BLOCK CIPHERS

Generally, ciphers transform pieces of plaintext of a fixed size into ciphertext. In older, manual systems, these pieces were usually single letters or characters (or sometimes, as in the Playfair cipher, digraphs), since these were the largest units that could be easily encrypted or decrypted by hand. Although systems which operated on sets of three characters and other, larger groups of numbers, were proposed and understood to potentially be more secure they were never implemented because of the extra difficulty in the manual encryption or decryption process. In modern, single key cryptography however, the units of information can be much larger.

A block cipher is a type of symmetric-key encryption algorithm that changes a fixed-length block of the plaintext into the same length of ciphertext. The encryption works by means of a key. Decryption is simply the reverse of the encryption process using the same secret key. The fixed length is called the block size and for modern block ciphers is usually 64 bits. As processors become more sophisticated, however, it is likely that this block size will increase to 128 bits.

Since different plaintext blocks are mapped to different ciphertext blocks, a block cipher effectively provides a permutation of the set of all possible messages. The actual permutation produced during any particular operation is of course secret, and determined by the key.

An iterated block cipher encrypts a plaintext block using a process with several stages (rounds). At each stage the same process (known as a round function) is applied to the data using a subkey (the set of subkeys usually being derived from a user provided key). The number of rounds in an iterated block cipher depends on the desired security level of the encrypted ciphertext and the trade-off that must be made with performance; fairly obviously a iterated block cipher with a large number of rounds will require more processing time. It is worth noting that in some cases the number of rounds required to provide an accurate level of security will be too large for the cipher to be practical.

An example of an iterated block cipher is a Feistel cipher. Feistel ciphers are a special class of iterated block ciphers. In this type of cipher the ciphertext is calculated from the repeated application of the same round function.