Hi all,
I would like to state up front that I am not disputing the sentence "Implementations MUST support TLS 1.2 [RFC5246]." If that's consensus, I guess I'm just in the rough, and at least the text covering implementations is much more encouraging than I expected.
But the document is still not consistent, because the rationale says "when the recommendations in this document are followed [...] the use of TLS 1.2 is as safe as the use of TLS 1.3." But the document also states many valid reasons for going TLS 1.3-only, not least of which is attack surface. I think the document might be better if it said "Implementations SHOULD support TLS 1.2, [for all of the reasons in the rationale below]". It seems to be almost the textbook example of when SHOULD as defined in RFC2119 is appropriate (almost never, imho).
There are so many details to track in supporting both versions that I just can't see using MUST here. For example, something close to this TLS 1.3 extension came up at the last TLS WG meeting:
But this was in the context of assuming TLS 1.2, and that's a different protocol, whatever you think of the tradeoffs.
So, I've said my piece here, and will leave it to the authors.
thanks,
Rob
On Sun, Aug 14, 2022 at 1:27 PM Yaron Sheffer <yaronf.ietf@xxxxxxxxx> wrote:
Hi Peter,
Thank you for your comments and sorry for the delayed response.
The abstract of the Racoon paper mentions TLS-DH(E) five times, so clearly the authors believe it applies to both TLS-DH and DHE. I think the disconnect is that Racoon is about public key reuse which you would characterize as static-ephemeral, but to most users this is just normal DHE with a very common (though dangerous) optimization.
Section 4.1 now includes references to both Sec. 7.3 and 7.4 (see editor's draft [1]).
I agree we should add a SHOULD NOT for TLS-ECDH, referencing [Jager2015], "Practical invalid curve attacks on TLS-ECDH". See pull request [2].
The internal implementation of ECC algorithms is out of scope of the BCP, which focuses on the use of algorithms within TLS. In other words, the target audience for the BCP is TLS administrators more than the people who write the internals of crypto libraries. This is why we never mentioned [Brumley11] in RFC 7457 and I don't think we should add it here.
Thanks,
Yaron
[1] https://www.sheffer.org/I-D/draft-ietf-uta-rfc7525bis.html
[2] https://github.com/yaronf/I-D/pull/480
On 04/08/2022, 13:00, "Peter Gutmann" <pgut001@xxxxxxxxxxxxxxxxx> wrote:
Peter Saint-Andre <stpeter@xxxxxxxxxx> writes:
>Given that we already discuss these matters in Section 7.4, I don't see the
>need for additional text.
The issue that I pointed out is in section 4.1, "General Guidelines", while
what you're referring to is buried in the security considerations right at the
end. What's in 4.1 at the moment is wrong (Raccoon is static-ephemeral DH,
not ephemeral-ephemeral DH), so I think it needs to be changed. 4.1 refers to
7.3 but not 7.4, so anyone reading the doc who doesn't read into every corner,
including the parts buried after the IANA considerations at the end, will get
an incorrect idea of what the issues are.
Also what 4.1 is in effect saying is "implementations MUST use ECC algorithms"
(via SHOULD NOT RSA (key transport), SHOULD NOT DH, SHOULD NOT DHE), and that
includes TLS_ECDH_*, not just TLS_ECDHE_* (section 4.1 says, about *_DH_*,
"These cipher suites, which have assigned values prefixed by 'TLS_DH_*', have
several drawbacks, especially the fact that they do not support forward
secrecy", but omits any mention of the equivalent *_ECDH_* which is no
better).
Since the ECC algorithms are notoriously vulnerable to nonce issues as well as
various others (e.g. forgetting to perform validity checks on received
values), it's just moving the insecurity from one algorithm over to another.
There's no mention in the security considerations of any issues with replacing
DHE with ECC algorithms, it's just "badly implemented DH is insecure" but no
mention that badly-implemented ECC is also insecure. For example Brumley et
al's attack on ECDHE takes advantage of the same not-really-ephemeral nature
that Raccoon uses against DHE, but it's never mentioned. Anyone reading the
draft would be forgiven for thinking that all you need to do to magically make
all security problems go away is switch to ECC (and GCM, the other universal
solution to all problems, despite it also having led to endless
vulnerabilities around nonce reuse).
Peter.
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