TABLE OF CONTENTS Block ciphers are the most used symmetric key enciphering schemes. Their name comes from the fact that block ciphers encrypt blocks of plaintext into blocks of ciphertext. Two important characteristics of a block cipher are the size of the key and the size of the blocks on which the cipher operates.
Before addressing the basic structure of block ciphers and explaining a few practical examples, we offer a review of important concepts and techniques that are used when designing block ciphers.
Two important techniques often used when designing symmetric key cryptosystems are permutation and substitution.
In permutation, the letters of a message are simply rearranged, effectively generating an anagram. In this case of cryptosystem, the key will be the transposition rules. It is clear that for very short messages this system is insecure, because there are only a limited number of ways to rearrange a handful of letters. However, as the number of letters increase, so do the number of possibilities exponentially increase, making it impossible to recover the message without the scrambling rules. It is true that a random transposition offers a high security level, but the catch is that in order to be effective, the transposition rules must follow a straightforward system. It restricts many sets of possible keys (transpositions).
Besides transposition, substitution is also another widely used technique to scramble a message into a ciphertext. In a substitution, each character of the message is substituted by another one from the...
