Computer Security and Cryptography

1.2 CRYPTOGRAPHIC SYSTEMS

When a pair of users encipher the data they exchange over a network, the cryptographic transformation they use must be specific to the users. A cryptographic system is a family = {T_k: k ∈ } of cryptographic transformations. A key k is an identifier specifying a transformation T_k in the family . The key space is the totality of all key values. In some way the sender and receiver agree on a particular k and encipher their data with the enciphering transformation T_k.

Encipherment originally involved pen-and-pencil calculations. Mechanical devices were introduced to speed up encipherment in the eighteenth century, and they in turn were replaced by electromechanical devices a century later. Encipherment today is often implemented in software (Fig. 1.4); T_k is an algorithm whose input consists of plaintext x and key k and with ciphertext y as output.