# Copyright (c) 2013-2022 by Ron Frederick <ronf@timeheart.net> and others.
#
# This program and the accompanying materials are made available under
# the terms of the Eclipse Public License v2.0 which accompanies this
# distribution and is available at:
#
#     http://www.eclipse.org/legal/epl-2.0/
#
# This program may also be made available under the following secondary
# licenses when the conditions for such availability set forth in the
# Eclipse Public License v2.0 are satisfied:
#
#    GNU General Public License, Version 2.0, or any later versions of
#    that license
#
# SPDX-License-Identifier: EPL-2.0 OR GPL-2.0-or-later
#
# Contributors:
#     Ron Frederick - initial implementation, API, and documentation

"""SSH Diffie-Hellman, ECDH, and Edwards DH key exchange handlers"""

from hashlib import sha1, sha224, sha256, sha384, sha512
from typing import TYPE_CHECKING, Callable, Mapping, Optional, cast
from typing_extensions import Protocol

from .constants import DEFAULT_LANG
from .crypto import curve25519_available, curve448_available
from .crypto import Curve25519DH, Curve448DH, DH, ECDH
from .crypto import sntrup761_available
from .crypto import sntrup761_pubkey_bytes, sntrup761_ciphertext_bytes
from .crypto import sntrup761_keypair, sntrup761_encaps, sntrup761_decaps
from .gss import GSSError
from .kex import Kex, register_kex_alg, register_gss_kex_alg
from .misc import HashType, KeyExchangeFailed, ProtocolError
from .misc import get_symbol_names
from .packet import Boolean, MPInt, String, UInt32, SSHPacket
from .public_key import SigningKey, VerifyingKey


if TYPE_CHECKING:
    # pylint: disable=cyclic-import
    from .connection import SSHConnection, SSHClientConnection
    from .connection import SSHServerConnection


class DHKey(Protocol):
    """Protocol for performing Diffie-Hellman key exchange"""

    def get_public(self) -> bytes:
        """Return the public key to send to the peer"""

    def get_shared(self, peer_public: bytes) -> int:
        """Return the shared key from the peer's public key"""


_ECDHClass = Callable[..., DHKey]


# pylint: disable=line-too-long

# SSH KEX DH message values
MSG_KEXDH_INIT             = 30
MSG_KEXDH_REPLY            = 31

# SSH KEX DH group exchange message values
MSG_KEX_DH_GEX_REQUEST_OLD = 30
MSG_KEX_DH_GEX_GROUP       = 31
MSG_KEX_DH_GEX_INIT        = 32
MSG_KEX_DH_GEX_REPLY       = 33
MSG_KEX_DH_GEX_REQUEST     = 34

# SSH KEX ECDH message values
MSG_KEX_ECDH_INIT          = 30
MSG_KEX_ECDH_REPLY         = 31

# SSH KEXGSS message values
MSG_KEXGSS_INIT            = 30
MSG_KEXGSS_CONTINUE        = 31
MSG_KEXGSS_COMPLETE        = 32
MSG_KEXGSS_HOSTKEY         = 33
MSG_KEXGSS_ERROR           = 34
MSG_KEXGSS_GROUPREQ        = 40
MSG_KEXGSS_GROUP           = 41

# SSH KEX group exchange key sizes
KEX_DH_GEX_MIN_SIZE        = 1024
KEX_DH_GEX_PREFERRED_SIZE  = 2048
KEX_DH_GEX_MAX_SIZE        = 8192

# SSH Diffie-Hellman group 1 parameters
_group1_g = 2
_group1_p = 0xffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024e088a67cc74020bbea63b139b22514a08798e3404ddef9519b3cd3a431b302b0a6df25f14374fe1356d6d51c245e485b576625e7ec6f44c42e9a637ed6b0bff5cb6f406b7edee386bfb5a899fa5ae9f24117c4b1fe649286651ece65381ffffffffffffffff

