Computer Security and Cryptography

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, …, k_r−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 ...