mcrypt is intended to be a replacement of the old unix crypt under the GNU General Public License. Unix Crypt was a popular file encryption program in unix boxes. mcrypt was based on the enigma encryption algorithm but it was considerable trivialized.
Since this was not adequate, even for individual privacy needs, I decided to create a similar program using some modern block encryption algorithms. Mcrypt also has a compatibility mode with unix crypt(1) and with solaris des. It supports all the algorithms and modes found in libmcrypt and it is very extendable.
At the time writing this, it supports the algorithms: BLOWFISH, TWOFISH, DES, TripleDES, 3-WAY, SAFER, LOKI97, GOST, RC2, MARS, RIJNDAEL, SERPENT, CAST, ARCFOUR and WAKE.
Block algorithms are implemented in modes: CFB, CBC, CTR, ECB, OFB (8 bit and n bit, where n is the size of the algorithm's block length). For a brief description of the algorithms and the modes look at the mcrypt manpage (this may be out of date). In mcrypt it is on the user to decide which algorithm he considers best for encrypting his data.
Since mcrypt 2.6.0, the OpenPGP (RFC2440) encrypted file format is supported.
Mcrypt should work fine in all unix systems. It may work (probably with changes) in other non-unix systems. Mcrypt 2.6.x can also been ported to Win32 using the Cygwin compiler.