Key Destruction
Once a key has been created, it's sometimes important to be able to destroy it as well. One example is if a user is going to sell, surplus, or recycle a computer and wants to make sure data that was encrypted on that system with that key is no longer available. TPMs provide this facility in a number of easy ways.
If the key used is a primary key, the easiest way to destroy it is to ask the TPM to change its copy of the seed of the hierarchy on which it was created (usually the storage hierarchy). TPM2_Clear does this for the storage hierarchy. Clearing the TPM destroys all non-duplicable keys that are associated with the hierarchy, evicts all keys in the hierarchy from the TPM, and changes the seed, preventing any primary keys previously associated with that hierarchy from being re-generated. It also flushes the endorsement hierarchy, but it doesn't change that seed. [1] Duplicable keys can no longer be loaded into the system, although if they have been duplicated to a different system, they may not be destroyed.
If such a drastic step isn't necessary (perhaps the machine is only going to be loaned for a time to a different employee, or multiple employees are using the machine), other things can be done, if preparations are made ahead of time. For example, if a primary key is generated with secret entropy in the template and then made into a persistent key, then the only thing that needs to be done to destroy the key is to destroy the copies of the template and evict the primary key from the persistent storage. Once this is done, the key is gone and can't be re-generated. Because there can be multiple trees underneath different primary keys, this provides a way to destroy a particular tree of keys without destroying all the trees of keys in a TPM. This may be important if multiple users are using the same TPM.
It's even possible to destroy keys that are generated outside the TPM, imported into the TPM, and then made persistent. If the copies outside the TPM are destroyed (which may be possible if the import was done in a controlled facility), then merely evicting the key from persistent memory also destroys the key.
Putting It All Together
This section provides two examples of how different types of businesses might decide to manage TPM entities. We start with a simple case, which might apply to a small business, and then consider a large enterprise.
- [1] This way, the EK and TPM vendor certificates are still valid.
