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Building on top of the previously mentioned signing of existing PDF files work, one more PDF feature coming in LibreOffice 5.3 is initial support for the PDF Advanced Electronic Signatures (PAdES) standard. First, thanks to the Dutch Ministry of Defense in cooperation with Nou&Off who made this work possible.

Results

PAdES is an extension of the ISO PDF signature with additional constraints, so that it conforms to the requirements of the European eIDAS regulation, which in turns makes it more likely that your signed PDF document will be actually legally binding in many EU member states.

The best way to check if LibreOffice produces such PDF signatures is to use a PAdES validator. So far I found two of them:

As it can be seen above, the PDF signature produced by LibreOffice 5.3 by default conforms to the PAdES baseline spec.

Implementation

I implemented the followings in LO to make this happen:

  • PDF signature creation now defaults to the stronger SHA-256 (instead of the previously used weaker SHA-1), and the PDF verifier understands SHA-256

  • the PDF signature creation now embeds the signing certificate into the PKCS#7 signature blob in the PDF, so the verifier can check not only the key used for the signing, but the actual certificate as well

  • the PDF signature import can now detect if such an embedded signing certificate is present in the signature or not

Note
Don’t get confused, LO does signature verification (checks if the digest matches and validates the certificate) and now shows if the signing certificate is present in the signature or not, but it doesn’t do more than that, the above mentioned DSS tool is still superior when it comes to do a full validation of a PAdES signature.

As usual, this works both with NSS and MS CryptoAPI. In the previous post I noted that one task was easier with CryptoAPI. Here I experienced the opposite: when writing the signing certificate hash, I could provide templates to NSS on how the ASN.1 encoding of it should happen, and NSS did the actual ASN.1 DER encoding for me. In the CryptoAPI case there is no such API, so I had to do this encoding manually (see CreateSigningCertificateAttribute()), which is obviously much more complicated.

Another pain was that the DSS tool doesn’t really separate the validation of the signature itself and of the certificate. The above screenshot was created using a non-self-signed certificate, hence the unclear part in the signed-by row.

If you want to try these out yourself, get a daily build and feel free to play with it. This work is part of both master or libreoffice-5-3, so those builds are of interest. Happy testing! :-)

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