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

[Daniel Borkmann]

On 02/11/2013 07:37 PM, Andy Lutomirski wrote:
> On 02/10/2013 02:00 PM, Daniel Borkmann wrote:
> > If you need to compare a password or a hash value, the timing of the
> > comparison function can give valuable clues to the attacker. Let's
> > say the password is 123456 and the attacker tries abcdef. If the
> > comparision function fails at the first byte without looking at the
> > other bytes, then the attacker can measure the difference in runtime
> > and deduce which byte was wrong, reducing the attack space from
> > exponential to polynomial. [Daniel J. Bernstein]
> >
> > Therefore add memcmp_nta ({n}o {t}iming {a}ttacks) in order to avoid
> > such scenarios and to facilitate development by providing a generic
> > function for (e.g.) the crypto and networking subsystems.
> >
> > Signed-off-by: Daniel Borkmann <dborkman@xxxxxxxxxx>
> > ---
>
> I read this as "compare memory with non-temporal access".  Perhaps
> something like "memcpy_constant_time" would be less confusing.

You probably mean "memcmp_constant_time".

Well, this could probably be misinterpreted, that for every possible input
it will take only O(1), which of course it doesn't. It's simply that for both
results (``equals to'', ``does not equal to'') it will take the same amount of
*operations* to achieve this in order to not leak any time information of a
successful or not successful comparison, where the attacker could draw
conclusions if he might have gotten parts of the hash/key/.. right or wrong.
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