Polyalphabetic cipher
A polyalphabetic cipher switches among multiple substitution alphabets as it processes a message.
Definition
A polyalphabetic cipher uses more than one substitution alphabet. A keystream selects the alphabet or shift for each position, so the same plaintext letter can encrypt to different ciphertext letters.
How it works
Vigenère repeats a keyword to form a keystream and applies the corresponding rows of a tabula recta. Autokey extends a short key with message material, while Beaufort changes the arithmetic relationship between key, plaintext, and ciphertext.
Cryptanalysis
Changing alphabets obscures the simple frequency profile of monoalphabetic substitution. A short repeating key, however, creates periodic structure: Kasiski examination and the index of coincidence can estimate its length, after which each position class can be analyzed separately.
Yes. Each keystream letter selects a different shifted substitution alphabet.
No. Classical designs with repeated or predictable keystreams leak structure and are not suitable for modern protection.