/**
* Calculates the shared secret for Diffie-Hellman key exchange.
*
* This is the second step in the Diffle-Hellman key exchange process. The other
* party (in OpenID 1.0 terms, the consumer) has already generated the public
* key ($dh_consumer_public) and sent it to this party (the server).
*
* @param string $their_public the other party's public key, in Base64 representation
* @return BigNum the shared secret
*
* @see generateKeyPair()
* @link http://www.ietf.org/rfc/rfc2631.txt RFC 2631
*/
protected function getSharedSecret($their_public)
{
// Decode the keys
$their_y = new BigNum(base64_decode($their_public), 256);
// Generate the shared secret = their public ^ my private mod p = my public ^ their private mod p
$ZZ = $their_y->powmod($this->x, $this->p);
return $ZZ;
}