# SSH Diffie-Hellman group 14 parameters
_group14_g = 2
_group14_p = 0xffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024e088a67cc74020bbea63b139b22514a08798e3404ddef9519b3cd3a431b302b0a6df25f14374fe1356d6d51c245e485b576625e7ec6f44c42e9a637ed6b0bff5cb6f406b7edee386bfb5a899fa5ae9f24117c4b1fe649286651ece45b3dc2007cb8a163bf0598da48361c55d39a69163fa8fd24cf5f83655d23dca3ad961c62f356208552bb9ed529077096966d670c354e4abc9804f1746c08ca18217c32905e462e36ce3be39e772c180e86039b2783a2ec07a28fb5c55df06f4c52c9de2bcbf6955817183995497cea956ae515d2261898fa051015728e5a8aacaa68ffffffffffffffff

# SSH Diffie-Hellman group 15 parameters
_group15_g = 2
_group15_p = 0xffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024e088a67cc74020bbea63b139b22514a08798e3404ddef9519b3cd3a431b302b0a6df25f14374fe1356d6d51c245e485b576625e7ec6f44c42e9a637ed6b0bff5cb6f406b7edee386bfb5a899fa5ae9f24117c4b1fe649286651ece45b3dc2007cb8a163bf0598da48361c55d39a69163fa8fd24cf5f83655d23dca3ad961c62f356208552bb9ed529077096966d670c354e4abc9804f1746c08ca18217c32905e462e36ce3be39e772c180e86039b2783a2ec07a28fb5c55df06f4c52c9de2bcbf6955817183995497cea956ae515d2261898fa051015728e5a8aaac42dad33170d04507a33a85521abdf1cba64ecfb850458dbef0a8aea71575d060c7db3970f85a6e1e4c7abf5ae8cdb0933d71e8c94e04a25619dcee3d2261ad2ee6bf12ffa06d98a0864d87602733ec86a64521f2b18177b200cbbe117577a615d6c770988c0bad946e208e24fa074e5ab3143db5bfce0fd108e4b82d120a93ad2caffffffffffffffff

# SSH Diffie-Hellman group 16 parameters
_group16_g = 2
_group16_p = 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

# SSH Diffie-Hellman group 17 parameters
_group17_g = 2
_group17_p = 0xffffffffffffffffc90fdaa22168c234c4c6628b80dc1cd129024e088a67cc74020bbea63b139b22514a08798e3404ddef9519b3cd3a431b302b0a6df25f14374fe1356d6d51c245e485b576625e7ec6f44c42e9a637ed6b0bff5cb6f406b7edee386bfb5a899fa5ae9f24117c4b1fe649286651ece45b3dc2007cb8a163bf0598da48361c55d39a69163fa8fd24cf5f83655d23dca3ad961c62f356208552bb9ed529077096966d670c354e4abc9804f1746c08ca18217c32905e462e36ce3be39e772c180e86039b2783a2ec07a28fb5c55df06f4c52c9de2bcbf6955817183995497cea956ae515d2261898fa051015728e5a8aaac42dad33170d04507a33a85521abdf1cba64ecfb850458dbef0a8aea71575d060c7db3970f85a6e1e4c7abf5ae8cdb0933d71e8c94e04a25619dcee3d2261ad2ee6bf12ffa06d98a0864d87602733ec86a64521f2b18177b200cbbe117577a615d6c770988c0bad946e208e24fa074e5ab3143db5bfce0fd108e4b82d120a92108011a723c12a787e6d788719a10bdba5b2699c327186af4e23c1a946834b6150bda2583e9ca2ad44ce8dbbbc2db04de8ef92e8efc141fbecaa6287c59474e6bc05d99b2964fa090c3a2233ba186515be7ed1f612970cee2d7afb81bdd762170481cd0069127d5b05aa993b4ea988d8fddc186ffb7dc90a6c08f4df435c93402849236c3fab4d27c7026c1d4dcb2602646dec9751e763dba37bdf8ff9406ad9e530ee5db382f413001aeb06a53ed9027d831179727b0865a8918da3edbebcf9b14ed44ce6cbaced4bb1bdb7f1447e6cc254b332051512bd7af426fb8f401378cd2bf5983ca01c64b92ecf032ea15d1721d03f482d7ce6e74fef6d55e702f46980c82b5a84031900b1c9e59e7c97fbec7e8f323a97a7e36cc88be0f1d45b7ff585ac54bd407b22b4154aacc8f6d7ebf48e1d814cc5ed20f8037e0a79715eef29be32806a1d58bb7c5da76f550aa3d8a1fbff0eb19ccb1a313d55cda56c9ec2ef29632387fe8d76e3c0468043e8f663f4860ee12bf2d5b0b7474d6e694f91e6dcc4024ffffffffffffffff

