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Re: [TLS] Hybrid key exchange in TLS 1.3 and variable-length secrets

Nimrod Aviram <nimrod.aviram@gmail.com> Fri, 25 September 2020 08:47 UTC

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From: Nimrod Aviram <nimrod.aviram@gmail.com>
Date: Fri, 25 Sep 2020 11:47:38 +0300
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To: Douglas Stebila <douglas@stebila.ca>
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Subject: Re: [TLS] Hybrid key exchange in TLS 1.3 and variable-length secrets
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Thanks!
The PR is here, happy to hear comments and corrections:
https://github.com/dstebila/draft-ietf-tls-hybrid-design/pull/1

best,
Nimrod


On Fri, 18 Sep 2020 at 12:04, Nimrod Aviram <nimrod.aviram@gmail.com> wrote:

> Sounds good to me.
> I'm happy to send a PR making these changes, but couldn't find the
> repository for the document.
> Could you please point me to it?
>
> best,
> Nimrod
>
>
> On Thu, 17 Sep 2020 at 17:12, Douglas Stebila <douglas@stebila.ca> wrote:
>
>> Given that all the finalists and alternate candidates have fixed
>> length shared secrets, and your observations on the potential for
>> timing attacks, I'm fine with dealing with only fixed length secrets,
>> removing the paragraph discussing the possibility for variable-length
>> shared secrets from the TLS 1.3 hybrid KEX draft, and adding a note
>> regarding the risks as identified by the Raccoon attack.
>>
>> Douglas
>>
>>
>> On Wed, Sep 16, 2020 at 12:27 PM Nimrod Aviram <nimrod.aviram@gmail.com>
>> wrote:
>> >
>> > Dear all,
>> >
>> > We are writing to ask about the possible security impact of
>> > variable-length secrets on the "Hybrid key exchange in TLS 1.3" RFC
>> > [1].
>> >
>> > As you probably know, when using key material of variable length
>> > and processing this material using hash functions, a timing side
>> > channel may arise. In broad terms, when the secret is longer, the hash
>> > function may need to process more blocks internally. In some unfortunate
>> > circumstances, this has led to timing attacks, e.g. Lucky Thirteen [2]
>> > and the newly-disclosed Raccoon Attack [3]. To be clear, we are not
>> > aware of any attack on the proposed standard. Rather, we view this as
>> > an opportunity to defend-in-depth against such attacks, while work on
>> > the standard is in progress.
>> >
>> > Our proposal is to add language to the RFC explaining that
>> variable-length
>> > secrets may enable such attacks, and should therefore be avoided when
>> > possible.  Currently, the language seems to allow for variable-length
>> > secrets, should the need arise:
>> > "Variable-length shared secrets ... if it is envisioned that this
>> specification
>> > be used with algorithms which do not have fixed-length shared secrets
>> ..."
>> >
>> > We also note that a related RFC exists, "Hybrid Post-Quantum Key
>> > Encapsulation Methods (PQ KEM) for Transport Layer Security 1.2"
>> > [4]. However, that RFC apparently only uses BIKE, Kyber or SIKE as the
>> > PQ KEM. To our knowledge, all three KEMs have fixed-length secrets. It
>> > may be prudent to add cautionary language to that document as well,
>> > in case other KEMs are used in the future.
>> >
>> > Kind regards,
>> > The Raccoon Team
>> >
>> > [1]
>> https://www.ietf.org/id/draft-ietf-tls-hybrid-design-00.html#name-open-questions
>> > [2] https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6547131
>> > [3] https://raccoon-attack.com/
>> > [4]
>> https://tools.ietf.org/id/draft-campagna-tls-bike-sike-hybrid-05.html#name-key-exchange-algorithms
>> >
>> >
>> > _______________________________________________
>> > TLS mailing list
>> > TLS@ietf.org
>> > https://www.ietf.org/mailman/listinfo/tls
>>
>

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