/**
* Generates a random integer, which will be used to derive a private key
* for Diffie-Hellman key exchange. The integer must be less than $stop
*
* @param BigNum $stop a prime number as a bignum
* @return BigNum the random integer as a bignum
*/
private function generateRandom($stop)
{
$duplicate_cache = array();
$rand = new Random();
// Used as the key for the duplicate cache
$rbytes = $stop->val(256);
if (array_key_exists($rbytes, $duplicate_cache)) {
list($duplicate, $nbytes) = $duplicate_cache[$rbytes];
} else {
if ($rbytes[0] == "") {
$nbytes = strlen($rbytes) - 1;
} else {
$nbytes = strlen($rbytes);
}
$mxrand = new BigNum(256);
$mxrand = $mxrand->pow(new BigNum($nbytes));
// If we get a number less than this, then it is in the
// duplicated range.
$duplicate = $mxrand->mod($stop);
if (count($duplicate_cache) > 10) {
$duplicate_cache = array();
}
$duplicate_cache[$rbytes] = array($duplicate, $nbytes);
}
do {
$bytes = "" . $rand->bytes($nbytes);
$n = new BigNum($bytes, 256);
// Keep looping if this value is in the low duplicated range
} while ($n->cmp($duplicate) < 0);
return $n->mod($stop);
}
