Re: [PATCH] lib: memcmp_nta: add timing-attack secure memcmp

From: Daniel Borkmann
Date: Mon Feb 11 2013 - 18:28:40 EST


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
> name.

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

>> + 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);
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