auth_handler.py

# Copyright (C) 2003-2007  Robey Pointer <robeypointer@gmail.com>
#
# 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.,
# 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA.

"""
`.AuthHandler`
"""

import weakref
import time

from paramiko.common import (
    cMSG_SERVICE_REQUEST, cMSG_DISCONNECT, DISCONNECT_SERVICE_NOT_AVAILABLE,
    DISCONNECT_NO_MORE_AUTH_METHODS_AVAILABLE, cMSG_USERAUTH_REQUEST,
    cMSG_SERVICE_ACCEPT, DEBUG, AUTH_SUCCESSFUL, INFO, cMSG_USERAUTH_SUCCESS,
    cMSG_USERAUTH_FAILURE, AUTH_PARTIALLY_SUCCESSFUL,
    cMSG_USERAUTH_INFO_REQUEST, WARNING, AUTH_FAILED, cMSG_USERAUTH_PK_OK,
    cMSG_USERAUTH_INFO_RESPONSE, MSG_SERVICE_REQUEST, MSG_SERVICE_ACCEPT,
    MSG_USERAUTH_REQUEST, MSG_USERAUTH_SUCCESS, MSG_USERAUTH_FAILURE,
    MSG_USERAUTH_BANNER, MSG_USERAUTH_INFO_REQUEST, MSG_USERAUTH_INFO_RESPONSE,
    cMSG_USERAUTH_GSSAPI_RESPONSE, cMSG_USERAUTH_GSSAPI_TOKEN,
    cMSG_USERAUTH_GSSAPI_MIC, MSG_USERAUTH_GSSAPI_RESPONSE,
    MSG_USERAUTH_GSSAPI_TOKEN, MSG_USERAUTH_GSSAPI_ERROR,
    MSG_USERAUTH_GSSAPI_ERRTOK, MSG_USERAUTH_GSSAPI_MIC, MSG_NAMES,
    cMSG_USERAUTH_BANNER
)
from paramiko.message import Message
from paramiko.py3compat import b
from paramiko.ssh_exception import (
    SSHException, AuthenticationException, BadAuthenticationType,
    PartialAuthentication,
)
from paramiko.server import InteractiveQuery
from paramiko.ssh_gss import GSSAuth, GSS_EXCEPTIONS


class AuthHandler (object):
    """
    Internal class to handle the mechanics of authentication.
    """

    def __init__(self, transport):
        self.transport = weakref.proxy(transport)
        self.username = None
        self.authenticated = False
        self.auth_event = None
        self.auth_method = ''
        self.banner = None
        self.password = None
        self.private_key = None
        self.interactive_handler = None
        self.submethods = None
        # for server mode:
        self.auth_username = None
        self.auth_fail_count = 0
        # for GSSAPI
        self.gss_host = None
        self.gss_deleg_creds = True

    def _log(self, *args):
        return self.transport._log(*args)

    def is_authenticated(self):
        return self.authenticated

    def get_username(self):
        if self.transport.server_mode:
            return self.auth_username
        else:
            return self.username

    def auth_none(self, username, event):
        self.transport.lock.acquire()
        try:
            self.auth_event = event
            self.auth_method = 'none'
            self.username = username
            self._request_auth()
        finally:
            self.transport.lock.release()

    def auth_publickey(self, username, key, event):
        self.transport.lock.acquire()
        try:
            self.auth_event = event
            self.auth_method = 'publickey'
            self.username = username
            self.private_key = key
            self._request_auth()
        finally:
            self.transport.lock.release()

    def auth_password(self, username, password, event):
        self.transport.lock.acquire()
        try:
            self.auth_event = event
            self.auth_method = 'password'
            self.username = username
            self.password = password
            self._request_auth()
        finally:
            self.transport.lock.release()

    def auth_interactive(self, username, handler, event, submethods=''):
        """
        response_list = handler(title, instructions, prompt_list)
        """
        self.transport.lock.acquire()
        try:
            self.auth_event = event
            self.auth_method = 'keyboard-interactive'
            self.username = username
            self.interactive_handler = handler
            self.submethods = submethods
            self._request_auth()
        finally:
            self.transport.lock.release()

