Retrieves the SecretKey for the given label.
This method queries for a SecretKey with the given label and no passphrase.
Subclasses override this method if additional properties are needed to retrieve the key.
label - The label to query
InvalidKeyException - If there is a Key but it is not a SecretKey
NoSuchAlgorithmException - If the recovery algorithm is not supported
UnrecoverableKeyException - If the key could not be recovered for some reason
The key for the given label, or
null if not present