The other poster was talking about a stream cipher... However, I'd point out that DES is a block cipher (thought it can be turned in to streaming cipher much l

The other poster was talking about a stream cipher... However, I'd point out that DES is a block cipher (thought it can be turned in to streaming cipher much like AES), and its decryption is encryption is with the keys in reverse order. This is how triple-DES can work as it does.