Here is a list of ciphers which are currently supported by the mcrypt extension. For a complete list of supported ciphers, see the defines at the end of mcrypt.h. The general rule with the mcrypt-2.2.x API is that you can access the cipher from PHP with MCRYPT_ciphername. With the libmcrypt-2.4.x and libmcrypt-2.5.x API these constants also work, but it is possible to specify the name of the cipher as a string with a call to mcrypt_module_open. MCRYPT_3DES MCRYPT_ARCFOUR_IV (libmcrypt > 2.4.x only) MCRYPT_ARCFOUR (libmcrypt > 2.4.x only) MCRYPT_BLOWFISH MCRYPT_CAST_128 MCRYPT_CAST_256 MCRYPT_CRYPT MCRYPT_DES MCRYPT_DES_COMPAT (libmcrypt 2.2.x only) MCRYPT_ENIGMA (libmcrypt > 2.4.x only, alias for MCRYPT_CRYPT) MCRYPT_GOST MCRYPT_IDEA (non-free) MCRYPT_LOKI97 (libmcrypt > 2.4.x only) MCRYPT_MARS (libmcrypt > 2.4.x only, non-free) MCRYPT_PANAMA (libmcrypt > 2.4.x only) MCRYPT_RIJNDAEL_128 (libmcrypt > 2.4.x only) MCRYPT_RIJNDAEL_192 (libmcrypt > 2.4.x only) MCRYPT_RIJNDAEL_256 (libmcrypt > 2.4.x only) MCRYPT_RC2 MCRYPT_RC4 (libmcrypt 2.2.x only) MCRYPT_RC6 (libmcrypt > 2.4.x only) MCRYPT_RC6_128 (libmcrypt 2.2.x only) MCRYPT_RC6_192 (libmcrypt 2.2.x only) MCRYPT_RC6_256 (libmcrypt 2.2.x only) MCRYPT_SAFER64 MCRYPT_SAFER128 MCRYPT_SAFERPLUS (libmcrypt > 2.4.x only) MCRYPT_SERPENT(libmcrypt > 2.4.x only) MCRYPT_SERPENT_128 (libmcrypt 2.2.x only) MCRYPT_SERPENT_192 (libmcrypt 2.2.x only) MCRYPT_SERPENT_256 (libmcrypt 2.2.x only) MCRYPT_SKIPJACK (libmcrypt > 2.4.x only) MCRYPT_TEAN (libmcrypt 2.2.x only) MCRYPT_THREEWAY MCRYPT_TRIPLEDES (libmcrypt > 2.4.x only) MCRYPT_TWOFISH (for older mcrypt 2.x versions, or mcrypt > 2.4.x ) MCRYPT_TWOFISH128 (TWOFISHxxx are available in newer 2.x versions, but not in the 2.4.x versions) MCRYPT_TWOFISH192 MCRYPT_TWOFISH256 MCRYPT_WAKE (libmcrypt > 2.4.x only) MCRYPT_XTEA (libmcrypt > 2.4.x only) You must (in CFB and OFB mode) or can (in CBC mode) supply an initialization vector (IV) to the respective cipher function. The IV must be unique and must be the same when decrypting/encrypting. With data which is stored encrypted, you can take the output of a function of the index under which the data is stored (e.g. the MD5 key of the filename). Alternatively, you can transmit the IV together with the encrypted data (see chapter 9.3 of &book.applied.cryptography; for a discussion of this topic).