“While One certainly does not condone your skullduggerious activities, One would like to present you with this royal cipher disc, should you find it necessary to send Us any missives of a clandestine nature. ”
— HRH ER II
This is a slightly silly take on a cipher disk used to encode and decode secret messages (think secret decoder ring). You would spin the inner disc until the queen was saying your predetermined Britishism. Then use the pointer to find the correct encryption for each letter of your message. Decoding works the same way, only in reverse.
This is called a substitution cipher. As you might have guessed, it's a very poor encryption scheme, and easy for an attacker to see through. However I really like that it can be done with a simple paper-craft device.
I tried a number of different layouts for the disc. You can see some of my design sketches below.