Introduction to Cryptography with Open-Source Software

Chapter 8

Block ciphers and the data encryption

standard

Block ciphers, which operate on an n-bit block of plaintext (the length of which

is a function o f the cipher), are some of the most powerful, fastest, and most

used cryptosystems in existence. Unlike public-key systems, which obtain

their security from a well-known ha rd mathematical problem, block ciphers

are secret key systems based on bit operatio ns, and obtain their strengths from

a mixture of non-linear operations, such as substitutions, and permutations.

This chapter introduces the block ciphers, and investigates one of the his-

torically most important such ciphers. In particular, the