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 17:46 UTC

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To: mrex@sap.com
Date: Tue, 23 May 2017 19:46:27 +0200
CC: Eric Rescorla <ekr@rtfm.com>, tls-chairs@ietf.org, draft-ietf-tls-ecdhe-psk-aead@ietf.org, The IESG <iesg@ietf.org>, tls@ietf.org
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Subject: Re: [TLS] Eric Rescorla's Discuss on draft-ietf-tls-ecdhe-psk-aead-04: (with DISCUSS and COMMENT)
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It seems I had a typo in the new text.

Martin Rex 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

That line should say
                                         through ClientHello.client_version

>>    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 change 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 have been needed, either.