Cryptographic Verification of Backwards Time Travel

In every sci-fi show where a character goes back in time, a whole bunch of time is spent convincing everyone that he is indeed from the future. In the unlikely event that this sort of thing happens in real life, we should have a way to quickly determine whether someone is truly from the future without all the hassles we see on TV.

I propose a small government ministry, possibly a service or subsidiary of NIST, whose sole purpose is to generate, store, and release asymmetric-cryptographic keys. Each day, this agency generates several asymmetric key pairs and publishes all the public keys. The private keys are kept private for a time, then released after the following intervals:

• 1 day
• 3 days
• 1 week
• 1 month
• 6 months
• 1 year
• 2 years
• 5 years
• 20 years
• 50 years
• 100 years
• 200 years

Everybody should keep (or have a device that keeps) the most recently released private key for each day. Upon being transported back in time, the traveler can use the private key for the day in which he finds himself to encrypt a challenge text from those who wish to verify his traveling. The challengers can then decrypt the challenge text with the day's public key, and if the decryption succeeds, the traveler holds the private key and did indeed go back in time. It is possible to verify about how far in the future the traveler was by the last private key he has for the day.

The entire system relies on the security and longevity of the time service. Everybody would have to carry their key management device with them all the time, but it could be as unobtrusive as a small flashlight on a key ring. Of course, the entire idea of time travel is kind of far-fetched, so implementing this system be entirely pointless. You never know, though...