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From: Emil Lenngren <emil.lenngren@gmail.com>
Date: Sun, 11 Jun 2017 23:30:33 +0200
Message-ID: <CAO1O6see6yDr-hDVNPi9Kv9wO6p-98w7YhsnnwnVOVCSEGVksw@mail.gmail.com>
Subject: Re: [PATCH 0/6] Constant Time Memory Comparisons Are Important
To: 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>,
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	Anna Schumaker <anna.schumaker@netapp.com>, David Howells <dhowells@redhat.com>,
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	Gilad Ben-Yossef <gilad@benyossef.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Gustavo Padovan <gustavo@padovan.org>, "J. Bruce Fields" <bfields@fieldses.org>,
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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.