ABSTRACT
A cipher is a secret way of writing in which plaintext is enciphered into ciphertext under the control of a key. Those who know the key can easily decipher the ciphertext back into the plaintext. Cryptanalysis is the study of breaking ciphers, that is, finding the key or converting the ciphertext into the plaintext without knowing the key. For a given cipher, key, and plaintext, let M, C, and K denote the plaintext, the corresponding
ciphertext, and the key, respectively. If EK and DK represent the enciphering and deciphering functions when the key is K, then we may write C = EK(M) andM = DK(C). For allM and K, we must have DK(EK(M)) = M. There are four kinds of cryptanalytic attacks. All four kinds assume that the forms of the enci-
phering and deciphering functions are known. 1. Ciphertext only: Given C, find K or at leastM for which C = EK(M). 2. Known plaintext: GivenM and the corresponding C, find K for which C = EK(M). 3. Chosen plaintext: The cryptanalyst may choose M. He is told C and must find K for
which C = EK(M). 4. Chosen ciphertext: The cryptanalyst may choose C. He is told M and must find K for
which C = EK(M). The ciphertext only attack is hardest. To carry it out, one may exploit knowledge of the language
of the plaintext, its redundancy, or its common words.
