Instead of there being a one-to-one relationship between each letter and its substitute, there is a one-to-many relationship between each letter and its substitutes. A polyalphabetic substitution cipher involves the use of two or more cipher alphabets. One of the most common approaches is to suppress the normal frequency data by using more than one alphabet to encrypt the message. Therefore, to make ciphers more secure, cryptographers have long been interested in developing enciphering techniques that are immune to frequency analysis. Given a sufficiently large ciphertext, it can easily be broken by mapping the frequency of its letters to the know frequencies of, say, English text. One of the main problems with simple substitution ciphers is that they are so vulnerable to frequency analysis. However, in this case, it is the key, not the cipher, which provides cryptographic strength, and such systems are correctly referred to collectively as one-time pad systems, irrespective of which ciphers are employed.Cryptography - Vigenere Cipher The Vigenere Cipher - A Polyalphabetic Cipher If using a truly random key, which is at least as long as the encrypted message and is used only once, the Vigenère cipher is theoretically unbreakable. The problem with the running key Vigenère cipher is that the cryptanalyst has statistical information about the key elements(assuming that the block of text is in a known language) and that information will be reflected in the ciphertext. This version uses as the key a block of text as long as the plaintext. The running key variant of the Vigenère cipher was also considered unbreakable at one time. If a cryptanalyst correctly guesses the key's length, then the ciphertext can be treated as interwoven Caesar ciphers, which individually are easily broken. The primary weakness of the Vigenère cipher is the repeating nature of its key. However, using the Vigenère cipher, E can be enciphered as different ciphertext letters at different points in the message, thus defeating simple frequency analysis. For instance, if P is the most frequent letter in a ciphertext whose plaintext is in English, one might suspect that P corresponds to E because E is the most frequently used letter in English.
The idea behind the Vigenère cipher, like all polyalphabetic ciphers, is to disguise plaintext letter frequencies, which interferes with a straightforward application of frequency analysis. Phrase LEMON, for example, defines the sequence of ROT11-ROT4-ROT12-ROT14-ROT13, which is repeated until all block of text is encrypted.Īs wikipedia tells us, it is a simple form of polyalphabetic substitution. The sequence is defined by keyword, where each letter defines the needed shift.
For example, the first letter of text is transformed using ROT5, second - using ROT17, et cetera. Vigenère cipher is the sequence of Caesar ciphers with different transformations (ROTX, see Caesar cipher). Here is the calculator, which transforms entered text (encrypt or decrypt) using Vigenere cipher. Since we already have Caesar cipher, it seems logical to add the Vigenère cipher as well.