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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.

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