8.1.2 Phone Security Code

To defend against MitM attacks, Zoom Phone provides a “phone security code” that has a similar format to the meeting leader security code (Section 7.7), but that is derived from the ephemeral public keys of both parties. Since the set of participants is fixed, there is no concern about this code changing too frequently. The security code is computed as Digits(SHA256(Context||pkCaller||pkCallee||CallSessionID)), where Context is the string "Zoombase-2-ClientOnly-KDF-PhoneSecurityCode". The user who initiated the call is designated the Caller, and the other user is the Callee.