# SSH Diffie-Hellman group 18 parameters
_group18_g = 2
_group18_p = 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

_dh_gex_groups = ((1024, _group1_g,  _group1_p),
                  (2048, _group14_g, _group14_p),
                  (3072, _group15_g, _group15_p),
                  (4096, _group16_g, _group16_p),
                  (6144, _group17_g, _group17_p),
                  (8192, _group18_g, _group18_p))

# pylint: enable=line-too-long


class _KexDHBase(Kex):
    """Abstract base class for Diffie-Hellman key exchange"""

    _init_type: int = 0
    _reply_type: int = 0

    def __init__(self, alg: bytes, conn: 'SSHConnection', hash_alg: HashType):
        super().__init__(alg, conn, hash_alg)

        self._dh: Optional[DH] = None
        self._g = 0
        self._p = 0
        self._e = 0
        self._f = 0
        self._gex_data = b''

    def _init_group(self, g: int, p: int) -> None:
        """Initialize DH group parameters"""

        self._g = g
        self._p = p

    def _compute_hash(self, host_key_data: bytes, k: bytes) -> bytes:
        """Compute a hash of key information associated with the connection"""

        hash_obj = self._hash_alg()
        hash_obj.update(self._conn.get_hash_prefix())
        hash_obj.update(String(host_key_data))
        hash_obj.update(self._gex_data)
        hash_obj.update(self._format_client_key())
        hash_obj.update(self._format_server_key())
        hash_obj.update(k)
        return hash_obj.digest()

    def _parse_client_key(self, packet: SSHPacket) -> None:
        """Parse a DH client key"""

        if not self._p:
            raise ProtocolError('Kex DH p not specified')

        self._e = packet.get_mpint()

    def _parse_server_key(self, packet: SSHPacket) -> None:
        """Parse a DH server key"""

        if not self._p:
            raise ProtocolError('Kex DH p not specified')

        self._f = packet.get_mpint()

    def _format_client_key(self) -> bytes:
        """Format a DH client key"""

        return MPInt(self._e)

    def _format_server_key(self) -> bytes:
        """Format a DH server key"""

        return MPInt(self._f)

    def _send_init(self) -> None:
        """Send a DH init message"""

        self.send_packet(self._init_type, self._format_client_key())

    def _send_reply(self, key_data: bytes, sig: bytes) -> None:
        """Send a DH reply message"""

        self.send_packet(self._reply_type, String(key_data),
                         self._format_server_key(), String(sig))

    def _perform_init(self) -> None:
        """Compute e and send init message"""

        self._dh = DH(self._g, self._p)
        self._e = self._dh.get_public()

        self._send_init()

    def _compute_client_shared(self) -> bytes:
        """Compute client shared key"""

        if not 1 <= self._f < self._p:
            raise ProtocolError('Kex DH f out of range')

        assert self._dh is not None
        return MPInt(self._dh.get_shared(self._f))

    def _compute_server_shared(self) -> bytes:
        """Compute server shared key"""

        if not 1 <= self._e < self._p:
            raise ProtocolError('Kex DH e out of range')

        self._dh = DH(self._g, self._p)
        self._f = self._dh.get_public()

        return MPInt(self._dh.get_shared(self._e))

    def _perform_reply(self, key: SigningKey, key_data: bytes) -> None:
        """Compute server shared key and send reply message"""

        k = self._compute_server_shared()
        h = self._compute_hash(key_data, k)
        self._send_reply(key_data, key.sign(h))

        self._conn.send_newkeys(k, h)

    def _verify_reply(self, key: VerifyingKey, key_data: bytes,
                      sig: bytes) -> None:
        """Verify a DH reply message"""

