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Re: [Sframe] Key ratchting

Richard Barnes <rlb@ipv.sx> Thu, 06 October 2022 12:40 UTC

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From: Richard Barnes <rlb@ipv.sx>
Date: Thu, 06 Oct 2022 08:39:59 -0400
Message-ID: <CAL02cgTfh8=j=t_MZNTw+Mpmyym8UXr=rkjMNUKK_P2iH_F4DA@mail.gmail.com>
To: Sergio Garcia Murillo <sergio.garcia.murillo@gmail.com>
Cc: sframe@ietf.org
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Subject: Re: [Sframe] Key ratchting
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Hey Sergio,

It looks like there is still some mention of ratcheting in the document:

https://github.com/sframe-wg/sframe/blob/main/draft-ietf-sframe-enc.md#sender-keys

Maybe that needs to be rephrased somehow?

If we're going to keep / adjust / extend that text, some suggested
improvements:
1. Instead of allowing every sender to use the whole KID space, you could
segment the KID space as in the MLS-based case (so that KID would indicate
sender as well as generation)
2. Instead of ratcheting the keys directly, there should probably be a
ratchet_secret that gets ratcheted, and from which keys are derived.  This
would avoid the keys being used with two algorithms (HMAC and AEAD).

--Richard

On Thu, Oct 6, 2022 at 5:15 AM Sergio Garcia Murillo <
sergio.garcia.murillo@gmail.com> wrote:

> Hi all,
>
> As we prepare to publish the new version of the draft (currently available
> on https://github.com/sframe-wg/sframe/blob/main/draft-ietf-sframe-enc.md),
> I would like to bring back a topic for discussion: key ratcheting.
>
> If I recall correctly, key ratcheting was removed from the SFrame draft as
> this feature would be provided by MLS, which would just generate a new key
> internally and set it in SFrame when ratcheting is needed.
>
> However, SFrame must be usable without MLS, so I think we should bring
> back the ratcheting feature to support other keying mechanisms. What
> are your thoughts?
>
> Best regards
> Sergio
> --
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