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

Richard Barnes <rlb@ipv.sx> Fri, 23 April 2021 01:24 UTC

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From: Richard Barnes <rlb@ipv.sx>
Date: Thu, 22 Apr 2021 21:24:03 -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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On Thu, Apr 22, 2021 at 11:39 AM Théophile Wallez <theophile.wallez@inria.fr>
wrote:

> Hi Richard,
>
> On 22/04/2021 14:29, Richard Barnes wrote:
>
> > It is used the next time there's a group operation and an UpdatePath
> > is sent.
>
> I don't get why this is true?
>  From my understanding, a new leaf will be in the `unmerged_leaves` list
> of all the non-blank nodes above it [0].


Ah, this might not be obvious from the spec, but the order of operations
for the committer and their tree is as follows:

1. Apply proposals - including adding joiners to unmerged_leaves
2. Generate and apply an UpdatePath

In that stage 2, the nodes on the committer's direct path are overwritten
-- including clearing unmerged_leaves, because the corresponding secret is
sent to all the leaves underneath that node.  Continuing members get the
path secret from the UpdatePath struct; new members get it from
GroupSecrets.path_secret.

As usual, happy to clarify the spec if it can be clearer on this point.

--RLB


> Given this fact, the new leaf
> will be in the resolution of all the nodes above it, therefore the new
> leaf will be able to decipher the path secrets for the following
> UpdatePath without using the `path_secret` of the Welcome message.
> Also, in the worst case the `path_secret` might correspond to the path
> secret of the root node (since it is the path secret of the lowest
> common ancestor of the leaf sending the Welcome message and the leaf
> receiving the Welcome message [1]), which would not be useful to process
> an UpdatePath message.
>
>
> [0]
>
> https://messaginglayersecurity.rocks/mls-protocol/draft-ietf-mls-protocol.html#section-11.1.1-5.2
> [1]
>
> https://messaginglayersecurity.rocks/mls-protocol/draft-ietf-mls-protocol.html#section-11.2.2-9.4.2.2
>
> Thanks,
> Théophile.
>
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