        k = self._compute_client_shared()
        h = self._compute_hash(key_data, k)

        if not key.verify(h, sig):
            raise KeyExchangeFailed('Key exchange hash mismatch')

        self._conn.send_newkeys(k, h)

    def _process_init(self, _pkttype: int, _pktid: int,
                      packet: SSHPacket) -> None:
        """Process a DH init message"""

        if self._conn.is_client():
            raise ProtocolError('Unexpected kex init msg')

        self._parse_client_key(packet)
        packet.check_end()

        server_conn = cast('SSHServerConnection', self._conn)
        host_key = server_conn.get_server_host_key()
        assert host_key is not None

        self._perform_reply(host_key, host_key.public_data)

    def _process_reply(self, _pkttype: int, _pktid: int,
                       packet: SSHPacket) -> None:
        """Process a DH reply message"""

        if self._conn.is_server():
            raise ProtocolError('Unexpected kex reply msg')

        host_key_data = packet.get_string()
        self._parse_server_key(packet)
        sig = packet.get_string()
        packet.check_end()

        client_conn = cast('SSHClientConnection', self._conn)
        host_key = client_conn.validate_server_host_key(host_key_data)
        self._verify_reply(host_key, host_key_data, sig)

    def start(self) -> None:
        """Start DH key exchange"""

        if self._conn.is_client():
            self._perform_init()


class _KexDH(_KexDHBase):
    """Handler for Diffie-Hellman key exchange"""

    _handler_names = get_symbol_names(globals(), 'MSG_KEXDH_')

    _init_type = MSG_KEXDH_INIT
    _reply_type = MSG_KEXDH_REPLY

    def __init__(self, alg: bytes, conn: 'SSHConnection',
                 hash_alg: HashType, g: int, p: int):
        super().__init__(alg, conn, hash_alg)

        self._init_group(g, p)

    _packet_handlers: Mapping[int, Callable]= {
        MSG_KEXDH_INIT:     _KexDHBase._process_init,
        MSG_KEXDH_REPLY:    _KexDHBase._process_reply
    }


class _KexDHGex(_KexDHBase):
    """Handler for Diffie-Hellman group exchange"""

    _handler_names = get_symbol_names(globals(), 'MSG_KEX_DH_GEX_')

    _init_type = MSG_KEX_DH_GEX_INIT
    _reply_type = MSG_KEX_DH_GEX_REPLY
    _request_type = MSG_KEX_DH_GEX_REQUEST
    _group_type = MSG_KEX_DH_GEX_GROUP

    def __init__(self, alg: bytes, conn: 'SSHConnection', hash_alg: HashType,
                 preferred_size: Optional[int] = None,
                 max_size: Optional[int] = None):
        super().__init__(alg, conn, hash_alg)

        self._pref_size = preferred_size
        self._max_size = max_size

    def _send_request(self) -> None:
        """Send a DH gex request message"""

        if self._pref_size and not self._max_size:
            # Send old request message for unit test
            pkttype = MSG_KEX_DH_GEX_REQUEST_OLD
            args = UInt32(self._pref_size)
        else:
            pkttype = self._request_type
            args = (UInt32(KEX_DH_GEX_MIN_SIZE) +
                    UInt32(self._pref_size or KEX_DH_GEX_PREFERRED_SIZE) +
                    UInt32(self._max_size or KEX_DH_GEX_MAX_SIZE))

        self._gex_data = args
        self.send_packet(pkttype, args)

    def _process_request(self, pkttype: int, _pktid: int,
                         packet: SSHPacket) -> None:
        """Process a DH gex request message"""

        if self._conn.is_client():
            raise ProtocolError('Unexpected kex request msg')

        self._gex_data = packet.get_remaining_payload()

        if pkttype == MSG_KEX_DH_GEX_REQUEST_OLD:
            preferred_size = packet.get_uint32()
            max_size = KEX_DH_GEX_MAX_SIZE
        else:
            _ = packet.get_uint32()
            preferred_size = packet.get_uint32()
            max_size = packet.get_uint32()

        packet.check_end()

