added v3

venv/lib/python3.11/site-packages/paramiko/kex_gss.py

@@ -0,0 +1,686 @@
+# Copyright (C) 2003-2007  Robey Pointer <robeypointer@gmail.com>
+# Copyright (C) 2013-2014 science + computing ag
+# Author: Sebastian Deiss <sebastian.deiss@t-online.de>
+#
+#
+# This file is part of paramiko.
+#
+# Paramiko is free software; you can redistribute it and/or modify it under the
+# terms of the GNU Lesser General Public License as published by the Free
+# Software Foundation; either version 2.1 of the License, or (at your option)
+# any later version.
+#
+# Paramiko is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with Paramiko; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301 USA.
+
+
+"""
+This module provides GSS-API / SSPI Key Exchange as defined in :rfc:`4462`.
+
+.. note:: Credential delegation is not supported in server mode.
+
+.. note::
+    `RFC 4462 Section 2.2
+    <https://tools.ietf.org/html/rfc4462.html#section-2.2>`_ says we are not
+    required to implement GSS-API error messages. Thus, in many methods within
+    this module, if an error occurs an exception will be thrown and the
+    connection will be terminated.
+
+.. seealso:: :doc:`/api/ssh_gss`
+
+.. versionadded:: 1.15
+"""
+
+import os
+from hashlib import sha1
+
+from paramiko.common import (
+    DEBUG,
+    max_byte,
+    zero_byte,
+    byte_chr,
+    byte_mask,
+    byte_ord,
+)
+from paramiko import util
+from paramiko.message import Message
+from paramiko.ssh_exception import SSHException
+
+
+(
+    MSG_KEXGSS_INIT,
+    MSG_KEXGSS_CONTINUE,
+    MSG_KEXGSS_COMPLETE,
+    MSG_KEXGSS_HOSTKEY,
+    MSG_KEXGSS_ERROR,
+) = range(30, 35)
+(MSG_KEXGSS_GROUPREQ, MSG_KEXGSS_GROUP) = range(40, 42)
+(
+    c_MSG_KEXGSS_INIT,
+    c_MSG_KEXGSS_CONTINUE,
+    c_MSG_KEXGSS_COMPLETE,
+    c_MSG_KEXGSS_HOSTKEY,
+    c_MSG_KEXGSS_ERROR,
+) = [byte_chr(c) for c in range(30, 35)]
+(c_MSG_KEXGSS_GROUPREQ, c_MSG_KEXGSS_GROUP) = [
+    byte_chr(c) for c in range(40, 42)
+]
+
+
+class KexGSSGroup1:
+    """
+    GSS-API / SSPI Authenticated Diffie-Hellman Key Exchange as defined in `RFC
+    4462 Section 2 <https://tools.ietf.org/html/rfc4462.html#section-2>`_
+    """
+
+    # draft-ietf-secsh-transport-09.txt, page 17
+    P = 0xFFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE65381FFFFFFFFFFFFFFFF  # noqa
+    G = 2
+    b7fffffffffffffff = byte_chr(0x7F) + max_byte * 7  # noqa
+    b0000000000000000 = zero_byte * 8  # noqa
+    NAME = "gss-group1-sha1-toWM5Slw5Ew8Mqkay+al2g=="
+
+    def __init__(self, transport):
+        self.transport = transport
+        self.kexgss = self.transport.kexgss_ctxt
+        self.gss_host = None
+        self.x = 0
+        self.e = 0
+        self.f = 0
+
+    def start_kex(self):
+        """
+        Start the GSS-API / SSPI Authenticated Diffie-Hellman Key Exchange.
+        """
+        self._generate_x()
+        if self.transport.server_mode:
+            # compute f = g^x mod p, but don't send it yet
+            self.f = pow(self.G, self.x, self.P)
+            self.transport._expect_packet(MSG_KEXGSS_INIT)
+            return
+        # compute e = g^x mod p (where g=2), and send it
+        self.e = pow(self.G, self.x, self.P)
+        # Initialize GSS-API Key Exchange
+        self.gss_host = self.transport.gss_host
+        m = Message()
+        m.add_byte(c_MSG_KEXGSS_INIT)
+        m.add_string(self.kexgss.ssh_init_sec_context(target=self.gss_host))
+        m.add_mpint(self.e)
+        self.transport._send_message(m)
+        self.transport._expect_packet(
+            MSG_KEXGSS_HOSTKEY,
+            MSG_KEXGSS_CONTINUE,
+            MSG_KEXGSS_COMPLETE,
+            MSG_KEXGSS_ERROR,
+        )
+
+    def parse_next(self, ptype, m):
+        """
+        Parse the next packet.
