Re: [MLS] Functional Definition of End-to-End Secure Messaging

Raphael Robert <> Fri, 07 May 2021 08:40 UTC

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From: Raphael Robert <>
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Date: Fri, 7 May 2021 10:40:34 +0200
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Subject: Re: [MLS] Functional Definition of End-to-End Secure Messaging
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Thanks for sharing!

You mention the prior art by Knodel et al. at the end and I’m wondering if it wouldn’t be a better starting point for discussion? Especially since it has the advantage of building upon notions that are well-defined and established in the industry.

I’m also still struggling with your definition of a “backdoor” in 4.4:

'A "backdoor" is any intentional or unintentional mechanism, in
respect of a given message and that message's set of participants,
where some PCASM of that message MAY become available to a non-
participant without the intentional action of a participant.’

You previously mentioned that intent is hard to measure and I agree with that. But I’m quite convinced a design or implementation flaw shouldn’t be called a “backdoor” until there is sufficient proof of intent. If you call it a “backdoor” right from the beginning, most people will assume that intent was there. This would be very discouraging for everyone who contributes to E2EE protocols, because it introduces the risk of being stigmatised as someone with (criminal) intent just because an honest mistake was made. Long story short, I strongly reject that notion of a “backdoor” for being intimidating.

That being said, discussion around these topics is a good thing and I’m curious to hear other people’s thoughts!


> On 6. May 2021, at 19:56, Alec Muffett <> wrote:
> Hi All,
> I am drafting an I-D to offer a measurable definition of End-to-End Secure Messaging, including End-to-End Encrypted Messaging.
> It's basically a "Duck Test" for End-to-End Secure Messengers: "Does <this software> quack like an E2EE-Secure Messenger, should?"
> I believe that such a definition is a long-overdue necessity for settling arguments such as "Would adding a Ghost participant *really* break End-to-End Security" (answer: "yes").
> I think that the MLS group might be a good workgroup for such a document. 
> What do you think, please?
> Current draft text:
> <>
> As HTML:
> <>
>     - Alec
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