        g, p = _group1_g, _group1_p

        for gex_size, gex_g, gex_p in _dh_gex_groups:
            if gex_size > max_size:
                break
            else:
                g, p = gex_g, gex_p

                if gex_size >= preferred_size:
                    break

        self._init_group(g, p)
        self._gex_data += MPInt(p) + MPInt(g)
        self.send_packet(self._group_type, MPInt(p), MPInt(g))

    def _process_group(self, _pkttype: int, _pktid: int,
                       packet: SSHPacket) -> None:
        """Process a DH gex group message"""

        if self._conn.is_server():
            raise ProtocolError('Unexpected kex group msg')

        p = packet.get_mpint()
        g = packet.get_mpint()
        packet.check_end()

        self._init_group(g, p)
        self._gex_data += MPInt(p) + MPInt(g)
        self._perform_init()

    def start(self) -> None:
        """Start DH group exchange"""

        if self._conn.is_client():
            self._send_request()

    _packet_handlers: Mapping[int, Callable] = {
        MSG_KEX_DH_GEX_REQUEST_OLD: _process_request,
        MSG_KEX_DH_GEX_GROUP:       _process_group,
        MSG_KEX_DH_GEX_INIT:        _KexDHBase._process_init,
        MSG_KEX_DH_GEX_REPLY:       _KexDHBase._process_reply,
        MSG_KEX_DH_GEX_REQUEST:     _process_request
    }


class _KexECDH(_KexDHBase):
    """Handler for elliptic curve Diffie-Hellman key exchange"""

    _handler_names = get_symbol_names(globals(), 'MSG_KEX_ECDH_')

    _init_type = MSG_KEX_ECDH_INIT
    _reply_type = MSG_KEX_ECDH_REPLY

    def __init__(self, alg: bytes, conn: 'SSHConnection', hash_alg: HashType,
                 ecdh_class: _ECDHClass, *args: object):
        super().__init__(alg, conn, hash_alg)

        self._priv = ecdh_class(*args)
        pub = self._priv.get_public()

        if conn.is_client():
            self._client_pub = pub
        else:
            self._server_pub = pub

    def _parse_client_key(self, packet: SSHPacket) -> None:
        """Parse an ECDH client key"""

        self._client_pub = packet.get_string()

    def _parse_server_key(self, packet: SSHPacket) -> None:
        """Parse an ECDH server key"""

        self._server_pub = packet.get_string()

    def _format_client_key(self) -> bytes:
        """Format an ECDH client key"""

        return String(self._client_pub)

    def _format_server_key(self) -> bytes:
        """Format an ECDH server key"""

        return String(self._server_pub)

    def _compute_client_shared(self) -> bytes:
        """Compute client shared key"""

        try:
            return MPInt(self._priv.get_shared(self._server_pub))
        except ValueError:
            raise ProtocolError('Invalid ECDH server public key') from None

    def _compute_server_shared(self) -> bytes:
        """Compute server shared key"""

        try:
            return MPInt(self._priv.get_shared(self._client_pub))
        except ValueError:
            raise ProtocolError('Invalid ECDH client public key') from None

    def start(self) -> None:
        """Start ECDH key exchange"""

        if self._conn.is_client():
            self._send_init()

    _packet_handlers: Mapping[int, Callable] = {
        MSG_KEX_ECDH_INIT:  _KexDHBase._process_init,
        MSG_KEX_ECDH_REPLY: _KexDHBase._process_reply
    }


class _KexSNTRUP761(_KexECDH):
    """Handler for Streamlined NTRU Prime post-quantum key exchange"""

    def __init__(self, alg: bytes, conn: 'SSHConnection', hash_alg: HashType,
                 *args: object):
        super().__init__(alg, conn, hash_alg, Curve25519DH)

        if conn.is_client():
            sntrup_pub, self._sntrup_priv = sntrup761_keypair()
            self._client_pub = sntrup_pub + self._client_pub

    def _compute_client_shared(self) -> bytes:
        """Compute client shared key"""

        ciphertext = self._server_pub[:sntrup761_ciphertext_bytes]
        curve25519_pub = self._server_pub[sntrup761_ciphertext_bytes:]