+
+        :param ptype: The (string) type of the incoming packet
+        :param `.Message` m: The packet content
+        """
+        if self.transport.server_mode and (ptype == MSG_KEXGSS_INIT):
+            return self._parse_kexgss_init(m)
+        elif not self.transport.server_mode and (ptype == MSG_KEXGSS_HOSTKEY):
+            return self._parse_kexgss_hostkey(m)
+        elif self.transport.server_mode and (ptype == MSG_KEXGSS_CONTINUE):
+            return self._parse_kexgss_continue(m)
+        elif not self.transport.server_mode and (ptype == MSG_KEXGSS_COMPLETE):
+            return self._parse_kexgss_complete(m)
+        elif ptype == MSG_KEXGSS_ERROR:
+            return self._parse_kexgss_error(m)
+        msg = "GSS KexGroup1 asked to handle packet type {:d}"
+        raise SSHException(msg.format(ptype))
+
+    # ##  internals...
+
+    def _generate_x(self):
+        """
+        generate an "x" (1 < x < q), where q is (p-1)/2.
+        p is a 128-byte (1024-bit) number, where the first 64 bits are 1.
+        therefore q can be approximated as a 2^1023.  we drop the subset of
+        potential x where the first 63 bits are 1, because some of those will
+        be larger than q (but this is a tiny tiny subset of potential x).
+        """
+        while 1:
+            x_bytes = os.urandom(128)
+            x_bytes = byte_mask(x_bytes[0], 0x7F) + x_bytes[1:]
+            first = x_bytes[:8]
+            if first not in (self.b7fffffffffffffff, self.b0000000000000000):
+                break
+        self.x = util.inflate_long(x_bytes)
+
+    def _parse_kexgss_hostkey(self, m):
+        """
+        Parse the SSH2_MSG_KEXGSS_HOSTKEY message (client mode).
+
+        :param `.Message` m: The content of the SSH2_MSG_KEXGSS_HOSTKEY message
+        """
+        # client mode
+        host_key = m.get_string()
+        self.transport.host_key = host_key
+        sig = m.get_string()
+        self.transport._verify_key(host_key, sig)
+        self.transport._expect_packet(MSG_KEXGSS_CONTINUE, MSG_KEXGSS_COMPLETE)
+
+    def _parse_kexgss_continue(self, m):
+        """
+        Parse the SSH2_MSG_KEXGSS_CONTINUE message.
+
+        :param `.Message` m: The content of the SSH2_MSG_KEXGSS_CONTINUE
+            message
+        """
+        if not self.transport.server_mode:
+            srv_token = m.get_string()
+            m = Message()
+            m.add_byte(c_MSG_KEXGSS_CONTINUE)
+            m.add_string(
+                self.kexgss.ssh_init_sec_context(
+                    target=self.gss_host, recv_token=srv_token
+                )
+            )
+            self.transport.send_message(m)
+            self.transport._expect_packet(
+                MSG_KEXGSS_CONTINUE, MSG_KEXGSS_COMPLETE, MSG_KEXGSS_ERROR
+            )
+        else:
+            pass
+
+    def _parse_kexgss_complete(self, m):
+        """
+        Parse the SSH2_MSG_KEXGSS_COMPLETE message (client mode).
+
+        :param `.Message` m: The content of the
+            SSH2_MSG_KEXGSS_COMPLETE message
+        """
+        # client mode
+        if self.transport.host_key is None:
+            self.transport.host_key = NullHostKey()
+        self.f = m.get_mpint()
+        if (self.f < 1) or (self.f > self.P - 1):
+            raise SSHException('Server kex "f" is out of range')
+        mic_token = m.get_string()
+        # This must be TRUE, if there is a GSS-API token in this message.
