Pure-PHP implementations of:

BigIntegers
RSA
SSH2
SFTP
X.509
Symmetric key encryption
    AES
    Rijndael
    Twofish
    Blowfish
    DES
    3DES
    RC4
    RC2

phpseclib is designed to be ultra-compatible. It works on PHP4+ (PHP4, assuming the use of PHP_Compat) and doesn't require any extensions. For purposes of speed, mcrypt is used if it's available as is gmp or bcmath (in that order), but they are not required.