Re: [TLS] Eric Rescorla's Discuss on draft-ietf-tls-ecdhe-psk-aead-04: (with DISCUSS and COMMENT)

mrex@sap.com (Martin Rex) Tue, 23 May 2017 13:35 UTC

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To: Eric Rescorla <ekr@rtfm.com>
Date: Tue, 23 May 2017 15:34:41 +0200 (CEST)
CC: The IESG <iesg@ietf.org>, tls@ietf.org, tls-chairs@ietf.org, draft-ietf-tls-ecdhe-psk-aead@ietf.org
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From: mrex@sap.com (Martin Rex)
Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/d4RpQCWcn28cwXrsUSWqwtrmnmw>
Subject: Re: [TLS] Eric Rescorla's Discuss on draft-ietf-tls-ecdhe-psk-aead-04: (with DISCUSS and COMMENT)
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Eric Rescorla wrote:
> draft-ietf-tls-ecdhe-psk-aead-04: Discuss
> 
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
> 
> The following text appears to have been added in -04
> 
>    A server receiving a ClientHello and a client_version indicating
>    (3,1) "TLS 1.0" or (3,2) "TLS 1.1" and any of the cipher suites from
>    this document in ClientHello.cipher_suites can safely assume that
> the
>    client supports TLS 1.2 and is willing to use it.  The server MUST
>    NOT negotiate these cipher suites with TLS protocol versions earlier
>    than TLS 1.2.  Not requiring clients to indicate their support for
>    TLS 1.2 cipher suites exclusively through ClientHello.client_hello
>    improves the interoperability in the installed base and use of TLS
>    1.2 AEAD cipher suites without upsetting the installed base of
>    version-intolerant TLS servers, results in more TLS handshakes
>    succeeding and obviates fallback mechanisms.
> 
> This is a major technical change from -03, which, AFAIK, prohibited
> the server from negotiating these algorithms with TLS 1.1 and below
> and maintained the usual TLS version 1.2 negotiation rules.

This change _still_ prohibits the server from negotiating these algorithms
with TLSv1.1 and below.

Could you elaborate a little on where and why you see a problem with this?

As this changes tries to explain, had such a text been used for all
TLSv1.2 AEAD cipher suite code points, then browsers would have never
needed any "downgrade dance" fallbacks, POODLE would have never
existed as a browser problem, and the TLS_FALLBACK_SCSV band-aid
would not been needed, either.

-Martin