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8.9. EXERCISES 159
13. It has been remarked that one can detect an Auto Key Cipher (or a
Vigenère Cipher with long keyword) from the large number of VAISX’s
and the lack of DQNJ’s in the ciphertext. Explain this remark.
14. Break the following Auto-Key Ciphers. A hint has been given.
(a) KFGWW SRVVV HYDWZ PXLSR VFMFY Hint: priming key was length 1
and message rhymes with “true”
(b) RZDVQ BUXLR HGKXS SGSTS SHUDE SWMIY QFTZM VXSSG STSSA UWE Hint:
priming key had length 2, and message contains several the’s
(c) ORMMI MWIAD HTBAW I. The priming key had length 1, and ee appears
somewhere in the plaintext.
(d) SNGHC UDRYI MCFLS UEZAN AGUP. The priming key had length 2 and
one of the letters is N.
15. There are many ways of combining the several types of ciphers we have
seen into new, and possibly more difficult ciphers. Here is one way. Pick a
Vigenère keyword. For each letter in this keyword pick another keyword.
Use these secondary keywords to form mixed alphabets using the Keyword
Mixed method. Then to encipher the message use the Vigenère technique,
but use the mixed alphabet corresponding to the primary keyword letter.
For example, we pick Suit as our primary key, and Club, Diamond, Heart
and Spade as secondary keys. Then when enciphering the 1st, 5th, 9th,
etc. letters of the message we use as cipheralphabet the one formed from
Club and Vigenère key S. For the 2nd, 6th, 10th, etc. letters we use as
alphabet the one formed from Diamond and Vigenère key U. Similarly for
the rest of the message.
(a) Does this system have an increase of security over a regular Vigenère
system with keyword Suit If so, where is the increase For example,
is it because the Kasiski test fails Or for other reasons If the
security has not increased, why not
(b) What if we make five secondary keys, perhaps Wild as the fifth, and
cyclically use both the Vigenère key and the five cipheralphabets
16. Suppose you capture a Vigenère-enciphered text, and somehow you know
that it was enciphered using a mixed cipher alphabet. Will or will not a
Kasiski test still work Explain.
17. How does the use of a keyword with repeated letters effect the accuracy of
Φ (Hint: What sort of encipherment and roughness would the keyword
TTTATTT give)