Date: Mon, 6 Jan 2014 17:28:03 -0500
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Security concern for Catena

On Mon, Jan 6, 2014 at 10:15 AM, Christian Forler > I hope not, because I
just switched NoelKDF to use something very
>
>  > similar to Catena 2 to eliminate timing attacks.  If Catena 2 has a
> > weakness, I would appreciate hearing about it.
>
> Catena-\lambda, and especially Catena-2, is *NOT*
> sequential-memory-hard. So you do always benefit from multiple cores.
> For c cores you can theoretically speed up the performance to a factor
> of O(c^(1/\lambda+1)). So, for c=1,000,000  you can speedup the
> computation of Catena-2 by a factor of 100.
>
> I'm mot sure if this is either acceptable or not. What do you think?
>

By "speeding up", I assume you mean for an attacker who wants to use less
memory than there are nodes in the graph.  Otherwise, since every hash has
to be computed sequentially, more cores wont help.  Because of the ability
to pre-compute data many cycles in advance due to the fixed memory read
pattern, you can get about the same speed with Catena-1 using 1% of the
memory and 100 cores as with using full memory and 1 core.  For Catena-2,
an attacker with 1% of the memory needs 10,000 cores to get the same speed.
 I don't know if Catena-3 offers enough security benefit to warrant the
extra delay in the algorithm vs Catena-2, so I'll put in a vote for
Catena-3 rather than Catena-4.  This is perfectly acceptable behavior, IMO.

Your proof is really helping me think about my NoelKDF, which now has
Catena-inspired password-independent memory access patterns.  When it
depended on the password, it was easy to show that it was sequential hard,
and in fact better off than scrypt in terms of the benefit an attacker can
gain with more cores and less memory.  This ability to get cores working
ahead of when their data will be needed makes it much harder to think about.

The simplified loop for NoelKDF for m memory locations looks like:

v(0) = H(x)
for(i = 0; i < m-1; i++)
    j = H(i) % (i+1)
    v(i+1) = H( v(i), v(j) )
output v(m-1)

This creates a random-ish DAG where like Catena, every node points to the
prior node , and there's an edge pointing to another random-ish node.

The weak hash function potentially introduces vulnerabilities, but let's
assume H is solid for this analysis.  In NoelKDF, partition memory into 4
equal size sequential groups g1...g4.  In this case over 1/4th of the edges
from group g4 point to g3.  If the memory size is M, then g4 has M/4 nodes
and 1/4th of them point to g3, so M/16 edges exist between g3 and g4.  The
edges between the other adjacent groups will be at least this high.

This subgraph forms a Catena-3 graph with M/16 nodes in each row!  Well,
almost.  The edges are random rather than bit-reversal ordered.  Does this
matter?  Therefore, I can say NoelKDF has at least as strong resistance to
TMTO as Catena-3 when using 8X as much memory?  An attacker trying to use
less than 1/8th the memory will get hammered with extra computational
effort, just like Catena-3.

This still doesn't take into account 3/4ths of the edges, though.  It's
nice to start with a good lower bound.

Bill

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