Subclasses need to override this method to return the Key that this
keyIdentifier refers to. Although a EncryptedKeyIdentifer refers to a an EncryptedKey, clients
need not store the EncryptedKey, they might just maintain a hash table of the encryptedKeySHA1
and the key
Returns:
the Key after decrypting the EncryptedKey that this EncryptedKeyIdentifier refers to