![]() We can now observe that the plaintext letter T is represented by both L and Y in the ciphertext and that the ciphertext letter L represents both I and T. Using the Vigenère Square to encrypt P with the key letter fĪny reader who completes this process should conclude that the complete cryptogram for PLAINTEXT with keyword fred is The process for encrypting P with key letter f is illustrated here. Similarly we encrypt L by taking the letter underneath it in the row determined by r, which is C. Thus to encrypt P we go to the row of the square determined by f and read off the letter beneath P, which is U. To encrypt the initial letter P we use the key letter beneath it which, in this case, is f. We now use the square to encrypt the message as follows. Thus, for example, if our message is PLAINTEXT and our keyword is 'fred' we obtain: We then write this sequence of letters beneath our message. If the plaintext message is longer than the keyword, then, by repeating the key as often as is necessary, we obtain a sequence of letters which is as long as the message. One of the most common methods for using the square to obtain a cipher involves choosing a keyword (or key phrase) with no repeated letters. Thus, for example, the letter g gives the Caesar Cipher with shift 6. So each letter in the left-hand column gives a Caesar Cipher whose shift is determined by that letter. The left-hand (key) column of this square contains the English alphabet and, for each letter, the row determined by that letter contains a rotation of the alphabet with that letter as the leading character. The Vigenère Cipher uses a Vigenère Square to perform encryption. Grant: 'It would sometimes take too long to make translations of intercepted dispatches for us to receive any benefit from them, but sometimes they gave useful information.' This is illustrated by the quotation by General Ulysses S. The Civil War occurred after the cipher had been broken. It is interesting to note that the Vigenère Cipher was used by the Confederate Army in the American Civil War. Although it was published in 1586, it was not widely recognized until nearly 200 years later and was finally broken by Babbage and Kasiski in the middle of the 19th century. The Vigenère Cipher is probably the best known of the 'manual' polyalphabetic ciphers and is named after Blaise de Vigenère, a 16thcentury French diplomat. As with our earlier examples, we include them to illustrate a number of cryptanalytic techniques and because they enable us to set exercises which are both informative and fun. ![]() However we discuss them in some detail as we can then illustrate a number of weaknesses that the modern algorithm designer has to avoid. Once again we must point out that the simple examples of these ciphers that we describe are no longer used. For these ciphers it is now true that the same ciphertext character may represent different plaintext letters. ![]() ![]() When a polyalphabetic cipher is used, the ciphertext character replacing a particular plaintext letter may vary through the cryptogram and might, for instance, depend on its position in the plaintext message or the content of the plaintext that precedes it. However, it is still true that each ciphertext character represents a unique plaintext character and there is always the danger of an attacker compiling a dictionary of known plaintext and ciphertext pairs for a given key.Īnother way of achieving the objective of flattening the frequency histogram is by the use of a polyalphabetic cipher. This ensures that more than one ciphertext character may represent the same plaintext character. When a homophonic cipher is used, the frequency histogram of the cryptogram is made flatter by increasing the size of the alphabet.
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