        try:
            sntrup_secret = sntrup761_decaps(ciphertext, self._sntrup_priv)
        except ValueError:
            raise ProtocolError('Invalid SNTRUP server ciphertext') from None

        try:
            priv = cast(Curve25519DH, self._priv)
            curve25519_shared = priv.get_shared_bytes(curve25519_pub)
        except ValueError:
            raise ProtocolError('Invalid ECDH server public key') from None

        return String(self._hash_alg(sntrup_secret +
                                     curve25519_shared).digest())

    def _compute_server_shared(self) -> bytes:
        """Compute server shared key"""

        sntrup_pub = self._client_pub[:sntrup761_pubkey_bytes]
        curve25519_pub = self._client_pub[sntrup761_pubkey_bytes:]

        try:
            sntrup_secret, ciphertext = sntrup761_encaps(sntrup_pub)
        except ValueError:
            raise ProtocolError('Invalid SNTRUP client public key') from None

        try:
            priv = cast(Curve25519DH, self._priv)
            curve25519_shared = priv.get_shared_bytes(curve25519_pub)
        except ValueError:
            raise ProtocolError('Invalid ECDH client public key') from None

        self._server_pub = ciphertext + self._server_pub

        return String(self._hash_alg(sntrup_secret +
                                     curve25519_shared).digest())


class _KexGSSBase(_KexDHBase):
    """Handler for GSS key exchange"""

    def __init__(self, alg: bytes, conn: 'SSHConnection',
                 hash_alg: HashType, *args: object):
        super().__init__(alg, conn, hash_alg, *args)

        self._gss = conn.get_gss_context()
        self._token: Optional[bytes] = None
        self._host_key_data = b''

    def _check_secure(self) -> None:
        """Check that GSS context is secure enough for key exchange"""

        if (not self._gss.provides_mutual_auth or
                not self._gss.provides_integrity):
            raise ProtocolError('GSS context not secure')

    def _send_init(self) -> None:
        """Send a GSS init message"""

        if not self._token:
            raise ProtocolError('Empty GSS token in init')

        self.send_packet(MSG_KEXGSS_INIT, String(self._token),
                         self._format_client_key())

    def _send_reply(self, key_data: bytes, sig: bytes) -> None:
        """Send a GSS reply message"""

        if self._token:
            token_data = Boolean(True) + String(self._token)
        else:
            token_data = Boolean(False)

        self.send_packet(MSG_KEXGSS_COMPLETE, self._format_server_key(),
                         String(sig), token_data)

    def _send_continue(self) -> None:
        """Send a GSS continue message"""

        if not self._token:
            raise ProtocolError('Empty GSS token in continue')

        self.send_packet(MSG_KEXGSS_CONTINUE, String(self._token))

    def _process_token(self, token: Optional[bytes] = None) -> None:
        """Process a GSS token"""

        try:
            self._token = self._gss.step(token)
        except GSSError as exc:
            if self._conn.is_server():
                self.send_packet(MSG_KEXGSS_ERROR, UInt32(exc.maj_code),
                                 UInt32(exc.min_code), String(str(exc)),
                                 String(DEFAULT_LANG))

            if exc.token:
                self.send_packet(MSG_KEXGSS_CONTINUE, String(exc.token))

            raise KeyExchangeFailed(str(exc)) from None

    def _process_init(self, _pkttype: int, _pktid: int,
                      packet: SSHPacket) -> None:
        """Process a GSS init message"""

        if self._conn.is_client():
            raise ProtocolError('Unexpected kexgss init msg')

        token = packet.get_string()
        self._parse_client_key(packet)
        packet.check_end()

        server_conn = cast('SSHServerConnection', self._conn)
        host_key = server_conn.get_server_host_key()

        if host_key:
            self._host_key_data = host_key.public_data
            self.send_packet(MSG_KEXGSS_HOSTKEY, String(self._host_key_data))
        else:
            self._host_key_data = b''

        self._process_token(token)

        if self._gss.complete:
            self._check_secure()
            self._perform_reply(self._gss, self._host_key_data)
            self._conn.enable_gss_kex_auth()
        else:
            self._send_continue()

