Block Cipher Modes

Block cipher is one of the most widely-used tool in cryptography, and its mode of operation features the use of a symmetric-key block-cipher algorithm in providing information security such as confidentiality or authenticity. Five modes of operation with AES algorithm will be discussed in this paper: Electronic Codebook (ECB), Cipher Block Chaining (CBC), Cipher Feedback (CFB), Output Feedback (OFB), and Counter (CTR). These modes can provide cryptographic protection for sensitive, but classified, computer data.

Keywords: Computer security; cryptography; block cipher; mode of operation

Block cipher is one of the most widely-used tool in cryptography, but itself is only used of one fixed-length group of bits called a block [1]. A mode of operation describes how to repeatedly apply a cipher's single-block operation to securely transform amounts of data larger than a block [2].

The five modes—the Electronic Codebook (ECB), Cipher Block Chaining (CBC), Cipher Feedback (CFB), Output Feedback (OFB), and Counter (CTR) modes can provide data confidentiality.

The electronic codebook (ECB) is the simplest encryption mode, which is named after conventional physical codebooks. For the given key, ECB features the same assignment of a fixed ciphertext block to each plaintext. Each block is been manipulated separately.

In ECB encryption and ECB decryption, multiple forward cipher functions and inverse cipher functions can be computed in parallel under a given key.

The ECB mode has an disadvantages—a lack of diffusion, under a given key, any given plaintext block always gets encrypted to the same cyphertext if they are identical.

The cipher block chaining (CBC) mode of operation is introduced in 1976 [3]. The CBC mode features the combining of the plaintext blocks with the previous ciphertext blocks. The combination method is XOR. To make each message unique, an initialization vector are introduced in the first block. This initialization vector need not be secret, but must be unpredictable.

The main drawback of the CBC mode is sequential. In CBC encryption, the input block to each forward cipher operation (except the first) depends on the result of the previous forward cipher operation, so process cannot be parallelized. And only a one-bit change in a plaintext or initialization vector affects all following ciphertext blocks.

[1] Plass, Jan L., Roxana Moreno, and Roland Brünken. Cognitive Load Theory. Cambridge University Press, 2010.

[2] Mayer, Richard E. The Cambridge handbook of multimedia learning. Cambridge University Press, 2005.

[3] Kirschner, P. A, Sweller, J. and Clark, R. E. Why minimal guidance during instruction does not work: An analysis of the failure of constructivist, discovery, problem-based, experiential, and inquiry-based teaching. Educational psychologist 41, no. 2, pp 75-86, 2006