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90 CHAPTER 6. DECRYPTING MONOALPHABETIC CIPHERS
6.1 Letter Interactions
We have previously given the frequencies of letters, initial letters and final letters.
Now we must look in more detail at the characteristics of the letters: how
they behave and interact with each other.
Approximately 35% of letters in English are the vowels aeio and about 35%
of letters are the dominant consonants hnrst. So if we can determine which
ciphertext letters represent these nine letters about 70% of the decrypting will
be done. (And the rest will usually be fill in the blank.) Thus the main challenge
when tackling a monoalphabetic cipher is telling the etaoinshr letters apart.
First, these nine letters tend to group themselves into three sets of three by
the likelihood they are initial or final letters, as can be seen in Figures 5.6 and
5.6.
Initial and Final Letters:
(1) t, o and s appear frequently as both initial and final letters.
(2) a, i, and h appear frequently as initial letters, but much less so as final
letters.
(3) e, n and r appear frequently as final letters but much less so as initial letters.
While the doubling of a letter is fairly rare, in a long message there will
almost certainly be some, and this help us tell the letters apart.
Doublings:
(1) ee, tt, oo and ss are common doubles.
(2) aa, ii, hh, nn and rr are less common.
Next, the vowels.
Vowels:
(1) Vowels like to combine with consonants, but not with each other.
(2) Vowels are friendly, willing to combine with many different letters, including
the low frequency ones.
(3) The only common pairs of vowels are ou, ea and io.
(4) e is the easiest to find: it is very common and ends many words.
(5) a frequently follows e but never precedes it.
(6) The pair ie and ei is the only common vowel-vowel reversal.
(7) e and o almost never touch each other and both will make doubles.
Finally, the consonants.
Consonants:
(1) Consonants like vowels, but, as there are so many of them and so few vowels,
they often combine with one another as well.