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

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Date:	Mon, 11 Feb 2013 20:39:20 +0100
From:	Daniel Borkmann <dborkman@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	gregkh@...uxfoundation.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib: memcmp_nta: add timing-attack secure memcmp

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@...hat.com>
>> ---
>
> 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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