Polyalphabetic Cipher

A polyalphabetic cipher is a substitution cipher that uses multiple substitution alphabets to encrypt a message. Each letter of the key specifies a different shift amount, cycling through the key as the message is encrypted.

Polyalphabetic ciphers were first described in the 15th century by Leon Battista Alberti. The concept was refined by Johannes Trithemius and later by Giovan Battista Bellaso in 1553. These ciphers were considered unbreakable for centuries until frequency analysis techniques were developed.

Polyalphabetic ciphers were used extensively for diplomatic and military communication from the 16th to 19th centuries. Today they are used in educational contexts and as a building block for understanding more complex ciphers.

Vulnerable to Kasiski examination (finding repeated sequences to determine key length) and frequency analysis once key length is known. Short keys make it especially weak. The Index of Coincidence method can also reveal key length.