Repairing Security: The Cipher Disk
By: Benjamin Rhodes
Tags: Digital Life, History, Science, Gadgets, Cybersecurity

In a previous article, I explored how Al-Kindi broke all known methods of encryption by the process of frequency analysis. Today, we will discover how one man attempted to toughen data security and encryption. His name is Leon Battista Alberti.

Leon Battista Alberti was the son of an Italian noble (Servos, “The Alberti Cipher”) and came from a wealthy family involved in banking and commerce (Garfinkel and Grunspan 26). Leon took an early interest in mathematics, perhaps explaining his interest in encryption and cryptography (although, cryptography was more of a passing interest). His real interests were more broad than just mathematics. He held a degree in law and was involved in the fields of linguistics, poetry, philosophy, and architecture. Alberti was even appointed as an architectural advisor to the Vatican in 1447 by Pope Nicholas V.

Leon Alberti was a renaissance man, literally. Some have compared him to Lenardo Di Vinci (Servos, “The Alberti Cipher”). Like Di Vinci, Alberti loved to invent and explore new boundaries of science and math. His most famous invention was by far, “the formula,” or as it is known today, the cipher disk. The cipher disk is the first mechanical device invented to encrypt a language (Garfinkel and Grunspan 26).

Prior to the cipher disk’s invention, secure encryption was time consuming and open to many flaws.

For one thing, in order to effectively use encryption for the transfer of secure information, the sender and receiver had to both know the method of encryption and decryption used to encode the message. This often involved carrying compromising documents that explained how the information was coded, an obvious flaw. The cipher disk did away with this flaw. It was small, could be carried, and, even if it was found right alongside the message it coded, would not reveal the secret message.

This invention is so simple, yet so powerful. The initial cipher disk was composed of two…well…disks. One larger and one smaller. The smaller disk fit on top of the larger disk and rotated while the larger disk remained stationary. The larger disk had the alphabet written all the way around the disk. The smaller disk would have the same alphabet printed on it, only the alphabet on the smaller disk was scrambled (Servos, “The Alberti Cipher”).

The encoder would then choose an index. For simplicity, I will choose the index letter “A” for my examples. The encoder would use a secret symbol or message to tell the decoder that the index was the letter “A.” Both the encoder and decoder would align the letter “A” on both disks. Now it’s as simple as substituting the letters on the outside (the letters of the message) for the letters on the inside (Garfinkel and Grunspan 26).

Now, I know what you’re saying, this is only another way to substitute letters! Al-Kindi defeated this method of encryption hundreds of years before the cipher disk was invented! And to that I’ll say, you’re completely right!

However, there are a few facts regarding the cipher disk that still make it secure:

One, although Al-Kindi’s method was old, it was not widely known at the time. The cipher disk was used for quick encryption. If the message was intercepted, then it could be broken. However, hopefully by the time the enemy intercepted the message, determined which method of encryption was used, and deciphered the message, the message would have already served its purpose. The coder could change the index letter by a secret sign the decoder would recognize. This means that while A used to mean A, (the old index letter), A now means B. This would throw off any frequency analysis encryption attempts. A third disk could also be added to the cipher disk! Later versions of the cipher disk involved a third disk with numbers or symbols. The scrambled message could be further complicated by coding the coded message with numbers. Another disk of symbols could also be added. The message could now be completely obscured and become extremely difficult to crack.

I’ve prepared an example of how the cipher disk worked. We’re going to use this short sentence:

This sentence is encrypted using the cipher disk.

The first cipher disk would have been a simple substitution. I’m going to use the letter “A” as my index. Here’s the coded sentence with the first simple substitution:

Ybzv vqiyqidq zv qiduxhyqo mvzij ybq dzhbqu ozvk.

Now, let’s complicate the text a bit by using a third wheel that has numbers. I’ll align the number “1” (our second index) with the letter “A” (our first index). Here’s the new coded text:

20 8 9 19 19 5 14 20 5 14 3 5 9 19 5 14 3 18 25 16 20 5 4 21 19 9 14 7 20 8 5 18 4 9 19 11

Now, our above message isn’t all that secure. I never changed the index and I aligned the second index with the first index. If I had changed the index of both encodes multiple times then the message would be quite secure.

The cipher disk was originally invented to be used for the Latin language, however, its simplicity transfers to nearly any language and nearly any time.

Although it was invented around 1467, it experienced a resurgence in its use during the Civil War and the Spanish-American War in the mid-1800s to early 1900s (“The Cipher Disk”).

Both the Confederate States of America and the United States issued cipher disks to their troops. These cipher disks were small, pocket sized, and made of metal (“Confederate Cipher Disc”). Soldiers could quickly encode documents or even prepare encrypted signals sent from towers to prevent the enemy from easily learning important communication. The cipher disks issued by the CSA are now incredibly rare, there are only five known to exist in the world: two are in the hands of private collectors, one is at the Smithsonian in Washington, DC, and two are held by the Museum of the Confederacy in Richmond, VA (“Confederate Cipher Disc”).

At the time of its invention, the cipher disk revolutionized the world of cryptography.

For the first time since Al-Kindi messed everything up, the world had a secure way of encrypting and decrypting messages. The cipher disk provided not only a more secure method of encrypting data, it provided a much easier way of encrypting data. The cipher disk was the first mechanical method to use a polyalphabetic substitution cipher.

Because it is regarded as the first mechanical device for encryption, we should compare the cipher disk to the first encryption device ever, the Scytale. I discussed the Scytale in detail just a few weeks ago, so be sure to check out that article, but I will review it briefly:

The Scytale presented a method of encryption that was closer to that of transposition. It used the diameter of a wooden rod to scramble letters. I’m sure you can see now how insecure that device really was. If the enemy captured the pole used to read the message, or happened to have a similar object lying around, they could easily read the message.

The cipher disk is many times more secure. The disk could have been transported with the message and the message would have still been secure. The enemy could have even owned the same cipher disk and would have had difficulty reading the message.

I’ve spent enough time over the past few weeks pounding at the importance of cybersecurity. I’m sure you understand, so I won’t fill these pages with it yet again. However, if you still don’t realize the importance, reread both articles mentioned (The Scytale and Code Breakers).

But here’s what you need to know and this is the take-away lesson: technology must be secured.

Our computers today would not be the powerful, daily tools we use without being secured. These early inventions and processes highlight the importance of security and a worldwide, multi-generational understanding of the need for security. The cipher disk changed the way data was secured and heavily influenced the encryption on the devices you use, even without your knowledge or input.

The cipher disk was a tool for important diplomats, militaries, and individuals. It fixed many of the issues with other methods of encryption. The cipher disk is the sixth major milestone in the history of computing.

Make your own Cipher Disk! https://tinyurl.com/cipher-disk

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