phpseclib is designed to be ultra-portable. The 3.0 version works on PHP 5.6+ and doesn't require any extensions. For purposes of speed, OpenSSL, GMP, libsodium or mcrypt are used, if they're available, but they are not required. phpseclib is designed to be fully interoperable with standardized cryptography libraries and protocols. MIT-licensed pure-PHP implementations of SSH-2, SFTP, X.509, an arbitrary-precision integer arithmetic library, Ed25519 / Ed449 / Curve25519 / Curve449, ECDSA /...