I am not convinced that the extra effort is justified.
However, I am convinced that the proposed construction is complex.
combined_key = H(HMAC(key=H1(k1), data=2||F(k2)) xor HMAC(key=H2(k2),
data=1||F(k1)))
H1(k) = H('derive1' || k)
H2(k) = H('derive2' || k)
F(m) =
H(0||m1)||H(1||m1)||...||H(j-1||m1)||H(0||m2)||H(1||m2)||...||H(j-1||m2)||H(0||mn)||H(1||mn)||...||H(j-1||mn)
for m = m1||m2||...||mn and j =~ 3
It's nice that this is a dual PRF; that's something I think we've wanted for a
number of other reasons in TLS. I might have preferred a more efficient option
though.
Comparing that to k1 || k2 means - for me - this needs much stronger
justification.
Perhaps if the CFRG were to standardize a dual (or multi) PRF that were more
efficient I would be more favourably inclined toward its inclusion - in a
revision of the core specification.
The nice thing about the hybrid draft is that it isn't a firm commitment to any
particular combination method. Each new key exchange "group" can define its
own combination method. It only suggests a method. So I don't agree that
"[m]issing this opportunity would effectively further embed the problem" (or
maybe "effectively" is doing a little too much work there).
On Wed, Jan 19, 2022, at 22:21, Nimrod Aviram wrote:
> Hi Everyone,
>
>
> As Douglas wrote, we have discussed the issues together at length, and
> we thank him for the productive (and friendly :-)) conversation.
>
>
> Our paper, which describes our concerns, can be found here:
> https://eprint.iacr.org/2022/065
>
> And a reference implementation of our proposed KDF:
> https://github.com/nimia/kdf_reference_implementation/blob/main/kdf_reference_implementation.py#L60
>
>
> A few points from our side:
>
> Firstly, our proposed construction is simple to implement (see the
> Python code above), and adds a modest overhead of a few microseconds
> (see the paper).
>
>
> Re: point a) from Douglas’ first mail: Admittedly, our concerns are
> broader than Hybrid Key Exchange in TLS. However, we view the
> standardization of Hybrid Key Exchange as an opportunity to add defense
> in depth. Missing this opportunity would effectively further embed the
> problem. We don’t see another such opportunity on the horizon: If we
> standardize a TLS extension in a few years, getting everyone to deploy
> the extension would be hard. Whereas here everyone has to deploy the
> new thing anyway, so we might as well make it as robust as we can.
>
>
> Consider the following: SHA-1 weaknesses to collisions were first
> really highlighted in 2005. TLS version 1.0 was standardised in 2006
> and hardcoded the use of SHA-1, and MD5 (admittedly, for use in HMAC).
> TLS 1.2 was standardised in 2008, and formal deprecation of SHA-1
> occurred in 2011 by NIST. The standard deprecating the use of SHA-1 in
> TLS 1.2 digital signatures occurred in 2021. In 2016, TLS support
> (according to Qualys SSL Labs SSL survey) was over 90%. In 2020, TLS
> 1.0 support was still above 50%, despite practical chosen-prefix
> collision attacks against SHA-1 being possible. Being robust against
> future threats when given the option is something that we should
> seriously take time to consider.
>
>
> As to ekr’s response that the standard already states we need a
> collision-resistant hash function: Brendel et al. [1] proved that the
> TLS 1.3 ECDHE handshake survives losing the collision resistance of the
> hash function, as long as HKDF retains its pseudorandomness property.
> However, HKDF does not provably possess this property to begin with,
> with respect to the (EC)DH shared secret input, since this input is fed
> as the message input to HMAC, and HMAC/HKDF is not a dual PRF.
>
>
> To summarize, we recommend using our new proposed construction. It’s
> fast, easy to implement, and provides provable security. We see no
> reason to entrench a problem if we’re already changing the protocol in
> this area, and requiring implementation changes anyway.
>
>
> Best,
>
> Nimrod, Ben, Ilan, Kenny, Eyal, and Eylon
>
>
> [1] https://www.felixguenther.info/publications/ESORICS_BreFisGun19.pdf
>
>
>
>
> On Tue, 11 Jan 2022 at 21:08, Douglas Stebila <[email protected]> wrote:
>> Hello TLS working group,
>>
>> We've posted a revised version of "Hybrid key exchange in TLS 1.3" [1].
