T. Beth (Ed.): Cryptography - EUROCRYPT '82, LNCS 149, pp. 1-28, 1983. 0 Springer-Verlag Berlin Heidelberg 1983 3 Having all of a sudden left the shady corner of semi-military art, modern cryptography has become a central topic of research in all areas of communication science. Definitions (cf. Bauer pp. 31 - 48) Cryptographic measures are applied to p- tect valuable data during transmission against unwanted interception INTERCEPTOR Fig. A: passive violation and (possibly undectable) forgery . Fig. 2: acti-se violation In accordance with the subsequent paper of Bauer (pp. 31 - 481, the technique applied to meet these requirements is called en- tion. In this process the transmitter enciphers (or encrypts) a plaintext message into a ciphertext. 4 ciphertexc ciphering Fig. 3: The Wire-tap-channel This transformation is called a cipher(function) which the au- rized receiver deciphers (decrypts). An enemy is a person or institution who wants illegal access to the messages. Assuming that the enemy can only get hold of the ciph- texts, he has to perform a cryptanalysis in order to reconstitute the plaintexts. To add to the difficulties for a cryptanalyst, the cipher functions are chosen to a varying parameter, called the key. A generator cryptosystem consists of a class of injective cipher functions ES: M-C, mapping plaintext messages(EM) into ciphertexts(EC) . The parameter s runs through the set K of keys. These formulations are best demonstrated by the basic, classical examples.