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

If then we try subtracting THE from the cipher at every possible position
and look at the trigraphs obtained we might find plausible looking parts
of English words which we might then be able to complete, thus adding a
few more letters in the other text, preceding or following THE. Other
common trigraphs can be tried and so the two texts might begin to
‘unravel’, so to speak. If X is used as a separator we can extend THE to
THEX, or possibly even to XTHEX, although in doing so we might fail to
pick up a word such as THERE. Even a short word, such as A, might be
helpful if it occurs as XAX.
If some unusual words can be found in the key text it might be possible
to deduce the type of book being used and even to identify the book,
which would make subsequent cryptanalysis much easier. In practice we
might at first only be able to recover occasional words or parts of words in
the two texts but even partial recovery could be informative and subsequent
messages might provide further useful ‘cribs’. As an indication of
the method, on a small scale (just 50 letters):
Example 7.4
The following 10 groups have been enciphered using a book cipher. Use
the technique of crib-dragging to try to recover the texts of the key and
message.
FLIQT NYQFK VACEH UCUAC MOXRG EYYQJ BNOEQ
FJXUL ILREJ ATVQB
Ciphers for spies 81
We try some common words as possible cribs, THE being the most
obvious. Crib-dragging is undeniably tedious since we must try the crib
at all possible positions of the cipher text. Since any of the letters that we
try, T, H and E in this case, may occur in other cribs we might save some
effort overall if we first see what the resultant plain letter would be if we
assume that T is the letter to be subtracted from the cipher at each position,
then if H is the letter and finally if E is the letter. If we now write out
the three resulting streams of ‘putative plaintext’ with the first line (corresponding
to T) offset two places to the right and the second line ( the ‘H’
line) offset one place to the right relative to the third (‘E’) line then any
possible ‘words’ will appear as three letters in a vertical line; thus:
Cipher BHFLIQT NYQFK VACEH UCUAC MOXRG EYYQJ BNOEQ FJXUL ILREJ ATVQB
Tline BHMSPXA UFXMR CHJLO BJBHJ TVEYN LFFXQ IUVLX MQEBS PSYLQ HACXI
Hline HYEBJMG RJYDO TVXAN VNTVF HQKZX RRJCU GHXJY CQNEB EKXCT MOJUH
Eline BHEMPJU MBGRW YADQY QWYIK TNCAU UMFXJ KAMBF TQHEH NAFWP RMXHH