Lecture 17: Introduction to Encryption

DES In Practice

The ECB mode of operation is now rarely used, since it is now generally agreed that it is breakable given sufficient resources.

In the Chain Block Cipher (CBC) mode, each block of plaintext is exclusive-ORed with the ciphertext output from the previous encryption operation. Thus, the next block of ciphertext is a function of its corresponding plaintext, the 56-bit key and the previous block of ciphertext. Identical blocks of plaintext no longer generate identical ciphertext, which makes this system much more difficult to break.

The CBC mode of DES is the normal technique used for encryption in modern business data communications. 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 Cypher Feedback Mode (CBM), and uses shift registers to permit one byte at a time to be encrypted or decrypted.

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