Classical Cryptography

Cryptography is a study of secret writing. It allows two people, usually referred to as Alice and Bob, to communicate over an insecure channel in such a way that an opponent, Eve, cannot understand what is being said. For many centuries people have used different cryptographic algorithms. The most well known historical cryptosystems are: • The Shift Cipher • The Substitution Cipher • The Affine Cipher • The Vigenére Cipher • The Hill Cipher • The Permutation Cipher • Rotor machines All these ciphers were once considered to be secured. With the development of cryptoanalysis techniques and tools it became clear that they are easily breakable. There are several kinds of attacks that one can use in order to break the ciphers: • Ciphertext only attack • Known plaintext attack • Chosen plaintext attack • Adaptive-chosen plaintext attack • Chosen ciphertext attack • Adaptive-chosen ciphertext attack To break modern cryptosystems more sophisticated and complex techniques are used. Often they require specially designed algorithms and hardware. Yet there is also a universal technique, which sometimes may be the only feasible way of breaking a cipher. It is a so-called ”rubber hose” cryptology. This method includes bribery, blackmail, etc.