Article · October 008 citations reads 5,930 authors

Figure 1: 
Running key cipher.
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October 2008
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www.ddj.com
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Dr. Dobb’s Journal
49
good because the main problem with the
Book cipher is the need for a long book. To
encode a 5000-character plaintext, you need
at least a 5000-word book. In fact, it should
be much longer because you cannot expect
the frequency of all letters in the text to
match the frequency of initial letters of words
in the book. 
Due to the disproportion in frequencies
of all letters and initial letters, the coding
process may become impossible, and you
could run out of some letters even before
you start coding. For example, there are
practically no English words beginning with
the letter “X”—in the Harry Potter book,
there is only one such “word” in the phrase
“a fiery X appeared on the door.” In other
books, the only “words” beginning with “X”
are chapter numbers written as Roman
numerals. In the second Beale cipher (the
only one that was successfully decoded),
every “X” was coded as 1005, probably
pointing to the word “sexes” in the
Declaration of Independence, which was
used as a key (“sexes” obviously does not
start with “x,” but does sound like it). Some
books also contain zero words beginning
with the letter “Z”, unless they contain the
word “zero” itself.
Therefore, it seems like a good idea to
investigate the frequencies of first, second,
third, or even last letters in words, to find out
which of them matches the frequency of all
letters. We analyzed 14 books, from Pride
and Prejudice to The Da Vinci Code, and
found out that the distribution of frequen-
cies for the third letter in the word has the
best correlation with frequencies of all let-
ters. A possible solution is to let users choose
which letter in the word to use, by setting an
appropriate parameter.
To get the system rolling, you also have
to agree with the correspondent about the
books you will use to encode the mes-
sages. Using “real” books isn’t very practi-
cal (OCRing them on both sides intro-
duces different errors). So you will proba-
bly use some texts available on the
Internet, and you might even consider
promoting some of your own essays into
“books” for additional security. (If we were
running the NSA, we would make sure we
had a machine-readable copy of the