phc-discussions - Re: [PHC] Tortuga issues

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Date: Thu, 03 Apr 2014 04:03:23 -0700
From: Jeremi Gosney <epixoip@...dshell.nl>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Tortuga issues

On 4/2/2014 9:26 PM, Bill Cox wrote:
> Tortuga fails on both windows and Linux for > 1MiB m_cost, due to
> allocating hashing memory on the stack.  

Just a heads-up, the optimized implementation of Pufferfish has this
`issue' as well, as it calls alloca() to dynamically allocate the sbox
buffers on the stack. The reference implementation allocates memory on
the heap with calloc() so this is not a problem there, but you'll blow
out the stack on the optimized implementation if using an m_cost > 10
(it doesn't "go to 11.")

And yes, this was done intentionally. Since it is unlikely that anyone
will be using an m_cost > 10, it's a mostly-safe optimization
(especially for attackers, which is largely what the optimized
implementation was, rewriting the algorithm from an attacker's perspective.)

For optimized defender code, where one might just be crazy enough to use
an m_cost of 11, there might be some benefit in writing a custom malloc
implementation that can quickly allocate heap memory without the
unnecessary overhead, not unlike JTR's mem_calloc_tiny(). But I think
this is implementation-specific detail that is outside the scope of the
PHC. Ideally implementers should be coding to the reference
implementation and making their own optimizations, using the optimized
code only as, erm, a reference.