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Re: [Teep] Use of AES-CTR in TEEP?

Hannes Tschofenig <hannes.tschofenig@gmx.net> Mon, 09 October 2023 14:35 UTC

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Date: Mon, 09 Oct 2023 16:35:46 +0200
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To: Dave Thaler <dthaler=40microsoft.com@dmarc.ietf.org>, "TEEP@ietf.org" <teep@ietf.org>, "suit@ietf.org" <suit@ietf.org>
References: <PH7PR21MB3878F05953BAF6113F429396A3C9A@PH7PR21MB3878.namprd21.prod.outlook.com>
From: Hannes Tschofenig <hannes.tschofenig@gmx.net>
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Subject: Re: [Teep] Use of AES-CTR in TEEP?
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We should create two algorithm profiles, one for firmware updates of IoT
devices and another one for use with TA/config data update in TEEs.


I believe these use cases are different enough to justify having two
algorithm profiles.


Ciao

Hannes



Am 06.10.2023 um 18:29 schrieb Dave Thaler:
> In the TEEP WG, I believe we had consensus to reuse profiles from draft-ietf-suit-mti
> in the TEEP protocol (not just for SUIT manifests).
>
> draft-ietf-suit-mti recently replaced the use of GCM with AES-CTR, and Ken filed
> https://github.com/ietf-teep/teep-protocol/issues/356
> accordingly, proposing replacing GCM with AES-CTR in TEEP, which we discussed at
> the interim last month.  The minutes say at
> https://datatracker.ietf.org/meeting/interim-2023-suit-01/materials/minutes-interim-2023-suit-01-202309111500-00:
>
>> Noting: #356, there are no objections to replacing each of them.
>> Brendan: for the use case of SUIT (TEEP+RATS), using AES-CTR makes
>> sense, but in general, this is not the right mode.
>> * https://mailarchive.ietf.org/arch/msg/cose/9smwFXNpbd6Fci8-mIeaT2xPP-E/
>> * https://mailarchive.ietf.org/arch/msg/cose/0Dj5yp4mptZUBiw7FWb5PcXGJAg/
>> RH: APIs are not supposed to return any plaintext if the integrity check
>> fails, so the ability to break the image into chunks is an important
>> aspect, so that's why it does not fit into AES-GCM.
> However, in a discussion with Hannes this week, he said
>> We should not use AES-CTR mode in TEEP. The registration of AES-CTR / AES-CBC in COSE was done
>> to support low-end IoT devices that use flash memory. It is not an algorithm that should be used in
>> other places where not needed. I don't see a use case for it in TEEP
> I look to others to provide guidance here... should we
> a) make TEEP and SUIT diverge so that a TEEP implementation has to implement two different things
> b) use AES-CTR in TEEP
> c) add GCM back to suit-mti
> d) something else because Dave is confused :)
>
> Dave
>
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