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92 CHAPTER 6. DECRYPTING MONOALPHABETIC CIPHERS
Let us illustrate these steps with the example given earlier. We start with
an example that has word breaks because these are a bit easier. To help us later
lets also number the words
(1)KNHHXKK (2)QS (3)PXTDQSB (4)YQFJ (5)NSISCYS (6)HQEJXUK (7)QK
(8)LXTKNUXP (9)AO (10)FJXKX (11)RCNU (12)FJQSBK (13)QS (14)FJX (15)CUPXU
(16)STLXP (17)EXUKXVXUTSHX (18)HTUXRND (19)LXFJCPK (20)CR (21)TSTDOKQK
(22)QSFNQFQCS (23)DNHI (24)FJX (25)TAQDQFO (26)TF (27)DXTKF (28)FC
(29)UXTP (30)FJX (31)DTSBNTBX (32)CR (33)FJX (34)CUQBQSTD (35)QK (36)VXUO
(37)PXKQUTADX (38)ANF (39)SCF (40)XKKXSFQTD
Step 1) Make a Frequency Chart:
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
4 5 10 10 2 17 0 6 2 9 17 3 0 9 4 7 18 4 16 16 12 2 0 27 2 0
Step 2) Is it a Caesar Cipher
This does not look like a Caesar cipher. X is probably e, but then U is a, T is z,
S is y, etc., which looks bad.
Step 3) Find the common letters:
The most common letters are C, D, F, J, K, Q, S, T, and X. (We usually pick the
most common letters seven to ten letters.)
Step 4) Make a Digraph Chart:
For each letter from Step 3) we build a column that gives each appearance of
that letter. Let’s work on C’s column. C’s first appearance is in word (5), where
it is preceded by S and followed by Y. So we put an SY in the C column. Similarly,
an RN will come from C’s next appearance, in word (11). We will use “.” to
indicate a space, or no letter. So word (15) provides with a .U, in word (28)
with a , we put F. Doing this for each appearance of each common letter results
in Figure 6.2.
Step 5) Make a Letter Behavior Table:
The Digraph Table took some time, but makes it easy to build a Behavior Table.
For example, there are four times .* appears in C’s column (here * just means
“some letter”), so C must begin four words. Likewise one *. shows C ends one
word.
C D F J K Q S T X
count 10 10 17 9 17 18 16 16 27
start 4 3 7 1 1 5 2 3 1
end 1 3 3 1 7 0 4 0 8
mates 9 6 9 5 10 14 10 13 15
doubles 0 0 0 0 0 2 0 0 0