This enumeration type defines the policy used to derive a seal key.
Key is derived from a measurement of the enclave.
Under this policy, the sealed secret can only be unsealed by an instance of the exact enclave code that sealed it.
Key is derived from the signer of the enclave.
Under this policy, the sealed secret can be unsealed by any enclave signed by the same signer as that of the sealing enclave.
Unused.