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The alphabet used at each point depends on a repeating keyword. At different points in the encryption process, the cipher uses a different alphabet from one of the rows. It has the alphabet written out 26 times in different rows, each alphabet shifted cyclically to the left compared to the previous alphabet, corresponding to the 26 possible Caesar ciphers. To encrypt, a table of alphabets can be used, termed a tabula recta, Vigenère square or Vigenère table. The Vigenère cipher has several Caesar ciphers in sequence with different shift values. For example, in a Caesar cipher of shift 3, a would become D, b would become E, y would become B and so on. In a Caesar cipher, each letter of the alphabet is shifted along some number of places.
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The Vigenère square or Vigenère table, also known as the tabula recta, can be used for encryption and decryption. Kasiski entirely broke the cipher and published the technique in the 19th century, but even in the 16th century, some skilled cryptanalysts could occasionally break the cipher. Charles Babbage is known to have broken a variant of the cipher as early as 1854 but did not publish his work. In 1917, Scientific American described the Vigenère cipher as "impossible of translation". Noted author and mathematician Charles Lutwidge Dodgson ( Lewis Carroll) called the Vigenère cipher unbreakable in his 1868 piece " The Alphabet Cipher" in a children's magazine. The Vigenère cipher gained a reputation for being exceptionally strong. David Kahn, in his book, The Codebreakers lamented this misattribution, saying that history had "ignored this important contribution and instead named a regressive and elementary cipher for him though he had nothing to do with it". In the 19th century, the invention of Bellaso's cipher was misattributed to Vigenère. As it is relatively easy to secure a short key phrase, such as by a previous private conversation, Bellaso's system was considerably more secure. Bellaso's method thus required strong security for only the key. Keys were typically single words or short phrases, known to both parties in advance, or transmitted "out of band" along with the message.
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Whereas Alberti and Trithemius used a fixed pattern of substitutions, Bellaso's scheme meant the pattern of substitutions could be easily changed, simply by selecting a new key. He built upon the tabula recta of Trithemius but added a repeating "countersign" (a key) to switch cipher alphabets every letter. The cipher now known as the Vigenère cipher, however, is that originally described by Giovan Battista Bellaso in his 1553 book La cifra del Sig. In 1586 Blaise de Vigenère published a type of polyalphabetic cipher called an autokey cipher – because its key is based on the original plaintext – before the court of Henry III of France. The Trithemius cipher, however, provided a progressive, rather rigid and predictable system for switching between cipher alphabets. Later, Johannes Trithemius, in his work Polygraphiae (which was completed in manuscript form in 1508 but first published in 1518), invented the tabula recta, a critical component of the Vigenère cipher. Alberti's system only switched alphabets after several words, and switches were indicated by writing the letter of the corresponding alphabet in the ciphertext. In 1863, Friedrich Kasiski first published that successful attacks are possible on the Vigenere cipher.The very first well-documented description of a polyalphabetic cipher was by Leon Battista Alberti around 1467 and used a metal cipher disk to switch between cipher alphabets. Vigenere cipher was first described in 1553 and it was considered indecipherable till 1863. Where C i is the i th letter in the ciphertext, P i is the i th letter of the plaintext and K i is the i th letter of the extended key.
VIGENERE CIPHER MOD
P i = D(C i) = (C i – K i + 26) mod symbol size In other words, if E and D are the encryption and decryption operations respectively, then: C i = E(P i) = (P i + K i) mod symbol size So, the ciphertext now becomes: Plaintext: a t t a c k a t d a w n And, we can then encrypt each letter of the plaintext with the corresponding integer using the Caesar cipher. Now, we can map each letter of the key to an integer as discussed. So, we would need to repeat the letters of the key. But, the length of the plaintext is 12 and the length of the key is 4. Now, we would encrypt each letter of the plaintext using one letter of the key. For example, ‘a’ will map to 0, ‘b’ will map to ‘1’, and so on. We are encrypting the plaintext “attackatdawn” with the key “road”.įirstly, we would map each letter of the alphabet to an integer between 0 and 25.
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Let’s say the symbol size is 26 and we are encrypting English text. Let’s understand the Vigenere cipher with an example first.