+        bool = m.get_boolean()
+        srv_token = None
+        if bool:
+            srv_token = m.get_string()
+        K = pow(self.f, self.x, self.P)
+        # okay, build up the hash H of
+        # (V_C || V_S || I_C || I_S || K_S || e || f || K)
+        hm = Message()
+        hm.add(
+            self.transport.local_version,
+            self.transport.remote_version,
+            self.transport.local_kex_init,
+            self.transport.remote_kex_init,
+        )
+        hm.add_string(self.transport.host_key.__str__())
+        hm.add_mpint(self.e)
+        hm.add_mpint(self.f)
+        hm.add_mpint(K)
+        H = sha1(str(hm)).digest()
+        self.transport._set_K_H(K, H)
+        if srv_token is not None:
+            self.kexgss.ssh_init_sec_context(
+                target=self.gss_host, recv_token=srv_token
+            )
+            self.kexgss.ssh_check_mic(mic_token, H)
+        else:
+            self.kexgss.ssh_check_mic(mic_token, H)
+        self.transport.gss_kex_used = True
+        self.transport._activate_outbound()
+
+    def _parse_kexgss_init(self, m):
+        """
+        Parse the SSH2_MSG_KEXGSS_INIT message (server mode).
+
+        :param `.Message` m: The content of the SSH2_MSG_KEXGSS_INIT message
+        """
+        # server mode
+        client_token = m.get_string()
+        self.e = m.get_mpint()
+        if (self.e < 1) or (self.e > self.P - 1):
+            raise SSHException('Client kex "e" is out of range')
+        K = pow(self.e, self.x, self.P)
+        self.transport.host_key = NullHostKey()
+        key = self.transport.host_key.__str__()
+        # okay, build up the hash H of
+        # (V_C || V_S || I_C || I_S || K_S || e || f || K)
+        hm = Message()
+        hm.add(
+            self.transport.remote_version,
+            self.transport.local_version,
+            self.transport.remote_kex_init,
+            self.transport.local_kex_init,
+        )
+        hm.add_string(key)
+        hm.add_mpint(self.e)
+        hm.add_mpint(self.f)
+        hm.add_mpint(K)
+        H = sha1(hm.asbytes()).digest()
+        self.transport._set_K_H(K, H)
+        srv_token = self.kexgss.ssh_accept_sec_context(
+            self.gss_host, client_token
+        )
+        m = Message()
+        if self.kexgss._gss_srv_ctxt_status:
+            mic_token = self.kexgss.ssh_get_mic(
+                self.transport.session_id, gss_kex=True
+            )
+            m.add_byte(c_MSG_KEXGSS_COMPLETE)
+            m.add_mpint(self.f)
+            m.add_string(mic_token)
+            if srv_token is not None:
+                m.add_boolean(True)
+                m.add_string(srv_token)
+            else:
+                m.add_boolean(False)
+            self.transport._send_message(m)
+            self.transport.gss_kex_used = True
+            self.transport._activate_outbound()
+        else:
+            m.add_byte(c_MSG_KEXGSS_CONTINUE)
+            m.add_string(srv_token)
+            self.transport._send_message(m)
+            self.transport._expect_packet(
+                MSG_KEXGSS_CONTINUE, MSG_KEXGSS_COMPLETE, MSG_KEXGSS_ERROR
+            )
+
+    def _parse_kexgss_error(self, m):
+        """
+        Parse the SSH2_MSG_KEXGSS_ERROR message (client mode).
+        The server may send a GSS-API error message. if it does, we display
+        the error by throwing an exception (client mode).
+
+        :param `.Message` m: The content of the SSH2_MSG_KEXGSS_ERROR message
+        :raise SSHException: Contains GSS-API major and minor status as well as
+                             the error message and the language tag of the
+                             message
+        """
+        maj_status = m.get_int()
+        min_status = m.get_int()
+        err_msg = m.get_string()
+        m.get_string()  # we don't care about the language!
+        raise SSHException(
+            """GSS-API Error:
+Major Status: {}
+Minor Status: {}
+Error Message: {}
+""".format(
+                maj_status, min_status, err_msg
+            )
+        )
+
+
+class KexGSSGroup14(KexGSSGroup1):
+    """
+    GSS-API / SSPI Authenticated Diffie-Hellman Group14 Key Exchange as defined
+    in `RFC 4462 Section 2
+    <https://tools.ietf.org/html/rfc4462.html#section-2>`_
+    """
+
+    P = 0x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noqa
+    G = 2
+    NAME = "gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g=="
+
+
+class KexGSSGex:
+    """
+    GSS-API / SSPI Authenticated Diffie-Hellman Group Exchange as defined in
+    `RFC 4462 Section 2 <https://tools.ietf.org/html/rfc4462.html#section-2>`_
+    """
+
+    NAME = "gss-gex-sha1-toWM5Slw5Ew8Mqkay+al2g=="
+    min_bits = 1024
+    max_bits = 8192
+    preferred_bits = 2048
+
+    def __init__(self, transport):
+        self.transport = transport
+        self.kexgss = self.transport.kexgss_ctxt
+        self.gss_host = None
+        self.p = None
+        self.q = None
+        self.g = None
+        self.x = None
+        self.e = None
+        self.f = None
+        self.old_style = False
+
+    def start_kex(self):
+        """
+        Start the GSS-API / SSPI Authenticated Diffie-Hellman Group Exchange
+        """
+        if self.transport.server_mode:
+            self.transport._expect_packet(MSG_KEXGSS_GROUPREQ)
+            return
+        # request a bit range: we accept (min_bits) to (max_bits), but prefer
+        # (preferred_bits).  according to the spec, we shouldn't pull the
+        # minimum up above 1024.
