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c) A capability to revoke passphrase from Controller, in case the EVE node is found to be compromised, which will render the contents of the drive permanently unrecoverable (Remote-Wipe)
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Key Revocation)
Controller-Managed Key with TPM
On EVE nodes with TPM devices without TPM, we do not have a way to measure the software state or seal the encryption key outside the hard drive. The only other place for creating and storing the keys is the Controller. Therefore for EVE nodes without TPM, it is proposed that encryption keys will be stored in the Controller. After device registers and gets UUID from the Controller, it would fetch either new encryption key(if booting up for the first time) or get the stored encryption key from the Controller. After this EVE will either setup the directory for encryption (if booting up for the first time) or unlock the encrypted directory with the key fetched from the Controller.
Physical Isolation: Yes
Node Authentication & Attestation: No
Remote-Wipe: Yes
EVE node with a TPM
On EVE nodes with TPM devices, TPM can be used for sealing the master passphrase against a set of PCR values, so the master passphrase will be unsealed only when the PCRs values indicate that there is no change in the software state. Once EVE supports measured boot(that measures all the components in the boot chain, and storing its measurements in the TPM PCRs), we can start using those PCRs to seal the master passphrase. Also, if we store passphrase in TPM, if the hard disk alone is physically compromised, the keys are still not available on the hard disk, so there will not be a way to decrypt the data on the disk, but if the entire device is physically compromised(which is more likely than just the drive being physically compromised), TPM will automatically unseal the encryption key.
Physical Isolation: Yes
Node Authentication & Attestation: Yes
Remote-Wipe: No
EVE node with a TPM - An Enhanced Approach using Two-Factor Authentication
To address the Remote-Wipe concern in the above approach, we can come-up with an enhanced model for EVE nodes with a TPM: Use a key from TPM and a key from Controller to arrive at the master encryption key. It is the best of both of the above approaches put together. With this, even if the entire EVE node is physically compromised, one can not access the complete encryption key unless the EVE node is authenticated and attested by the Controller, which can be advised to flag a given EVE node as black-listed, and hence revoke it's authentication and hence, the encryption key as well, TPM can be used for sealing the master passphrase against a set of PCR values, so the master passphrase will be unsealed only when the PCRs values indicate that there is no change in the software state. Once EVE supports measured boot(that measures all the components in the boot chain, and storing its measurements in the TPM PCRs), we can start using those PCRs to seal the master passphrase. Also, if we store passphrase in TPM, if the hard disk alone is physically compromised, the keys are still not available on the hard disk, so there will not be a way to decrypt the data on the disk, but if the entire device is physically compromised(which is more likely than just the drive being physically compromised), TPM will automatically unseal the encryption key.
Therefore, we can enhance this and arrive at a better model for EVE nodes with a TPM: Use a key from TPM and a key from Controller to arrive at the master encryption key. It is the best of both of the above approaches put together. With this, even if the entire EVE node is physically compromised, one can not access the complete encryption key unless the EVE node is authenticated and attested by the Controller, which can be advised to flag a given EVE node as black-listed, and hence revoke it's authentication and hence, the encryption key as well.
Physical Isolation: Yes
Node Authentication & Attestation: Yes
Key Revocation: Yes
Controller-Managed Key without TPM
On devices without TPM, we do not have a way to measure the software state or seal the encryption key outside the hard drive. The only other place for creating and storing the keys is the Controller. Therefore for EVE nodes without TPM, it is proposed that encryption keys will be stored in the Controller. After device registers and gets UUID from the Controller, it would fetch either new encryption key(if booting up for the first time) or get the stored encryption key from the Controller. After this EVE will either setup the directory for encryption (if booting up for the first time) or unlock the encrypted directory with the key fetched from the Controller.
Physical Isolation: Yes
Node Authentication & Attestation: YesRemote-Wipe No
Key Revocation: Yes
Master Key Rotation
Fscrypt supports changing the protectors password without re-encrypting all the files in the encrypted directory. If required, we can change the protector password, if we think that protector keys might have been compromised. In future we can consider rotating these protectors periodically (say every week) as a precautionary measure to enhance the security. The frequency can be either on-demand, or a configured time-interval which can be configured for each EVE node from the Controller. However care needs to be taken to not miss any rotation from Controller, as this will render the whole data locked forever.(i.e. Controller moved from key 2 to key 3, but EVE is yet to process the last config change from key 1 to key 2)
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