Even if a multitude of independent alphabets is used, periodic polyalphabetic encryption contains one element which is difficult to hide: the number of keys in the period of the encryption. This is based on the following stationariness property of stochastic sources: If P is a plaintext (of length M) from a source Q, then P (s), the plaintext P shifted cyclically by any number s of positions, is from the same source.
KeywordsPeri Patent 1838389
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