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Comment by yardstick

1 day ago

There used to be - probably still are - cameras that would digitally sign all their images. Used in crime scenes? Maybe we will end up seeing wider adoption of this, despite the privacy implications. Hackers attention then will focus (once again) on the certificate supply chain and crypto hardware.

31 comments

yardstick

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ChrisMarshallNY  1 day ago

I worked for a company that made these. We sold expensive software to the FBI.

Took about six months for someone to crack the hash.

  • deepserket  1 day ago

    What about a system that saves in some way the hash in a Blockchain, and if you, eg, XOR the hash of the video with the hash of the previous block you will "certainly" know that the video was created between the previous block and the block where the hash is saved in. That's a starting point.

  • EPWN3D  1 day ago

    "Crack the hash"? Does this mean you were employing some novel hashing algorithm and relying on its secrecy? If so your employer were never serious about security in the first place. Hardware attestation is more or less a solved problem, and that solution does not involve secret algorithms.

    • ChrisMarshallNY  1 day ago

      Eh. It was some kind of hash of the image. I was not involved in that project, so can't tell you exactly how it worked, but the images were "signed," and someone figured out how to "re-sign" an altered image.

      I think it was a fairly well-known technique.

      6 replies →

aorloff  1 day ago

I imagine in this age of blockchains you could embed into a media file a signature that proved it was no older than the timestamp of when it occurred, the digital equivalent of a hostage-proof-of-life photo with a recent newspaper

But I don't know of a cryptographic mechanism to ensure that a digital image is not more recent than a particular time

  • dspillett  1 day ago

    > 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).

    ----------------

    [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

  • gcr  1 day ago

    Publish hash(image) on the blockchain at a verifiable time, then publish the image itself.

    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?

    • bigiain  20 hours ago

      You don't need a blockchain for that. You just need some reliable-enough way to publish hash(image) with a timestamp - some way that it's infeasible enough as to be considered impossible for thepublisher to change the hash or the date.

      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).

      1 reply →

  • __del__  1 day ago

    wouldn't that be a hash of the image signed by a trusted entity and stored on a chain? maybe i'm overlooking why this doesn't work

  • catlikesshrimp  1 day ago

    Interesting, There aren't any newspapers left in my country, neither printed nor not printed. The closest you can find is the weekly advertising booklet here and there. Which is irrelevant now because a computer can either stich new content to an old picture, or entirely producing a custom picture.

    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]