A polyalphabetic cipher is any cipher where a given letter of the alphabet is not always encrypted by the same ciphertext letter.
To put it simply, a polyalphabetic cipher is a combination of different monoalphabetic ciphers.
It is a substitution cipher with an alphabet that changes. And a key is used to define encryption mappings per alphabet.
Are you interested in finding out more about ciphers and codes?
The Codebreakers – The Story of Secret Writing book by David Kahn is what I would start with.
What is the difference between a monoalphabetic and a polyalphabetic cipher
A monoalphabetic cipher mixes up letters of the alphabet and keeps that same arrangement for the entire message while a polyalphabetic cipher uses more that one monoalphabetic cipher with the same message.
To put it simply, polyalphabetic ciphers utilizes multiple, changing ciphertext alphabets to encrypt the plaintext while monoalphabetic ciphers use a single method throughout the entire message.
A brief history of polyalphabetic ciphers
Development of polyalphabetic substitution ciphers was to counter frequency analysis attacks.
Alberti cipher is recognized as the first polyalphabetic cipher, invented by Leon Battista in the 14the century.
He used a mixed alphabet to encrypt the plaintext, changing to a different mixed alphabet at random points by use of an uppercase.
How to solve a polyalphabetic cipher
Alberti used a cipher disc to show how plaintext letters are related to ciphertext letters.
For example, from the disk above, you can see that the plaintext letter “E” (on the outside ring) is encrypted to “Z” (on the inside ring).
This disc set would be used for a few letters of the message, and then the inner disc is rotated to a different setting for the next few letters, and so on.
Trithemius cipher is another polyalphabetic cipher that was invented by Johannes Trithemius in the 15th century.
It worked by changing the ciphertext alphabet after each letter was encrypted.
Trithemius cipher is an example of a progressive key cipher that uses a tabula recta to show all the different alphabets.
The only drawback of this cipher is that it lacks any key, meaning every message encrypted using it uses the same method.
Thus making it prone to frequency analysis attacks.
Vigenere cipher is yet another example of a polyalphabetic cipher. It was one of the biggest advances in cryptography before computers were made.
It was invented by a French mathematician, Blaise de Vigenère in the 16th century.
It used a string of text or a keyword as a key which was used to do multiple alphabetic shifts on the plaintext to encrypt it to plaintext.
Are you interested in learning how to break codes?
The Elementary Cryptanalysis – A Mathematical Approach book by Abraham Sinkov is what I would recommend.