code: function gPrga(key, s) {
  
    var keystream = [];
    var k = 0;
    var j = 0;
    var len = key.length;
    for (var i = 0; i < len; ++i) {
      k = (k + 1) % 256;
      j = (j + s[k]) % 256;
      s[j] = [ s[k], s[k] = s[j] ][0];
      keystream[i] = s[(s[k] + s[j]) % 256];
    }
  
    return keystream;
  }
docs: Generate PRGA.
  
  @function gPrga
  @param {Array} key - user key
  @param {Array} s - s1 vector
  @return {Array}