With the aid of a little hardware, DES can be made pretty hard to penetrate. If you run the serialized cypher through a CPLD that implements one-bit cypher feedback, for example, essentially encrypting the key with a bit selected from the cyher by the cypher content, the process is entirely reversible at the other end, but, if it's several layers deep, and uses either one, two, or three bits of the cypher, depending on a parameter contained in the key, the thing becomes a lot of trouble to crack. The guy(s) doing the cracking have to know quite a bit more than you do about how your scheme works, though, else they'll be tearing out what little hair they have left.

A cheap 44-pin CPLD will do this little job several layers deep at nearly any rate at which a '52 family member can produce it serially.

RE