Cryptography

7 Block Ciphers: Basics, TDEA, and AES

This chapter focuses on block ciphers, which are the most commonly used algorithms to encrypt confidential data. In addition to ciphering, block ciphers can be used as stream ciphers and pseudorandom number generators or used to build hash functions and MACs (Message Authentication Codes). Therefore, block ciphers are of prime importance to build cryptosystems.

A huge number of block ciphers are published in literature; but a very small number of them are standards used in operational cryptosystems. This chapter aims at introducing the basics of construction of block ciphers and present in detail the standard block ciphers, currently in use, namely TDEA (Triple Data Encryption Algorithm) and AES (Advanced Encryption Standard). Both ciphers are NIST (National Institute of Standards and Technology, US) standards. It is worth noticing that, because of some reported attacks (even if they are theoretical) against TDEA, AES would be the dominating block cipher in the near future and for a long time.

7.1 Construction Principles for Block Cipher Design

Definition 7.1 Block cipher: it is an encryption–decryption scheme where a block of plaintext is treated as a single block and is used to obtain a block of ciphertext with the same size.

Modern ciphering was inspired by mechanical ciphering machines such as Enigma, presented in Section 4.5. The most commonly used cyphers, called block ciphers, operate as follows (see Figure 7.1):

Figure 7.1 Block-based ...