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

- 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.

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

A solution requires you to identify the period *r*, key *k* = (*k*_{0}, *k*_{1}, …, *k*_{r−1}), and derive the plaintext. This requires a student

- To compute the
-value and infer the most*κ**likely*period*r*; - To compute the correlation values and infer the most
*likely*key; - 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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