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

Table S.5
R H A P S
O D Y B C
E F G I K
L M N Q T
U V W X Z
and, breaking the cipher text into digraphs, we have
OX BG IH PE OK GH MT TR OI UE VG KG NC
We convert these to plaintext using the Playfair square, which produces
BU YI FP RI CE FA LQ LS BE LO WF IF TY
or, after running the text together, inserting spaces between words and
deleting the single dummy Q,
BUY IF PRICE FALLS BELOW FIFTY.
Chapter 6
6.1 (Fibonacci key)
(1) Starting with 0 and 2 as the first two terms produces a sequence which
repeats after 20 digits:
0, 2, 2, 4, 6, 0, 6, 6, 2, 8, 0, 8, 8, 6, 4, 0, 4, 4, 8, 2, 0, 2, 2, ...
All the terms are even and the sequence is quite unsuitable as a key.
(2) Starting with 1 and 3 produces a sequence which repeats after 12
digits:
1, 3, 4, 7, 1, 8, 9, 7, 6, 3, 9, 2, 1, 3, 4,
6.2 (Code plus additive)
Cipher text 86 69 42 19 60 35 08 13 76 48 23 02 50 91
Key 12 31 35 45 84 94 37 37 18 07 98 74 86 15
Difference 74 38 17 74 86 41 71 86 68 41 35 38 74 86
Text T H A T X I S X R I G H T X
i.e. THAT IS RIGHT with X separating the words.
Solutions to problems 223