# Proving you're from the year 2069

Just a question I thought of while playing an amazing game about time travel (13 sentinals aegis rim). What are the most bullet-proof cryptographic methods for proving you've come from beyond a particular year? (e.g. 2069)

My current idea goes something like this:

1. A huge trust-worthy company (e.g. Google) creates a super secret key and a public key.
2. Every year, they put out the message "Hello *current year*!" (e.g. in 2069, they put out the message "Hello 2069!")
3. Using the secret key, they publish a digital signature for their annual statement

If this scheme was implemented, and a time-traveler came back to me and showed "Hello 2069!" with a matching digital signature, I'd have 1 of two choices:

1. Believe the time-traveler
2. Google has been compromised, and their secret key has been leaked

Obviously, not a perfect scheme, but still quite decent I think. My question is: is there a more full-proof cryptographic scheme so that a time traveler can prove they've come from the distant year of 2069? Perhaps using some decentralized public consensus instead of a single point-of-failure like Google. For example, using a joint signature by 4 different companies (e.g. Google, Amazon, Apple, and Microsoft).

To be clear, a time-traveler can do a ton of things to prove they're from the future (e.g. pointing out exact weather/solar flare patterns), but I'm talking about more mathematical/cryptographic proof methods.

• A plausible proof'd be the following. Let's assume we can use the moduli from the RSA factoring challenge, i.e., no one, especially the time-traveller knows the factors. For instance, let's use the 2048-bit modulus (en.wikipedia.org/wiki/RSA_numbers#RSA-2048). The time-travellers should compute a Verifiable Delay Function (VDF) on the input of one of the Bitcoin blocks from 2022 (or some string which is verifiably from 2022). The time-delay parameter of the VDF should be 47 years. The time-traveller can prove that they correctly computed the VDF. Alternatively class groups can be used. Apr 27, 2022 at 7:44