    def _process_continue(self, _pkttype: int, _pktid: int,
                          packet: SSHPacket) -> None:
        """Process a GSS continue message"""

        token = packet.get_string()
        packet.check_end()

        if self._conn.is_client() and self._gss.complete:
            raise ProtocolError('Unexpected kexgss continue msg')

        self._process_token(token)

        if self._conn.is_server() and self._gss.complete:
            self._check_secure()
            self._perform_reply(self._gss, self._host_key_data)
        else:
            self._send_continue()

    def _process_complete(self, _pkttype: int, _pktid: int,
                          packet: SSHPacket) -> None:
        """Process a GSS complete message"""

        if self._conn.is_server():
            raise ProtocolError('Unexpected kexgss complete msg')

        self._parse_server_key(packet)
        mic = packet.get_string()
        token_present = packet.get_boolean()
        token = packet.get_string() if token_present else None
        packet.check_end()

        if token:
            if self._gss.complete:
                raise ProtocolError('Non-empty token after complete')

            self._process_token(token)

            if self._token:
                raise ProtocolError('Non-empty token after complete')

        if not self._gss.complete:
            raise ProtocolError('GSS exchange failed to complete')

        self._check_secure()
        self._verify_reply(self._gss, self._host_key_data, mic)
        self._conn.enable_gss_kex_auth()

    def _process_hostkey(self, _pkttype: int, _pktid: int,
                         packet: SSHPacket) -> None:
        """Process a GSS hostkey message"""

        self._host_key_data = packet.get_string()
        packet.check_end()

    def _process_error(self, _pkttype: int, _pktid: int,
                       packet: SSHPacket) -> None:
        """Process a GSS error message"""

        if self._conn.is_server():
            raise ProtocolError('Unexpected kexgss error msg')

        _ = packet.get_uint32()         # major_status
        _ = packet.get_uint32()         # minor_status
        msg = packet.get_string()
        _ = packet.get_string()         # lang
        packet.check_end()

        self._conn.logger.debug1('GSS error: %s',
                                 msg.decode('utf-8', errors='ignore'))

    def start(self) -> None:
        """Start GSS key exchange"""

        if self._conn.is_client():
            self._process_token()
            super().start()


class _KexGSS(_KexGSSBase, _KexDH):
    """Handler for GSS key exchange"""

    _handler_names = get_symbol_names(globals(), 'MSG_KEXGSS_')

    _packet_handlers = {
        MSG_KEXGSS_INIT:     _KexGSSBase._process_init,
        MSG_KEXGSS_CONTINUE: _KexGSSBase._process_continue,
        MSG_KEXGSS_COMPLETE: _KexGSSBase._process_complete,
        MSG_KEXGSS_HOSTKEY:  _KexGSSBase._process_hostkey,
        MSG_KEXGSS_ERROR:    _KexGSSBase._process_error
    }


class _KexGSSGex(_KexGSSBase, _KexDHGex):
    """Handler for GSS group exchange"""

    _handler_names = get_symbol_names(globals(), 'MSG_KEXGSS_')

    _request_type = MSG_KEXGSS_GROUPREQ
    _group_type = MSG_KEXGSS_GROUP

    _packet_handlers = {
        MSG_KEXGSS_INIT:     _KexGSSBase._process_init,
        MSG_KEXGSS_CONTINUE: _KexGSSBase._process_continue,
        MSG_KEXGSS_COMPLETE: _KexGSSBase._process_complete,
        MSG_KEXGSS_HOSTKEY:  _KexGSSBase._process_hostkey,
        MSG_KEXGSS_ERROR:    _KexGSSBase._process_error,
        MSG_KEXGSS_GROUPREQ: _KexDHGex._process_request,
        MSG_KEXGSS_GROUP:    _KexDHGex._process_group
    }


class _KexGSSECDH(_KexGSSBase, _KexECDH):
    """Handler for GSS ECDH key exchange"""

    _handler_names = get_symbol_names(globals(), 'MSG_KEXGSS_')

