Re: [TLS] Adoption call for Deprecating Obsolete Key Exchange Methods in TLS

Peter Gutmann <pgut001@cs.auckland.ac.nz> Sat, 31 July 2021 12:54 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: "Scott Fluhrer (sfluhrer)" <sfluhrer=40cisco.com@dmarc.ietf.org>, "tls@ietf.org" <tls@ietf.org>
Thread-Topic: [TLS] Adoption call for Deprecating Obsolete Key Exchange Methods in TLS
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Date: Sat, 31 Jul 2021 12:54:02 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/SC6l3RxefOmJxUAyVxyJWk8tRxs>
Subject: Re: [TLS] Adoption call for Deprecating Obsolete Key Exchange Methods in TLS
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Scott Fluhrer (sfluhrer) <sfluhrer=40cisco.com@dmarc.ietf.org> writes:

>The problem is that it is hard for the client to distinguish between a well
>designed server vs a server that isn't as well written, and selects the DH
>group in a naïve way.

What should the client do if it could detect this?  And how would it
distinguish between a server that selects bad DH parameters, a server that
uses time() to seed its RNG for prime generation, a server that has a buffer
overflow allowing RCE, and a server whose ACLs allow read/write access to any
file on the filesystem including its private key(s)?

>Now, as I mentioned in the WG meeting, it would be possible to detect if the
>server proposes a safe prime (it's not especially cheap, being several times
>as expensive as the rest of the DH operations, but it's possible),

Or you could use TLS-LTS, which sends { p, q, g } allowing the client to
verify certain properties about the primes being used at next to no cost.

Peter.