What are some good methods to prove current time?

The easy thing to do is prove a message existed prior to some point in time. That's a timestamp, you can do that easily for free with my OpenTimestamps project, which uses Bitcoin for the timestamp proof: https://opentimestamps.org/

The way it works is pretty simple: your message is hashed with a series of cryptographically secure hash/append/prepend operations, leading to a Bitcoin block. Since they are one way hashes, your message must have existed prior to that Bitcoin block.

The hard part is actually proving a message existed after some point in time. To do that, you need a random beacon: a large number (also known as a nonce) that we know was created at some point in time, and prior to that point in time, was impossible to predict. Newspaper headlines are a weak form of random beacon, as it's hard to predict the news in advance. Bitcoin block hashes are even better, as the proof-of-work ensures that even trying to force a single bit of the block hash is extremely expensive. I also run a project to inject other random beacons into the Bitcoin blockchain, such as the NIST Random Beacon: https://github.com/opentimestamps/nist-inject

But in the age of deep fakes, what does any of this actually prove? See, while a timestamp proof mathematically depends on your message, with a random beacon it's the opposite: your message needs to depend on the random beacon. For human meaningful messages that's not easy to achieve. A photo can be photoshopped, a video deepfaked, etc.

The best you can do in general is try to make the random beacon time, and the timestamp, be as close together as possible to make it as difficult as possible to do a photoshop, deepfake, etc. But deepfakes are good enough these days that IMO that's dubious security.

Probably the best approach here is to actually think about what exactly you are trying to achieve - are you kidnapping someone? proving you've recovered from the plague? launching a cryptocurrency? There isn't a one-size-fits-all solution to this problem.

Back in 1990, Bellcore researchers Haber and Stornetta set up a system (surety.com) where documents were hashed and then the hash value was published as a classified ad in the New York Times every week. This provided a solid proof that the document was published as of that date. (Unless someone visited all the world's libraries and swapped in fake New York Times copies.)

An article showing one of the classified ads: https://www.vice.com/en/article/j5nzx4/what-was-the-first-bl...

You can do the opposite, include your message (or a digest of it) in a medium that is hard to alter, for example a newspaper, notaries, Twitter, a blockchain, etc (or preferably a combination of those). Some of those solutions don't prevent you from publishing multiple versions of the message and only revealing one however.

On the other hand if you're trying to show that you did something (not just authored a message) at a given time, it's a bit harder, because you can always notarize the video long after it has been recorded. The best way to deal with this one (in the absence of trusted parties that might attend in-person) might be to do it in a public place e.g. in the library while a specific event is happening, on the street while some specific work is being done, in front of a newspaper stand in a shop etc. However I can see any of those faked using video editing.

I looked into this a few years ago[1]. The solution I came to involves putting the hash of the most recent Bitcoin block in your document, and then signing the hash of your document into the Bitcoin blockchain.

This cryptographically proves that the document was created/published within those two time bounds.

This of course doesn't help in all cases, i.e. you could edit an old document to make it look newer, but I'm not aware of any way to properly solve that particular problem without a trusted third-party.

[1] https://www.jamieweb.net/blog/proof-of-timestamp/

To show that a message existed at a particular date and time, you could use a time-stamping service operated by a trusted third party, in which they digitally sign a timestamp and the cryptographic digest of your message.

Several public certification authorities offer these services, described in RFC 3161, at no charge, because they’re used by code signing schemes, to prove that a digital signature on an object file was made during the validity period of the signer’s identity certificate. Here’s Microsoft’s explanation:

https://docs.microsoft.com/en-us/windows/win32/seccrypto/tim...

You don’t have to use an identity certificate, however; OpenSSL, for example, can work work with timestamps directly:

https://www.openssl.org/docs/manmaster/man1/tsget.html

https://www.openssl.org/docs/manmaster/man1/openssl-ts.html

Some examples of time-stamping servers:

https://knowledge.digicert.com/generalinformation/INFO4231.h...

https://www.entrust.com/knowledgebase/ssl/time-stamp-url

https://sectigo.com/resource-library/time-stamping-server

Whether such a timestamp would be acceptable proof, on its own or accompanied by an explanation from an independent expert, would depend on the audience and the situation, of course.

I found the problem statement and every reply remarkably confusing, because I am not smart. I'd like to give an answer to help fellow not-smart people.

The goal is to prove that you did something in the past -- for example, that you proved a theorem, or that you predicted that COVID would become an issue way back in 2018.

Interestingly, patio11 solved this for COVID: He published a hash of his article, calling out COVID in Japan, to Twitter, which has no edit button. Then a month or so later, he published the article itself, proving that he did in fact write that article a month prior.

It's an old solution, not his. But that's the general idea here.

>In other words how can I prove the current time of message?

In many countries you can have a statement you wrote signed, dated and stamped by the state authorities that verifies not the truth of the content, but that _you_ wrote it (and the specific time). You need to fill a form, take it to a state office, and present your ID to them for them to stamp and approve it.