    _packet_handlers = {
        MSG_KEXGSS_INIT:     _KexGSSBase._process_init,
        MSG_KEXGSS_CONTINUE: _KexGSSBase._process_continue,
        MSG_KEXGSS_COMPLETE: _KexGSSBase._process_complete,
        MSG_KEXGSS_HOSTKEY:  _KexGSSBase._process_hostkey,
        MSG_KEXGSS_ERROR:    _KexGSSBase._process_error
    }


if curve25519_available: # pragma: no branch
    if sntrup761_available: # pragma: no branch
        register_kex_alg(b'sntrup761x25519-sha512@openssh.com', _KexSNTRUP761,
                         sha512, (), True)

    register_kex_alg(b'curve25519-sha256', _KexECDH, sha256,
                     (Curve25519DH,), True)
    register_kex_alg(b'curve25519-sha256@libssh.org', _KexECDH, sha256,
                     (Curve25519DH,), True)
    register_gss_kex_alg(b'gss-curve25519-sha256', _KexGSSECDH, sha256,
                         (Curve25519DH,), True)

if curve448_available: # pragma: no branch
    register_kex_alg(b'curve448-sha512', _KexECDH, sha512,
                     (Curve448DH,), True)
    register_gss_kex_alg(b'gss-curve448-sha512', _KexGSSECDH, sha512,
                         (Curve448DH,), True)

for _curve_id, _hash_name, _hash_alg, _default in (
        (b'nistp521',     b'sha512', sha512, True),
        (b'nistp384',     b'sha384', sha384, True),
        (b'nistp256',     b'sha256', sha256, True),
        (b'1.3.132.0.10', b'sha256', sha256, True)):
    register_kex_alg(b'ecdh-sha2-' + _curve_id, _KexECDH, _hash_alg,
                     (ECDH, _curve_id), _default)
    register_gss_kex_alg(b'gss-' + _curve_id + b'-' + _hash_name, _KexGSSECDH,
                         _hash_alg, (ECDH, _curve_id), _default)

for _hash_name, _hash_alg, _default in (
        (b'sha256',         sha256, True),
        (b'sha224@ssh.com', sha224, False),
        (b'sha384@ssh.com', sha384, False),
        (b'sha512@ssh.com', sha512, False),
        (b'sha1',           sha1,   False)):
    register_kex_alg(b'diffie-hellman-group-exchange-' + _hash_name,
                     _KexDHGex, _hash_alg, (), _default)

    if not _hash_name.endswith(b'@ssh.com'):
        register_gss_kex_alg(b'gss-gex-' + _hash_name,
                             _KexGSSGex, _hash_alg, (), _default)

for _name, _hash_alg, _g, _p, _default in (
        (b'group14-sha256',         sha256, _group14_g, _group14_p, True),
        (b'group15-sha512',         sha512, _group15_g, _group15_p, True),
        (b'group16-sha512',         sha512, _group16_g, _group16_p, True),
        (b'group17-sha512',         sha512, _group17_g, _group17_p, True),
        (b'group18-sha512',         sha512, _group18_g, _group18_p, True),
        (b'group14-sha256@ssh.com', sha256, _group14_g, _group14_p, True),
        (b'group14-sha224@ssh.com', sha224, _group14_g, _group14_p, False),
        (b'group15-sha256@ssh.com', sha256, _group15_g, _group15_p, False),
        (b'group15-sha384@ssh.com', sha384, _group15_g, _group15_p, False),
        (b'group16-sha384@ssh.com', sha384, _group16_g, _group16_p, False),
        (b'group16-sha512@ssh.com', sha512, _group16_g, _group16_p, False),
        (b'group18-sha512@ssh.com', sha512, _group18_g, _group18_p, False),
        (b'group14-sha1',           sha1,   _group14_g, _group14_p, True),
        (b'group1-sha1',            sha1,   _group1_g,  _group1_p,  False)):
    register_kex_alg(b'diffie-hellman-' + _name, _KexDH, _hash_alg,
                     (_g, _p), _default)

    if not _name.endswith(b'@ssh.com'):
        register_gss_kex_alg(b'gss-' + _name, _KexGSS, _hash_alg,
                             (_g, _p), _default)
