[Cryptography] Best internet crypto clock

[John Kelsey]

You can solve one end of this problem with beacons--nobody could have known this information before this time.  You can do the same thing with public information that's unpredictable, like the complete contents of the New York Times front page, or today's sports scores or stock prices.

You can use a digital timestamping service to solve the other end--this information had to be available by this time.

I don't know about the kidnapping scenario, but consider some program that takes an RNG seed, or some experiment which requires some random inputs.  I use the beacon values for today at noon to run the experiment, and as soon as I have the results, I get them digitally timestamped--say, at 1PM today.  This binds the experiment in time--it can't have happened before noon today or after 1 PM today.  

--John