Block ciphers

Block ciphers are encryption algorithms that break up the text to be encrypted (plaintext) into blocks of a fixed length and apply the encryption block-by-block. Block ciphers are generally built using a design strategy known as a Feistel cipher. Recent block ciphers, such as AES (Rijndael) have been built using a combination of substitution and permutation called a Substitution-Permutation Network (SPN).

Feistel ciphers are based on the Feistel network, which is a structure developed by Horst Feistel. This structure is based on the idea of combining multiple rounds of repeated operations to achieve desirable cryptographic properties known as confusion and diffusion. Feistel networks operate by dividing data into two blocks (left and right) and processing these blocks via keyed round functions in iterations to provide sufficient pseudorandom permutation.

Confusion makes the relationship between the encrypted text and plaintext complex. This is achieved by substitution. In practice, A in plaintext is replaced by X in encrypted text. In modern cryptographic algorithms, substitution is performed using lookup tables called S-boxes. The diffusion property spreads the plaintext statistically over the encrypted data. This ensures that even if a single bit is changed in the input text, it results in changing at least half (on average) of the bits in the ciphertext. Confusion is required to make finding the encryption key very difficult, even if many encrypted and decrypted data pairs are created using the same key. In practice, this is achieved by transposition or permutation.

A key advantage of using a Feistel cipher is that encryption and decryption operations are almost identical and only require a reversal of the encryption process to achieve decryption. DES is a prime example of Feistel-based ciphers:

Various modes of operation for block ciphers are Electronic Code Book (ECB), Cipher Block Chaining (CBC), Output Feedback (OFB) mode, and Counter (CTR) mode. These modes are used to specify the way in which an encryption function is applied to the plaintext. Some of these modes of block cipher encryption are introduced here.