polyalphabetic

Polyalphabetic refers to a class of substitution ciphers that use multiple alphabet mappings to disguise plaintext. In a polyalphabetic cipher, the substitution used for a given letter depends on its position in the message, typically by consulting a sequence of alphabets rather than a single fixed mapping. This approach reduces the effectiveness of frequency analysis compared with monoalphabetic ciphers, where a single substitution applies to all instances of a letter.

The best-known example is the Vigenère cipher, which uses a tabula recta to apply shifts according to a keyword. Other early polyalphabetic methods include Alberti's cipher, Beaufort, and autokey variants. In practice, the plaintext letter is paired with a key letter that determines which alphabet is used, effectively applying a Caesar shift that varies with the key.

Security and limitations: Polyalphabetic methods improve security against simple frequency analysis, but they are not unbreakable. If the key length is short or repeats in a detectable way, cryptanalysis techniques such as Kasiski examination or Friedman tests can reveal the key length and allow decryption. A key that is as long as the plaintext and truly random—an ideal one-time pad—would be theoretically unbreakable, but distribution and use pose practical challenges. Modern ciphers use more sophisticated polyalphabetic concepts within larger encryption schemes.

In contemporary cryptography, polyalphabetic ideas underpin many stream ciphers and the use of multiple alphabets or states, though classical polyalphabetic ciphers themselves are largely of historical interest. They illustrate how varying the substitution over a message improves security and laid groundwork for modern cryptographic techniques.