Re: [bitcoin-dev] SHA1 collisions make Git vulnerable to attakcs by third-parties, not just repo maintainers

[Aymeric Vitte <vitteaymeric@gmail.com>]

??? apparently we are not discussing the same thing

Maybe I did not provide the right links (reading them again I myself
don't find them so clear), see maybe again
https://github.com/whatwg/streams/issues/33#issuecomment-28045860

a - b - c -d

hash(a)

hash(a+b)

etc

But you are not going to rehash from the beginning, then:

update a --> keep the remaining bytes a_ (+ hash state 1) --> digest
a=hash(a)

update a_+b from hash state 1--> keep the remaining bytes b_ (+ hash
state 2) --> digest a_+b=hash(a+b)

etc

Basically that's similar to a real time progressive hash of chunks of a
file that you are streaming and therefore don't know what will come next
(per opposition to hashing a file that you already have), this could
apply to trees

This is different from something like:

hash(a)

hash(hash(a) +hash(b))

etc

There is no initial state, and the attacker can't modify what was
already hashed, to make it more difficult you can probably modify the
hash state N


Le 24/02/2017 à 17:30, Tim Ruffing via bitcoin-dev a écrit :
> On Fri, 2017-02-24 at 16:18 +0100, Aymeric Vitte via bitcoin-dev wrote:
>> Not sure that you really read deeply what I sent, because stating
>> that
>> hashing files continuously instead of hashing the intermediate steps
>> just gives more latitude to the attacker can't be true when the
>> attacker
>> has absolutely no control over the past files
> What prevents the attacker to provide different past files when talking
> to parties who are still in the initial state?
>
> Then the question is: knowing the hash state, is it as easy to find a
>> collision between two files that will be computed in the next round
>> than
>> finding a collision between two files only?
> With the original usage of the hash function, the hash state is always
> the initial state. Now that the attacker has some control over the hash
> state even. In other words, if the original use of the hash function
> was vulnerable, then your scheme is vulnerable for the initial state.
>
> Concrete attack: If you can find x != y with H(x) = H(y), then you can
> also find m, x != y, with H(m||x) = H(m||y), just by setting m = "". 
>
> Not sure if this is the right place to discuss that issue though...
>
> Best,
> Tim
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