    def auth_gssapi_with_mic(self, username, gss_host, gss_deleg_creds, event):
        self.transport.lock.acquire()
        try:
            self.auth_event = event
            self.auth_method = 'gssapi-with-mic'
            self.username = username
            self.gss_host = gss_host
            self.gss_deleg_creds = gss_deleg_creds
            self._request_auth()
        finally:
            self.transport.lock.release()

    def auth_gssapi_keyex(self, username, event):
        self.transport.lock.acquire()
        try:
            self.auth_event = event
            self.auth_method = 'gssapi-keyex'
            self.username = username
            self._request_auth()
        finally:
            self.transport.lock.release()

    def abort(self):
        if self.auth_event is not None:
            self.auth_event.set()

    # ...internals...

    def _request_auth(self):
        m = Message()
        m.add_byte(cMSG_SERVICE_REQUEST)
        m.add_string('ssh-userauth')
        self.transport._send_message(m)

    def _disconnect_service_not_available(self):
        m = Message()
        m.add_byte(cMSG_DISCONNECT)
        m.add_int(DISCONNECT_SERVICE_NOT_AVAILABLE)
        m.add_string('Service not available')
        m.add_string('en')
        self.transport._send_message(m)
        self.transport.close()

    def _disconnect_no_more_auth(self):
        m = Message()
        m.add_byte(cMSG_DISCONNECT)
        m.add_int(DISCONNECT_NO_MORE_AUTH_METHODS_AVAILABLE)
        m.add_string('No more auth methods available')
        m.add_string('en')
        self.transport._send_message(m)
        self.transport.close()

    def _get_session_blob(self, key, service, username):
        m = Message()
        m.add_string(self.transport.session_id)
        m.add_byte(cMSG_USERAUTH_REQUEST)
        m.add_string(username)
        m.add_string(service)
        m.add_string('publickey')
        m.add_boolean(True)
        # Use certificate contents, if available, plain pubkey otherwise
        if key.public_blob:
            m.add_string(key.public_blob.key_type)
            m.add_string(key.public_blob.key_blob)
        else:
            m.add_string(key.get_name())
            m.add_string(key)
        return m.asbytes()

    def wait_for_response(self, event):
        max_ts = None
        if self.transport.auth_timeout is not None:
            max_ts = time.time() + self.transport.auth_timeout
        while True:
            event.wait(0.1)
            if not self.transport.is_active():
                e = self.transport.get_exception()
                if (e is None) or issubclass(e.__class__, EOFError):
                    e = AuthenticationException('Authentication failed.')
                raise e
            if event.is_set():
                break
            if max_ts is not None and max_ts <= time.time():
                raise AuthenticationException('Authentication timeout.')

        if not self.is_authenticated():
            e = self.transport.get_exception()
            if e is None:
                e = AuthenticationException('Authentication failed.')
            # this is horrible.  Python Exception isn't yet descended from
            # object, so type(e) won't work. :(
            if issubclass(e.__class__, PartialAuthentication):
                return e.allowed_types
            raise e
        return []

    def _parse_service_request(self, m):
        service = m.get_text()
        if self.transport.server_mode and (service == 'ssh-userauth'):
            # accepted
            m = Message()
            m.add_byte(cMSG_SERVICE_ACCEPT)
            m.add_string(service)
            self.transport._send_message(m)
            banner, language = self.transport.server_object.get_banner()
            if banner:
                m = Message()
                m.add_byte(cMSG_USERAUTH_BANNER)
                m.add_string(banner)
                m.add_string(language)
                self.transport._send_message(m)
            return
        # dunno this one
        self._disconnect_service_not_available()