+        self.gss_host = self.transport.gss_host
+        m = Message()
+        m.add_byte(c_MSG_KEXGSS_GROUPREQ)
+        m.add_int(self.min_bits)
+        m.add_int(self.preferred_bits)
+        m.add_int(self.max_bits)
+        self.transport._send_message(m)
+        self.transport._expect_packet(MSG_KEXGSS_GROUP)
+
+    def parse_next(self, ptype, m):
+        """
+        Parse the next packet.
+
+        :param ptype: The (string) type of the incoming packet
+        :param `.Message` m: The packet content
+        """
+        if ptype == MSG_KEXGSS_GROUPREQ:
+            return self._parse_kexgss_groupreq(m)
+        elif ptype == MSG_KEXGSS_GROUP:
+            return self._parse_kexgss_group(m)
+        elif ptype == MSG_KEXGSS_INIT:
+            return self._parse_kexgss_gex_init(m)
+        elif ptype == MSG_KEXGSS_HOSTKEY:
+            return self._parse_kexgss_hostkey(m)
+        elif ptype == MSG_KEXGSS_CONTINUE:
+            return self._parse_kexgss_continue(m)
+        elif ptype == MSG_KEXGSS_COMPLETE:
+            return self._parse_kexgss_complete(m)
+        elif ptype == MSG_KEXGSS_ERROR:
+            return self._parse_kexgss_error(m)
+        msg = "KexGex asked to handle packet type {:d}"
+        raise SSHException(msg.format(ptype))
+
+    # ##  internals...
+
+    def _generate_x(self):
+        # generate an "x" (1 < x < (p-1)/2).
+        q = (self.p - 1) // 2
+        qnorm = util.deflate_long(q, 0)
+        qhbyte = byte_ord(qnorm[0])
+        byte_count = len(qnorm)
+        qmask = 0xFF
+        while not (qhbyte & 0x80):
+            qhbyte <<= 1
+            qmask >>= 1
+        while True:
+            x_bytes = os.urandom(byte_count)
+            x_bytes = byte_mask(x_bytes[0], qmask) + x_bytes[1:]
+            x = util.inflate_long(x_bytes, 1)
+            if (x > 1) and (x < q):
+                break
+        self.x = x
+
+    def _parse_kexgss_groupreq(self, m):
+        """
+        Parse the SSH2_MSG_KEXGSS_GROUPREQ message (server mode).
+
+        :param `.Message` m: The content of the
+            SSH2_MSG_KEXGSS_GROUPREQ message
+        """
+        minbits = m.get_int()
+        preferredbits = m.get_int()
+        maxbits = m.get_int()
+        # smoosh the user's preferred size into our own limits
+        if preferredbits > self.max_bits:
+            preferredbits = self.max_bits
+        if preferredbits < self.min_bits:
+            preferredbits = self.min_bits
+        # fix min/max if they're inconsistent.  technically, we could just pout
+        # and hang up, but there's no harm in giving them the benefit of the
+        # doubt and just picking a bitsize for them.
+        if minbits > preferredbits:
+            minbits = preferredbits
+        if maxbits < preferredbits:
+            maxbits = preferredbits
+        # now save a copy
+        self.min_bits = minbits
+        self.preferred_bits = preferredbits
+        self.max_bits = maxbits
+        # generate prime
+        pack = self.transport._get_modulus_pack()
+        if pack is None:
+            raise SSHException("Can't do server-side gex with no modulus pack")
+        self.transport._log(
+            DEBUG,  # noqa
+            "Picking p ({} <= {} <= {} bits)".format(
+                minbits, preferredbits, maxbits
+            ),
+        )
+        self.g, self.p = pack.get_modulus(minbits, preferredbits, maxbits)
+        m = Message()
+        m.add_byte(c_MSG_KEXGSS_GROUP)
+        m.add_mpint(self.p)
+        m.add_mpint(self.g)
+        self.transport._send_message(m)
+        self.transport._expect_packet(MSG_KEXGSS_INIT)
+
+    def _parse_kexgss_group(self, m):
+        """
+        Parse the SSH2_MSG_KEXGSS_GROUP message (client mode).
