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Date: Thu, 11 Sep 2014 20:27:52 -0400
From: Bill Cox <waywardgeek@...hershed.org>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] A review per day - Lyra2

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On 09/11/2014 01:00 PM, Marcos Simplicio wrote:
>> The value of register "a" is simply hashed into the state values
>> with Blake2s after hashing each 16KiB block.  With alternating
>> multiplies and XORs, all of it reversible, I believe that an
>> attacker will have to duplicate these sequential operations.
>> TwoCats relies entirely on the underlying cryptographic hash to
>> provide irreversibly and solid final hashes.
> 
> I really can't blame your writing as much as my reading, so let's
> be gentlemen and say we are even :)
> 
> Anyhow, I'm not sure you are achieving the goal of forcing the
> attacker to perform the multiplies, at least not always. My point
> is: since you get the most significant portion of the
> multiplication (that is what ">> 32" does, right?), you get a
> biased distribution for the result. Hence, there may be some
> shortcuts here, such as a simple (?) test that eliminates the need
> of doing multiplications because you already now the result by
> looking at (a part of) the inputs.

There may be enough bias for 0 in the MSB of the output to enable a
bit of pre-computation in the next stage with this assumption, but
even the next lower bit has too low predictability to aid in a
speed-up, IMO.  Basically, any pre-compputation based on an assumption
does introduce the speed penalty of the MUX that gets put in to select
between the result given the guess was right or wrong.  Knowing the
MSB is 0 likely would reduce the delay through the multiplier by one
single-bit adder stage, and you might be able to use that to justify a
MUX, but the next bit down is less predictable.

> For the sake of the discussion, take a look at the attached excel
> file: it shows what happens for multiplications between numbers of
> 4 bits. In this scenario, about 30%  of the multiplications will
> give 0 as a result (namely, when both operands are small enough),
> and there is a reasonably high probability that only a few bits on
> the right will be 1. The same file also shows that the least
> significant portion is less biased, while the distribution when one
> at least one of the operands is odd is almost unbiased (the latter
> is actually a result from number theory: when one of the operands
> is co-prime to the modulus, you get all possible values between 0
> and the modulus-1).
> 
> With 32 bits, you will get 0 out of your multiplications much less
> often than 30% of the time, but the bias toward small numbers
> (including 0) will still be there. Is it worthy exploring such bias
> to avoid those multiplications (e.g., using a look-up circuit)? I'm
> not sure, but if that is the case, there will be passwords that can
> be tested with fewer multiplications than usual.
> 
> I apologize if you already have considered this attack venue and
> deemed it unpractical. However, in that case it may be worthy to
> add this discussion to your security analysis to avoid anyone else
> thinking it may be worthy exploring it.
> 
> BTW, is there a good reason why you are using the most significant 
> rather than the least significant bits? The latter seems better 
> considering that it leads to less predictable outputs...

Alexander pointed out that the LSBs of a multiplier can be computed
more quickly than the MSBs, and outlined what looked like about a 2X
faster multiplier that takes advantage of this.  To keep
multiplication chain hardness high, we both decided to use the upper
bits.  I think Alexander uses the whole 64-bit value, but in the
scalar unit, I found it a bit faster to just use the upper bits.

>> Sweet.  The cache-timing defense in Lyra2 is one of the things I
>> like best about it, though I am hoping you will wind up with a
>> better memory access pattern, and I bet you will.  Let me know if
>> you would like me to test pebbling a new memory pattern.
> 
> Please feel free to do so if you wish. The description of the
> pebbling strategy is on http://lyra-kdf.net/Lyra2ReferenceGuide.pdf
> (table 2 has some examples). The idea is to use a step is
> approximately half the window containing all rows that have already
> been initialized. More precisely, for a window of wnd, we have
> step=wnd/2+(-1)^{lg(wnd)}, where wnd is a power of 2 as in the
> original submission (but there the step was always -1).
> 
> Among many alternatives we tested, this led to the best results
> when you consider that we write on the rows we revisit. I'm not
> sure we would beat bit-reversal if we were only reading rows,
> though (that is a merit of Catena that TwoCats seems to have
> employed quite well).

