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Dr. Dobb’s Journal
51
supply of at least one of the letters in
allice.cod is consumed), the appropriate
message is generated and the file allice.pos
is unchanged. Bob should add another
book to allice.cod using the command
bkadd allice myotherbook.txt 1 and repeat
the bkcode command.
To decode a message, Allice uses the
command bkdecode allice message1.cry,
producing the message1.txt file. Figure 3
shows the complete process. In a two-way
communication, by using different books,
Allice should generate the file bob.cod and
use it to encode messages to Bob.
Eve is welcome to intercept any of the
.cry files but without knowledge of the books
used, she is clueless even if her other name
is “Susan Fletcher.”
How Strong Is It?
Cryptanalysts mostly agree that the Book
cipher, if used properly, is practically
unbreakable; nearly as good as the one-time
pad. Why isn’t it used every day? Maybe
because of that “if used properly” clause—
the complete algorithm is somehow “pri-
vate.” The next time you bury a treasure, you
can describe its location within an encrypt-
ed message and be reasonably sure that it
will not be decoded for the next 150 years,
but if you have to organize a secure corre-
spondence for a web of spies all over the
world, finding, deploying, and protecting
adequate books might prove very difficult.
By implementing the Book cipher in your
applications, you don’t meddle with powers
you cannot comprehend—you leave the
meddling to users of your software. The
average user will probably go to www
.gutenberg.org, download the first book, and
use it as a key without even bothering to
delete the copyright message (which is basi-
cally the same for every book on that site).
On the other hand, if users of your soft-
ware contribute ideas of their own—using
texts from the Internet instead of books, using
different languages, pre-encoding messages
using other algorithms, and so on—then
potential attackers would be facing many
groups of short messages encrypted using (at
least slightly) different algorithms, which
might present the ultimate challenge.