I seem to recall that there was at least some discussion at one point
about adding some extra fields to the commit object in a backwards
compatible way by adding it after the trailing NUL. We didn't end up
doing it, but I could see it being a useful thing nonetheless (for
example, we could potentially put the backup SHA-2/SHA-3 pointer there).
What if we explicitly allow a length plus SHA-2/3 hash of the commit
plus the fields after the SHA-2/3 hash as an extension? This would
allow a secure way of adding an extension, including perhaps adding
backup SHA-2/3 parent pointers, which is something that would be
useful to do from a security perspective if we really are worried
about a catastrophic hash failure.
The one reason why we *might* want to use SHA-3, BTW, is that it is a
radically different design from SHA-1 and SHA-2. And if there is a
crypto hash failure which is bad enough that the security of git would
be affected, there's a chance that the same attack could significantly
affect SHA-2 as well. The fact that SHA-3 is fundamentally different
from a cryptographic design perspective means that an attack that
impacts SHA-1/SHA-2 will not likely impact SHA-3, and vice versa.
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