10.2 Modes of enciphering

An actual message is typically made up of many message blocks. One simple way to use the symmetric encryption would be to encrypt each block individually using E as above. The disadvantage of this is that after a suitable interval an attacker with access to plaintext-ciphertext combinations can set up a dictionary. Thus it is beneficial to consider other modes of functioning. The simplest method described above (encrypting block by block) is called Electronic Code-Book since it functions effectively like a (very large) code book.

In another mode, an additional ``key''-like entity called Initialisation Vector is used which consists of one bit for each message block. This bit is added in to the key (in some specified way) to affect the encryption performed. This way the different blocks correspond to different keys and so the size of the dictionary required for a dictionary attack is much larger even if the initialisation vector is well-known. This method is known as Cipher Block Chaining, since the message cannot be decrypted unless the blocks are arranged in the sequence in which they were encrypted.

Other modes which depend on ``feedback'' from the output of the previous encryption block can also be used to chain the encryption procedure.

In addition one can construct ciphers with longer keys by constructing of chain of distinct ciphers each of which uses a sub-key. The two notable variants are Double encryption (usually performed with different algorithms) and Triple encryption (usually performed with the same algorithm but one of the three encryptions is actually a ``decryption'' operation (i. e. uses D rather than E).