The previous change was done in the optimistic hope that NID_subject_alt_name
could be handled in the same fashion as all the rest of the attributes we match
against. However, X509 is not a place for optimisim. Instead, it turns out
that the Subject Alternative Name is an X509v3 extension, and needs to be
handled separately.
Therefore, this change...
* Introduces the idea of Criteria matching against a location in the
certificate, and not just against the entirety of the certificate.
* Extracts the Subject Alternative Name extension, and allows iteration and
matching against its components.
* Extends our constraint language to sensibly match against SubjectAlternativeNames.
The `S.subjectAltName` syntax has been kept, but the value is now required to
provide what type of field the rest of the value is intended to match against.
The code currently supports DNS, EMAIL, URI, and IP. Prefix and suffix
matching is supported.
Both verify-test and plugin-test were updated to cover Subject Alternative Name
matching. I've additionally run plugin-test under valgrind to verify that I've
understood object lifetimes correctly.
This extends our language for specifying verification rules from, e.g.
S.O=XYZCorp
to also include two more operators
S.O>=XYZ # Prefix
S.O<=Corp # Suffix
both of which would match against an Organization of XYZCorp (among others).
The TLS code understands three different things it can verify: the subject, the
issuer, and the root. The existing code assumes that any attribute we can
verify against one of these is also verifyable against the others.
For Subject Alternative Name, this might not be true. There exists both
Subject Alternative Name and Issuer Alternative Name. This code change allows
one to write "I.subjectAltName=Foo", and we'll verifiy a Subject Alt Name
against the Issuer, which wouldn't be right. Issuer Alternative Name isn't a
requested feature (yet?), so I'm punting on this problem.
Evan Tschannen