I'm a bit under the gun at the moment, but I surely can find time in
the next couple month to do some more pebbling.  By the way, I deleted
a ton of great comments in your reply above.  It's not that I didn't
feel like they deserve to be re-posted, it's just that I had nothing
to add at the moment.

>> You and I never had a chance to beat Alexander in this category
>> :-)  I thought about adding ROM support just so I could check
>> that box, but I don't know how best to do that.  I'd mess it up,
>> and it would look dorky.  If Yescrypt does not wind up as a
>> winner, perhaps Alexander can help the winning entry add decent
>> ROM support.
> 
> I'm not sure (Alexander certainly grasps the details much better
> than I do), but it seems to me that the same idea that allows more 
> pseudorandomly picked rows to be visited in Lyra2 (again, section
> 6.3) would allow one or more of these rows to come from ROM rather
> than RAM. Probably the same idea could be included in other entries
> too. But again, this is an educated guess...

That would work fine, I think.  However, he gets details like
preferring that speed always increase with increased threads, up to
the max supported by the CPU, to help keep the authentication server
from getting stuck in an inefficient high parallel authentication
state.  I never would have though of that.

>> This is actually something I would love to see in Lyra2.  If
>> there is any concern about entropy loss, this should keep Lyra2
>> secure.  It makes it easier, IMO, to prove the algorithm is
>> secure.  I am biased here, but after reading all three sets of
>> code, Lyra2, IMO, wins at simplicity and elegance, but left me a
>> be worried about its reliance on a reduced-round Blake2b to not
>> lose entropy.  I believe it's not a problem, but why not just
>> hash in the original derived key before returning the result?
> 
> The last squeeze operation is not round-reduced: it does employ the
> full Blake2b to produce the keys. We do not fell comfortable about
> doing only a reduced-round operation here either! Actually, I
> sincerely doubt that two consecutive squeezes at this point would
> result in unrelated outputs.

Nice.  So long as the repeated Blake2b rounds lose insignificant
entropy, that should be fine.

>> First, a full Blake2s Hash round is called after each block is
>> hashed, cryptographically scrambling the state often.  This is
>> the only place memory hashing in TwoCats can lose entropy.
> 
> I'm not sure we agree with the concept of "entropy loss". Entropy
> is measured by how much "disorganized" things are. From that
> perspective, any biased operation leads to entropy loss. To give
> extreme case examples: a logical AND with zeros just wipes out
> every entropy of the system, because it is fully biased toward
> zero; on the other hand, a random oracle preserves entropy, since
> for each entry there will be an unpredictable result (and that is
> why cryptographer like to model hash functions as random oracles
> when analyzing the security of a system).

If we disagree on something like this, then I'm probably wrong.  By
"entropy", I mean the set of reachable values divided by the total
states.  So long as it remains close to 1, there's no major issue.

Blake2b maps 512 bits to an unbiased 512 bits.  If you keep applying
Blake2b over and over, eventually you fall into a cycle regardless of
the starting point.  If you consider all possible inputs, and then
consider only states that are in a cycle, it is reduced quite a bit.
Applying Blake2 or any other cryptographic one-way hash should reduce
the entropy from the full set of possible inputs to just the states in
cycles.

> Then, I would say that the part in which you do a cryptographic
> hash is the only place in TwoCats that you more are likely to _not_
> lose entropy. Was that I typo or I misunderstood something?

When you use a permutation for mixing, you never lose a single
possible state.  If you take all 2^256 input states to TwoCat's ARX
hashing loop, every one of those states maps to a unique output state.
 By my definition of entropy, that means there is zero entropy loss.
Even if my hash function were just the identity, or x + 1, or any lame
permutation, no information is lost.  It makes it to the next call to
Blake2s where any bias, such as the output looking a lot like the
input, is erased, though with some actual entropy loss.

The multiplication loop in TwoCats is not a permutation and the set of
possible output states is smaller than the input states.  However, I
just feed that into Blake2s at the end of the cycle.  Entropy was
preserved by the ARX loop, so the multiplication loop does not have to
be perfect.  It simply has to be hard to skip and difficult to speed up.

There is the possibility that some correlation exists between inputs
to the ARX loop.  XOR does nasty things to correlated inputs :-)
However, all the data feeding the ARX function is isolated by at least
one, and typically many, full Blake2s hashes.  Even if Blake2s is
found to have a weakness, I can easily switch to SHA256, or SHA512,
both of which are compiled in already as options.

