Code and ciphers: Julius Caesar, the Enigma and the internet

80
chapter 7
Table 7.4 Letter frequencies in unenciphered
English and in book cipher
Frequency (per 1000)
Letter Normal English Book cipher
A 64 31
B 14 30
C 27 29
D 35 55
E 100 40
F 20 32
G 14 39
H 42 46
I 63 34
J 3 26
K 6 36
L 35 37
M 20 45
N 56 38
O 56 28
P 17 26
Q 4 37
R 49 46
S 56 52
T 71 38
U 31 26
V 10 39
W 18 34
X 3 32
Y 18 25
Z 2 52
Space etc. 166 47
Source: The data in the left-hand column are
quoted in [1.2, Appendix 2]; the data in the
right-hand column were derived
mathematically from those in the left-hand
column (for the details see M6).
the cipher might have helped him to the conclusion that a book cipher is
being used, the individual letter frequencies will not be so different from
random that they will be of much help, nor will the digraphs etc. There is,
however, an attack to which a book cipher is vulnerable: it might be called
‘crib-dragging’. Suppose that either the message or the key text contains
some common English word such as THE. This is added to 3 letters of
another English word in the other text to produce 3 letters of the cipher.