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Computer Security and Cryptography by Alan G. Konheim

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3.10 MONOALPHABETIC SUBSTITUTION PROBLEMS

The ciphertext files cipherPr3.1-cipherPr3.6 and the table of one-gram probabilities (Table 3.6) may be downloaded from the following ftp address: ftp://ftp.wiley.com/public/sci_tech_med/computer_security.

3.1

cipherPr3.1 results from a Caesar substitution on plaintext written using the alphabet AB … Z. Find the key.

___________________________________________________
                   cipherPr3.1
___________________________________________________
znkyzgzksktzzngzznkqtgvygiqvxuhrksoyngxjotgtgyykxz
outghuazznkmktkxgrqtgvygiqvxuhrksgcuxyzigykgyykxzo
utznkyurazoutluxikxzgotirgyykyulqtgvygiqvxuhrksyoy
waozkyzxgomnzlucgxjluxkdgsvrkolznkqtgvygiqbkizux
___________________________________________________
3.2

The term autokey refers to the use of the plaintext to modify the key. cipherPr3.2 has been enciphered by an autokey Caesar system with key k as follows:

1. The first letter of plaintext x0 of the plaintext x = (x0, x1,…, xn−1) is enciphered by the Caesar substitution x0y0 = (x0 + k) (modulo 26);

2. The plaintext letter xi with 1 ≤ i < n is enciphered by a Caesar substitution xiyi = (xi + xi − 1) (modulo 26).

Develop a non exhaustive method for the cryptanalysis for the autokey Caesar cryptosystem and test the method using the ciphertext cipherPr3.2 containing 293 lowercase letters.

____________________________________________________________ cipherPr3.2 ____________________________________________________________ ldttnrxpkfbcgtavrzwimcsvqvsrvgwlivrgejgvrbfalxrpgsfzvgaltgfq ...

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