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

If we generate the same sequence (mod 100) it begins
00 01 01 02 04 07 13 24 44 81 49 74 ...
and is found to repeat after 1240 terms.
It is possible to modify the Fibonacci sequence so that the odd–even
ratio is somewhat reduced, making it slightly better for encryption. A
modest step in this direction is shown by
Example 6.4
Generate 20 terms of the Fibonacci sequence (mod 100) starting with 13
and 21 as the first 2 terms then interchange the second and third digits in
each group of four to give 20 terms of a two-digit key stream.
Solution
The first 20 terms of the Fibonacci Sequence (mod 100) starting with 13
and 21 are
13 21 34 55 89 44 33 77 10 87 97 84 81 65 46 11 57 68 25 93
We interchange the second and third digits in each group of four –
12 31 35 45 84 94 37 37 18 07 98 74 86 15 41 61 56 78 29 53
– and this is the resultant key. The bias of odd: even numbers has now been
reduced (from 2 : 1 to about 7 : 5) and the key, though still unsatisfactory, is
stronger for that.
Problem 6.2
A two-digit code represents the letters of the alphabet as follows:
A�17, B�20, C�23, ..., Z�92,
each number being 3 more than the one before it. A message is then enciphered
using this code and the additive key (12 31 35...) obtained in the
example above, the addition being digit by digit with no carrying. The
resultant cipher text is
86 69 42 19 60 35 08 13 76 48 23 02 50 91.
Decrypt the message.
Codes 71