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Block Ciphers and Their Modes of Operation

The TLS Record protocol protects application data sent by Alice and Bob using shared secret keys established during their TLS handshake. We will start our study of the TLS Record protocol with its most fundamental building block: block ciphers. In this chapter, we discuss in detail what block ciphers are, what design principles govern the construction of secure block ciphers, what mathematical objects are used to model block ciphers, and what actual block cipher algorithms used in practice look like.

Block ciphers form an integral part of modern cryptography. However, RFC 8446, the IETF standard specifying TLS 1.3, does not explicitly treat block ciphers. Rather, it points to some related references, including NIST Special Publication 800-38D and IETF’s RFC 5116, RFC 6655, and RFC 8439. In contrast, we will cover block ciphers in quite some detail because cryptography of the TLS Record protocol cannot be understood in depth without...