one of over 290 texts published in his lifetime [50]. This book forms the
first mention of cryptology in an empirical sense, predating all other known
references by 300 years [28]. The focus of this work was the application of
probability theory (predating Fermat and Pascal by nearly 800 years!) to
letters, and is now called frequency analysis [41].
The roots of al-Kindi’s insight into frequency analysis began while he
was studying the Koran. Theologians at the time had been trying to piece
together the exact order in which the Koran was assembled by counting
the number of certain words in each sura. After continual examination it
became clear that a few words appeared much more often in comparison to
the rest and, after even closer study in phonetics, it became more evident
that letters themselves appeared at set frequencies also. In his treatise on
cryptanalysis, al-Kindi wrote in [50]:
One way to solve an encrypted message, if we know its
language, is to find a different plaintext of the same lan-
guage long enough to fill one sheet or so, and then we
count the occurrences of each letter. We call the most
frequently occurring letter the “first”, the next most oc-
curring letter the “second”, the following most occurring
letter the “third”, and so on, until we account for all the
different letters in the plaintext sample. Then we look at
the cipher text we want to solve and we also classify its
symbols. We find the most occurring symbol and change
it to the form of the “first” letter of the plaintext sample,
the next most common symbol is changed to the form of
the ’‘second” letter, and the following most common sym-
bol is changed to the form of the “third” letter, and so
on, until we account for all symbols of the cryptogram we
want to solve.
Interest in and support for cryptology faded away after the fall of the
Roman Empire and during the Dark Ages. Suspicion of anything intellec-
tual caused suffering and violence, and intellectualism was often labeled as
mysticism or magic. The fourteenth century revival of intellectual interests
became the Renaissance, or rebirth, and allowed for the opening and use of
the old libraries, which provided access to documents containing the ancient
ciphers and their solutions and other secret writings. Secret writing was at
first banned in many places, but then restored and supported. Nomencla-
tors (from the Latin nomen for name and calator for caller) were used until
the nineteenth century for concealment. These were pairs of letters used to
refer to names, words, syllables, and lists of cipher alphabets.
It’s easy to create your own nomenclator for your own code. Write a list
of the words you most frequently use in correspondence. Create codewords
or symbols for each one and record them in a list. Then create an alphabet,
which you will use for all the words that are not included in your list. Try
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