How to use the pgpy.constants.SymmetricKeyAlgorithm function in PGPy

m = bytearray(self.ct.decrypt(decrypter, *decargs))

        """
        The value "m" in the above formulas is derived from the session key
        as follows.  First, the session key is prefixed with a one-octet
        algorithm identifier that specifies the symmetric encryption
        algorithm used to encrypt the following Symmetrically Encrypted Data
        Packet.  Then a two-octet checksum is appended, which is equal to the
        sum of the preceding session key octets, not including the algorithm
        identifier, modulo 65536.  This value is then encoded as described in
        PKCS#1 block encoding EME-PKCS1-v1_5 in Section 7.2.1 of [RFC3447] to
        form the "m" value used in the formulas above.  See Section 13.1 of
        this document for notes on OpenPGP's use of PKCS#1.
        """

        symalg = SymmetricKeyAlgorithm(m[0])
        del m[0]

        symkey = m[:symalg.key_size // 8]
        del m[:symalg.key_size // 8]

        checksum = self.bytes_to_int(m[:2])
        del m[:2]

        if not sum(symkey) % 65536 == checksum:  # pragma: no cover
            raise PGPDecryptionError("{:s} decryption failed".format(self.pkalg.name))

        return (symalg, symkey)

def gen_key(username):
    print("generating OpenPGP key pair")
    key = pgpy.PGPKey.new(PubKeyAlgorithm.RSAEncryptOrSign, 4096)
    uid = pgpy.PGPUID.new(username, email='%s@%s' % (username, _provider))
    key.add_uid(
        uid,
        usage={KeyFlags.EncryptCommunications},
        hashes=[HashAlgorithm.SHA512],
        ciphers=[SymmetricKeyAlgorithm.AES256],
        compression=[CompressionAlgorithm.Uncompressed]
    )
    return key

def test_encrypt_message(self, pub, cipher):
        if pub.key_algorithm in {PubKeyAlgorithm.DSA}:
            pytest.skip('Asymmetric encryption only implemented for RSA/ECDH currently')

        if cipher in {SymmetricKeyAlgorithm.Plaintext, SymmetricKeyAlgorithm.Twofish256, SymmetricKeyAlgorithm.IDEA}:
            pytest.xfail('Symmetric cipher {} not supported for encryption'.format(cipher))

        # test encrypting a message
        mtxt = "This message will have been encrypted"
        msg = PGPMessage.new(mtxt)
        with warnings.catch_warnings():
            warnings.simplefilter('ignore')
            emsg = pub.encrypt(msg, cipher=cipher)
        self.msgs[(pub.fingerprint, cipher)] = emsg

def test_protect_protected_key(self, rsa_enc, recwarn):
        rsa_enc.protect('QwertyUiop', SymmetricKeyAlgorithm.CAST5, HashAlgorithm.SHA1)

        w = recwarn.pop(UserWarning)
        assert str(w.message) == "This key is already protected with a passphrase - " \
                                 "please unlock it before attempting to specify a new passphrase"
        assert w.filename == __file__

def test_encrypt_passphrase_2(self):
        mtxt = "This message is to be encrypted"
        msg = PGPMessage.new(mtxt)
        assert not msg.is_encrypted

        sk = SymmetricKeyAlgorithm.AES256.gen_key()
        encmsg = msg.encrypt("QwertyUiop", sessionkey=sk).encrypt("AsdfGhjkl", sessionkey=sk)

        assert isinstance(encmsg, PGPMessage)
        assert encmsg.is_encrypted
        assert encmsg.type == 'encrypted'

        # decrypt with PGPy only, since GnuPG can't do multiple passphrases
        for passwd in ["QwertyUiop", "AsdfGhjkl"]:
            decmsg = encmsg.decrypt(passwd)

            assert isinstance(decmsg, PGPMessage)
            assert decmsg.type == msg.type
            assert decmsg.is_compressed
            assert decmsg.message == mtxt

def test_add_altuid(self, pkspec):
        if pkspec not in self.keys:
            pytest.skip('Keyspec {} not in keys; must not have generated'.format(pkspec))

        key = self.keys[pkspec]
        uid = PGPUID.new('T. Keyerson', 'Secondary UID', 'testkey@localhost.local')

        expiration = datetime.utcnow() + timedelta(days=2)

