Re: [MLS] UPKE and Epoch Forward Secrecy

Benjamin Beurdouche <benjamin.beurdouche@inria.fr> Thu, 31 October 2019 08:37 UTC

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From: Benjamin Beurdouche <benjamin.beurdouche@inria.fr>
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Date: Thu, 31 Oct 2019 09:37:44 +0100
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To: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>
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Subject: Re: [MLS] UPKE and Epoch Forward Secrecy
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I think I perfectly agree with most of what’s been discussed in the mailing list about rtreekem but even though, I agree with Karthik that I don’t fully understand how to break epoch level FS here...

I might not have understood something either :) input very very welcome !
B.

> On Oct 31, 2019, at 8:28 AM, Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>; wrote:
> 
> I must begin by saying that I have been enjoying reading Alwen et al’s work [1] and they make some excellent points.
> I particularly like the idea of using a primitive like UPKE (or SkuPke as [2] calls it) to improve the forward secrecy guarantees of TreeKEM.
> If this can be made to work with standards-compliant EC implementations, we should definitely consider adding this mechanism to MLS.
> 
> For my own better understanding, however, I am trying to figure out the exact forward secrecy improvement this will bring to the protocol.
> It is clear from [1] that the *update secret* and each *subgroup secret* in TreeKEM provides weak Forward Secrecy (since each update
> only modifies one leaf key, leaving the attacker N-1 members to compromise.)
> 
> However, the public key part of TreeKEM is only part of the Forward Secrecy story, we must also account for the “init_secret” which
> changes with every update. As far as I can see, the discussion in [1] appears to ignore the “init_secret -> update_secret -> epoch_secret+init_secret”
> ratchet which has always been part of MLS. So I don’t fully see how the attack of [1] works, and maybe someone can explain.
> 
> One may argue that the goal of TreeKEM is to provide FS and PCS for the epoch_secret, not the update_secret.
> If every member of the group is honest, then A sends an update , then B accepts the update (ratcheting forward its init_secret), 
> and then B is compromised, then how can the attacker learn the new epoch secret?
> 
> Perhaps we are worried about post-compromise forward secrecy (PCFS), but I don’t see any attack on that either.
> It is likely I am missing something, so do chime in and explain.
> 
> Best,
> Karthik
> 
> 
> [1] https://eprint.iacr.org/2019/1189.pdf
> [2] https://eprint.iacr.org/2018/954
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