Definition
Polyalphabetic Cipher
A polyalphabetic cipher is a substitution cipher in which the substitution applied to each plaintext symbol depends on the symbol’s position in the plaintext, typically according to a repeating key.
Security
Resistance to Frequency Analysis
A polyalphabetic cipher hides the frequency distribution of individual letters because the same plaintext symbol may be mapped to different ciphertext symbols, and the same ciphertext symbol maycorrespond to different plaintext symbols.
Key Length Vulnerability
For longer texts, statistical analysis can still be effective once the key length is determined (e.g. via the Kasiski test or the Friedman test). The ciphertext can then be split into groups, each encrypted as a simple shift cipher.