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The cipher now known as the Vigenère cipher, however, is based on that originally described by Giovan Battista Bellaso in his 1553 book La cifra del Sig. Giovan Battista Bellaso. [8] He built upon the tabula recta of Trithemius but added a repeating "countersign" (a key) to switch cipher alphabets every letter.
Kasiski actually used "superimposition" to solve the Vigenère cipher. He started by finding the key length, as above. Then he took multiple copies of the message and laid them one-above-another, each one shifted left by the length of the key. Kasiski then observed that each column was made up of letters encrypted with a single alphabet. His ...
travel down that column to find key "m", travel to the left edge of the tableau to find the ciphertext letter ("K" in this case). To decrypt, the process is reversed. Unlike the otherwise very similar Vigenère cipher, the Beaufort cipher is a reciprocal cipher, that is, decryption and encryption algorithms are the same. This obviously reduces ...
This technique is used to cryptanalyze the Vigenère cipher, for example. For a repeating-key polyalphabetic cipher arranged into a matrix, the coincidence rate within each column will usually be highest when the width of the matrix is a multiple of the key length, and this fact can be used to determine the key length, which is the first step ...
In 1553, an important extension to Trithemius's method was developed by Giovan Battista Bellaso, now called the Vigenère cipher. [3] Bellaso added a key, which is used to dictate the switching of cipher alphabets with each letter. This method was misattributed to Blaise de Vigenère, who published a similar autokey cipher in 1586.
With reciprocal machine ciphers such as the Lorenz cipher and the Enigma machine used by Nazi Germany during World War II, each message had its own key. Usually, the transmitting operator informed the receiving operator of this message key by transmitting some plaintext and/or ciphertext before the enciphered message.
Consider a block cipher with a unicity distance of three ciphertext blocks. Although there is clearly enough information for a computationally unbounded adversary to find the right key (simple exhaustive search), this may be computationally infeasible in practice. The unicity distance can be increased by reducing the plaintext redundancy.
The list of ciphers in this work included both substitution and transposition, and for the first time, a cipher with multiple substitutions for each plaintext letter. Charles Babbage, UK, 19th century mathematician who, about the time of the Crimean War, secretly developed an effective attack against polyalphabetic substitution ciphers.