CIS 3362 Homework #1 Solutions
1) Trying all 26 possible shifts, we find that the encryption key was 17, thus to decrypt,
we can subtract 17 (or add 9) to each letter. Here is the message:
Welcome to the cryptography class. Hopefully you figured out this message quickly.
2) Trying all 312 possible keys, we find the following:
Today is game day. It is time to tailgate. Go UCF!
The encryption keys are (a = 7, b= 22). The decryption keys are (a = 15, b = 8).
3) Trying all 312 possible keys, we find the following:
Sometimes I hide a prize and reveal where it is in one of these messages, but not this
one, sorry!
The encryption keys are (a = 19, b = 2). The decryption keys are (a = 11, b = 4).
4) The highest letter frequencies are Z (15.2%), T (9.6%), O (8.3%), Q (7.6%) and G
(7.2%). Since E and T are the two most frequent letters, it is likely that Z maps to either E
or T. After some trial and error (looking for the, where a t is followed by an e after two
characters), we settle upon Z = T and T = E. Utlizing the repeated trigrams that are most
likely the, we find I = H. It is also likely that O and Q represent vowels, one of which is
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 Fall '08
 Staff
 Information Security, Cryptography, Encryption, Decryption Keys

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