Subject: [MLS] draft structure feedback and suggestions

From: Franziskus Kiefer <[email-masked]>
Date: Thu, 17 Sep 2020 13:18:52 +0200

Hi all,

While trying to implement the MLS protocol from scratch I ran into a couple
issues and I think there’s a general problem regarding the structure of the
draft. Note that these are all editorial issues.

Right now I see no straightforward way to implement the spec as there’s no
clear structure. To implement even the basic tree structure it’s necessary
to jump through the entire document. There’s also a lot of prose and
examples and very little implementable parts.

Let me give some examples:

Section 5 (Ratchet Trees) reads like a non-normative section (or paper).
The really interesting bit related to trees is in Appendix A. I suggest
moving the formal description of the tree from Appendix A to this section.
It’s impossible to implement this from the hand full examples here.

There’s no description in here on how the tree actually works. For example,
adding a node to the tree is defined in 10.1.1 (Add).

The ratchet tree nodes as described in Section 5 don't hold a KeyPackage.
But Section 7 says “As the KeyPackage is a structure which is stored in the
Ratchet Tree and updated depending on the evolution of this tree”.

The tree nodes have a parent hash field, which is of course part of the
ratchet tree but shouldn’t be necessary for the description of the tree
structure.

Also, the description of the tree hash (Section 7.5) follows the definition
of the parent_hash extension in Section 7.4 such that it is not entirely
clear which parent_hash is used there (extension or node value).

Section 5 also talks about commit messages without linking to them or
introducing them. Having to talk about commit messages in this section is
also a little confusing.

The term “handshake message” is first used in Section 7.8 (Key Schedule)
but never introduced. This should probably happen in 10 (Group Evolution),
which doesn’t talk about handshake messages at all until 10.3 (Ratchet Tree
Extension).

There are probably plenty more issues like this. But I don’t think
addressing any of these issues individually makes sense. I therefore
suggest having an overhaul of the document structure.

The way I see MLS there are a set of high-level structures and components
that should be clearly described and built upon each other. There’s a data
structure (tree) that is used to represent a group. Then there is the MLS
group that implements a specific semantic on the tree. Operations to modify
the group are specified in the handshake protocol. And application messages
define messages encrypted with keys derived in the handshake protocol with
the *key schedule*.

The tree is a basic left-balanced binary tree and defines functions to
create a tree, and add and remove nodes.

A group is the main part of MLS, a tree with nodes of a certain type. So
this part should describe what nodes look like and how the tree is used to
compute tree invariants and keys.

The handshake protocol defines all messages needed to perform operations on
the MLS group.

This is certainly not exhaustive and would need further refinement. But
it’s the high-level structure I’d expect from the document. I’m sure other
structures would work as well. But what we currently have isn’t what is
needed to implement MLS imho.

Cheers,

Franziskus