Contest: Cory Doctorow's Cipher Wheel Rings
Cory Doctorow wanted a secret decoder wedding ring, and he asked me to help design it. I wanted something more than the standard secret decoder ring, so this is what I asked for: “I want each wheel to be the alphabet, with each letter having either a dot above, a dot below, or no dot at all. The first wheel should have alternating above, none, below. The second wheel should be the repeating sequence of above, above, none, none, below, below. The third wheel should be the repeating sequence of above, above, above, none, none, none, below, below, below.” (I know it sounds confusing, but here’s a chart.)
So that’s what he asked for, and that’s what he got. And now it’s time to create some cryptographic applications for the rings. Cory and I are holding an open contest for the cleverest application.
I don’t think we can invent any encryption algorithms that will survive computer analysis—there’s just not enough entropy in the system—but we can come up with some clever pencil-and-paper ciphers that will serve them well if they’re ever stuck back in time. And there are certainly other cryptographic uses for the rings.
Here’s a way to use the rings as a password mnemonic: First, choose a two-letter key. Align the three wheels according to the key. For example, if the key is “EB” for eBay, align the three wheels AEB. Take the common password “PASSWORD” and encrypt it. For each letter, find it on the top wheel. Count one letter to the left if there is a dot over the letter, and one letter to the right if there is a dot under it. Take that new letter and look at the letter below it (in the middle wheel). Count two letters to the left if there is a dot over it, and two letters to the right if there is a dot under it. Take that new letter (in the middle wheel), and look at the letter below it (in the lower wheel). Count three letters to the left if there is a dot over it, and three letters to the right if there is a dot under it. That’s your encrypted letter. Do that with every letter to get your password.
“PASSWORD” and the key “EB” becomes “NXPPVVOF.”
It’s not very good; can anyone see why? (Ignore for now whether or not publishing this on a blog makes it no longer secure.)
How can I do that better? What else can we do with the rings? Can we incorporate other elements—a deck of playing cards as in Solitaire, different-sized coins to make the system more secure?
Post your contest entries as comments to Cory’s blog post—you can post them here, but they’re not going to count as contest submissions—or send them to firstname.lastname@example.org. Deadline is October 1st.
Good luck, and have fun with this.