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78 CHAPTER 5. MONOALPHABETIC CIPHERS
2.9 0.2 0.6 10.0 20.3 4.5 2.8 2.5 0.4 0 0.1 3.7 1.3
a b c d e f g h i j k l m
9.7 4.5 0.5 0 5.5 12.7 9.7 0.2 0.1 1.0 0.2 5.5 0
n o p q r s t u v w x y z
Figure 5.3: Letter Frequencies – Final Letters.
a e i o t r n h s
common x X x x x
start x x x X x
end X x x x x x
doubles x x x x
Figure 5.4: Characteristics of etaoinshr.
We also list the most common short words in English in Figure 5.5 and one
list of the 100 most common English words, with the number of times they
appeared out of 1, 000, 000 words, in Figure 5.6.
1 letter words: a i
2 letter words: an at as he be in is it on or to of
3 letter words: the and (both really common,) was for his had
4 letter words: that with this from have
Figure 5.5: Most Common Short Words
Notice that the most common words by far, are non-context words: articles,
prepositions, conjunctions and other auxiliary particles. In fact, in some lists
the of and to a in make up nearly 20% of all words, and up to 70 of the 100
most used words are non-context words.
5.7 Aristocrats
We begin our breaking of general monoalphabetic ciphers with Aristocrats.
These are short quotations enciphered with a monoalphabetic substitution.
They appear in almost every newspaper, usually in the comics section. We
start with them since they tend to be not terribly difficult, mainly because they
keep word divisions and punctuation, and are frequently given with a hint.
Decrypting aristocrats is mostly simple hard work, although some of our
frequency information will be of service. Some suggested steps are