4.1 Zoom Transparency Tree

Note: The Zoom Transparency Tree is not currently available. We plan to release it in a future update. In this section, we describe a mechanism that expands the authentication guarantees from Sections 3 and 5 to ensure that all Zoom users have a consistent view of each others’ devices and keys. Imagine an insider, Mallory, who wants to eavesdrop on a meeting between honest users Alice and Bob, who have never interacted on Zoom before and haven’t checked the meeting leader security code. To succeed in this attack, Mallory could instruct the Zoom server to lie to Alice about Bob’s keys and to Bob about Alice’s keys, replacing them with keys she controls. If Bob’s client is the only one to see the fake key for Alice, and similarly Alice’s is the only client who gets the fake key for Bob, then such an attack would be hard to detect after the fact. Some possible countermeasures for such attacks require trusted external entities or manual validation steps (such as checking the security codes described in Section 7.7) that potentially have to be performed out-of-band. Instead, we will be able to detect equivocation by Zoom servers and identity providers while minimizing active checking by the user. To do so, we will ensure that Zoom servers provide the same mapping between user accounts and public keys to all clients, sign such a mapping, and are held accountable for these signed statements. This way, in order to compromise a single meeting, Zoom would have to lie not only to Alice about Bob’s keys (and vice versa), but also to every other Zoom user about those keys, including lying to Bob about his own keys. Bob’s client can thus easily review the list of his devices and discover any suspicious activity. External auditors will routinely verify that the server’s mapping is consistent over time. Thus, key fingerprint comparisons and other related warnings can be demoted in the user experience, to be replaced with targeted security alerts (which we expect never to be triggered). Key security becomes virtually invisible to the user.