Re: [PATCH 0/6] Constant Time Memory Comparisons Are Important

[Emmanuel Grumbach]

On Mon, Jun 12, 2017 at 12:30 AM, Emil Lenngren <emil.lenngren@xxxxxxxxx> wrote:
> 2017-06-11 22:48 GMT+02:00 Emmanuel Grumbach <egrumbach@xxxxxxxxx>:
>> On Sun, Jun 11, 2017 at 4:36 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>>
>>> On Sun, Jun 11, 2017 at 1:13 AM, Kalle Valo <kvalo@xxxxxxxxxxxxxx> wrote:
>>> > "Jason A. Donenfeld" <Jason@xxxxxxxxx> 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.

Obviously... Sorry for the noise.