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Date: Mon, 04 May 2015 18:42:05 -0300
From: Marcos Simplicio <mjunior@...c.usp.br>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Maximising Pseudo-Entropy versus resistance to Side-Channel
 Attacks

On 04-May-15 14:36, Krisztián Pintér wrote:
> 
> Solar Designer (at Thursday, April 30, 2015, 5:55:05 PM):
> 
>> While you're mostly correct, you're somehow omitting important detail.
>> As discussed before, salting may defeat side-channel attacks on password
>> hashes, as long as the salts are not known/predictable by the attacker.
> 
> this wording is actually accurate, because even secret salts does not
> guarantee safety against side channel attacks. they *may* defeat them,
> but may not.
> 
> side channel attacks, obviously, have two phases: 1, gathering data
> that is in some relation with the secret and 2, evaluating said data
> to gain knowledge about secret.
> 
> the question now is the second phase. can you simply calculate the
> secret from the gathered data? or you can just exclude some
> candidates? establish constraints?
> 
> for example if we can reduce the search space significantly, like for
> example by 50 bit, it can be a break against regular passwords,
> reducing a strong 60 bit passphrase to a trivial 10 bit. however,
> with 128 bit unknown salt, this is still unbreakable 138 bit.
> 
> but what if the attack is stronger? what if we can actually calculate
> the secret precisely from the acquired data? in this case, we can
> simply infer both the password and the salt at the same time.
> 
> this latter scenario is made more "probable" by the fact that we emit
> a *lot* of data during the processing. this is what password hashing
> is about: filling megabytes of memory with secret-dependent data, and
> doing billions of operations. if at all possible to do exact secret
> recovery without brute force, we can expect password hashing be the
> most susceptible of all use cases.
> 
> i understand that this scenario still isn't exactly "probable". and
> using irreversible mixing functions early can thwart such attempts.
> however for example lyra2 or pomelo, i would not tell at a glance
> whether they can be traced backwards if the internal state is
> partially recovered.
> 

Regarding this last paragraph: if r rounds of the underlying hash
function are invertible, then r iterations of Lyra2 can be inverted
basically with the same effort, as each iteration does 1 round of
Blake2. With Blake2, after r=10 this should be computationally unfeasible.

Anyhow, recovering the password should be infeasible from the start: the
attacker needs to invert the initial (full) hash to get it, and this
happens much before any Lyra2-specific password-dependent operation
starts (i.e., much before the Wandering phase)... Well, that unless you
mean that the internal state recovered is one from the first absorb
operation performed, in which case you are talking about the
side-channel resistance of Blake2's permutation, not Lyra2's.

Marcos.

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