Propagating Cipher Block Chaining;Kerberos;intialization vector;protocols;CBC;plaintext;ciphertext">


Question 84. What are the Counter and PCBC Modes?

Due to shortcomings in OFB mode (see Question 83) Diffie has proposed [Bra88] an additional mode of operation, termed the counter mode. It differs from OFB mode in the way the successive data blocks are generated for subsequent encryptions. Instead of deriving one data block as the encryption of the previous data block, Diffie proposed encrypting the quantity i + IV (mod 264) for the ith data block, where IV is some initialization vector.

The Propagating Cipher Block Chaining (PCBC) mode of encryption is another mode of operation using block ciphers. It is used in protocols such as Kerberos version 4 (see Question 144). The PCBC mode of encryption has not been formally published as a federal or national standard, and it does not have widespread general support. The PCBC mode is a variation on the CBC mode of operation and is designed to extend or propagate a single bit error in the ciphertext. This allows errors in transmission to be captured and the resultant plaintext to be rejected. The method of encryption is given by

and decryption is achieved by computing

where m0 c0 is the initialization vector.