[MLS] Sending secrets in WelcomeInfo instead of keys

Michael Rosenberg <micro@fastmail.com> Wed, 07 August 2019 17:58 UTC

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From: Michael Rosenberg <micro@fastmail.com>
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Date: Wed, 7 Aug 2019 13:58:41 -0400
To: Messaging Layer Security WG <mls@ietf.org>
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Subject: [MLS] Sending secrets in WelcomeInfo instead of keys
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Problem
-------

The special-casing required to use WelcomeInfo::add_key_nonce in unframing Add messages nontrivially increases code complexity.

Framing ordinarily depends on just a handshake_secret and sender_data_secret. From those, the framer/unframer derives the handshake key/nonce and sender data key/nonce.

But in the case of an Addee processing an incoming Add, the Addee's unframer needs to be told to use the handshake key / nonce from the previous WelcomeInfo, and be told to not attempt to decrypt MLSCiphertext::encrypted_sender_data, because

1) The only reason it would need to do this is to derive the handshake key/nonce, which it already has, and
2) the Addee doesn't even know sender_data_key.

This is complex enough that it becomes hard to reason about and painful to implement.


Proposal
--------

To reduce the amount of special casing necessary, the handshake_secret and sender_data_secret be included in WelcomeInfo instead of handshake_key and handshake_nonce. This way, the only steps in the algorithm that differ between the Addee and non-Addees is where the inputs to the unframer's constructor come from, i.e., Unframer(group_ctx.handshake_secret, group_ctx.sender_data_secret) versus Unframer(welcome_info.handshake_secret, welcome_info.sender_data_secret).

Benjamin mentioned that this is technically a violation of the secrecy invariant of TreeKEM: a user who is not a member of a group at epoch n cannot know secrets from epoch n. It should be noted though that this is already violated by the existence of WelcomeInfo::init_secret and WelcomeInfo::add_key_nonce.

-Michael