Premise: Depending on the ROI in each specific case, SHA1 is either broken
today or will be broken at some indefinite (but finite) time in the future.
The cost of attack will continue to decrease for many years to come, so the
number of repositories in danger of attack will continue increase.
Given this, does the delta compression scheme give a way around the transition
to SHA-3 checksums?
Consider a 2-line file checked into Fossil with a SHA-1 hash:
hello
world
Then I check the following version of that same file in with a SHA3 checksum:
hello
Bob
Now imagine that I somehow replace the first artifact in the repository without
modifying the SHA-1 hash. This would almost certainly require a larger file,
but since real repos will have larger files, and there’s no point complicating
the example, that is not an important detail.
My question is, does the new SHA-3 scheme protect us from that possibility, or
will a Fossil checkout of the tip of that repository replay the SHA-3 delta on
top of the tampered SHA-1 checkin and be thereby tainted?
If so, should Fossil checkin and rebuild detect these SHA-1 to -3 transitions
and store whole files at the transition point to firewall the new-hashed
artifacts from the old?
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