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  • Sunday, 23 September 2018
    odfsig: an Open Document Format (ODF) digital signatures tool

    I created a toy project to experiment with a few technologies I wanted to try out (cmake, googletest, xmlsec outside LibreOffice, libzip and AppVeyor). The result is a tool with a similar interface as pdfsig from poppler (for PDF files), just for ODF: a cmdline executable to verify the digital signature(s) in an ODF document.

    The source code now has CI on Linux and Windows, so perhaps in a shape that is interesting for others to have a look as well. And if not, then no problem, it was interesting to put together these building blocks and see them working as expected. :-)


  • Thursday, 05 July 2018
    Improved ECDSA handling in LibreOffice

    I wrote about ECDSA handling in LibreOffice last year, back then the target was to be able to verify signatures using the ECDSA algorithm on Linux.

    Lots of things happened since then, this post is meant to summarize those improvements. My personal motivation is that Hungarian eID cards come with a gov-trusted ECDSA (x509) cert, so handling those in LibreOffice would be nice. My goals were:

    • platforms: handling Windows as well, not only Linux/macOS

    • operations: handling signing as well, not only verification

    • formats: cover all of ODF, OOXML and PDF

    Let’s see what has happened:

    • Linux, ODF, sign: we had hardcoded RSA algorithm when generating a signature, now we infer the sign algorithm from the signing cert algorithm (commit)

    • Linux, OOXML, sign: same problem with hardcoded RSA (commit)

    • Windows, PDF, sign: the certificate chooser had to be ported to from CryptoAPI to CNG (commit)

    • Windows, ODF, verify / sign: this was the largest problem, this required a whole new libxmlsec backend (interface, implementation, all in C90) and also required conditionally using that new backend in LibreOffice (commit)

    • Windows, OOXML, sign: this was almost functional, except that the UI recently regressed, now fixed (commit)

    • Finally now that everything is ported on Windows to use CNG, I could enable it by default yesterday.

    I could test hardware-based signing after this, which was fine out of the box on both platforms. Some screenshots:

    • Linux:

    https://farm2.staticflickr.com/1784/29321634078_8818b2d7ba_o.png
    • Windows:

    https://farm1.staticflickr.com/927/42288765505_db72ee48f2_o.png

    (There is no reason why this would not work on macOS, but I did not test that.)

    Thanks Gabor Kelemen who helped me to get a sane card reader that has reasonable driver support on Linux.

    All this is available in master (towards LibreOffice 6.2), or you can grab a daily build and try it out right now. :-)


  • Monday, 13 March 2017
    ECDSA support in xmlsec-nss, bundled by LibreOffice

    Last month a LibreOffice bugreport was filed, as the ODF signature created with Hungarian citizen eID cards is not something LibreOffice can verify. After a bit of research it seemed that LibreOffice and NSS (what we use for crypto work on Linux/macOS) is not a problem, but xmlsec’s NSS backend does not recognize ECDSA keys (RSA or DSA keys work fine).

    The xmlsec improvements happened in these pull requests:

    After this the xmlsec code looked good enough. I had to request an update of the bugdoc in the TDF bug twice, as the signature itself looked also incorrect initially:

    • an attribute type in the signature that had no official abbreviation was described as "UNDEF" instead of the dotted decimal form

    • RFC3279 specifies that an ECDSA signature value in general should be ASN1-encoded in general, but RFC4050 is specific to XML digital signatures and that one says it should not be ASN1-encoded. The bugdoc was initially ASN1-encoded.

    Finally a warning still remains: while trying to parse the text of the <X509IssuerName> element, the dotted decimal form is still not parsed (see this NSS bugreport). The bug is confirmed on the mailing list, but no other progress have been made so far.

    Oh, and of course: Windows is still untouched, there a bigger problem remains: we use CryptoAPI (not CNG) there, and that does not support ECDSA at all. Hooray for open-source libs where you can add such support yourself. ;-)


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