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80 CHAPTER 5. MONOALPHABETIC CIPHERS
Turning to the 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
0 5 2 8 3 7 6 5 0 5 5 3 4 0 0 3 6 0 1 1 1 0 3 1 1 2
the most common letters are BDFGHJKQ. Of these, F, J and Q each occur three
times as final letters. Let’s guess that one of these is e. It cannot be F, a
consonant, nor Q, as then the word TDQJ would end in an e-vowel combination.
So e must be J.
Next, BQJ=BQe, so Q must be a consonant, and from LBQ, B is probably a
vowel, hence a or o. Trying B=o doesn’t work. But when we try B=a, we quickly
see Q=r, making YDQ = YDr = Yor = for, as o is the only vowel then left.
At this point the cipher looks like *oli*i*s is *ore *an*ero*s **an *ar,
for in *ar *o* are onl* *ille* on*e. *in*on ***r**ill. From here the
quotation should be pretty easy to complete. 5
⋄
Clearly this example worked out a little too nicely, in part because we hid
some false starts and in part because time spent thinking and scratching one’s
head is hard to indicate in text. But it should still give an idea of how Aristocrats
are attacked.
5.8 Summary
A monoalphabetic cipher is one in which the each plaintext letter is replaced by
the same ciphertext letter throughout the entire message. The Caesar or Shift
Ciphers, Decimation Ciphers and Linear Ciphers are all monoalphabetic, as are
the various types of Keyword Ciphers introduced in this chapter.
The Keyword Ciphers all use a keyword to develop an ordering of the ciphertext
alphabet. The more important ones are the Keyword Mixed and Keyword
Transposed Ciphers. Select a keyword and drop any repeated letters in it. Then
write the remainder of the alphabet underneath in as many columns as remain
in the keyword. For a Mixed Cipher the ciphertext alphabet is then the columns
pulled off in order, left-to-right, while for a Transposed Cipher the alphabet is
pulled off in alphabetical order of the top row.
A popular type of decryption puzzle is an Aristocrat, which is generally
a short quote enciphered with a monoalphabetic cipher. Word divisions and
punctuation are generally kept, and a hint is often given. The keys to decrypting
these ciphers include the usual frequency count, some knowledge of initial and
final letters of words, and very often the use of common short words.
5 Politics is more dangerous than war, for in war you are only killed once.
Winston Churchill.