Crypto agile logs may contain digest types that we don't currently
handle. However, we still need to know how long each digest is in order
to read over the appropriate amount of the buffer. This information is
provided to us as part of the spec header - make use of it rather than
hardcoding the set of digests and lengths we know about.
@brandonweeks detected another case of the "make([]T, untrustedValue)"
pattern, which would allow an attacker to cause the parser to allocate
an unbounded amount of memory.
Fix this by reading one algorithm at a time instead of pre-allocating a
slice of algorithms.
This PR adds event log parsing logic. It's main goal is to require
validation at the same time as parsing, so structured events are always
verified against a quote. This new API replaces the exisitng "verifier"
package.
It's not a goal of this PR to parse the event data. This will be a
follow up, but since different users might want to parse different
events based on the OS, this API lets users of this package implement
custom event data parsing if they absolutely need to.
