Alberti cipher
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The Alberti Cipher was the first ever Polyalphabetic Cipher.
Created in the late 15th century, circa.1470, by Leon Battista Alberti, it was the peak of cryptography at that time.
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[edit] Leon Alberti
Its creator was Leon Battista Alberti, an illegitimate son of an Italian nobleman, was also interested in painting and writing, though he is probably best known for his architecture. He created the cipher after a chance encounter with Leonardo Dato in the gardens of the Vatican. [1]
[edit] Description
The Alberti Cipher initially consisted of two Caesar Shift ciphers performed on the plain text, alternating at random between the two shifts. The change between ciphers was marked by the capitalisation of the first letter of the section of text to be encrypted with the new shift.
The cipher was later made more complicated by using 3 or 4 different Caesar shifts alternating between them in alphabetical order, again highlighted by a Capital Letter.
Finally, Alberti started using more random substitutions - Affine Shifts and Keyword shifts, but the substitution cipher would be shifted about, once again denoted by the presence of a capital.
As holding several Caesar Alphabets in one's head at the same time is difficult at the least, Alberti used a Cipher Disc consisting of two metal discs, one mobile, and one fixed, both connected to a central spindle so that the inner disc may be rotated. Around the outside of the outer disc are inscribed the lowercase letters of the alphabet, which you would look at according to which letter came next in the text you were trying to encrypt. Around the inside, mobile, disc the UPPERCASE Letters the of the alphabet were written, so that when the inside disc was rotated to the relevant position, the cipher text character could be read off from the inner wheel.
To use this Cipher Disc to encrypt a message one would simply chose a starting letter, rotate the wheel until A was encrypted as that chosen letter, encrypt a set of letters, (normally about two sentences worth), then rotate the wheel to a new position and continue encryptng, remembering to write the first letter of the new cipher as a capital.
[edit] Cryptanalysis
Compared to previous ciphers of the time the Alberti Cipher was all but impossible to break without knowledge of the method.
This was because the Frequency Distribution of the letters - the only known technique for attacking ciphers at that time - was ruined. Thus Frequency Analysis was no help.
Each of the Caesar Shifts would have its only, highly recognisable, frequency pattern, but when the two were combined the result was all but meaningless. Thus for a long time Alberti had an unbreakable cipher.
Unfortunately the weakness of the cipher was the need to indicate to the intended recipient where the cipher alphabet was changed. This was done by using a Capital Letter at the start of each change of alphabet. However, if some one who knew of Alberti's method intercepted his message, he could decipher it even if he neither knew the specific Caesar shifts used, nor had any crib.
This deciphering of the text could be done by splitting it in two: You would take all the text up to the first Capital, then separate out all the text up to the next capital, then put the next section of text with the first, and the next section of text with the second, and so on until all the text had been separated into two groups. Now, each group has been encrypted with the same cipher, which can be very easily broken by the use of Frequency Analysis, as its Frequency Distribution is no longer disguised. Once each section has been decoded, the two sections can be "stuck" back together again, to give the entire decrypted message.
[edit] Recent Discoveries
Although Alberti is usually considered the father of polyalphabetic cipher, Prof. Ibrahim A. Al-Kadi's 1990 paper ( ref- 1) to the Swedish Royal Institute of Technology in Stockholm regarding the Arabic contributions to cryptology reported (based on a recently discovered ancient script) the knowledge of polyalphabetic ciphers 500 years before Alberti.
Dr. Al-Kadi reported on the Arabic scientist by the name of Abu Yusuf Yaqub ibn Is-haq ibn as Sabbah ibn 'omran ibn Ismail Al- Kindi, who authored a book on cryptology the "Risalah fi Istikhraj al-Mu'amma" (Manuscript for the Deciphering Cryptographic Messages) circa 750 AD. Al-Kindi introduced cryptanalysis techniques (including those for polyalphabetic ciphers) , classification of ciphers, Arabic Phonetics and Syntax and most importantly described the use of several statistical techniques for cryptanalysis. [This book apparently antedates other cryptology references by 300 years.] [It also predates writings on probability and statistics by Pascal and Fermat by nearly 800 years.]
[edit] External Links
- Example of a Cipher Disc, and a Description of the difference between the Alberti Cipher and the Vigenère Cipher
[edit] References
- Ibrahim A. Al-Kadi "The origins of cryptology: The Arab contributions”, Cryptologia, 16(2) (April 1992) pp. 97–126.
[edit] Notes
- ^ Singh, S: "The Code Book", page 45. Fourth Estate, 1999
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