3.11 Security Properties

The identity and key management system described in this section is leveraged by multiple Zoom products, and it provides some lower level security properties that these products build upon. First, we note that the secret keys corresponding to any device public keys included in any sigchain are known only to the device that generated them (unless an attacker has somehow gained access to the device’s memory or storage): secret keys never leave the device,6 and are only used to perform encryption and signing. A consequence is that ciphertexts/signatures for/by each of the device keys can only be decrypted/created by the device that generated these keys. Virtual device keys are an exception. For example, backup keys are shown once to the user as text strings that can be written down, and therefore our guarantees depend on these keys being kept securely by the user, and not

Except possibly when the user voluntarily sends crash reports to Zoom. We try to minimize this risk, but cannot exclude it. Being available to an attacker. A second exception is escrow keys, where the escrow device secret key is possibly generated by one of the user’s own devices and encrypted for the account’s escrow admin’s PUK: this gives the EA’s devices the same access to the user’s keys as the user’s own devices. Similarly, consider any valid user sigchain. The secret keys corresponding to any per-user public key that appears in that sigchain are only known to any devices that were added but not revoked before the per-user key was added to that sigchain, plus any devices that were added afterwards and approved by one of those devices (as recorded in each device’s own view of the user’s sigchain). If escrow is enabled or the user creates backup keys, some EA devices and those with access to the backup keys might also learn these PUKs. Also, note that the Zoom server cannot force any devices to forget identity updates like device revocations: when receiving sigchains, devices only accept new sigchain links that extend the ones they are already aware of.