  The classification of quantum symmetric-key encryption protocol is presented.
According to five elements of a quantum symmetric-key encryption protocol:
plaintext, ciphertext, key, encryption algorithm and decryption algorithm,
there are 32 different kinds of them. Among them, 5 kinds of protocols have
already been constructed and studied, and 21 kinds of them are proved to be
impossible to construct, the last 6 kinds of them are not yet presented
effectively. That means the research on quantum symmetric-key encryption
protocol only needs to consider with 5 kinds of them nowadays.
