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The wheel cipher, and in the process the polyalphabetic cipher, was created by the Leon Battista Alberti. Alberti was a quintessential renaissance man. He excelled in everything he did, and he did everything. Music. Architecture. Law. Art. Athletics. And Cryptology. He has been called a 'truly all-sided man who towered over his many-sided contemporaries." And while some may remember Alberti for his influential treatises on gothic architecture, and some may remember him for his first study of perspective in art, cryptology will remember him for the polyalphabetic wheel cipher.
Now, if you've been reading these lessons in the proper order, you may be wondering why we already discussed polyalphabetic ciphers in some depth during the classical ciphers section yet never mentioned Alberti. The reason is because his polyalphabetic cipher, the first, used a mechanism. Those who came after him, namely Trittenheim, Porta, Cardano and Viginere all developed polyalphabetic systems which were made for hand written work. The principles already discussed in those lessons for polyalphabetic ciphers remain true for this lesson, and to spare repetition, let me say simply that you should read the polyalphabetic section if you haven't done so yet.
So, what is this wheel cipher that started the polyalphabetic craze? It was simply two disks and a revolutionary method.
Alberti's system used two disks of different sizes, the larger being the stationary disk and the smaller being movable. Each disk's circumference was divided into 24 (26 should be used for the English alphabet) equal parts. In each division of the larger stationary disk, the letters of the alphabet were written in the proper order. The smaller disk also contained the letters of the alphabet, though the order was mixed.
With the two disks properly aligned, a plaintext letter on the larger stationary disk would correspond to a cipertext letter on the smaller disk. However, since the inner disk can move, the ciphertext letter may correspond to more than one plaintext letter, and vice versa. To prevent this, the encoder and sender must agree on an 'index' letter on the small disk which can be aligned to the plaintext 'a' so that the disks will be aligned in the same way. In the disk system's current status, we have described an elaborate method for a simple monoalphabetic substitution.
Alberti, however, changed the course of cryptography when he wrote that following: "After writing three or four words, I shall change the position of the index." By changing the index to another specific letter known by the sender and reciever after a certain number of characters, all plaintext letters will receive 'new meanings', as he call them. In essence, the cipheralphabet was infact changed by changing the index and the first polyalphabetic cipher system was created.
If you have a newer browser that supports DHTML, than you can play with a replica of Alberti's original cipher disks. Just click on the small circle and move your mouse up or down to spin the circle. Fun, eh…
We discussed the security improvements that polyalphabetic ciphers give to cryptography in our previous polyalphabetic cipher page in the classical section. We also discussed how to identify them. Know we'll show you how to beat them.