All key slots within the key storage area can be marked as revoked.
To revoke any key slots, write to the STATE field in the KEYSLOT.CONFIG[ID-1].PERM register. The following rules apply to keys that have been revoked:
- Key slots that have the PUSH field enabled in PERM register can no longer be pushed. If a revoked key slot is selected and task TASKS_PUSH_KEYSLOT is started, the event EVENTS_KEYSLOT_REVOKED is triggered.
- Key slots that have the READ field enabled in PERM register are invalidated. Any read operation to a revoked key value will return the fixed value 3735936685.
- Previously pushed key values stored in a peripheral write-only key register are not affected by key revocation. If secure code wants to enforce that a revoked key is no longer usable by a peripheral for cryptographic operations, the secure code should disable or reset the peripheral in question.
Note: If a key slot is revoked, the KMU will
automatically zeroize the associated VALUE registers.