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Book Ciphers

*
Burton Rosenberg
*

Last Update: Tue Apr 1 01:44:21 EST 2003

Encryption is an old art. Here is perhaps
the simplest encryption. I leave the reader to
try her hand at breaking it:

12,15,22,5;9,19;20,8,5;12,1,19,20;12,9,7,8,20;19,16,15,11,5,14;

A more interesting process is called the *book cipher*.
Each letter is given by a row, column pair as it is found
in an agreed upon text. So the cipher would read:
row,column,row,column,row,column,...

Note all such ciphers would have an even number of numbers in them,
which might be a hint that one is facing a book cipher.
Here is the above text as a book cipher:
1,1,3,10,5,6,1,16,2,5,2,6,1,14,4,7,1,10,3,7,2,4,3,5

and so on. Spaces, punctuation, etc., if the text is carefully
numbered, can all be represented in a book cipher.
For instance, you should be able to guess which pair of numbers
represent the first space in the above cipher, if you have
broken the easy cipher version of the same phrase.
This is a lot like taking a page from a book and circling
letters to be read.
The page to use, the page with the circled letters, must
be shared ahead of time.
This is often a weakness of a book cipher, since it leaves
you the *problem of distributing the page covertly*
but definitively.

In practice, people miscount, and that can lead to indecipherables.
However, most errors seem to be in forgetting a comma, so it might
be better to work with letters instead of numbers, where a=1, b=2,
and so on. As long as you don't go past column x=24 there is no problem.
(Else you have to work base 25, where A=25, B=50, C=75, etc., So that
27 would be Ab (25+2), 53 would be Bc (50+3).
The letter z=0, so 75 is Cz (75+0).
The letter y is unused.)

*Squared paper is a great help to cryptographers.*
You can write a, b, c, etc., across the top, a, b, c, etc., down the
left margin, and then write the book into the squares, so there is
no further need to count.

aebcbebrcdcecidwfqftfxgqiljljtjvkgkplbljnxokqprrswvjvuwewnwsyr
brcbevewoasbsctvujefeqetevfafufxgjglhshthuiyjlmlmunpnvokpxxp
pyqbqgqhqjbrcdcedcdidrfqfygcgegqiljljtlhncnqokoopxqgqprrrtrurvshsisj
spsqsrtstwuevovpvswewnxcxezdzrztbcbfbgbobqcaccdwevfafgfifugmgyhajbjpkbkjmx

Traditionally, once a letter is used from a text it is crossed
off, so that no pattern of repeating integer pairs will aid the
cryptanalysis.
If, for instance, which ever letter is represented by 1,1 is always
represented by 1,1, then the cipher degenerates to a so called
simple substitution cipher, which is easy enough to break.