Re: [TLS] Finished stuffing/PSK Binders

Eric Rescorla <ekr@rtfm.com> Fri, 07 October 2016 20:42 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 7 Oct 2016 13:41:48 -0700
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To: Benjamin Kaduk <bkaduk@akamai.com>
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Subject: Re: [TLS] Finished stuffing/PSK Binders
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On Fri, Oct 7, 2016 at 1:39 PM, Benjamin Kaduk <bkaduk@akamai.com>; wrote:

> On 10/07/2016 12:08 PM, Eric Rescorla wrote:
>
>
>
> On Fri, Oct 7, 2016 at 10:03 AM, Ilari Liusvaara <ilariliusvaara@welho.com
> > wrote:
>
>> On Fri, Oct 07, 2016 at 09:35:40AM -0700, Eric Rescorla wrote:
>> > On Fri, Oct 7, 2016 at 8:26 AM, Ilari Liusvaara <
>> ilariliusvaara@welho.com>;
>> > wrote:
>> >
>> > > On Fri, Oct 07, 2016 at 08:01:43AM -0700, Eric Rescorla wrote:
>> > > > 4. I've taken a suggestion from David Benjamin to move the
>> negotiation
>> > > > of the PSK key exchange parameters out of the PSK itself and into a
>> > > > separate message. This cleans things up and also lets us drop the
>> > > > currently non-useful auth_mode parameter.
>> > >
>> > > Eeh... From the text, it seems to currently require the kex modes
>> > > extension if PSK extension is present. Which seems worse than useless
>> > > if the meaning is to get rid of the kex mode parameter from PSK
>> > > extension (since you will have the value anyway, but need to dig it
>> > > from another extension... Blech).
>> >
>> > I guess this is a matter of taste, but what convinced me was that:
>> >
>> > 1. It put all the logic on the server side.
>>
>
> I was going to ask whether this also includes the decision on whether to
> send a Certificate to authenticate the server (even for PSK modes), but it
> looks like this change is intentionally removing the ability to do PSK
> keyex and auth with a certificate?
>

Yes, while preserving the ability to add it later by adding a PskAuthMode
extension parallel to this one.

-Ekr


> > 2. It removed the auth mod parameter.
>> >
>> > Maybe david can say more.
>>
>> I mean if server is to accept PSK, it must now go fishing for another
>> extension, check that it is present and pay attention to values there.
>> As opposed to having the data in where it is needed.
>>
>
> This is a reasonable argument (and the reason I stuffed the binder here).
> However, David's argument was that this applied to *all* PSKs even new
> ones.
>
>
> Implementations already have to do some amount of "scan through all
> extensions to detect certain things, do some initial processing, and then
> finish process everything [else]", not least because SNI can change what
> cert types you have, configuration knobs, etc..  So, yes it's bad, but how
> bad is it in a relative sense?
>
> I think there is some value in the client indicating to the server whether
> it doesn't want to do psk_dhe (or doesn't want to do pure psk) for future
> NSTs, though it's perhaps not of the utmost importance.  That property does
> support a separate psk_key_exchange_modes extension in preference  to
> rolling it into pre_shared_keys (as a separate list next to identities and
> binders), but it seems to only be a weak argument for separation.  I do
> think that keeping the psk kex mode orthogonal to the individual keys is
> helpful, though.
>
> Anyway, maybe it's time to bite the bullet and actually write up the
> description of the proper procedure for ClientHello processing (scan
> extensions for SNI, load up servername-specific config+cert/key, etc.).
> Which would make this stuff any prettier, but would at least help people
> not get it wrong.
>
> -Ben
>