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The Vigenere cipher is a plain-text form of encoding that uses alphabetical substitution to encode text. This ancient form of cryptography dates back to the 1400s and was documented in the works of famous writers of the era such as Trithemius.
The Vigenere cipher, like other contemporary cryptographic ciphers, uses something called a tabula recta, a grid of alphabetic characters where encoders can shift lines for alphabetic substitution. This basic strategy is also part of the Trithemius cipher, and the Caesar cipher, named after Julius Caesar.
Instead of doing a consistent shift alphabetically, the Vigenere shifts letters according to a repeating keyword, which serves to make the encryption more complex and more difficult to decode.
As a "transposition" code or a code involving the substitution of alphabetic letters, the Vigenere cipher represented an improvement from codes that simply shifted letters consistently. These primitive codes were easy to break through processes like letter frequency. Still, even the Vigenere code is not considered a very strong code, and is easily broken with modern tools.