verify: Support XAdES encapsulated certificates

[peterkelly]

This change modifies the logic used to find X509 certificates within a Signature element to include EncapsulatedX509Certificate elements.

This makes it possible to use the XMLVerifier with documentsignatures.xml files generated by LibreOffice, which places all certificates in the chain of trust from the signer to the root in EncapsulatedX509Certificate elements.

Related: #139

[kislyuk]

Thank you for your contribution. To proceed with merging, we need unit tests to cover all new code paths introduced by this PR. Could you please add unit tests? Let me know if you run into any difficulties running the test suite.

[peterkelly]

I've added a unit test now (see description in latest commit). Please let me know if this is sufficient.

[kislyuk]

Thanks again for adding a test, and apologies for the delay in getting your contribution reviewed.

I'm OK with merging partial/incremental support for XAdES, especially if it enables specific use cases that people have (like the LibreOffice one you point out). However from a security risk management standpoint, I'd like to sequester XAdES-specific functionality into subclasses that have to be specifically imported and used instead of being on by default. I want to do this because we don't have a full XAdES conformance test suite in place, and I don't have a good enough security analysis of XAdES in hand to be sure that this is safe from new types of substitution/wrapping attacks, etc. for general purpose XMLDSIG verification.

If you can separate this functionality into XAdES specific classes, I'm happy to merge this now, otherwise I'll use your branch as a starting point for a more comprehensive XAdES implementation using e.g. https://github.com/esig/dss/tree/master/dss-xades/src/test/resources/validation and https://signatures-conformance-checker.etsi.org/pub/documents/XAdESConformanceTestChecker_v1.0.pdf as starting points.

[peterkelly]

I think the latter is probably better.

One simple thing I could do if you wanted is to create a subclass of XMLVerifier, say XAdESVerifier, which overrides the _get_certificates added in 908029b. The base XMLVerifier class could provide a version of this method that just returns the certificates that it did before, while XAdESVerifier could override the method to supply the extra certificates.

An alternative approach would be to allow an optional certificate_extractor parameter, with a base class of CertificateExtractor (used by default) and a subclass XAdESCertificateExtractor that can be specified by clients if they need this functionality. I don't know enough about XAdES to say whether this or the former is the better approach.

I want to avoid doing a full implementation of XAdES since it's far beyond the scope of what I need (and couldn't justify the time for), so something like this would be good. I suggest you go ahead using my branch as a starting point, perhaps using the ideas above if appropriate.