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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.

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    $\begingroup$ 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. $\endgroup$ Apr 27, 2022 at 7:44

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This can be done using the RFC 3161 standard, for example. There are similarities with your idea, using a PKI and digital signatures. The keyword is Trusted Timestamping and it requires a trusted third party, referred to as a Time Stamping Authority (TSA).

Basically, create a one way hash of the data that needs to be timestamped (could be the headline from a major newspaper on the date, or any other data which should not be possible to repudiate). To this is anppended the official timestamp from the TSA, which is digitally signed by the TSA.

This can then be checked by anyone.

There are also distributed schemes that can achieve the same goal.

Edit: You could use something like OpenGPG. Alternatively one way to go (though not yet standardized) is using distributed Ledgers, such as Blockchain. See an IETF draft here.

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  • $\begingroup$ Thanks so much for referring me to Trusted Timestamping! Also, the thing I'm mainly concerned with is removing the need for a trusted third-party. I'd love more elaboration on distributed schemes that allow for that. $\endgroup$
    – chausies
    Apr 27, 2022 at 7:49

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