Information Security: Principles and Practices

A Vigenre cipher uses a sequence of "shift-by-n" simple substitutions,

where the shifts are indexed using a keyword, with "A" representing

a shift-by-0, "B" representing a shift-by-l, etc. For example, if the

keyword is "DOG," then the first letter is encrypted using a simple

substitution with a shift-by-3, the second letter is encrypted using a

shift-by-14, the third letter is encrypted using a shift-by-6, and the

pattern is repeatedthe fourth letter is encrypted using a shift-by-3,

the fifth letter is encrypted using a shift-by-14, and so on. Cryptanalyze

the following ciphertext, i.e., determine the plaintext and the key. This

particular message was encrypted using a Vigenre cipher with a 3-letter

English keyword:

CTMYR DOIBS RESRR RIJYR EBYLD IYMLC CYQXS RRMLQ FSDXF

OWFKT CYJRR IQZSM X