In the US I find that this is called a "statutory declaration":

A statutory declaration is a document that formalizes matters to be made known publicly. It is a solemn statement made by plaintiff or witnesses instead of the oath, but equally binding.

Statutory declaration is a legal document based on statute law as to their format and content requirements. Statutory declarations are of the same force and effect as if sworn under oath or affirmation.

Rephrasing and reframing: You want to prove that you did something before and after certain points of times, and you have solutions for proving that you did something after a certain point of time (filming current newspaper or other similar methods). So you want to prove "the other side": That is, you want to prove that you did something BEFORE a certain point of time.

Here are two low-tech solutions almost anyone can do:

1. Record yourself standing under a well-known landmark, which has a digital or analog display of time and date. Movie theaters have displays showing time and date. Malls have them too.

2. Record yourself doing a Google search for "time now" and record the search result showing the time.

You're looking for: https://opentimestamps.org

It builds a Merkle tree of hashed documents, timestamps it, and puts it on the blockchain.

Very good question. I have also occasionally wondered about this and haven't come up with a satisfying solution. I guess it boils down to 1) reference in your message something that didn't exist before your point-in-time 2) let a trusted third party publish the hash of the message you compiled. A third party seems necessary because time isn't a technical concept, but time can be described as a sequence of events - so some trusted event needs to reference your event/document(its hash or equivalent). As a comment here mention, blockchain could be used as a solution. Really interested to see other solutions in this thread.

I think there is an inherent asymmetry to the problen. You can prove that something (a headline) exists when you do your message, but you cannot easily prove that it does not yet exist. You could, theoretically, use something that one day will cease to exist to show that you signed your message before that point. However, digital things (information) normally do not cease to exist.

Assuming you don't want to involve any third party, things get difficult. To be honest I think there is always a way to make a message look older.

Considering the extent of our abilities to modify videos after a recording is made (After Effect, Blender and all those post-editing softwares out there), it's hard to prove in-video other than a livestream as you mentioned.

The only solution I can see would be to use an impartial third-party with legal weight to attest the current time to be as declared on video and on paper.

Another idea would be to make and film a transaction on a public system (Bitcoin blockchain?) with a proof of the transaction and its timestamp?

Encode your message as a hostname, like “the-answer-is-42.yourdomain.com”

Buy or request a SSL certificate for that CN. it’ll be from a notarized source, with a date on it, proving you knew that info at least as early as that, and recorded in the certificate transparency streams.

Also encode today’s lottery numbers to prove you knew the info after that date.

Many are mentioning digital timestamps. There's a pretty universal physical one, too -- a postmark. So enclose your message and a newspaper front page in an envelope addressed to yourself and sealed in a trustable way, and drop it in the mail. But then opening it can only happen once and has to be witnessed.

A system like Bitcoin has this functionality. "Time" is essentially represented as "depth" of a block, and a block can only be buried deep into the chain with sufficient time (due to the nature of proof-of-work). A cryptographic primitive called "verifiable delay function" provides a good abstraction for this problem.

Specifically, you may put the hash of your message into a bitcoin transaction, and submit this transaction. If you later want to prove its age, just point to the bitcoin block containing your transaction. The guy verifying it is sure that you have got this message before the block is mined, because otherwise you would not have the hash of the message at that time and successfully embed it into a block.

Are there no cosmic sources of randomness, for example variations in pulsar frequency?

That could solve the "No older than this" part, by imprinting a sufficiently long (unique) sequence from a particular time on your asset. Although I suppose it would be possible for whoever keeps the recorded "event-log" to just alter it.

For the "No younger than this" part, it seems you need to do the reverse, imprint the universe with your signature, which can then be verified by anyone who looks.

I guess blockchain can work as "universe" to some extent, but a more natural/absolute solution would be neat.

You could use a trusted / centralised source to produce a TOTP or some equivalent code, and generate some code alongside a timestamp, which would be verifiable. (You can't get the code ahead of time from the service, but can later verify a timestamp code combination.)

Honestly, just emit the message in the Bitcoin blockchain and it will be there for perpetuity with a timestamp that is fully verifiable.

Post online. Twitter, Facebook, your favorite messenger won't allow changing the time.

When renting a car from a sketchy place, I take photos and send them either to myself or to the owner. In cases of liability, a public medium would be preferable so you can just share a link to let anyone validate it.

https://blog.cloudflare.com/roughtime/

https://github.com/Merovius/notary

Reading something allows one to create a proof of being after. Writing something allows one to create a proof of being before.

Are there other ways of conceptualizing this problem?

Blockchain would be your best bet, but even that could be scheduled for the future, all depends on context.

http://surety.com/

Any service that timestamps uploads will do, github for example.

Make it known at the time (possibly via something likely strongly dependent like a hash or encryption). If it is not observable, it may just as well not have happened. Call it quantum mechanics or simply common sense.

You can ask the person you wish to prove current time to, to provide a "challenge" that you will film ("put two fingers up", "hold up a USB cable").