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

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chapter 7
been enciphered using a book as key, a fact that is not immediately apparent.
Indicators
These have already been referred to in Chapter 3 but they are sufficiently
important to warrant mention again, since provision of the indicator may
prove to be the Achilles heel of any cipher system. When a book cipher is
used the sender has to let the receiver know at what line on what page of
the key book he is beginning the encipherment. If, for example, he begins
with line 15 of page 216 he may preface the message with the number
15216. This is, however, rather too obvious and he would be more likely
to disguise it. There are numerous ways of doing this such as:
(1) transposing the digits according to an agreed rule so that 15216
becomes, say,
65121;
(2) adding an agreed number, such as 59382, digit by digit, thus
producing
64598;
(3) converting the digits of the indicator to letters and hiding the
resulting five-letter group at an agreed place within the cipher text of
the message; thus 15216 becomes
BFCBG;
(4) a combination of any of these.
Provision of an indicator is not, of course, unique to book ciphers; it is
an essential part of many cipher systems, but the basic principle is the
same: the sender must somehow communicate it to the receiver in such a
way as to make it as difficult as possible for a cryptanalyst to find it. The
cryptanalyst, conversely, will give high priority to identifying the location
of the indicator and discovering its method of encryption .
Disastrous errors in using a book cipher
A significant risk for the sender of a book cipher message is that if he
makes a mistake and has to re-send the message he may provide the cryptanalyst
with a relatively simple route into decryption. Such a crucial