Syntax:
DES_DECRYPT(crypt_str[,key_str])

Decrypts a string encrypted with DES_ENCRYPT(). If an error occurs,
this function returns NULL.

This function works only if MySQL has been configured with SSL support.
See http://dev.mysql.com/doc/refman/5.6/en/ssl-connections.html.

If no key_str argument is given, DES_DECRYPT() examines the first byte
of the encrypted string to determine the DES key number that was used
to encrypt the original string, and then reads the key from the DES key
file to decrypt the message. For this to work, the user must have the
SUPER privilege. The key file can be specified with the --des-key-file
server option.

If you pass this function a key_str argument, that string is used as
the key for decrypting the message.

If the crypt_str argument does not appear to be an encrypted string,
MySQL returns the given crypt_str.

URL: http://dev.mysql.com/doc/refman/5.6/en/encryption-functions.html