+
+        :param `Message` m: The content of the SSH2_MSG_KEXGSS_GROUP message
+        """
+        self.p = m.get_mpint()
+        self.g = m.get_mpint()
+        # reject if p's bit length < 1024 or > 8192
+        bitlen = util.bit_length(self.p)
+        if (bitlen < 1024) or (bitlen > 8192):
+            raise SSHException(
+                "Server-generated gex p (don't ask) is out of range "
+                "({} bits)".format(bitlen)
+            )
+        self.transport._log(
+            DEBUG, "Got server p ({} bits)".format(bitlen)
+        )  # noqa
+        self._generate_x()
+        # now compute e = g^x mod p
+        self.e = pow(self.g, self.x, self.p)
+        m = Message()
+        m.add_byte(c_MSG_KEXGSS_INIT)
+        m.add_string(self.kexgss.ssh_init_sec_context(target=self.gss_host))
+        m.add_mpint(self.e)
+        self.transport._send_message(m)
+        self.transport._expect_packet(
+            MSG_KEXGSS_HOSTKEY,
+            MSG_KEXGSS_CONTINUE,
+            MSG_KEXGSS_COMPLETE,
+            MSG_KEXGSS_ERROR,
+        )
+
+    def _parse_kexgss_gex_init(self, m):
+        """
+        Parse the SSH2_MSG_KEXGSS_INIT message (server mode).
+
+        :param `Message` m: The content of the SSH2_MSG_KEXGSS_INIT message
+        """
+        client_token = m.get_string()
+        self.e = m.get_mpint()
+        if (self.e < 1) or (self.e > self.p - 1):
+            raise SSHException('Client kex "e" is out of range')
+        self._generate_x()
+        self.f = pow(self.g, self.x, self.p)
+        K = pow(self.e, self.x, self.p)
+        self.transport.host_key = NullHostKey()
+        key = self.transport.host_key.__str__()
+        # okay, build up the hash H of
+        # (V_C || V_S || I_C || I_S || K_S || min || n || max || p || g || e || f || K)  # noqa
+        hm = Message()
+        hm.add(
+            self.transport.remote_version,
+            self.transport.local_version,
+            self.transport.remote_kex_init,
+            self.transport.local_kex_init,
+            key,
+        )
+        hm.add_int(self.min_bits)
+        hm.add_int(self.preferred_bits)
+        hm.add_int(self.max_bits)
+        hm.add_mpint(self.p)
+        hm.add_mpint(self.g)
+        hm.add_mpint(self.e)
+        hm.add_mpint(self.f)
+        hm.add_mpint(K)
+        H = sha1(hm.asbytes()).digest()
+        self.transport._set_K_H(K, H)
+        srv_token = self.kexgss.ssh_accept_sec_context(
+            self.gss_host, client_token
+        )
+        m = Message()
+        if self.kexgss._gss_srv_ctxt_status:
+            mic_token = self.kexgss.ssh_get_mic(
+                self.transport.session_id, gss_kex=True
+            )
+            m.add_byte(c_MSG_KEXGSS_COMPLETE)
+            m.add_mpint(self.f)
+            m.add_string(mic_token)
+            if srv_token is not None:
+                m.add_boolean(True)
+                m.add_string(srv_token)
+            else:
+                m.add_boolean(False)
+            self.transport._send_message(m)
+            self.transport.gss_kex_used = True
+            self.transport._activate_outbound()
+        else:
+            m.add_byte(c_MSG_KEXGSS_CONTINUE)
+            m.add_string(srv_token)
+            self.transport._send_message(m)
+            self.transport._expect_packet(
+                MSG_KEXGSS_CONTINUE, MSG_KEXGSS_COMPLETE, MSG_KEXGSS_ERROR
+            )
+
+    def _parse_kexgss_hostkey(self, m):
+        """
+        Parse the SSH2_MSG_KEXGSS_HOSTKEY message (client mode).
