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Re: [TLS] DTLS 1.3

Hannes Tschofenig <hannes.tschofenig@gmx.net> Tue, 05 July 2016 08:59 UTC

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To: Eric Rescorla <ekr@rtfm.com>, Ilari Liusvaara <ilariliusvaara@welho.com>
References: <577A38A2.2090209@gmx.net> <20160704140312.GC4287@LK-Perkele-V2.elisa-laajakaista.fi> <577ABCE2.9050409@gmx.net> <20160704204603.GA4837@LK-Perkele-V2.elisa-laajakaista.fi> <CABcZeBMYQ=SWfEwFVjpmO3Pzh78VTrdqXKF26TDnSA-nR-k=rQ@mail.gmail.com>
From: Hannes Tschofenig <hannes.tschofenig@gmx.net>
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Cc: "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] DTLS 1.3
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Hi Ekr,

~snip~

>     I think the obvious way is:
> 
>     - Cookies from HelloVerifyRequest go to legacy cookie field.
>     - Cookies from HelloRetryRequest go to extension cookie field.
> 
>     If you don't do the first, you can't downnegotiate successfully. And
>     if you don't do the second, you would have to have special case with
>     cookie sizes (since HRR can transmit >255 byte cookie, which would be
>     too big for legacy cookie).
> 
> 
> IMO we should just forbid HVR for DTLS 1.3. I.e., you should just send
> HRR.

That makes sense to me.

Btw, Section 11 "IANA Considerations" says that the Cookie extension is
encrypted in the HelloRetryRequest message. I wonder whether this is a
mistake particularly in the combination where the server returns the
cookie as part of the incorrect key share (and therefore does not yet
have a key to create the EncryptedExtension). The table should also say
that the cookie is also contained in the ClientHello (in clear, unless
it is a 0-RTT handshake).

> 
> 
>     > > - The handshake retransmit scheme doesn't seem to work that
>     > >   well with post-handshake auth, and even less well with
>     > >   session tickets.
>     > Why do you think so? Of course, unreliable transports creates
>     > inconvenience; is it that what you are referring to?
> 
>     DTLS assumes handshake messages are reliable, and that reliability is
>     implemented via handshake messages ACKing one another.
> 
>     - Session tickets have no ACK at all.
> 
> 
> DTLS 1.3 should add an ACK, IMO.

Sounds reasonable.

> 
>  
> 
>     - CertificateRequest can have very slow ACK.
>     - KeyUpdate has no real ACK (and isn't idempotent either).
> 
> 
> Yes, I think we should remove KeyUpdate for DTLS 1.3 and just use epoch
> instead.

Ok.

Ciao
Hannes

> 
> -Ekr
> 
> 
> 
> 
>     -Ilari
> 
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