ABSTRACT
The classical model of a crypto-system represents the simplest protocol possible. It is a one-way transmission that involves two parties, the sender and the recipient. The channel is a common object for which the sender can write to and the recipient can read from. The goal is to communicate while keeping the information on the channel incomprehensible to outside parties that have read access to the channel. A common secret cryptographic key makes it possible to carry this through. Whoever acquires the key will be capable of computing the cleartext from the ciphertext. The cryptographic function must at least be designed to withstand a ciphertext-only attack. If side-information might be available then the function must be analyzed with respect to a known-plaintext attack .