    def _parse_service_accept(self, m):
        service = m.get_text()
        if service == 'ssh-userauth':
            self._log(DEBUG, 'userauth is OK')
            m = Message()
            m.add_byte(cMSG_USERAUTH_REQUEST)
            m.add_string(self.username)
            m.add_string('ssh-connection')
            m.add_string(self.auth_method)
            if self.auth_method == 'password':
                m.add_boolean(False)
                password = b(self.password)
                m.add_string(password)
            elif self.auth_method == 'publickey':
                m.add_boolean(True)
                # Use certificate contents, if available, plain pubkey
                # otherwise
                if self.private_key.public_blob:
                    m.add_string(self.private_key.public_blob.key_type)
                    m.add_string(self.private_key.public_blob.key_blob)
                else:
                    m.add_string(self.private_key.get_name())
                    m.add_string(self.private_key)
                blob = self._get_session_blob(
                    self.private_key, 'ssh-connection', self.username)
                sig = self.private_key.sign_ssh_data(blob)
                m.add_string(sig)
            elif self.auth_method == 'keyboard-interactive':
                m.add_string('')
                m.add_string(self.submethods)
            elif self.auth_method == "gssapi-with-mic":
                sshgss = GSSAuth(self.auth_method, self.gss_deleg_creds)
                m.add_bytes(sshgss.ssh_gss_oids())
                # send the supported GSSAPI OIDs to the server
                self.transport._send_message(m)
                ptype, m = self.transport.packetizer.read_message()
                if ptype == MSG_USERAUTH_BANNER:
                    self._parse_userauth_banner(m)
                    ptype, m = self.transport.packetizer.read_message()
                if ptype == MSG_USERAUTH_GSSAPI_RESPONSE:
                    # Read the mechanism selected by the server. We send just
                    # the Kerberos V5 OID, so the server can only respond with
                    # this OID.
                    mech = m.get_string()
                    m = Message()
                    m.add_byte(cMSG_USERAUTH_GSSAPI_TOKEN)
                    try:
                        m.add_string(sshgss.ssh_init_sec_context(
                            self.gss_host,
                            mech,
                            self.username,))
                    except GSS_EXCEPTIONS as e:
                        return self._handle_local_gss_failure(e)
                    self.transport._send_message(m)
                    while True:
                        ptype, m = self.transport.packetizer.read_message()
                        if ptype == MSG_USERAUTH_GSSAPI_TOKEN:
                            srv_token = m.get_string()
                            try:
                                next_token = sshgss.ssh_init_sec_context(
                                    self.gss_host,
                                    mech,
                                    self.username,
                                    srv_token)
                            except GSS_EXCEPTIONS as e:
                                return self._handle_local_gss_failure(e)
                            # After this step the GSSAPI should not return any
                            # token. If it does, we keep sending the token to
                            # the server until no more token is returned.
                            if next_token is None:
                                break
                            else:
                                m = Message()
                                m.add_byte(cMSG_USERAUTH_GSSAPI_TOKEN)
                                m.add_string(next_token)
                                self.transport.send_message(m)
                    else:
                        raise SSHException(
                            "Received Package: {}".format(MSG_NAMES[ptype])
                        )
                    m = Message()
                    m.add_byte(cMSG_USERAUTH_GSSAPI_MIC)
                    # send the MIC to the server
                    m.add_string(sshgss.ssh_get_mic(self.transport.session_id))
                elif ptype == MSG_USERAUTH_GSSAPI_ERRTOK:
                    # RFC 4462 says we are not required to implement GSS-API
                    # error messages.
                    # See RFC 4462 Section 3.8 in
                    # http://www.ietf.org/rfc/rfc4462.txt
                    raise SSHException("Server returned an error token")
                elif ptype == MSG_USERAUTH_GSSAPI_ERROR:
                    maj_status = m.get_int()
                    min_status = m.get_int()
                    err_msg = m.get_string()
                    m.get_string()  # Lang tag - discarded
                    raise SSHException("""GSS-API Error:
Major Status: {}
Minor Status: {}
Error Message: {}
""".format(maj_status, min_status, err_msg))
                elif ptype == MSG_USERAUTH_FAILURE:
                    self._parse_userauth_failure(m)
                    return
                else:
                    raise SSHException(
                        "Received Package: {}".format(MSG_NAMES[ptype]))
            elif (
                self.auth_method == 'gssapi-keyex' and
                self.transport.gss_kex_used
            ):
                kexgss = self.transport.kexgss_ctxt
                kexgss.set_username(self.username)
                mic_token = kexgss.ssh_get_mic(self.transport.session_id)
                m.add_string(mic_token)
            elif self.auth_method == 'none':
                pass
            else:
                raise SSHException(
                    'Unknown auth method "{}"'.format(self.auth_method))
            self.transport._send_message(m)
        else:
            self._log(
                DEBUG,
                'Service request "{}" accepted (?)'.format(service))

