Hi Achim,
See below for a comment on your analysis.
-----Original Message-----
From: TLS <tls-boun...@ietf.org> on behalf of Achim Kraus <achimkr...@gmx.net>
Date: Monday, 9 September 2019 at 09:24
To: "tls@ietf.org" <tls@ietf.org>
Subject: [TLS] RFC 7457, Lucky 13, mitigation, DTLS 1.2
RFC 7457, Lucky 13, mitigation, DTLS 1.2
Dear List,
currently I try to do some investigation about the simplest way to
mitigate the “lucky 13” attack without using RFC 7366.
Therefore I read the slides in [1] and also the recommended mitigation
in [2], which is cited in RFC 7457.
From the slides, my impression is, that the “defined padding & padding
check” was used to reduce the “timing side channel” of MAC depending on
the data fragment size. Lucky 13 demonstrates, that this “defined
padding” could be tricked out.
The recommended mitigation in [2] describes on page 539 to do,
a) the padding check “time side channel” free by using always “256 compares”
b) and the MAC check “time side channel” free, by adjust the number of
compression function evaluations with extra evaluations on dummy data to
achieve always the same number of evaluations.
FMPOV b) is the one, which closes the “time side channel”.
But a) seems to be more a left over. It doesn’t protect enough, as lucky
13 shows, and complicated algorithms, as “always 256 compares” even on
shorter messages, may harm more.
So, why should that “defined padding” check be done, if b) is applied?
Wouldn’t a simple check, if the padding length exceeds the amount of
data, and on failure, set it to 0, simplify the mitigation?
That would still leave a timing side channel which would reveal whether the
padding length exceeds the amount of data. I'd need to dig into it more to be
certain, but my sense is that such a side channel could be turned into at least
a partial plaintext recovery attack, and possibly a full plaintext recovery
attack.
You might want to read Adam Langley's account of how L13 was addressed in
OpenSSL:
https://www.imperialviolet.org/2013/02/04/luckythirteen.html
Regards,
Kenny
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