    var tmp = range;
    /**
     * mask is a binary string of 1s that we can & (binary AND) with our random
     * value to reduce the number of lookups
     */
    var mask = 1;
    while (tmp > 0) {
      if (bits % 8 === 0) {
        bytes++;
      }
      bits++;
      mask = mask << 1 | 1; // 0x00001111 -> 0x00011111
      tmp = tmp >>> 1;      // 0x01000000 -> 0x00100000
    }
    
    var values = new Uint8Array(bytes);
    do {
      window.crypto.getRandomValues(values);
      
      // Turn the random bytes into an integer
      rval = 0;
      for (i = 0; i < bytes; i++) {
        rval |= (values[i] << (8 * i));
      }
      // Apply the mask
      rval &= mask;
      // We discard random values outside of the range and try again
      // rather than reducing by a modulo to avoid introducing bias
      // to our random numbers.
    } while (rval > range);
    
    // We should return a value in the interval [min, max]
    return (rval + min);
  } else {
    // CSPRNG not available, fail closed
    throw Error('placeholder; this should be customized with accordance to whatever practices are best for Node.js')
  }
}