Re: Git and SHA-1 security (again)

From: Junio C Hamano <gitster@pobox.com>
Date: Jul 22, 2016, 15:59 UTC
Message-ID: <xmqqvazxbr4h.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <alpine.DEB.2.20.1607211451060.14111@virtualbox>

Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:

Show 30 quoted lines

> Hi Junio,
>
> On Mon, 18 Jul 2016, Junio C Hamano wrote:
>
>> "brian m. carlson" <sandals@crustytoothpaste.net> writes:
>> 
>> > I will say that the pack format will likely require some changes,
>> > because it assumes ...  The reason is that we can't have an
>> > unambiguous parse of the current objects if two hash algorithms are in
>> > use....  So when we look at a new hash, we need to provide an
>> > unambiguous way to know what hash is in use.  The two choices are to
>> > either require all object use the new hash, or to extend the objects
>> > to include the hash.  Until a couple days ago, I had planned to do the
>> > former.  I had not even considered using a multihash approach due to
>> > the complexity.
>> 
>> Objects in Git identify themselves, but once you introduce the second
>> hash function (as opposed to replacing the hash function to a new one),
>> you would allow people to call the same object by two names.  That has
>> interesting implications.
>> 
>> [...]
>
> So essentially you are saying that the multi-hash approach has too many
> negative implications, right? At least that is what I understand.
>
> Looks more and more like we do need to convert repositories wholesale, and
> keep a two-way mapping for talking to remote repositories.
>
> Would you concur?

Not necessarily.

That was me thinking aloud, listing some issues that I would imagine to be tricky to solve, without even attempting to be exhaustive, that I expect to see solved in a good end-result implementation. For example, "I do not see a nice way to solve X myself without doing Y" in the message you are responding to does not necessarily mean there is no good solution to X (just "I do not think of any offhand"), and it does not mean I think it is terrible that we have to do Y to solve X.