    def _send_auth_result(self, username, method, result):
        # okay, send result
        m = Message()
        if result == AUTH_SUCCESSFUL:
            self._log(INFO, 'Auth granted ({}).'.format(method))
            m.add_byte(cMSG_USERAUTH_SUCCESS)
            self.authenticated = True
        else:
            self._log(INFO, 'Auth rejected ({}).'.format(method))
            m.add_byte(cMSG_USERAUTH_FAILURE)
            m.add_string(
                self.transport.server_object.get_allowed_auths(username))
            if result == AUTH_PARTIALLY_SUCCESSFUL:
                m.add_boolean(True)
            else:
                m.add_boolean(False)
                self.auth_fail_count += 1
        self.transport._send_message(m)
        if self.auth_fail_count >= 10:
            self._disconnect_no_more_auth()
        if result == AUTH_SUCCESSFUL:
            self.transport._auth_trigger()

    def _interactive_query(self, q):
        # make interactive query instead of response
        m = Message()
        m.add_byte(cMSG_USERAUTH_INFO_REQUEST)
        m.add_string(q.name)
        m.add_string(q.instructions)
        m.add_string(bytes())
        m.add_int(len(q.prompts))
        for p in q.prompts:
            m.add_string(p[0])
            m.add_boolean(p[1])
        self.transport._send_message(m)

    def _parse_userauth_request(self, m):
        if not self.transport.server_mode:
            # er, uh... what?
            m = Message()
            m.add_byte(cMSG_USERAUTH_FAILURE)
            m.add_string('none')
            m.add_boolean(False)
            self.transport._send_message(m)
            return
        if self.authenticated:
            # ignore
            return
        username = m.get_text()
        service = m.get_text()
        method = m.get_text()
        self._log(
            DEBUG,
            'Auth request (type={}) service={}, username={}'.format(
                method, service, username
            )
        )
        if service != 'ssh-connection':
            self._disconnect_service_not_available()
            return
        if ((self.auth_username is not None) and
                (self.auth_username != username)):
            self._log(
                WARNING,
                'Auth rejected because the client attempted to change username in mid-flight'  # noqa
            )
            self._disconnect_no_more_auth()
            return
        self.auth_username = username
        # check if GSS-API authentication is enabled
        gss_auth = self.transport.server_object.enable_auth_gssapi()

