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

[Eric Rescorla <[e-mail]>]

On Tue, May 23, 2017 at 9:34 PM, Martin Rex <[e-mail]> wrote:

> 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?
>

For starters, TLS 1.3 has already designed a completely independent
mechanism for doing version negotiation outside of ClientHello.version,
so doing another seems pretty odd. In any case, it's not something you
do between IETF-LC and IESG approval.


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.
>

I'm not sure this is true, because there were also servers which did
not understand extensions.

-Ekr


> -Martin
>