-----BEGIN PGP SIGNED MESSAGE----- Bryce wrote:
Can anyone explain what use this theoretical "time-sensitive" crypto box would be good for?
Sameer wrote:
Suppose you die.
Hey! Who do you think you are? :-) Just kidding. When I woke up this morning I realized what I was missing: the decryption might be out of your hands, such as when you die, or you might *want* it to be out of your hands for some other reason. With that in mind, I can think of only one unalterable lower-limit on the time of as decryption-- the speed of light. Suppose you encrypt your data with successive layers of keys, K1-Kn. Then you encrypt each key with its predecessor, encrypting Kn with Kn-1, encryping Kn-1 with Kn-2, etc. Destroy all copies of unencrypted keys except for K1, which has not been encrypted. Now put all odd-numbered keys in location A and all even-numbered keys in location B, which is 1 light minute from location A. Once an agent has received Key 1, it will take at least n minutes to decrypt the data. Of course, the agent could just take copies of all of the keys from location B on some physical media and transport the media to location A, which would make the lower bound on time to be "much longer than 1 minute". Hm. Suppose the n different keys are in n different physical locations, and the agent does not know where the k+1 location is until he decrypts the material at the k location. The "scavenger hunt" scheme for timed decryption. Of course this doesn't mean that you have to bury your crypto box and make a map with an "X" marking the spot. Each key could be held by a crypto box which is publically accessible on the Net. The important thing is that the decrypting agent can't retrieve the k+1 piece until he has decrypted the k piece. Then the lower bound on time of decryption is... um... Well it depends on the location of the decrypting agent with respect to the locations of the n pieces. (Neglecting, still, transmission overhead and decryption time.) I'm not sure what the lower bound actually is, but it can be increased simply by adding more pieces to the puzzle. A single station could serve up multiple pieces. It would only reveal the k piece if the querying agent can prove that he has the k-1 piece. Of course if the total number of stations is small then the "physically move the pieces" trick might work. Bryce signatures follow "To strive, to seek, to find and not to yield." <a href="http://www-ugrad.cs.colorado.edu/~wilcoxb/Niche.html"> bryce@colorado.edu </a> -----BEGIN PGP SIGNATURE----- Version: 2.6.2 Comment: Auto-signed under Unix with 'BAP' Easy-PGP v1.01 iQCVAwUBMKT/JPWZSllhfG25AQFDlwQAhWHB//NeYM8vylQcBDWbNmScrVoCjUdR TmXVDtnLCZcrAv233l+H3SpdEQmMwQwQCQrM52AreQWMYTSBLuxqr7j9SbpZjek2 FFCMDezbvBPX3ZIuX3SVwrdHa6dm4qgGtpKyfFHxDAn39p+T/HJ+uKaZbA7YVbTC U6NnnfYv1k8= =/2+H -----END PGP SIGNATURE-----