Maybe not, unlike frozen objects (certificates, etc), trees are supposed
to extend

Then you can perform progressive hash operations on the objects, ie
instead of hashing the intermediate hash of the objects you do it
continuously (ie instead of hashing the hash of hash file a + hash file
b + hash file c, wait for file d and then do the same, instead hash(file
a + file b + file c), when d comes compute the hash of (file a + file b
+ file c + file d), which implies each time to keep the intermediary
hash state because you are not going to recompute everything from the
beginning)

I have not worked on this since some time, so that's just thoughts, but
maybe it can render things much more difficult than computing two files
until the same hash is found

The only living example I know implementing this is the Tor protocol,
fact apparently unknown, this is probably why nobody cares and nobody is
willing to take it into account (please follow bwd/fwd [1] and see [2]),
this is not existing in any crypto implementations, unless you hack into
it, and this applies to progressive encryption too

[1]
https://lists.w3.org/Archives/Public/public-webcrypto-comments/2013Feb/0018.html


[2] https://github.com/whatwg/streams/issues/33#issuecomment-28554151


Le 23/02/2017 à 22:28, Peter Todd via bitcoin-dev a écrit :
> On Thu, Feb 23, 2017 at 01:14:09PM -0500, Peter Todd via bitcoin-dev wrote:
>> Worth noting: the impact of the SHA1 collison attack on Git is *not* limited
>> only to maintainers making maliciously colliding Git commits, but also
>> third-party's submitting pull-reqs containing commits, trees, and especially
>> files for which collisions have been found. This is likely to be exploitable in
>> practice with binary files, as reviewers aren't going to necessarily notice
>> garbage at the end of a file needed for the attack; if the attack can be
>> extended to constricted character sets like unicode or ASCII, we're in trouble
>> in general.
>>
>> Concretely, I could prepare a pair of files with the same SHA1 hash, taking
>> into account the header that Git prepends when hashing files. I'd then submit
>> that pull-req to a project with the "clean" version of that file. Once the
>> maintainer merges my pull-req, possibly PGP signing the git commit, I then take
>> that signature and distribute the same repo, but with the "clean" version
>> replaced by the malicious version of the file.
> Thinking about this a bit more, the most concerning avenue of attack is likely
> to be tree objects, as I'll bet you you can construct tree objs with garbage at
> the end that many review tools don't pick up on. :(
>
>
>
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