Cryptomator Hub facilitates asymmetric encryption to allow sharing the key material used in Cryptomator vaults between multiple parties.
Cryptomator Hub facilitates different keys types. Here is an overview of these types and how they are interconnected:
User Key Pair
The purpose of the user key is to access secrets that have been shared with this user using ECDH-ES-encrypted JWEs, most prominently the masterkey of shared vaults.
If users wish to rotate their keys, e.g. when a device may be compromised, they can simply re-roll the key pair, re-encrypt secrets that they whish to keep access to and delete the old key pair.
Device Key Pair
Every device requires a key pair, which is generated on first use. The private key is securely stored on-device and not intended to ever leave it. For example, on web browsers the private key is non-extractable and stored in the browser’s IndexedDB.
A device is any client that interacts with Cryptomator Hub on behalf of a user. This definition includes the web browser used to access the Hub web interface as well as the mobile app on a user’s smartphone. On multi-user systems, every user is expected to have a separate user account, in which case we’re talking about multiple devices with distinct key pairs, even if they share the same hardware.
Users can invalidate devices by simply deleting the device-specific JWE and rotating their user key.
When users attempt to access their account from a new device, there is no device-specific JWE yet. Instead they can then use the Account Key to decrypt the User Key. The Account Key acts as a password to derive a key for a PBES2-encrypted JWE.
The Account Key needs to be kept secret, as it is the only user-facing secret that allows anyone knowing it to authorize as the corresponding user.
When an Account Key is suspected of being compromised, it can and should be re-generated from the user’s profile page, which will immediately invalidate any circulating copies.
The Account Key itself is stored as an ECDH-ES-encrypted JWE, allowing its owner to view it from any authorized device. Regardless it should be securely stored independently.