Lecture 19: Introduction to Encryption

Cipher Feedback Mode DES, etc

A variation on CBC is used where the message may not be a multiple of 64 bits, or where interactive (character at a time) encryption and decryption is desired. This is called Cipher Feedback Mode (CBM), and uses shift registers to permit one byte at a time to be encrypted or decrypted.

DES has always been controversial:

The original (IBM) algorithm which was the basis of DES used a 128 bit key. The US government changed this to a 56 bit key. This made DES considerably less secure.
Many people believe that the US government has a "backdoor" (or trapdoor) decryption technique which is infeasible at 128 bits, but possible with 56. This has never been confirmed.
56 bit key DES is nowadays regarded as "probably crackable". A variation called triple encryption DES uses two keys (112 bits) and is still considered sufficiently secure.
The IDEA encryption algorithm, which uses a 128 bit key and did not come out of the US government, is suggested as a more suitable basis for current secure communications.
The US Government has mandated (in 2000) a new encryption algorithm which is intended to replace DES, but it is apparently not yet in wide use.

Some good Internet resources on cryptography are available here The tutorial for this lecture is Tutorial #19.
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