A key is an element from an alphabet (the key alphabet) that selects, resp. defines a particular encryption step. A keytext is a sequence of key elements from a key alphabet that select, resp. define a sequence of particular encryption steps.
A polyalphabetic encryption, which is also called a polyalphabetic substitution cipher, is a substitution (see substitutions and permutations) with more than one alphabet, each one designated by a key element.
A double key is a polyalphabetic encryption with shifted mixed alphabets (see Alberti encryption). It is cryptologically equivalent to a polyalphabetic encryption with a Vigen\‘ere table (“tabula recta”) whose plaintext standard alphabet is replaced by a mixed alphabet—the mixed alphabet being the \lq second key\rq. Moreover, a treble key is a double key with the additional proviso that the standard alphabet for the keys of a Vigen\‘ere table is replaced by a mixed alphabet—this mixed alphabet being the \lq third key\rq.
A periodic key or rep...
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References
Bauer, F.L. (1997). “Decrypted secrets.” Methods and Maxims of Cryptology. Springer-Verlag, Berlin.
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© 2005 International Federation for Information Processing
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Bauer, F.L. (2005). Key. In: van Tilborg, H.C.A. (eds) Encyclopedia of Cryptography and Security. Springer, Boston, MA . https://doi.org/10.1007/0-387-23483-7_217
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