[Cryptography] distrusted root CA: WoSign

[Florian Weimer]

* Georgi Guninski:

> Don't get "distrust future certs". Mozilla either trust root(s) or not.
> Root(s) can trivially sign "old" cert requests, requiring old date now.
> Having in mind the chinese have the Startcom, roots, they can issue
> essentially whatever chaining up to root as long as the roots are trusted.

The idea is to whitelist certificates which have been logged to the
Certificate Transparency servers prior to a cut-off date.  As long as
you trust the timestamps of those servers (or even just your previous
downloads you have kept), the CAs cannot backdate that.