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

6.2. DECRYPTING MONOALPHABETIC CIPHERS 91
(2) h can be the easiest consonant to identify. It precedes vowels, rarely follows
them, and can be found most often in th, he and ha.
(3) t acts like a vowel in that it likes to mate with many other letters. It has a
very strong desire to make th’s, and will make tt’s.
(4) r will appear next to any vowel, and likes the company of the other highfrequency
letters.
(5) n and s act behave the reverse of h. They prefer to follow vowels and precede
consonants. It can be hard to tell whether nst are vowels or consonants.
We summarize some of this information in Figure 6.1, using x and X to
indicate strong and stronger behaviors.
a e i o t r n h s
common x X x x x
starting words x x x X x
ending words X x x x x
lots of mates x X x x x
doubles x x x x
Figure 6.1: Some Basic Letter Behaviors
6.2 Decrypting Monoalphabetic Ciphers
Now its time we get decrypting. What do we do
1) Make a frequency chart.
2) Does the frequency count suggest a Caesar cipher Look for e and t, the aei
and rst triples, the uvwxyz string, either forwards or backwards. If you find
two or three of these patterns it may be a Caesar cipher and we can decrypt
these. If not, continue with step 3.
3) Find the most common letters: these are probably etaoinshr.
4) Develop a digraph table for the probable etaoinshr letters. For each
appearance of a common ciphertext letter list the letter that precedes it and
the letter that follows it in the cipher. These tables are a bit time consuming
to construct, but are very helpful, especially for ciphers without word breaks.
5) Use the letter behaviors table, Figure 6.1, to make initial guesses.
6) Work hard and be persistent.