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From: Jeff King <>
To: "brian m. carlson" <>
Cc: "Wu, Zhichen" <>,
	"" <>
Subject: Re: Question About Git V2 Protocol & SHA256
Date: Tue, 29 Sep 2020 20:46:30 -0400	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Tue, Sep 29, 2020 at 10:43:56PM +0000, brian m. carlson wrote:

> On 2020-09-29 at 22:13:11, Jeff King wrote:
> > On Tue, Sep 29, 2020 at 01:17:59AM +0000, Wu, Zhichen wrote:
> > > 2. I see v2 has a capability called “object-format” that provides SHA1
> > >    option. I’m wondering if that capability will be the only way for
> > >    client and server to start using SHA256? Or put it as another word,
> > >    will v2 protocol be the prerequisite of SHA256?
> > 
> > I think it would be impossible to handle object-format via v1, because
> > the v1 protocol writes the ref advertisement before any capabilities are
> > negotiated. So I think v1 must implicitly remain sha1-only (and a sha256
> > repository on the server side would need to either reject a v1 client,
> > or back-translate as it would for a v2 client which asks for sha1).
> I don't think that's the case.  You can indeed use v1 with SHA-256, but
> if you have a SHA-1-only Git, it will choke because the object ID is
> longer than it expects.  If you want to negotiate the algorithm when we
> support both and the client can't deal with translating the initial ref
> advertisement, then yes, you'll need v2.

I agree that we _could_ just dump sha256 within a v1 protocol and wait
for the client to choke. But that seems like an awfully lousy user
experience. By contrast, assuming that the client wants sha1 means that

  - we can reject it with a sensible ERR message that tells the user
    what is going on

  - we can serve them by talking in terms of sha1 if we're willing to
    do the extra conversion work server-side (and/or have a cache of
    sha1-format objects)

The only thing we lose is that a recent client who understands sha256
wouldn't be able to contact us and do a sha256-over-v1 transaction. But
why would they want to do so?

This is all hypothetical at this point, though, right? I tried finding
details in the hash transition document, but protocol changes are listed
as out of scope there. It does say that sha256 servers may just reject
sha1 clients; but even so I'd prefer if we could do it with a nice
message (i.e., my bullet one above).

My suggestion does also require that we have a v2 receive-pack protocol,
which does not yet exist (but following the blueprint for fetch, I don't
expect it to be a big deal).


  parent reply	other threads:[~2020-09-30  0:46 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-29  1:17 Question About Git V2 Protocol & SHA256 Wu, Zhichen
2020-09-29 22:13 ` Jeff King
2020-09-29 22:41   ` Wu, Zhichen
2020-09-29 22:43   ` brian m. carlson
2020-09-29 23:07     ` Wu, Zhichen
2020-09-30  0:46     ` Jeff King [this message]
2020-09-30  2:19       ` brian m. carlson
2020-09-30 12:20         ` Jeff King
2020-10-01 23:52           ` brian m. carlson

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