>> Based on revision requests from the last draft, the main change is removing
>> the unnecessary appendix of the past design considerations, and a few
>> wording changes.
>>
>> Last September, Nimrod Aviram, Benjamin Dowling, Ilan Komargodski, Kenny
>> Paterson, Eyal Ronen, and Eylon Yogev posted a note [2,3] with some concerns
>> about whether the approach for constructing the hybrid shared secret in this
>> document -- direct concatenation -- was risky in a scenario where the hash
>> function used in TLS key derivation and transcript hashing is not collision
>> resistant. Nimrod and his colleagues exchanged many emails with us over the
>> past few months to help us understand their concerns. In the end we think
>> the concerns are low and we have not made any changes in this draft,
>> although if we receive different guidance from the working group, we'll do
>> so.
>>
>> There were two types of concerns that Nimrod and his colleagues identified
>> [2,3]:
>>
>> a) An attacker who can find collisions in the hash function can cause
>> different sessions to arrive at the same session key. This concern is
>> largely independent of this hybrid key exchange draft, as it focuses on
>> collisions in the transcript hash, and affects existing TLS 1.3 even without
>> this draft being adopted. If the TLS working group thinks this is a concern
>> that should be addressed, it seems like it should be addressed at the
>> overall level of TLS 1.3, rather than for this specific hybrid key exchange
>> draft.
>>
>> b) An attacker who can find collisions in the hash function and has a
>> certain level of control over the first of the two shared secrets in the
>> hybrid shared secret concatenation may be able to carry out an iterative
>> attack to recover bytes of the second shared secret. The iterative is
>> similar to the APOP attacks [4,5] and also somewhat similar to the CRIME
>> attack [6]. After discussing further with Nimrod and his colleagues, we
>> identified that the following conditions need to be satisfied for this
>> attack:
>> i) Chosen-prefix collisions can be found in the hash function within
>> the lifetime of the TLS handshake timeout of the victim.
>> ii) The victim reuses ephemeral keying material several hundred
>> times and for a time lasting at least as long as the time for part (i) of
>> the attack.
>> iii) The attacker can learn or control the value of the first shared
>> secret in the hybrid shared secret concatenation.
>> iv) The attacker is able to control the length of the first shared
>> secret, so that -- for the iterative component of the attack -- the hash
>> block boundary lands at different positions within the second shared secret.
>>
>> Although different standardized groups do not all have the same shared
>> secret length, for all DH/ECDH groups for TLS 1.3 standardized in RFC 8446,
>> once the group is fixed (during negotiation), the shared secret is fixed
>> length, so condition (iv) is not satisfied for stock TLS 1.3. All NIST
>> Round 3 finalist and alternate candidate KEMs currently have fixed-length
>> shared secrets, so they would not satisfy condition (iv) either, if a
>> post-quantum KEM was used as the first component in concatenation. It may
>> be possible that other organizations have bespoke key exchange methods they
>> would want to use in a hybrid format, which might be variable length, but we
>> don't have any information about that. Even still, the three other
>> conditions of the attack would need to be satisfied. We think that's a
>> pretty high barrier and as such have decided not to incorporate
>> countermeasures at this time, but if the working group prefers otherwise, we
>> can do so. For example, Nimrod and his colleagues ha
>> ve proposed a KDF design that would be secure even in this scenario, but it
>> has substantially more hash function applications that the current
>> HKDF-based approach does.
>>
>> Douglas
>>
>>
>> [1] https://datatracker.ietf.org/doc/draft-ietf-tls-hybrid-design/
>> [2] https://mailarchive.ietf.org/arch/msg/tls/F4SVeL2xbGPaPB2GW_GkBbD_a5M/
>> [3] https://github.com/nimia/kdf_public#readme
>> [4] Practical key-recovery attack against APOP, an MD5-based
>> challenge-response authentication. Leurent, Gaetan.
>> [5] Practical Password Recovery on an MD5 Challenge and Response. Sasaki, Yu
>> and Yamamoto, Go and Aoki, Kazumaro.
>> [6] https://en.wikipedia.org/wiki/CRIME
>> _______________________________________________
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>> [email protected]
>> https://www.ietf.org/mailman/listinfo/tls
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