Re: [TLS] Singular or multiple NamedGroup(s) in the "HelloRetryRequest"

Eric Rescorla <ekr@rtfm.com> Fri, 16 January 2015 16:34 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 16 Jan 2015 08:33:07 -0800
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To: "Dang, Quynh" <quynh.dang@nist.gov>
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Subject: Re: [TLS] Singular or multiple NamedGroup(s) in the "HelloRetryRequest"
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On Fri, Jan 16, 2015 at 7:51 AM, Dang, Quynh <quynh.dang@nist.gov> wrote:

>  ​If a client offers all of the options it supports and a server sends
> back one group out of these options, it would guarantee a failure.
>

I don't understand. The client sends a list of all the groups it supports
and a ClientKeyShare with a subset of those groups. If the server doesn't
support any group in the ClientKeyShare it sends back HRR with a single
group out of the clients larger set that the client needs to respond with.
How does that guarantee a failure?

-Ekr

Quynh.
>  ------------------------------
> *From:* Eric Rescorla <ekr@rtfm.com>
> *Sent:* Friday, January 16, 2015 10:44 AM
> *To:* Dang, Quynh
> *Cc:* tls@ietf.org
> *Subject:* Re: Singular or multiple NamedGroup(s) in the
> "HelloRetryRequest"
>
>  Note that the client advertises all the groups it can support, so the
> server
> can just pick the "best" one on the clients list.
>
>  -Ekr
>
>
> On Fri, Jan 16, 2015 at 7:26 AM, Dang, Quynh <quynh.dang@nist.gov> wrote:
>
>>  Hi all,
>>
>>
>>  It seems to be fine to improve inter-operability (or efficiency: less
>> back and forth) by allowing multiple groups (multiple "NamedGroup"s) in the
>> "HelloRetryRequest" message since a server might be willing to accept 2 (or
>> more) groups for a particular key exchange method. For example,  a client
>> might offer only one FFDH group in its Client Key Share message and the
>> server is willing to offer two higher level security groups for the client
>> to choose from.
>>
>>
>>  The situation would be similar if we had 3 or more curves.
>>
>>
>>  Quynh.
>>
>
>