module.exports = function (seed, len, crypto) {
  var t = new Buffer('');
  var  i = 0, c;
  while (t.length < len) {
    c = i2ops(i++);
    t = Buffer.concat([t, crypto.createHash('sha1').update(seed).update(c).digest()]);
  }
  return t.slice(0, len);
};

function i2ops(c) {
  var out = new Buffer(4);
  out.writeUInt32BE(c,0);
  return out;
}