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/