Vigenère

The Vigenère cipher is a polyalphabetic substitution cipher that uses a keyword to determine the shift applied to each letter of the plaintext. It relies on the Vigenère tableau (tabula recta), a grid of 26 shifted alphabets arranged in rows. To encrypt, the plaintext is paired with a repeating keyword; for each position, the row corresponding to the key letter provides the ciphertext letter from the plaintext letter. Decryption uses the same keyword in reverse. In simple terms, the ciphertext letter is obtained by adding the numeric values of the plaintext letter and the key letter modulo 26, and the plaintext letter can be recovered by subtracting the key letter.

Historically, the cipher is named after Blaise de Vigenère, though earlier forms were described by Giovan Battista Bellaso. It gained prominence in the 19th century as a solution to the shortcomings of simple monoalphabetic ciphers, which could be broken by frequency analysis. Its polyalphabetic nature makes frequency-based attacks more difficult, though not impossible.

Security and limitations: while more resistant than a single Caesar shift, the Vigenère cipher is not secure by modern standards. It can be broken with repeated-key analysis, such as the Kasiski examination or Friedman’s methods, which reveal the key length and enable reconstruction of the keyword. In practice, it is now of historical and instructional interest rather than practical cryptographic use.

Variants and related forms include the Beaufort cipher, a reciprocal version, and autokey variants that use portions of the plaintext as part of the key. The Vigenère cipher remains a foundational concept in the study of classical cryptography.