But I don't know of a cryptographic mechanism to ensure that a digital image is not more recent than a particular time
Many (most?) blockchain mechanisms include a timestamp in each transaction on the chain, so while multiple records from the same owner prove little (the timestamps could be faked over a given period of time) the interaction with the wider network and the chain would give some confidence that the record happened between within a small amount of time.
The other possibility, that doesn't require a chain with many independent active participants, is to have things signed by an external trusted authority. Submit a hash of the content and appropriate metadata to them, and have them sign it with a signing timestamp. I've considered abusing ACME certificates for document signing like that: the hash of content (or some signature based upon it) becomes the subdomain to sign¹ and you get a certificate that even after expiry is evidence that the CA saw that value at the signing timestamp. Note of the signing will also be in the public certificate transparency log. This wouldn't, on its own, prove anything about the authenticity of the content, that could have been doctored before signing, but it does prove that the content+metadata existed at that time (so might be more useful in copyright claim type cases, or agreed contract situations where all parties have signed the content and the signatures are included in the metadata, than for proving authenticity).
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[1] based64²-ed with non-alphanumeric characters removed and truncated³ to fit or split, so acodha3sf7whsrhtqestkabtx0b4bbhyveee0ajnrpqcuxrjjvmhsujgcex.domain.tld or acodha3sf7whsrhtqestkabtx0b4bbhyveee0ajnrpqcuxrjjvmhsujgcex.w5jmmkpmyfgshx2jecsfordpnq.domain.tld
[2] names not being case-sensitive drops some of the entropy, if that is a concern use a 32-bits-per-character encoding instead and have names twice as long
A TXT record may be used in the signing process if using DNS verification, but you are getting a certificate for the name overall so using an A record to hold the hash you are looking to certify does the trick, then if you want to use HTTP-01 for verification rather than an extra DNS record you can.
The image contains the previous block’s hash.
Wouldn’t this establish both a lower bound and an upper bound on the time the image could have been produced?
Back when I was on Twitter and following a lot of infosec accounts, it was quite common to see tweets that were just a hash. Sometimes they'd have an explanation "Zero click RCE in Android 10 - {hash}"
In the past I've used gmail for this internally - email a hash of something critical (logs, configurations, decision docs, etc) to a dedicated gmail account - relying on the in feasibility of "faking" the date/time once it's onb Google's servers.
The important thing here would be to make sure those hashes are published somewhere where its technically infeasible for the police to change it after the fact, so not on a platform the police run or p-aid for (or that is run or paid for by an organization that the police can request or coerce the operators to make changes).
As a community service you need them to have enough scale that no individual hash or source can be tampered with without being likely to become known as unreliable to everyone else as well ala certificate transparency records.
(You could probably just bootstrap let's encrypt for this - issuing a certificate you use to sign a bunch of data would stamp several minimums on the order anything could have happened).
That would be a use case for a block chain. But I still don't understand how you are securing the integrity of the validity of the certificate stating the authenticity of the media. I only understand you are stamping media with a "at least as old as [timestamp]
[0] https://csrc.nist.gov/projects/interoperable-randomness-beac...
Embedding a public random number also doesn't resist tampering, unlike signed timestamps.