Gentoo Archives: gentoo-scm

From: Mike Frysinger <vapier@g.o>
To: gentoo-scm@l.g.o
Subject: [gentoo-scm] thin manifests
Date: Thu, 25 Aug 2011 05:05:48
Message-Id: 201108250023.41079.vapier@gentoo.org
In Reply to: [gentoo-scm] Re: [gentoo-dev] Progress on cvs->git migration by "Robin H. Johnson"
1 On Monday, August 22, 2011 15:28:57 Robin H. Johnson wrote:
2 > Unresolved items:
3 > - commit signing
4 > - thin Manifests
5
6 how exactly are these two supposed to interact ? the previous discussion
7 seemed to miss signing. if devs sign the thin manifests, when we go to
8 produce the full manifest for rsync, we invalidate the signature.
9
10 also, a previous assertion was made which i think is incorrect:
11 Due to the distributed nature of git, to do mischief, you need to
12 change every clone in the world to be successful
13 each new sha1 comes from the previous state + new data. so injecting code
14 into the tip and finding a collision is not impossible and does not require
15 modification of anything before it. it would only be detected automatically
16 by people who have the original commit, make new commits on top of that, and
17 then attempt to push back again to the modified tree. i.e. the attack is made
18 against the source Gentoo repo sitting on our machines.
19
20 the other attack we want to prevent is MITM when people sync. in this case,
21 someone who syncs over git:// is perpetually vulnerable with thin manifests as
22 the attacker can keep recomputing the collisions so that the modified tree
23 keeps ending up with the same digests as the public one. and the end user
24 never notices without manually reviewing everything themselves.
25
26 further, it was stated:
27 This has nothing to do with strength of the hash used by git
28 well, it sort of does. sha1 has been shown to be weaker than brute forcing,
29 and while right now it might not be computationally feasible to inject useful
30 code in realtime, that is not something we should be betting on. attacks only
31 get better over time ... even in 2004 security conscious people started
32 talking about migrating away from it. and now in 2012, we want to talk about
33 migrating purely to it ?
34 -mike

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Replies

Subject Author
Re: [gentoo-scm] thin manifests "Robin H. Johnson" <robbat2@g.o>