Cryptographic Wedding Rings

[Ehtyar]

Bruce Schneier helps a friend build a crypto system for his wedding rings, and is asking for help determining their most effective use.

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."

Full Story
Ideas Here (must read full story first)

Ehtyar.

[steeladept]

I am not a cryptographer by any stretch, but it seems to me this would be extremely cumbersome for any significant size of text.  Instead, I would do a double substitution cypher where the two parties have a defined and known key.  This key is used to decipher a second cypher that tells how to decipher the rest of the text.

Example:

Ring Cypher uses predefined key of first letter written.  Actual cypher key starts at second letter.  First line defines the cypher for the rest of the text using some substitution rule that can be predefined, or defined by the deciphered key.  It is still hardly secure, but it is a simple method to use the ring without being overly cumbersome.