CODEWITHSUNDEEP
cryptographycertificatedigital-signaturedigital-certificate
nemesifier
nemesifier

Reputation: 8509

Document signed and timestamped locally and then uploaded to the server, does it have same characteristics?

Immagine a web application that lets you digitally sign (with personal digital certificates pkcs12 released by trusted CAs) and timestamp PDF documents with a Java applet or Active X. This must obviously happen on the machine of the user because the private key of the certificate is stored locally.

So once the PDF is signed and timestamped it is uploaded on the server. Does the uploaded file have the same features of the one created locally? Does it have sense to talk about "the original version of the file"?

I'm a bit confused on this.

Correction: i mean digitally sign a document with the private key of a personal digital certificate (should be pkcs7, pkcs12) to ensure that it has really been signed by someone and not someone else.

Upvotes: 0

Views: 214

Answers (2)

emboss
emboss

Reputation: 39620

If by "the original version of the file" you mean that you intend to "freeze" the document so that nobody can ever make changes to it again - that is neither possible nor the purpose of a digital signature. Anyone could simply "cut out" the a signature embedded within a document, nobody would notice.

Protecting a document from subsequent modification involves some kind of DRM mechanism. For example, "watermarking" involving steganography is used to protect photos so that noone should be able to claim ownership of a photo, even after having modified it. But the technology is not very advanced yet, most algorithms can be easily broken.

This implies that the notion of "the original version of the file" in let's say a legal dispute is something that the involved parties have to agree upon in consent. There's no way to prove origin without either consent or a trusted third party that will attest the integrity of a document, e.g. if they have an independent copy of the document.

Apart from that, uploading a file should not change its contents. The file will have the exact same properties than the local one including the signature that was added on the client side.

The signature will only attest authenticity and integrity of the document. If it is vital for your application to be able to tell that the signed document received is actually the one that was expected, then I'd advise you to do the following:

  • Create the PDF on the server
  • Create a hash of the document (same algorithm that will be used by the signature applet)
  • Send the PDF to the client
  • Let the client sign it and send it back
  • Compare the client's hash with the one previously computed on the server
  • Validate the signature

Validating the signature will ensure integrity and authenticity, comparing the hashes will guarantee you that the signed document you received on the server is indeed a signed version of the original document previously created.

Concerning timestamps using local clocks: they're worthless, it's very easy to cheat. What you actually should use there is RFC 3161-compliant cryptographically secured timestamps, issued by a trusted third party. Currently that's the only reliable way to include the notion of time in PDF signatures. There's also built-in support for this in Adobe Reader for example. As these services are generally not available for free, it would make sense to add such a timestamp on the server after receiving the signed document. They are added as an unsigned attribute to the CMS (Adobe still speaks of PKCS7) signature, so it won't break the signature and can safely be added after signature creation.

Upvotes: 1

Paŭlo Ebermann
Paŭlo Ebermann

Reputation: 74750

Okay, let's try to answer your question (as I understand it).

You have some software which uses some private key (and a clock) to add a signature to a file.

This signature is depending on the contents of the file, and thus makes sure that the signer knew (or could have known) the contents of the file at the time it signed it. (There are some ways to have "blind signatures", but I assume this is not the case here.)

Uploading the signed file anywhere does not change anything here.

About the timestamp: The key holder can put in any timestamp it wants - so this only helps if you want to prove knowledge of the document at some point in time against the key holder, not if you are the key holder and want to prove that you signed at some point in time and not earlier or later. (Also, are you sure his clock is not skewed?)

About whether this is legally relevant, you will have to ask your lawyer. It might depend on

  • the jurisdiction in which the signature happened, and the one in which you want the signed document to be valid
  • whether the owner of the key had a chance to actually read the document before signing
  • whether the owner of the key had actually a choice of signing or not.

If you use some applet or ActiveX control in the user's browser, I would not be totally sure that the last two points really hold.

Upvotes: 1

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