The Polyalphabetic cipher is a classical encryption method that uses multiple substitution alphabets to encode plaintext, significantly increasing security compared to monoalphabetic ciphers. It was first formalized by the Italian cryptographer Giovan Battista Bellaso in 1553 and later popularized by Vigenère in 1586 through what is now called the Vigenère cipher. In a polyalphabetic system, a keyword determines which substitution alphabet is applied to each plaintext letter. This means the same letter in different positions may be encrypted differently, breaking the simple frequency patterns that make monoalphabetic ciphers vulnerable.
To encrypt, the plaintext is aligned with a repeating keyword. For example, using the keyword KEY and plaintext “HELLO,” the letters are paired as H-K, E-E, L-Y, L-K, O-E. Each letter is shifted according to the corresponding key letter: for H-K, shift H by K=10 positions to R; E-E shifts E by E=4 to I; L-Y shifts L by Y=24 to J (modulo 26); the second L-K becomes V; O-E becomes S. The resulting ciphertext is RIJVS. Decryption reverses the process by subtracting the key shifts rather than adding them.
The Polyalphabetic cipher is symmetric, fully reversible, and demonstrates early use of key-dependent letter variation. Its security improves as the keyword length increases, reducing repetition patterns and resisting frequency analysis attacks. Variants include autokey systems, which incorporate plaintext into the key, further increasing unpredictability. Historically, polyalphabetic ciphers were widely used in diplomatic and military communication before the digital age, forming the conceptual foundation for modern symmetric encryption. Practicing with examples like “HELLO” and keyword KEY highlights how the same plaintext letters can produce different ciphertext characters depending on their position, illustrating the key advantage of polyalphabetic substitution over single-alphabet systems.