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Re: [TLS] Choice of Additional Data Computation

Hannes Tschofenig <Hannes.Tschofenig@arm.com> Fri, 24 April 2020 14:48 UTC

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From: Hannes Tschofenig <Hannes.Tschofenig@arm.com>
To: Achim Kraus <achimkraus@gmx.net>, "tls@ietf.org" <tls@ietf.org>
Thread-Topic: [TLS] Choice of Additional Data Computation
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Date: Fri, 24 Apr 2020 14:48:07 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/UB8DTdemIfWNtiykKl8Ru7ZonBA>
Subject: Re: [TLS] Choice of Additional Data Computation
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Hi Achim,

My point is that we made decisions based on the best of our knowledge (and personal preferences) but I am wondering whether formal methods can tell us whether there is a preferred way.

Ciao
Hannes

-----Original Message-----
From: Achim Kraus <achimkraus@gmx.net>
Sent: Friday, April 24, 2020 3:07 PM
To: Hannes Tschofenig <Hannes.Tschofenig@arm.com>; tls@ietf.org
Subject: Re: [TLS] Choice of Additional Data Computation

Hi Hannes,
Hi list,

as input for the discussion:

https://github.com/tlswg/dtls-conn-id/issues/25

A longer "comment-flow", the conclusion was, the CID is on the wire, so it's in the MAC.
(ekr: "authenticating the whole header is just good practice.")

My arguments was, that the CID is always included in the MAC, either explicit, or implicit (implicit, because the CID selects the "mac-keys"
or "cipher-keys" and gets included equal to the address:port before).

best regards
Achim

Am 24.04.20 um 13:04 schrieb Hannes Tschofenig:
> Hi all,
>
> the thread on the AEAD commutation in DTLS 1.3 and the construction of
> the additional data raised two interesting questions. I believe those
> would benefit from a formal analysis or at least a security investigation.
>
> Here are the questions:
>
>  1. Generic question: Should the construction of the additional data be
>     dependent on what is transmitted over the wire or should it be based
>     on a "pseudo header"? DTLS 1.2 uses a pseudo header and DTLS 1.3 the
>     data transmitted over the wire in the additional data calculation.
>  2. Specific question: Should the CID be included in the additional data
>     calculation, particularly for the case where it is only implicitly
>     sent? Asked differently, are there attacks possible?
>
> Your feedback would be appreciated to advance the discussion. I
> believe there is a chance to provide generic guidance for security
> protocol designers here.
>
> Ciao
>
> Hannes
>
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