But then you have to find a place to store the key that can be securely erased. Perhaps there is some sort of hardware enclave you can misuse. Even a tiny amount of securely erasable flash would be the answer.
A TPM can only store a limited number of keys. You need a forseparate key for anything you want to securely delete and in a lot of applications you might have a lot of things you want to delete separately.
You can pretty easily expand one secure, rotatable key into N. 1. Don't use TPM key directly, use it to encrypt the list of working keys. 2. Store the TPM-encrypted list of working keys on disk. 3. When you need to drop a working key, remove it from the list, rotate the TPM key and reencrypt all the working keys, and store the new list on disk again. Remnants of the old list are irrecoverable because the old TPM key doesn't exist anymore, and the new list is inaccessible without the new TPM key. There, now you have an arbitrary number of secure keys and can drop them individually.
Great point. This assumes that the TPM does secure deletes. Their primary purpose is protect keys, not get rid of them. I think in practice a TPM is a small enough system that the deletion would be secure just because that is the simplest way to do that. If you do this enough then some overwriting will likely occur. I guess media endurance could be a problem in some cases.
This is the theory, where you never have to store the key on disk. In reality you store the key on disk while performing actions that would block the TPM chip from releasing the key, such as upgrading the firmware.
