Git Mailing List Archive mirror
 help / color / mirror / code / Atom feed
From: "brian m. carlson" <>
To: Felipe Contreras <>
Cc: Jeff King <>, Junio C Hamano <>,, Adam Majer <>
Subject: Re: Is GIT_DEFAULT_HASH flawed?
Date: Wed, 3 May 2023 22:54:17 +0000	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <6451a0ba5c3fb_200ae2945b@chronos.notmuch>

[-- Attachment #1: Type: text/plain, Size: 3096 bytes --]

On 2023-05-02 at 23:46:02, Felipe Contreras wrote:
> In my view one repository should be able to have part SHA-1 history,
> part SHA3-256 history, and part BLAKE2b history.

That is practically very difficult and it means that it's hard to have
confidence in the later history because SHA-1 is weak and you have to
rely on it to verify the SHA-256 history later.  Since attacks always
get better, SHA-1 will eventually be so weak that collisions can be
computed in the amount of time we now take for MD4 or MD5 collisions
(i.e., seconds), and with your plan, we'd have to retain that history
forever with the resulting lack of confidence in part of the history.

This also doesn't work with various structures like trees, the index,
and pack and index formats, which have no indication of the algorithm
used and simply rely on fixed-size, often 4-byte aligned object IDs
without any metadata.  In addition, the internals of the code often
don't pass around enough data to make these values variable and thus
this approach would substantially complicate the code in many ways.

Also, we've already decided on the current design a long time ago with
the transition plan after extensive, thoughtful discussion by many
people.  Very few people other than me have worked on sending patches to
work on the hash function transition, and that work up to now has all
been done on my personal time, without compensation of any sort, out of
a desire to improve the project.  Lots of people have opined on how it
should have been different without sending any patches.

If you would like to propose patches for the extensive amount of work to
implement your solution, then we could consider them, although I will
warn you that your approach will likely require at least several hundred
patches.  However, I refer you to the list archives to determine why
your approach is not the one we chose and is not, in my view, the best
path forward.  I should also be clear that I have no intention of
submitting patches to change our approach now or in the future, or
redoing the patches I've already sent.

> The fact that apparently it's so easy to clone a repository with
> the wrong hash algorithm should give developers pause, as it means the
> whole point of using cryptographic hash algorithms to ensure the
> integrity of the commit history is completely gone.

No, it doesn't.  It means that our empty repositories until recently
lacked any indication of the algorithm or other capabilities, which was
a mistake in our original protocol design that has now been corrected.

If you interact with the repository later on when it has data, then if
you're using the wrong hash algorithm, you'll find that you get a
helpful error message that that's not yet supported.  If you patched Git
to ignore that check, you'd find that your repository would just be very
broken in many ways with lots of random crashing and seemingly unrelated
error messages instead of subtly using the wrong algorithm.
brian m. carlson (he/him or they/them)
Toronto, Ontario, CA

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 263 bytes --]

  parent reply	other threads:[~2023-05-03 22:54 UTC|newest]

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-04-05 10:28 git clone of empty repositories doesn't preserve hash Adam Majer
2023-04-05 19:04 ` Junio C Hamano
2023-04-05 19:47   ` Adam Majer
2023-04-05 20:01     ` Jeff King
2023-04-05 20:40       ` Junio C Hamano
2023-04-05 21:15         ` Junio C Hamano
2023-04-05 21:26           ` Jeff King
2023-04-05 22:48           ` brian m. carlson
2023-04-06 13:11           ` Adam Majer
2023-04-25 21:35           ` brian m. carlson
2023-04-25 22:24             ` Junio C Hamano
2023-04-25 23:12             ` Junio C Hamano
2023-04-26  0:20               ` brian m. carlson
2023-04-26 11:25                 ` Jeff King
2023-04-26 15:08                   ` Junio C Hamano
2023-04-26 15:13                     ` [PATCH] doc: GIT_DEFAULT_HASH is and will be ignored during "clone" Junio C Hamano
2023-04-26 21:06                       ` brian m. carlson
2023-04-27  4:46                     ` git clone of empty repositories doesn't preserve hash Jeff King
2023-04-26 10:51               ` Jeff King
2023-04-26 15:42                 ` Junio C Hamano
2023-04-26 20:40                 ` brian m. carlson
2023-04-26 20:53                   ` [PATCH 0/2] Fix empty SHA-256 clones with v0 and v1 brian m. carlson
2023-04-26 20:53                     ` [PATCH 1/2] http: advertise capabilities when cloning empty repos brian m. carlson
2023-04-26 21:14                       ` Junio C Hamano
2023-04-26 21:28                         ` brian m. carlson
2023-04-27  5:00                           ` Jeff King
2023-04-27  5:30                       ` Jeff King
2023-04-27 20:40                         ` Junio C Hamano
2023-04-26 20:53                     ` [PATCH 2/2] Honor GIT_DEFAULT_HASH for empty clones without remote algo brian m. carlson
2023-04-26 21:18                       ` Junio C Hamano
2023-04-26 21:33                       ` Junio C Hamano
2023-04-27  5:43                         ` Jeff King
2023-05-02 23:46                           ` Is GIT_DEFAULT_HASH flawed? Felipe Contreras
2023-05-03  9:03                             ` Adam Majer
2023-05-03 15:44                               ` Felipe Contreras
2023-05-03 17:21                                 ` Adam Majer
2023-05-08  0:34                                   ` Felipe Contreras
2023-05-03  9:09                             ` demerphq
2023-05-03 18:20                               ` Felipe Contreras
2023-05-03 22:54                             ` brian m. carlson [this message]
2023-05-08  2:00                               ` Felipe Contreras
2023-05-08 21:38                                 ` brian m. carlson
2023-05-09 10:32                                   ` Oswald Buddenhagen
2023-05-09 16:47                                     ` Junio C Hamano
2023-04-26 21:12                     ` [PATCH 0/2] Fix empty SHA-256 clones with v0 and v1 Junio C Hamano
2023-04-27  4:56                   ` git clone of empty repositories doesn't preserve hash Jeff King
2023-05-01 17:00                   ` [PATCH v2 0/1] Fix empty SHA-256 clones with v0 and v1 brian m. carlson
2023-05-01 17:00                     ` [PATCH v2 1/1] upload-pack: advertise capabilities when cloning empty repos brian m. carlson
2023-05-01 22:40                       ` Jeff King
2023-05-01 22:51                         ` Junio C Hamano
2023-05-01 17:37                     ` [PATCH v2 0/1] Fix empty SHA-256 clones with v0 and v1 Junio C Hamano
2023-05-17 19:24                   ` [PATCH v3 " brian m. carlson
2023-05-17 19:24                     ` [PATCH v3 1/1] upload-pack: advertise capabilities when cloning empty repos brian m. carlson
2023-05-17 21:48                     ` [PATCH v3 0/1] Fix empty SHA-256 clones with v0 and v1 Junio C Hamano
2023-05-17 22:28                       ` brian m. carlson
2023-05-18 18:28                     ` Jeff King
2023-05-19 15:32                       ` brian m. carlson
2023-04-05 21:23         ` git clone of empty repositories doesn't preserve hash Jeff King

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
Code repositories for project(s) associated with this public inbox

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).