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The Vigenère cipher (French pronunciation: [viʒnɛːʁ]) is a method of encrypting alphabetic text where each letter of the plaintext is encoded with a different Caesar cipher, whose increment is determined by the corresponding letter of another text, the key. For example, if the plaintext is attacking tonight and the key is ...
Code sheets included alternative digits to modify the gender or letter case [10] and so the rules of French composition held true to encryptions as well. Since e is the most commonly used letter in French, the Cipher typically allocated the most code numbers to writing that vowel. In one nomenclature, 131 out of the 711 code numbers stood for e ...
Twenty-two years later Blaise de Vigenère described another form of autokey using a standard table primed by a single letter [Vigenère, f. 49.], which is more vulnerable than that of Bellaso's because of its regularity. Obviously by trying as primers all the alphabet letters in turn the cryptogram is solved after a maximum of 20 attempts.
The substitution alphabet could also be scrambled in a more complex fashion, in which case it is called a mixed alphabet or deranged alphabet. Traditionally, mixed alphabets may be created by first writing out a keyword, removing repeated letters in it, then writing all the remaining letters in the alphabet in the usual order.
The method creates a chain-reaction when a letter is decrypted, this means that after decrypting a word, the letters of that word can be used to decrypt other words. [ 9 ] Depending on the type of cipher, a brute force attack method can be used, which attempts to use all possible keys for the encryption. [ 10 ]
For instance, if all occurrences of the letter e turn into the letter X, a ciphertext message containing numerous instances of the letter X would suggest to a cryptanalyst that X represents e. The basic use of frequency analysis is to first count the frequency of ciphertext letters and then associate guessed plaintext letters with them.
The action of a Caesar cipher is to replace each plaintext letter with a different one a fixed number of places down the alphabet. The cipher illustrated here uses a left shift of 3, so that (for example) each occurrence of E in the plaintext becomes B in the ciphertext.
The four-square cipher is a manual symmetric encryption technique. [1] It was invented by the French cryptographer Felix Delastelle.. The technique encrypts pairs of letters (digraphs), and falls into a category of ciphers known as polygraphic substitution ciphers.