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Computer Security and Cryptography by Alan G. Konheim

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4.8 POLYALPHABETIC SUBSTITUTION PROBLEMS

Problems 4.1–4.5 provide examples to test your skill at the cryptanalysis of Vigenère enciphered plaintext.

  1. The plaintext is written using the full ASCII alphabet:

    (a) All ASCII characters other than upper- and lower-case letters were then deleted from the plaintext, and

    (b) every upper-case letter was replaced by its corresponding lower-case letter.

  2. Vigenère substitution was applied to the resulting modified plaintext file.
  3. The ciphertext is written in rows of 50 lower-case alphabetic ASCII characters.
  4. Bounds on the period r are given.

A solution requires you to identify the period r, key k = (k0, k1, …, kr−1), and derive the plaintext. This requires a student

  1. To compute the κ-value and infer the most likely period r;
  2. To compute the correlation values and infer the most likely key;
  3. To recover the plaintext.

The ciphertext files cipherPr4.1–cipherPr4.6 may be downloaded from the following ftp address: ftp://ftp.wiley.com/public/sci_tech_med/computer_security.

PROBLEMS

4.1 cipherPr4.1 containing 692 lower-case ASCII characters results from a Vigenère encipherment of plaintext. The period r satisfies 6 ≤ r ≤ 12; the subject of the plaintext is unknown.

 __________________________________________________ cipherPr4.1 __________________________________________________ vlphpwtbvwqtdpuwahwwecnhgfsemlexvbvxjedagvdemgxlcn ywtlvweziosjmtiwphzoctnmnlipcattsezrvzjerxspuvfhqj saphavnxyeahxszoivlpdihbqethacdacpsnjzclpfrpgwdndt ahrkglexvbjerlldpsolitiolgjvvxggefglgvqkplcivhuift ...

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