Chapter 7 - Block ciphers

Abstract

This chapter deals with symmetric encryption algorithms, focusing on block ciphers. Any block cipher can be operated in several modes. There are four modes that are more often used; electronic codebook (ECB), cipher block-chaining (CBC), output feedback (OFB), and cipher feedback (CFB). The ECB mode uses the simplest idea, that is, to have the message partitioned into several blocks of n-bit length padding if necessary, and then to apply the cipher to each block. The advantage of this mode is that one can decrypt the n-bit blocks independently, in parallel. Thus, if one error occurs in transmission or encryption, that error affects only that particular block, where the error occurred. The CBC mode encrypts the first block using the ECB method applied to the xor of the first block with a block IV. The OFB mode, which is an alternative to CBC to avoid error propagation, produces a unique stream independent from other streams produced by the same encryption key, without rekeying. It uses the output of the encryption function as a feedback, as opposed to the CFB mode, where the output of the ciphertext is used as a feedback.