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Comment by alexopensource | Hacker News Reader

Thanks for asking the right question. We are taking legal help to be compliant with various jurisdictions. Our solution is currently able to safely sign a document with a digital signature that will make it tamper-proof and show a geen tick in Adobe PDF while keeping track of incremental annotations added by multiple signers. We envision to add support for eIDAS and AADHAAR e-sign(widely accepted in India) very soon.

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> Our solution is currently able to safely sign a document with a digital signature that will make it tamper-proof

Who holds the secret key that actually signs the document? If this is in fact a self-hosted, open-source, project then clearly the user does, and they could sign a different, tampered, version of the document after the fact. I would hesitate to use the term "tamper-proof" in that situation. Right now your documentation doesn't make it clear how this actually works.

I'll also point out, that even if you were using my OpenTimestamps scheme or some other secure timestamping system, I would still hesitate to call the solution "tamper-proof". The problem is that even with timestamps someone can in many situations pre-generate alternate versions of a document in advance. Calling this type of system "tamper-resistant" is better IMO.

In the hosted version, we sign the document on behalf of the user using our own private key. Our roadmap also has the feature to bring your own cert(not relevant here). As soon as a user signs a document, a copy of the signed document is instantly sent to all the parties involved. This ensures that the signer cannot revoke the documents already signed. If the receiving party tries to modify the document, the signature becomes invalid. This is how we make sure that the docs are "tamper-proof" after signing.

That's a reasonable, and pragmatic, way to implement this. But I'd still call it "tamper-resistant". One reason why is in situations where senders or recipients have modified something, proving that the _keys_ used to sign the documents were the correct ones can itself be a difficult problem.

>This ensures that the signer cannot revoke the documents already signed. If the receiving party tries to modify the document, the signature becomes invalid

This,fortunately, is a feature of the PDF digital signatures standard.

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