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102 CHAPTER 6. DECRYPTING MONOALPHABETIC CIPHERS
6.6 Topics and Techniques
1. Of the etaoinshr letters
(a) Which three appear frequently both as initial and final letters
(b) Which three appear frequently as final letters, but less so as initial
letters
(c) Which three appear frequently as initial letters, but less so as final
letters
(d) Which four are most likely to make doubles
2. Which are the most common vowel-vowel combinations Are they actually
very common
3. Which letters do vowels prefer to associate with What about consonants
4. Which (type of) letters are the friendliest, associating with almost all
others Which are most discriminating, preferring the company of only a
few other letters
5. Are ciphertexts that retain their word breaks easier or harder to decrypt
Explain why.
6. What is a digraph table How to construct one What does it tell us
7. What is a homophone
8. Why can ciphers that involve homophones be more secure than those
without
9. What is a null How do nulls make the cryptanalyst’s life more difficult
6.7 Exercises
1. Decrypt the following cipher.
UN MGHHYV LNS VYETEHGUH HLY KVQFHNCVGM GDD HLGH TE VYGDDQ UYYRYR
TE GU YUHVQ HLY TRYUHTATKGHTNU NA NUY SNVR NV NA HLVYY NV ANPV
DYHHYVE LYDYU ANPKLY CGTUYE
Frequency count:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
5 0 2 6 6 1 9 15 0 0 3 7 2 12 0 2 3 4 2 8 10 12 0 0 19 0
2. Decrypt the following cipher.
GNLTA DTPUH IOHJQ BTCNJ QHCVN DQPIQ ZNDCN ZJTGD TQHIQ TWWHO TIGTH
IQBTF NDWLQ NLPSH QVDNL XGTJC XGBCN DTPIL CXGBC NDTQD XJQFN DQBSH
IZNDC PQHNI QBPIJ VHTJ