ABSTRACT
This chapter discusses the basic design considerations while constructing a block cipher, and some modes of operations of block ciphers, and introduces the Data Encryption Standard and Advanced Encryption Standard algorithms in respectively. Block ciphers are one of the most important tools in modern cryptography. They are used not only for encryption but also for construction of stream ciphers, hash functions, and many other primitives. The modes of operation are designed to achieve efficiency, security, ease of use, many other functionalities and their trade-off as per the requirements, and the block cipher in use. The electronic code book mode of operation is the simplest approach for encrypting messages longer than the block size. The cipher-block chaining (CBC) mode of operation is one of the oldest approaches which outputs pseudo-random ciphertexts. The cipher feedback mode of operation is a close relative of the CBC mode and shares many properties of the CBC mode.
