Re: Git and SHA-1 security (again)

[Herczeg Zsolt <zsolt94@gmail.com>]

2016-07-19 20:04 GMT+02:00 Duy Nguyen <pclouds@gmail.com>:
> On Tue, Jul 19, 2016 at 7:59 PM, David Lang <david@lang.hm> wrote:
>> On Tue, 19 Jul 2016, Duy Nguyen wrote:
>>
>>> On Tue, Jul 19, 2016 at 7:34 PM, David Lang <david@lang.hm> wrote:
>>>>
>>>> On Tue, 19 Jul 2016, Duy Nguyen wrote:
>>>>
>>>>> On Tue, Jul 19, 2016 at 9:18 AM, Johannes Schindelin
>>>>> <Johannes.Schindelin@gmx.de> wrote:
>>>>>>>
>>>>>>>
>>>>>>> But we can recreate SHA-1 from the same content and verify GPG, right?
>>>>>>> I know it's super expensive, but it feels safer to not carry SHA-1
>>>>>>> around when it's not secure anymore (I recall something about
>>>>>>> exploiting the weakest link when you have both sha1 and sha256 in the
>>>>>>> object content). Rehashing would be done locally and is better
>>>>>>> controlled.
>>>>>>
>>>>>>
>>>>>>
>>>>>> You could. But how would you determine whether to recreate the commit
>>>>>> object from a SHA-1-ified version of the commit buffer? Fall back if
>>>>>> the
>>>>>> original did not match the signature?
>>>>>
>>>>>
>>>>>
>>>>> Any repo would have a cut point when they move to sha256 (or whatever
>>>>> new hash), if we can record this somewhere (e.g. as a tag or a bunch
>>>>> of tags, or some dummy commits to mark the heads of the repo) then we
>>>>> only verify gpg signatures _in_ the repository before this point.
>>>>
>>>>
>>>>
>>>> remember that a repo doesn't have a single 'now', each branch has it's
>>>> own
>>>> head, and you can easily go back to prior points and branch off from
>>>> there.
>>>>
>>>> Since timestamps in repos can't be trusted (different people's clocks may
>>>> not be in sync), how would you define this cutoff point?
>>>
>>>
>>> The set of all heads at the time the conversion happens (maybe plus
>>> all the real tags). We can make an octopus merge commit to cover all
>>> the heads, then it can be the reference point.
>>
>>
>> so to make sure I'm understanding this, anything not reachable from that
>> merge must be the new hash, correct? Including forks, merges, etc that
>> happen from earlier points in the history.
>
> Yes everything except that merge and everything reachable from it, the
> whole old clone, basically.

It could work, but does it worth it?

1) If you use multihash, you should assume that anything with SHA1
could be manipulated. That means you can "inject" something later to
that "old clone" anyway.
2) Even if the content is re-hashed, it's hard to understand for a
user where the trust comes from. The user should decide weather he
trust (or not) the person who signed that octopus breakpoint.

Even without git you can achieve this security: Get the complete old
repository, make a signed tarball of it. If anytime later you want to
check that signatures, you can just use that tarball. I don't think
it's worth the trouble to create a native method for something which
is rare, and can be worked around easily. It's actually easier for a
user to understand the "trust relation" when using this workaround.

Referring to that signed-tarball approach, you may just as well drop
all signature data on conversion... As long as you can look up the
references to old hashes easily, I think it's usable enough.