+
+        :param `Message` m: The content of the SSH2_MSG_KEXGSS_HOSTKEY message
+        """
+        # client mode
+        host_key = m.get_string()
+        self.transport.host_key = host_key
+        sig = m.get_string()
+        self.transport._verify_key(host_key, sig)
+        self.transport._expect_packet(MSG_KEXGSS_CONTINUE, MSG_KEXGSS_COMPLETE)
+
+    def _parse_kexgss_continue(self, m):
+        """
+        Parse the SSH2_MSG_KEXGSS_CONTINUE message.
+
+        :param `Message` m: The content of the SSH2_MSG_KEXGSS_CONTINUE message
+        """
+        if not self.transport.server_mode:
+            srv_token = m.get_string()
+            m = Message()
+            m.add_byte(c_MSG_KEXGSS_CONTINUE)
+            m.add_string(
+                self.kexgss.ssh_init_sec_context(
+                    target=self.gss_host, recv_token=srv_token
+                )
+            )
+            self.transport.send_message(m)
+            self.transport._expect_packet(
+                MSG_KEXGSS_CONTINUE, MSG_KEXGSS_COMPLETE, MSG_KEXGSS_ERROR
+            )
+        else:
+            pass
+
+    def _parse_kexgss_complete(self, m):
+        """
+        Parse the SSH2_MSG_KEXGSS_COMPLETE message (client mode).
+
+        :param `Message` m: The content of the SSH2_MSG_KEXGSS_COMPLETE message
+        """
+        if self.transport.host_key is None:
+            self.transport.host_key = NullHostKey()
+        self.f = m.get_mpint()
+        mic_token = m.get_string()
+        # This must be TRUE, if there is a GSS-API token in this message.
+        bool = m.get_boolean()
+        srv_token = None
+        if bool:
+            srv_token = m.get_string()
+        if (self.f < 1) or (self.f > self.p - 1):
+            raise SSHException('Server kex "f" is out of range')
+        K = pow(self.f, self.x, self.p)
+        # okay, build up the hash H of
+        # (V_C || V_S || I_C || I_S || K_S || min || n || max || p || g || e || f || K)  # noqa
+        hm = Message()
+        hm.add(
+            self.transport.local_version,
+            self.transport.remote_version,
+            self.transport.local_kex_init,
+            self.transport.remote_kex_init,
+            self.transport.host_key.__str__(),
+        )
+        if not self.old_style:
+            hm.add_int(self.min_bits)
+        hm.add_int(self.preferred_bits)
+        if not self.old_style:
+            hm.add_int(self.max_bits)
+        hm.add_mpint(self.p)
+        hm.add_mpint(self.g)
+        hm.add_mpint(self.e)
+        hm.add_mpint(self.f)
+        hm.add_mpint(K)
+        H = sha1(hm.asbytes()).digest()
+        self.transport._set_K_H(K, H)
+        if srv_token is not None:
+            self.kexgss.ssh_init_sec_context(
+                target=self.gss_host, recv_token=srv_token
+            )
+            self.kexgss.ssh_check_mic(mic_token, H)
+        else:
+            self.kexgss.ssh_check_mic(mic_token, H)
+        self.transport.gss_kex_used = True
+        self.transport._activate_outbound()
+
+    def _parse_kexgss_error(self, m):
+        """
+        Parse the SSH2_MSG_KEXGSS_ERROR message (client mode).
+        The server may send a GSS-API error message. if it does, we display
+        the error by throwing an exception (client mode).
+
+        :param `Message` m:  The content of the SSH2_MSG_KEXGSS_ERROR message
+        :raise SSHException: Contains GSS-API major and minor status as well as
+                             the error message and the language tag of the
+                             message
+        """
+        maj_status = m.get_int()
+        min_status = m.get_int()
+        err_msg = m.get_string()
+        m.get_string()  # we don't care about the language (lang_tag)!
+        raise SSHException(
+            """GSS-API Error:
+Major Status: {}
+Minor Status: {}
+Error Message: {}
+""".format(
+                maj_status, min_status, err_msg
+            )
+        )
+
+
+class NullHostKey:
+    """
+    This class represents the Null Host Key for GSS-API Key Exchange as defined
+    in `RFC 4462 Section 5
+    <https://tools.ietf.org/html/rfc4462.html#section-5>`_
+    """
+
+    def __init__(self):
+        self.key = ""
+
+    def __str__(self):
+        return self.key
+
+    def get_name(self):
+        return self.key