Crypt(1) is a very old unix tool used for simple encryption of files.
It should not be used anymore; the algorithm is very weak.
In case you want to decrypt a file that still uses the crypt algorithm, install mcrypt:
% mcrypt -a enigma --keymode scrypt --bare --no-openpgp < encrypted_file.crypt > decrypted_file
Since the crypt algorithm is symmetric, encryption work exactly the same.