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From: Kees Cook <keescook@chromium.org>
Date: Sun, 11 Jun 2017 06:36:27 -0700
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Subject: Re: [PATCH 0/6] Constant Time Memory Comparisons Are Important
To: Kalle Valo <kvalo@codeaurora.org>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>,
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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

-Kees

-- 
Kees Cook
Pixel Security