What is a Block-Chaining Cipher?

A block-chaining, or feedback, cipher is a type of block cipher in which some portion of the previous encryption block is used to modify the current encryption block. Essentially it takes a monoalphabetic cipher with a large blocklength and makes it polyalphabetic. Generally polyalphabetic ciphers are more secure than monoalphabetic ciphers and generally longer blocklengths are more secure than shorter blocklengths so to combine a large block with a huge potential range of alphabets should provide a truly difficult challenge for a cryptanalyst.

The most common form of block-chaining cipher is known as CBC¹ and functions simply by carrying out an XOR with the current plaintext block and the previous ciphertext block. The result is then encrypted to become the next ciphertext block. This is a form of autokey encryption.

The vast majority of computer block ciphers use some form of block-chaining, even DES has that facility though most implementations have been in ECB² mode.

AES

AES was established as the winner of a competition to find a replacement for the ageing DES. The eventual winner was an algorithm called Rijndael, designed by two Belgian academics.

To a mathematician, AES is a beautiful algorithm, elegant and deceptively simple, but more than that AES is capable of resisting all attempts at cryptanalysis for the forseeable future. It is a 128bit cipher with a 256bit keyspace, where DES was only 64bit with a 56bit keyspace and it resists types of theoretical attack not even discovered when DES was selected.

The Statistics of AES

• Alphabets: Approaching 10³⁹
• Blocklength: 16
• Keyspace: Over 10⁷⁷