phpseclib

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 / ECDH (with support for 66 curves), RSA (PKCS#1 v2.2 compliant), DSA / DH, DES / 3DES / RC4 / Rijndael / AES / Blowfish / Twofish / Salsa20 / ChaCha20, GCM / Poly1305. The only requirement that phpseclib 3.0 has is that you must be using PHP 5.6+. Using phpseclib2_compat will actually bring a few enhancements to your dependency.