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Porting PHP code from Mcrypt to OpenSSL

With Mcrypt long ago abandoned, you might well ask: Can OpenSSL, the only other common PHP crypto extension, replace Mcrypt? The answer is obviously: It depends. I've been studying on what it depends.

Cipher algorithms

First, compatibility depends on the algorithm. OpenSSL supports:

  • Blowfish
  • CAST-128, i.e. CAST5
  • DES
  • Triple DES with 2 or 3 keys
  • AES, known as 'rijndael-128' in Mcrypt
  • RC4, known as 'arcfour' in Mcrypt

OpenSSL cannot substitute for these Mcrypt cipher algorithms:

  • cast-256
  • enigma
  • gost
  • loki97
  • rijndael-192
  • rijndael-256
  • saferplus
  • serpent
  • twofish
  • wake
  • xtea

Both claim to support RC2 but Mcrypt's 'rc2' seems to produce incorrect ciphertext relative to OpenSSL, C♯ and test vectors in RFC-2268.

Block modes

OpenSSL does not implement CTR mode or OFB mode with 8-bit feedback, which is called 'ofb' mode in Mcrypt. Mcrypt's modes 'cfb', 'ncfb' and 'nofb' correspond to OpenSSL's 'cfb8', 'cfb' and 'ofb' modes respectively.

Cipher and mode compatibility map

The results of my compatibility testing of all ciphers and modes is in GitHub, together with PHP scripts for preparing and running the tests.

The cipher and mode compatibility map is in the same repo.

Padding

I wrote about the padding behavior of Mcrypt and OpenSSL in detail in a previous article. Although they are different, whatever padding you were doing with Mcrypt should be adaptable to OpenSSL.