Merge "Fix typos in KM4 interface definition documentation"

     * device with a StrongBox Keymaster has two Keymasters instances, because there must be a TEE

     * Keymaster as well.  The HMAC key used to MAC and verify authentication tokens must be shared

     * between TEE and StrongBox so they can each validate tokens produced by the other.  This

     * method is the second and final step in the process for for agreeing on a shared key.  It is

     * method is the second and final step in the process for agreeing on a shared key.  It is

     * called by Keystore during startup if and only if Keystore loads multiple Keymaster HALs.

     * Keystore calls it on each of the HAL instances, and sends to it all of the

     * HmacSharingParameters returned by all HALs.

     *     Any method of securely establishing K that ensures that an attacker cannot obtain or

     *     derive its value is acceptable.  What follows is a recommended approach, to be executed

     *     during each factory reset.  It relies on use of the factory-installed attestation keys to

     *     mitigate man-in-the-middle attacks.  This protocol requires that one of the instancess

     *     mitigate man-in-the-middle attacks.  This protocol requires that one of the instances

     *     have secure persistent storage.  This model was chosen because StrongBox has secure

     *     persistent storage (by definition), but the TEE may not.  The instance without storage is

     *     assumed to be able to derive a unique hardware-bound key (HBK) which is used only for