Re: [PATCH] lib: memcmp_nta: add timing-attack secure memcmp
From: Florian Weimer
Date: Tue Feb 12 2013 - 05:23:42 EST
* Daniel Borkmann:
> On 02/11/2013 08:00 PM, Florian Weimer wrote:
>> * Daniel Borkmann:
> Thanks for your feedback, Florian!
>>> + * memcmp_nta - memcmp that is secure against timing attacks
>> It's not providing an ordering, so it should not have "cmp" in the
> I agree. What would you suggest? Probably, it would make sense to
> integrate this into the Linux crypto API and name it sth like ...
> crypto_mem_verify(const void *,const void *,__kernel_size_t)
> ... which returns:
> == 0 - mem regions equal each other
> != 0 - mem regions do not equal each other
crypto_mem_equal or crypto_mem_equals should be fine. Or anything
else which matches an existing function name with similar function.
>>> + for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--)
>>> + res |= (*su1 ^ *su2);
>> The compiler could still short-circuit this loop. Unlikely at
>> present, but this looks like a maintenance hazard.
> So then better we leave out '|' as a possible candidate and rewrite it as:
> + for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--)
> + res += (*su1 ^ *su2);
That will cause false matches for long inputs.
If we had only four platforms to support, I would write this function
in assembler because it will be considerably easier to read.
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