May 2014
Intermediate to advanced
350 pages
9h 55m
English
Content preview from Computing
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137
Chapter 17
Cryptography
fromthe Medieval
totheModern Ages
17.1 THE ARAB CRYPTANALYSTS
A key for the substitution cipher consists of a permutation of 26 letters in
the case of the Roman alphabet. As described in Chapter 7, the shi cipher
(i.e., the Caesar cipher) is a special case of the substitution cipher. e num-
ber of permutations of 26 letters is 26!, which is more than 4 ×10
26
, a very
large number. Even if we restrict them to a smaller class of possi ble permu-
tations, the number of these permutations is still very large. For example,
as described in Section 7.2, pairing letters in the secret writing given in
the Kama Sutra (w
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