[mike_hearn]

anilgulecha is correct. Consider the following attack:

You wish to undermine the security of an important codebase managed by git.

You write a valuable and useful contribution to the code. You also create another version of the commit that has the same SHA-1 as the first, which breaks the security of that code.

You submit the first commit, it is accepted and merged.

Now you wait for your target to do a git clone on the repository, perhaps because it's a build environment. You then MITM it (e.g. using infrastructure like QUANTUM INSERT) and redirect the clone to your own git server, which serves the bad commit.

The target compares the top commit hashes of what it expected to get and what it actually got, and is none the wiser. They now compile the code and produce a backdoored binary.

[rocqua]

You need enough leeway in the first commit though. As this is a collision attack, you need to be able to tweak both commits in order to get their hashes to collide.

The mangling in the first commit is probably going to look mighty-suspicious. You might be able to handle this if you get to mangle e.g. code comments. In any case, you need something similar like PDFs malleability where you can change one document without the change being noticeable.

[euyyn]

Trailing whitespace! :)

[Klathmon]

That you of course helpfully fix in the very next commit!