[MLS] UPKE and Epoch Forward Secrecy

[Karthikeyan Bhargavan <[e-mail]>]

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 <https://eprint.iacr.org/2019/1189.pdf>
[2] https://eprint.iacr.org/2018/954 <https://eprint.iacr.org/2018/954>