Re: How safe are signed git tags? Only as safe as SHA-1 or somehow safer?

> On Sun, Nov 16, 2014 at 03:31:10PM +0000, Patrick Schleizer wrote:
> 
>> How safe are signed git tags? Especially because git uses SHA-1. There
>> is contradictory information around.
>>
>> So if one verifies a git tag (`git tag -v tagname`), then `checksout`s
>> the tag, and checks that `git status` reports no untracked/modified
>> files, without further manually auditing the code, how secure is this
>> actually? Is it only as safe as SHA-1?
> 
> Yes, it is only as "safe as SHA-1" in the sense that you have GPG-signed
> only a SHA-1 hash. If somebody can find a collision with a hash you have
> signed, they can substitute the colliding data for the data you signed.
> 
> Of course, "safe as SHA-1" and "find a collision" are vague. If
> pre-image attacks are feasible (i.e., given already-published SHA-1, I
> can find a different input with the same SHA-1), then attacks are
> trivial. But when people talk about attacks on SHA-1, they are usually
> referring to finding a collision between two new pieces of data. You can
> also use that in an attack, but it's much less straightforward
> (basically, you need to get somebody to sign one of the colliding pieces
> of data and then replace it with the other).

> And of course there is the question of getting the colliding data to the
> victim. Git does collision checks whenever a remote (e.g., from a "git
> fetch") gives us data that we already have. So you could poison new
> cloners with bad data, but you could not convince a repository with the
> existing "good" half of the collision to fetch the "evil" half.