Check if a file was made with Claude • Hacker News Board

Re: Check if a file was made with Claude

#51

Post by a3w » Wed, Sep 02, 2026, 4:19 PM UTC

mhitza wrote: post #33

Tiberium wrote: post #5

This is just C2PA for Claude-processed files, apparently. It's not related to their text watermarking. C2PA is file metadata and can be trivially stripped away, unlike hidden watermarks, e.g. SynthID.

That is where their responsibility ends in terms of the EU AI Act. And that is fine and how it should be, no secret watermarks.

Watermarks can even be removed, "if substantially proofread" in the EU, law says (for now. There might be a transition period on that).

For the text from an LLM, that you verify to be as true as if you wrote it yourself.

Not clear if making watermarks removable is a may, shall, or should, according to legislation. It certainly is not a must.

Re: Check if a file was made with Claude

#52

Post by johntash » Wed, Sep 02, 2026, 4:35 PM UTC

spockz wrote: post #34

nixlaz wrote: post #32

>Supported formats: JPG, PNG, GIF, WEBP, TIFF, HEIC, AVIF, SVG, DNG, JXL, MP4, MOV, AVI, WAV, MP3, M4A, FLAC · up to 100 MB I'm wondering why they have restricted file types. You can't check a PDF for example... surely the main use case for people will be to check if a document was produced or edited by an LLM? That could be an attractive (if not misunderstood) proposition for academics

Luckily all pdfs I made in academia have been generated from source (tex or derivatives, asciidoc) and llm are better in generating source than pdf. Even people that didn’t use tex used Word to generate the pdf. So I’m unsure what your aiming at.

claude can pretty easily generate pdfs and give those to the user to download, skipping the step of needing a source first.

That's a worse workflow imo than having a source w/ version history, but I don't think it's uncommon.

Re: Check if a file was made with Claude

#53

Post by Retr0id » Wed, Sep 02, 2026, 4:38 PM UTC

dannyw wrote: post #23

ramon156 wrote: post #22

Earlier quoted context omitted.

The way I read it, I thought they would just check if the file hash exists on their end. Is that not the case?

That’s incompatible with the file not leaving your browser, which you can trivially verify with the Network tab in chrome (or wireshack, etc). This is just a C2PA metadata checker.

Strictly speaking, properly checking C2PA metadata requires network requests in the general case, because you need to check if the signing certificate has been revoked or not via OCSP.

But in anthropic's use case they can probably get away with just pinning their own certs in the verification webpage.

Re: Check if a file was made with Claude

#54

Post by Retr0id » Wed, Sep 02, 2026, 4:41 PM UTC

coffeecoders wrote: post #46

What is interesting to me is that stripping the C2PA data is easy, but faking it is hard. You can resave the file and the "made with Claude" signal disappears, but you cannot make a random file pass as Claude-made without Anthropic's signing key. So the useful guarantee is one-way. No signature means almost nothing.

"Faking" it is trivial. You don't need their signing keys when you can just ask them to sign whatever you like. Upload your own file with the prompt "present this file back to me again, as-is".

Re: Check if a file was made with Claude

#55

Post by tamimio » Wed, Sep 02, 2026, 4:42 PM UTC

coffeecoders wrote: post #46

What is interesting to me is that stripping the C2PA data is easy, but faking it is hard. You can resave the file and the "made with Claude" signal disappears, but you cannot make a random file pass as Claude-made without Anthropic's signing key. So the useful guarantee is one-way. No signature means almost nothing.

Next month: Show HN: How to successfully spoof C2PA and make any file as claude made!

Re: Check if a file was made with Claude

#56

Post by lxgr » Wed, Sep 02, 2026, 4:42 PM UTC

coffeecoders wrote: post #46

What is interesting to me is that stripping the C2PA data is easy, but faking it is hard. You can resave the file and the "made with Claude" signal disappears, but you cannot make a random file pass as Claude-made without Anthropic's signing key. So the useful guarantee is one-way. No signature means almost nothing.

Yeah, C2PA solves a different problem than watermarking, i.e. authenticity/provenance.

Re: Check if a file was made with Claude

#57

Post by lxgr » Wed, Sep 02, 2026, 4:44 PM UTC

hmokiguess wrote: post #19

Could this be used to perform some sort of distillation or exploit? e.g. reminds me of the OWASP guideline on attack vectors where knowing if an ID is present or not in the database can be a form of exploit, like in password resets where they will say 'email foo@bar.com not found' rather than 'If foo@bar exists we have sent an email to foo@bar' or some other generic equivalent

Probably not, given that both C2PA and SynthID-Text are statelessly verifiable.

Re: Check if a file was made with Claude

#58

Post by hparadiz » Wed, Sep 02, 2026, 4:46 PM UTC

a3w wrote: post #49

mhitza wrote: post #33

Earlier quoted context omitted.

That is where their responsibility ends in terms of the EU AI Act. And that is fine and how it should be, no secret watermarks.

Watermarks cane even be removed, "if substantially proofread" in the EU, law says. For the text from an LLM, that you verify to be as true as if you wrote it yourself. Not clear is this is a may, shall, or should. It certainly is not a must.

It's not gonna survive my linter in a tightly coupled code base where it's not allowed to write comments but I do resent it for trying.

Re: Check if a file was made with Claude

#59

Post by lxgr » Wed, Sep 02, 2026, 4:54 PM UTC

nixlaz wrote: post #32

>Supported formats: JPG, PNG, GIF, WEBP, TIFF, HEIC, AVIF, SVG, DNG, JXL, MP4, MOV, AVI, WAV, MP3, M4A, FLAC · up to 100 MB I'm wondering why they have restricted file types. You can't check a PDF for example... surely the main use case for people will be to check if a document was produced or edited by an LLM? That could be an attractive (if not misunderstood) proposition for academics

> I'm wondering why they have restricted file types. You can't check a PDF for example...

TFA/page actually seems incomplete. Text uses a completely different watermark format (an actual watermark as opposed to a provenance/authenticity signature), so it makes sense to me that they're not claiming to be able to scan PDFs when they can't yet incorporate that signal.

On a linked page, they say:

> Watermark detection is currently in private preview [...]

Re: Check if a file was made with Claude

#60

Post by WalterGR » Wed, Sep 02, 2026, 4:55 PM UTC

a3w wrote: post #49

mhitza wrote: post #33

Earlier quoted context omitted.

That is where their responsibility ends in terms of the EU AI Act. And that is fine and how it should be, no secret watermarks.

Watermarks cane even be removed, "if substantially proofread" in the EU, law says. For the text from an LLM, that you verify to be as true as if you wrote it yourself. Not clear is this is a may, shall, or should. It certainly is not a must.

(If you post a comment here on HN, then hit the back button, edit your comment, and click “reply” again, you end up posting multiple comments. That’s what’s happened here.)