Shannon 1945 (†851)Shannon, Claude E. "A mathematical theory of cryptography." Memorandum MM 45 (1945): 110-02.
- cryptography (p.657): A secrecy system is defined abstractly as a set of transformations of one space (the set of possible messages) into a second space (the set of possible cryptograms). Each particular transformation of the set corresponds to enciphering with a particular key. The transformations are supposed reversible (non-singular) so that unique deciphering is possible when the key is known. (†2381)