Message Signature Validation

[Pivotal Issue 78632442][1]

[1]: https://www.pivotaltracker.com/n/projects/1163646/stories/78632442
Interesting to see how you solved this, as compared to how I solved it for EB: https://github.com/OpenConext/OpenConext-engineblock/blob/master/library/EngineBlock/X509/CertificateFactory.php

One day this really should be it's own library.