Re: [MLS] Purpose of path_secret in the Welcome message?

Richard Barnes <rlb@ipv.sx> Thu, 22 April 2021 12:30 UTC

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From: Richard Barnes <rlb@ipv.sx>
Date: Thu, 22 Apr 2021 08:29:37 -0400
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To: =?UTF-8?Q?Th=C3=A9ophile_Wallez?= <theophile.wallez@inria.fr>
Cc: Messaging Layer Security WG <mls@ietf.org>
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Subject: Re: [MLS] Purpose of path_secret in the Welcome message?
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Hi Théophile,

You're correct that the `path_secret` isn't used in the process of joining
the group (decrypting the Welcome message and initializing group state).
It is used the next time there's a group operation and an UpdatePath is
sent.  It's optional because the sender of an all-Add Commit message
doesn't have to send an Update Path.  In that case, the path above a
joiner's leaf will be all-blank; there's no path secret to communicate.

Hope that helps,
--Richard

On Thu, Apr 22, 2021 at 7:49 AM Théophile Wallez <theophile.wallez@inria.fr>
wrote:

> Hello,
>
> I am a PhD student currently writing a formalization of MLS, and I was
> wondering about the use of the `path_secret` field in `GroupSecrets`.
> To communicate with the group during the current epoch, it is sufficient
> to know `joiner_secret`.
> Also, thanks to the concept of unmerged leaves, a newly added leaf don't
> need to know any path secret to handle an UpdatePath: knowing a path
> secret early would simply result in the fact that the leaf could decrypt
> two different `encrypted_path_secret` in `UpdatePathNode`, which does
> not seem useful.
>
> Can someone give me some intuition for when and why this optional
> `path_secret` is used?
>
>
> Best,
> Théophile.
>
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