[cryptography] GOST attack

Jean-Philippe Aumasson jeanphilippe.aumasson at gmail.com
Tue Jun 14 13:47:36 EDT 2011


On Tue, Jun 14, 2011 at 7:13 PM, Nico Williams <nico at cryptonector.com> wrote:
> On Tue, Jun 14, 2011 at 7:31 AM, Jean-Philippe Aumasson
> <jeanphilippe.aumasson at gmail.com> wrote:
>
> (...)
>
> It is not reasonable to consider an attack with a 2^228 work factor as
> breaking a cipher, nor is it reasonable to say that because this 2^28
> times faster than a brute force attack that this is a break (also, the
> 2^64 storage requirement means that this attack is only ~2^23 times
> faster than brute force, because the random access to that storage
> won't be free).  Perhaps that's a typo and the author meant 2^28?
> *That* would be a break, even with a 2^64 storage requirement.  But
> skimming the paper it does not seem to be a typo.

Your argument perfectly makes sense. However, academic attack models
generally consider a cipher as broken as soon as an attack is found
that contradicts the claim of X-bit security, even when the reduction
of complexity is practically insignificant. When the attack requires a
non-negligible amount of memory, yet does fewer than 2^X
"computations", it can be difficult to evaluate whether it really
outperforms (in theory) bruteforce methods.


>
> For me the problem with GOST is its block size.  I would much prefer a
> 128-bit block size for reasons having to do with re-key
> considerations.
>



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