>> In contrast, Lyra2 lightly scrambles the state in every
>> operation, with possible entropy loss in memory (but not the
>> sponge).
> 
> I'm not sure I get it. If one round of the underlying function
> loses entropy, then it is unlikely that the whole function will not
> lose entropy. I mean, each round should conserve entropy (and that
> is the idea behind unbiased operations of the ARX design) to ensure
> that the whole function does so too.

I do not try to produce unbiased data in my inner loop.  I just try
not to lose any, and count on the cryptographic hash to even out any
bias.  It's not much different than how we often feed salt + a counter
into a hash function to generate arbitrary key lengths.  Input bias
doesn't matter so long as there is enough entropy there, meaning
enough possible input states.

> If you mean that "Lyra2 leaves traits of the hash function in
> memory", then I fully agree with you. However, I do not see how
> this can be explored to reverse a round or to predict an output (it
> that was easy enough for one round, then it should still be
> reasonably easy for 10 rounds). But this is basically a conjecture
> backed up by the ALRED idea.

That is not a complaint I have about Lyra2.  You do a better job at
making it difficult to detect that memory is in fact Lyra2 data than
most entries, including TwoCats.  I decided to accept this weakness
and focus on speed instead.  Lyra2 gets similar speed per memory
operation, without completely dropping the desire to appear random.

>> 
>> Memory hashing in TwoCats and Yescrypt does not have to be 
>> cryptographically secure.  Our memory hashing output can be
>> highly biased with no loss of security.  All that is required is
>> that the hashing cannot be reversed and does not loose so much
>> entropy that an attacker can skip memory hashing and try to guess
>> it's output correctly.  Guessing the output correctly brute-force
>> for any given password guess would have to be faster than the
>> memory hashing, which is not likely.  The state kept by TwoCats
>> is parallelism times the width of the underlying hash function.
>> It is cryptographically hashed with non-entropy-losing hash data
>> once for every block hashed.  Trying to guess this state at the
>> end would not work out for an attacker trying to improve on 1
>> second  for a 4GiB memory hash.
>> 
>> In reality, I'm sure it would be fine to just return the state.
>> This would be more similar to Lyra2.  However, just for added
>> paranoia, I prefer to re-hash in the derived key at the end.  I
>> don't think this is a "real" issue.  It's more like why we prefer
>> to choose "safe" primes when hard-coding primes for Duffie-Helman
>> key exchange.  It's just one less thing for the public to worry
>> about.
> 
> 
> I do agree with this view that the middle of the memory hashing
> between the start and end of the algorithm does not need to be
> cryptographically secure in the sense of avoiding collisions and
> even second pre-image. Resistance to first pre-image (i.e., no
> reversibility) and "having no shortcuts" (e.g., cycles) do seem to
> be the main requirements here. I'm just not fully convinced that
> these two requirements are met with something that does not try to
> be unbiased and, thus, in the long run create something that would
> look like a hash.
> 
> That does not mean at all that TwoCats or Yescrypt are insecure.
> Your choice of "multiplies <=8", for example, does give me a
> certain confidence to say that the simple operations employed in
> TwoCats are unlikely to allow anyone to skip the whole
> multiplication chain too often, because if the attacker gets even
> one bit wrong that will mess up the full hash computation that
> comes right afterward. All I'm saying is that they need a more
> careful analysis to see how often one can run into trouble.

One reason I chose a 128 bit state for the multiplication loop is that
I was able to find rare cases where the state goes to 0 and stays
there when I only used 64 bits.  This is not a real problem, since it
happened so rarely, but it bothered me a bit.  Thus the 128 bit state.

TwoCat's multiplication loop does produce bias and entropy loss, but
that bias and loss in no way impacts the entropy or bias of the
Blake2s output in the next block.

>> When all the bells and whistles get added, and if I can brow-beat
>> you into offering fewer read/writes per memory location, I am
>> confident any of the three would be a real winner.
> 
> Well, most of the credit goes to our "parallelism guy" (Paulo
> Carlos, to be exact), but at least you have the right to say "I
> told you so" :)
> 
> 
> Marcos.
> 
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