Cryptology - Unofficial St. Mary's College of California Web Site

98 CHAPTER 6. DECRYPTING MONOALPHABETIC CIPHERS
crypting process. Areas of frequent interest are methods for decrypting monoalphabetic
ciphers more quickly. To give a flavor of these, we present a slight
variation on a method for recognizing the vowels in a short ciphertext due to
B. V. Sukhotin [Gray].
The idea behind this method is that vowels tend to have a wide variety of
partners, with the vast majority being consonants. We should be able to make
use of this to weed out the vowels from the consonants. The easiest way to
explain the method is to use it. Let’s use our first example from Section 6.2 as
the ciphertext.
Step 1: Use the digraph chart to count the number of times each proposed
etaonishr letter is in contact with another of these letters, ignoring any times
they contact themselves, and total the results.
C D F J K Q S T X total
C 0 2 1 0 1 3 0 0 5
D 0 0 0 0 3 0 5 2 10
F 2 0 8 1 5 2 1 1 20
J 1 0 8 0 1 0 0 6 16
K 0 0 1 0 5 0 2 7 15
Q 1 3 5 1 5 6 1 0 22
S 3 0 2 0 0 6 6 1 18
T 0 5 1 0 2 1 6 4 19
X 0 2 1 6 7 0 1 4 20
Step 2: The letter with the largest total is a vowel. Mark it so. Any letter
contacting a vowel is probably not one. So we go through the remaining rows,
and penalize each row by twice the number of times this vowel appears in their
list.
C D F J K Q S T X total total2
C 0 2 1 0 1 3 0 0 7 5
D 0 0 0 0 3 0 5 2 10 4
F 2 0 8 1 5 2 1 1 20 10
J 1 0 8 0 1 0 0 6 16 14
K 0 0 1 0 5 0 2 7 15 5
Q 1 3 5 1 5 6 1 0 22 vowel
S 3 0 2 0 0 6 6 1 18 6
T 0 5 1 0 2 1 6 4 19 17
X 0 2 1 6 7 0 1 4 20 20
Step 3: Again, the letter with the largest remaining count is a vowel. Mark it