attest: Add signal that Data section in an event corresponds to the event digest

[twitchy-jsonp]

Use case:

• Sometimes interesting information is put in the Data section of an event
• In these cases the event is such that the digest extended matches the digest of the Data section
• In this case, the consumer of the Event would like to use the contents of the data section if and only if the data section matches the digest of the event

Proposal:

• Add a method/field that would indicate whether the data field of an Event is verified by the digest or not.

[brandonweeks]

How about a method on EventType that takes does a map lookup?

Only problem I can see with that is if I recall correctly, whether or not the data should be validated is a tuple of the EventType and PCR index.

[twitchy-jsonp]

Is this set of event types (and their semantics around verifiying the data section) relatively static? is it something we could embed in this library?

[brandonweeks]

Is it statically bound to the "TCG PC Client Platform Firmware Profile
Specification" spec version. So yes but we might hit edge cases between 1.2 and 2.0.

https://trustedcomputinggroup.org/wp-content/uploads/TCG_PCClientSpecPlat_TPM_2p0_1p04_pub.pdf

[ericchiang]

I'd like to implement this using private methods while I work on #9 to see what works and what we need. Ideally, we could check the event type and reject invalid digests at the time of (*EventLog).Verify.

That work?