PEAR::Crypt_RSA allows you to use two-key strong cryptography like RSA with arbitrary key length. It uses one of the following extensions for math calculations: - PECL big_int extension version greater than or equal to 1.0.3 - PHP GMP extension - PHP BCMath extension for both PHP4 and PHP5 WWW: http://pear.php.net/package/Crypt_RSA/