Ask HN: 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&#x27;s a timestamp, you can do that easily for free with my OpenTimestamps project, which uses Bitcoin for the timestamp proof: <a href="https:&#x2F;&#x2F;opentimestamps.org&#x2F;" rel="nofollow">https:&#x2F;&#x2F;opentimestamps.org&#x2F;</a><p>The way it works is pretty simple: your message is hashed with a series of cryptographically secure hash&#x2F;append&#x2F;prepend operations, leading to a Bitcoin block. Since they are one way hashes, your message must have existed prior to that Bitcoin block.<p>The hard part is actually proving a message existed <i>after</i> 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&#x27;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: <a href="https:&#x2F;&#x2F;github.com&#x2F;opentimestamps&#x2F;nist-inject" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;opentimestamps&#x2F;nist-inject</a><p>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&#x27;s the opposite: your message needs to depend on the random beacon. For human meaningful messages that&#x27;s not easy to achieve. A photo can be photoshopped, a video deepfaked, etc.<p>The best you can do <i>in general</i> 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&#x27;s dubious security.<p>Probably the best approach here is to actually think about what exactly you are trying to achieve - are you kidnapping someone? proving you&#x27;ve recovered from the plague? launching a cryptocurrency? There isn&#x27;t a one-size-fits-all solution to this problem.