Re: [MLS] MLSPlaintext packets aren't authenticated using symmetric key schedule

[Joel Alwen <[e-mail]>]

> For context: this would only apply when applications use cleartext
MLSPlaintext for HS messages. The recommendation is still to encrypt them and
send them around as MLSCiphertext.

Yup, all this only applies for that case.

> Question: would this attack work with Commit messages? I’m thinking that they
would be rejected because the attacker cannot compute the confirmation_tag.

TBH for commit messages I haven't thought it through yet. (The attack on
Proposals looked like enough of an issue that we might wanna do something
already.) But on further thought we might be OK already for commit msgs. The
confirmation_tag is, in particular, also a MAC of the MLSPlaintext packet's
content as that's part of the confirmed_transcript_hash. That really doesn't
leave any room to reuse Alice's confirmation_tag values to forge a non-trivial
commit on her behalf. So a non-trivial forgery requires computing fresh tag
values but that won't work with out the symmetric key schedule which was the goal.

> As you mention in the PS, the easy target would be Proposal messages.

Indeed. As far as I can tell those simply having nothing at all that depends on
the symmetric key schedule.

> I’d be interested to see what exactly you would propose as a mitigation mechanism.

Given that Commit's seem ok as is, how about adding an opaque tag<0...255> to
MLSPlaintext struct if and only if content_type == proposal && sender_type ==
member. In that case we set tag := KDF.Extract(confirmation_key,
MLSPlaintextTBS). (Here, conf key is taken from the current epoch.)

- Joël


On 18/08/2020 16:55, Raphael Robert wrote:
> Hi Joel,
> 
> For context: this would only apply when applications use cleartext MLSPlaintext for HS messages. The recommendation is still to encrypt them and send them around as MLSCiphertext.
> That being said, we said we would like to support scenarios where HS messages are not necessarily encrypted.
> 
> Question: would this attack work with Commit messages? I’m thinking that they would be rejected because the attacker cannot compute the confirmation_tag.
> 
> As you mention in the PS, the easy target would be Proposal messages.
> 
> I’d be interested to see what exactly you would propose as a mitigation mechanism.
> 
> Raphael
> 
>> On 18 Aug 2020, at 16:36, Joel Alwen <[e-mail]> wrote:
>>
>> Hey everyone,
>>
>> Something thats been bugging Marta Mularczyk and Daniel Jost and me for a bit
>> now is that handshake messages sent out as MLSPlaintext packets are only
>> authenticated using signatures, but not using the group's key schedule. For
>> non-members that makes sense but for group members that's weaker than need be.
>>
>> Suppose Alice is in a group using signing key pair (spk, ssk). I corrupt her to
>> learn ssk. Now I loose access to her device again. Later she generates a fresh
>> key package with her same spk but a new HPKE key for her leaf. She sends out and
>> update proposal for her new key package and someone commits to the update.
>>
>> Expected result: she (and the group at large) has achieved PCS again.
>>
>> Actual result: using her stolen ssk I can still forge a new proposal's (sent as
>> MLSPlaintext packets) coming from Alice. Some things I could do with this power:
>> - I can generate a new key package kp for Alice using her spk and some HPKE key
>> she doesn't know. Then I forge an update proposal for Alice with kp. If it gets
>> committed I've effectively kicked her out of the group.
>> - I could forge Add's and Remove's coming from Alice, so I could trick the
>> group into thinking Alice is trying to Add my account to the group or remove
>> some other group member.
>>
>> Lemme know if I've missed something here in that scenario...
>>
>>
>> If I didn't miss anything and the attacks really work as advertised then IMO
>> this is kinda weak sauce and worth avoiding if possible. So to that end, how
>> about we modify MLS such that MLSPlaintext packets coming from group members
>> must also be authenticated using something from the application key schedule.
>> Now the above attacks fail. As soon as Alice's update is gets committed I no
>> longer know the group's key schedule and so can't forged packet from Alice. More
>> generally, this brings the PCS guarantees when using MLSPlaintexts frameing in
>> line with what we're getting from MLSCiphertext packets.
>>
>> Any thoughts?
>>
>> - Joël
>>
>>
>>
>> PS. For concreteness, we could probably extend the current mechanism for getting
>> concistancy (the confirmation_tag) to also provide symmetric key authentication.
>> E.g. include most of the MLSPlaintext content into whats being tagged by
>> confirmation_tag. That would cover the case of a commit packet and doesn't even
>> grow the size of MLSPlaintext packets over the current design.
>>
>> For a proposal packet we could also have a confirmation_tag but this one is
>> computed using the *current* epoch's confirmation_key and confirmed_transcript_hash.
>>
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