        # add all of the sbpackets that only work on self-certifications
        with warnings.catch_warnings():
            warnings.simplefilter('ignore')
            key.add_uid(uid,
                        usage=[KeyFlags.Certify, KeyFlags.Sign],
                        ciphers=[SymmetricKeyAlgorithm.AES256, SymmetricKeyAlgorithm.Camellia256],
                        hashes=[HashAlgorithm.SHA384],
                        compression=[CompressionAlgorithm.ZLIB],
                        key_expiration=expiration,
                        keyserver_flags={KeyServerPreferences.NoModify},
                        keyserver='about:none',
                        primary=False)

        sig = uid.selfsig

        assert sig.type == SignatureType.Positive_Cert
        assert sig.cipherprefs == [SymmetricKeyAlgorithm.AES256, SymmetricKeyAlgorithm.Camellia256]
        assert sig.hashprefs == [HashAlgorithm.SHA384]
        assert sig.compprefs == [CompressionAlgorithm.ZLIB]
        assert sig.features == {Features.ModificationDetection}
        assert sig.key_expiration == expiration - key.created
        assert sig.keyserver == 'about:none'

    @encalg.register(SymmetricKeyAlgorithm)
    def encalg_int(self, val):
        self._encalg = SymmetricKeyAlgorithm(val)

:type passphrase: ``str``, ``unicode``, ``bytes``

        :optional param sessionkey: Provide a session key to use when encrypting something. Default is ``None``.
                                    If ``None``, a session key of the appropriate length will be generated randomly.

                                    .. warning::

                                        Care should be taken when making use of this option! Session keys *absolutely need*
                                        to be unpredictable! Use the ``gen_key()`` method on the desired
                                        :py:obj:`~constants.SymmetricKeyAlgorithm` to generate the session key!

        :type sessionkey: ``bytes``, ``str``
        :raises: :py:exc:`~errors.PGPEncryptionError`
        :returns: A new :py:obj:`PGPMessage` containing the encrypted contents of this message.
        """
        cipher_algo = prefs.pop('cipher', SymmetricKeyAlgorithm.AES256)
        hash_algo = prefs.pop('hash', HashAlgorithm.SHA256)

        # set up a new SKESessionKeyV4
        skesk = SKESessionKeyV4()
        skesk.s2k.usage = 255
        skesk.s2k.specifier = 3
        skesk.s2k.halg = hash_algo
        skesk.s2k.encalg = cipher_algo
        skesk.s2k.count = skesk.s2k.halg.tuned_count

        if sessionkey is None:
            sessionkey = cipher_algo.gen_key()
        skesk.encrypt_sk(passphrase, sessionkey)
        del passphrase

        msg = PGPMessage() | skesk

def key_size(self):
        ks = {SymmetricKeyAlgorithm.IDEA: 128,
              SymmetricKeyAlgorithm.TripleDES: 192,
              SymmetricKeyAlgorithm.CAST5: 128,
              SymmetricKeyAlgorithm.Blowfish: 128,
              SymmetricKeyAlgorithm.AES128: 128,
              SymmetricKeyAlgorithm.AES192: 192,
              SymmetricKeyAlgorithm.AES256: 256,
              SymmetricKeyAlgorithm.Twofish256: 256,
              SymmetricKeyAlgorithm.Camellia128: 128,
              SymmetricKeyAlgorithm.Camellia192: 192,
              SymmetricKeyAlgorithm.Camellia256: 256}

        if self in ks:
            return ks[self]

        raise NotImplementedError(repr(self))

def decrypt_sk(self, passphrase):
        # derive the first session key from our passphrase
        sk = self.s2k.derive_key(passphrase)
        del passphrase

        # if there is no ciphertext, then the first session key is the session key being used
        if len(self.ct) == 0:
            return self.symalg, sk

        # otherwise, we now need to decrypt the encrypted session key
        m = bytearray(_decrypt(bytes(self.ct), sk, self.symalg))
        del sk

        symalg = SymmetricKeyAlgorithm(m[0])
        del m[0]

        return symalg, bytes(m)