Subject: Re: [PATCH 0/6] Constant Time Memory Comparisons Are Important
To: Emil Lenngren <emil.lenngren@gmail.com>,
        Emmanuel Grumbach <egrumbach@gmail.com>
Cc: Kees Cook <keescook@chromium.org>,
        Kalle Valo <kvalo@codeaurora.org>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "kernel-hardening@lists.openwall.com"
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From: Arend van Spriel <arend.vanspriel@broadcom.com>
Message-ID: <5595d4bc-a43e-7a89-eb86-fa77556cd7af@broadcom.com> (sfid-20170612_093411_447427_741273E8)
Date: Mon, 12 Jun 2017 09:33:24 +0200
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On 6/11/2017 11:30 PM, Emil Lenngren wrote:
> 2017-06-11 22:48 GMT+02:00 Emmanuel Grumbach <egrumbach@gmail.com>:
>> On Sun, Jun 11, 2017 at 4:36 PM, Kees Cook <keescook@chromium.org> wrote:
>>>
>>> On Sun, Jun 11, 2017 at 1:13 AM, Kalle Valo <kvalo@codeaurora.org> wrote:
>>>> "Jason A. Donenfeld" <Jason@zx2c4.com> writes:
>>>>
>>>>> Whenever you're comparing two MACs, it's important to do this using
>>>>> crypto_memneq instead of memcmp. With memcmp, you leak timing information,
>>>>> which could then be used to iteratively forge a MAC.
>>>>
>>>> Do you have any pointers where I could learn more about this?
>>>
>>> While not using C specifically, this talks about the problem generally:
>>> https://www.chosenplaintext.ca/articles/beginners-guide-constant-time-cryptography.html
>>>
>>
>> Sorry for the stupid question, but the MAC address is in plaintext in
>> the air anyway or easily accessible via user space tools. I fail to
>> see what it is so secret about a MAC address in that code where that
>> same MAC address is accessible via myriads of ways.
> 
> I think you're mixing up Media Access Control (MAC) addresses with
> Message Authentication Code (MAC). The second one is a cryptographic
> signature of a message.

While this may be obvious to those who are in the know this mixup is 
easily made outside the crypto domain and especially in the (wireless) 
networking domain (my mind wandered towards the same error path). As 
this series is touching stuff outside crypto it is good to be explicit 
and not use such abbreviations that can be misinterpreted. The article 
Kees referred to is also useful to get into the proper context here and 
at least worth mentioning this or other useful references in the cover 
letter.

Regards,
Arend