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The Alberti Cipher was the first ever Polyalphabetic Cipher. A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets. ...
Created in the late 15th century, circa.1470, by Leon Battista Alberti, it was the peak of cryptography at that time. Events May 15 - Charles VIII of Sweden who had served three terms as King of Sweden dies. ...
Leone Battista Alberti (February 1404 - 25th April 1472), Italian painter, poet, linguist, philosopher, cryptographer, musician, architect, and general Renaissance polymath . ...
The German Lorenz cipher machine, used in World War II for encryption of very high-level general staff messages Cryptography (or cryptology; derived from Greek κÏυπτός kryptós hidden, and γÏάφειν gráfein to write) is the study of message secrecy. ...
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] Leone Battista Alberti (February 1404 - 25th April 1472), Italian painter, poet, linguist, philosopher, cryptographer, musician, architect, and general Renaissance polymath . ...
Leone Battista Alberti (February 1404 - 25th April 1472), Italian painter, poet, linguist, philosopher, cryptographer, musician, architect, and general Renaissance polymath . ...
The Cipher 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 action of a Caesar cipher is to move each letter a number of places down the alphabet. ...
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. The Affine cipher is a special case of the more general substitution cipher. ...
A keyword is a monoalphabetic substitution. ...
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 chracter 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 encrypitng, remembering to write the first letter of the new cipher as a capital.
Attacking the Alberti Cipher 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. In mathematics, physics and signal processing, frequency analysis is a method to decompose a function, wave, or signal into its frequency components so that it is possible to have the frequency spectrum. ...
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 Casesar 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. In mathematics, physics and signal processing, frequency analysis is a method to decompose a function, wave, or signal into its frequency components so that it is possible to have the frequency spectrum. ...
External Links - Description of how to make and use a version of Alberti's Cipher Disc
- Example of a Cipher Disc, and a Description of the difference between the Alberti Cipher and the Vigenère Cipher
The Vigenère cipher is named for Blaise de Vigenère (pictured), although Giovan Batista Belaso had invented the cipher earlier. ...
Notes
- ^ Singh, S: "The Code Book", page 45. Fourth Estate, 1999
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