Julius Caesar
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Julius Caesar (100BC-44BC) is credited as the first person to use a cipher in military affairs. His cipher, a shift of the alphabet
by three characters, may seem like child's play today but was probably quite effective in that era. Most people were illiterate and the
enemy was not likely to be able to read Roman, even when unenciphered. Caesar enciphered an "a" to a "d", a "b" to an "e", etc.
but a shift of any amount is now referred to as a Caesar cipher. Caesar's nephew Augustus, first emperor of Rome, must have thought the
cipher was too complex because he used a cipher where each letter was shifted to the very next letter.
Any cipher which mixes up the characters of the alphabet and uses that same arrangement for the entire message is classified as a
monoalphabetic or substitution cipher, so the Caesar cipher is a simple example of this kind of cipher. Another example of its use is in
the Bible. The Old Testament, in Jeremiah 25:26 and 51:41, uses the name Sheshach in place of Babel. Also, in Jeremiah 51:1 the words
"Leb Kamai" are used for "Kashdim". The cipher is called atbash and substitutes the last character for the first, the second-to-last
character for the second, etc. The name atbash is derived from the first 2 and last 2 letters of the Hebrew alphabet, so "aleph" is
exchanged with "taw" and "beth" is exchanged with "shin".
One advantage of the Caesar or atbash cipher is the ability to use these ciphers without the need to send any key information or have the
cipher written down and subject to capture by the enemy. While other monoalphabetic ciphers would normally require that cipher to be
written down, they do offer a much stronger cipher. For example, there are only 25 Caesar cipher arrangements possible since each letter can
be shifted by 1 to 25 characters. By comparison, for any monoalphabetic cipher, the first letter can be exchanged for any letter, the second
letter can be exchanged with any of the remaining 25 letters, etc. So the number of possible monoalphabetic ciphers is 26 X 25 X 24 X ... X
2 X 1 = 26! = 4.03 X 1026. The number of arrangements available is called the key length and normally gives an indication of the
strength of the cipher. In the case of the monoalphabetic cipher, the large key length is misleading, since a systematic solution to this
cipher was discovered.
Al-Khalil
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The first recorded example of cryptanalysis, or codebreaking, was many hundreds of years later, by an Arab scholar named Al-Khalil
(c. 725-790 AD). He deciphered a Greek cryptogram sent to him by the Byzantine emperor. He explained that he guessed the message would
start with "In the name of God" or something similar and worked out the first few characters on that basis, but it took him a month to
decipher the message. The words expected in ciphered messages are called cribs, which has been used in cryptanalysis to this day.
Ibn ad-Duraihim
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It was another 600 years before a more systematic solution to the monoalphabetic cipher was discovered. Ibn ad-Duraihim (1312-1361)
was the first person known to use letter frequency analysis. Since the Koran was well-studied and the frequency of each letter was known,
he applied this frequency to the frequency of the letters in the ciphered message. He would substitute the most frequent letter in the
Koran with the most frequent letter in the ciphered message and then the second most frequent, etc. While doing this, he would see if
letter pairs that rarely appear together were being formed, which would mean he would have to pick the next most frequent letter to see
if that combination would work.
This is the accepted technique to solve monoalphabetic ciphers today, but this type of cipher is not really in use any more. These ciphers
can be solved in mere minutes if the message has enough characters.
See the Entire Collection of Cipher Machines
See detailed pictures of the "Code-Maker" slide rule toy
Slide rules, wheels, rings, etc. were popular toys in the early and mid 1900s for enciphering messages in a monoalphabetic cipher.
Also many daily newspapers continue to feature cryptograms, which are popular puzzles and another example of a monoalphabetic cipher.