Key revocation not only allows for good key management practices (periodic key changes) but also can serve as a tamper penalty. This dual role can be a very valuable addition to a secure system. In the event of a tamper event, the system can revoke the PPK or SPK currently being used and initiate a reset. This revocation invalidates the current boot image and prevents the system from booting, which halts operation and protects the system from additional threats. The system would then have to be taken back to the depot and flashed with an image signed by a different (valid) key. This method represents a temporary penalty. However, some systems might desire a more drastic response. In this case, the system that detects the tamper event can revoke all PPKs. This revocation essentially “bricks” the part as there is no longer a valid key with which to boot (all have been revoked). This is a permanent penalty and is typically used only in the most secure systems as there is no method to recover the use of the device.