        if method == 'none':
            result = self.transport.server_object.check_auth_none(username)
        elif method == 'password':
            changereq = m.get_boolean()
            password = m.get_binary()
            try:
                password = password.decode('UTF-8')
            except UnicodeError:
                # some clients/servers expect non-utf-8 passwords!
                # in this case, just return the raw byte string.
                pass
            if changereq:
                # always treated as failure, since we don't support changing
                # passwords, but collect the list of valid auth types from
                # the callback anyway
                self._log(
                    DEBUG,
                    'Auth request to change passwords (rejected)')
                newpassword = m.get_binary()
                try:
                    newpassword = newpassword.decode('UTF-8', 'replace')
                except UnicodeError:
                    pass
                result = AUTH_FAILED
            else:
                result = self.transport.server_object.check_auth_password(
                    username, password)
        elif method == 'publickey':
            sig_attached = m.get_boolean()
            keytype = m.get_text()
            keyblob = m.get_binary()
            try:
                key = self.transport._key_info[keytype](Message(keyblob))
            except SSHException as e:
                self._log(
                    INFO,
                    'Auth rejected: public key: {}'.format(str(e)))
                key = None
            except Exception as e:
                msg = 'Auth rejected: unsupported or mangled public key ({}: {})' # noqa
                self._log(INFO, msg.format(e.__class__.__name__, e))
                key = None
            if key is None:
                self._disconnect_no_more_auth()
                return
            # first check if this key is okay... if not, we can skip the verify
            result = self.transport.server_object.check_auth_publickey(
                username, key)
            if result != AUTH_FAILED:
                # key is okay, verify it
                if not sig_attached:
                    # client wants to know if this key is acceptable, before it
                    # signs anything...  send special "ok" message
                    m = Message()
                    m.add_byte(cMSG_USERAUTH_PK_OK)
                    m.add_string(keytype)
                    m.add_string(keyblob)
                    self.transport._send_message(m)
                    return
                sig = Message(m.get_binary())
                blob = self._get_session_blob(key, service, username)
                if not key.verify_ssh_sig(blob, sig):
                    self._log(
                        INFO,
                        'Auth rejected: invalid signature')
                    result = AUTH_FAILED
        elif method == 'keyboard-interactive':
            submethods = m.get_string()
            result = self.transport.server_object.check_auth_interactive(
                username, submethods)
            if isinstance(result, InteractiveQuery):
                # make interactive query instead of response
                self._interactive_query(result)
                return
        elif method == "gssapi-with-mic" and gss_auth:
            sshgss = GSSAuth(method)
            # Read the number of OID mechanisms supported by the client.
            # OpenSSH sends just one OID. It's the Kerveros V5 OID and that's
            # the only OID we support.
            mechs = m.get_int()
            # We can't accept more than one OID, so if the SSH client sends
            # more than one, disconnect.
            if mechs > 1:
                self._log(
                    INFO,
                    'Disconnect: Received more than one GSS-API OID mechanism')
                self._disconnect_no_more_auth()
            desired_mech = m.get_string()
            mech_ok = sshgss.ssh_check_mech(desired_mech)
            # if we don't support the mechanism, disconnect.
            if not mech_ok:
                self._log(
                    INFO,
                    'Disconnect: Received an invalid GSS-API OID mechanism')
                self._disconnect_no_more_auth()
            # send the Kerberos V5 GSSAPI OID to the client
            supported_mech = sshgss.ssh_gss_oids("server")
            # RFC 4462 says we are not required to implement GSS-API error
            # messages. See section 3.8 in http://www.ietf.org/rfc/rfc4462.txt
            m = Message()
            m.add_byte(cMSG_USERAUTH_GSSAPI_RESPONSE)
            m.add_bytes(supported_mech)
            self.transport.auth_handler = GssapiWithMicAuthHandler(self,
                                                                   sshgss)
            self.transport._expected_packet = (MSG_USERAUTH_GSSAPI_TOKEN,
                                               MSG_USERAUTH_REQUEST,
                                               MSG_SERVICE_REQUEST)
            self.transport._send_message(m)
            return
        elif method == "gssapi-keyex" and gss_auth:
            mic_token = m.get_string()
            sshgss = self.transport.kexgss_ctxt
            if sshgss is None:
                # If there is no valid context, we reject the authentication
                result = AUTH_FAILED
                self._send_auth_result(username, method, result)
            try:
                sshgss.ssh_check_mic(mic_token,
                                     self.transport.session_id,
                                     self.auth_username)
            except Exception:
                result = AUTH_FAILED
                self._send_auth_result(username, method, result)
                raise
            result = AUTH_SUCCESSFUL
            self.transport.server_object.check_auth_gssapi_keyex(
                username, result)
        else:
            result = self.transport.server_object.check_auth_none(username)
        # okay, send result
        self._send_auth_result(username, method, result)

    def _parse_userauth_success(self, m):
        self._log(
            INFO,
            'Authentication ({}) successful!'.format(self.auth_method))
        self.authenticated = True
        self.transport._auth_trigger()
        if self.auth_event is not None:
            self.auth_event.set()

    def _parse_userauth_failure(self, m):
        authlist = m.get_list()
        partial = m.get_boolean()
        if partial:
            self._log(INFO, 'Authentication continues...')
            self._log(DEBUG, 'Methods: ' + str(authlist))
            self.transport.saved_exception = PartialAuthentication(authlist)
        elif self.auth_method not in authlist:
            for msg in (
                'Authentication type ({}) not permitted.'.format(
                    self.auth_method
                ),
                'Allowed methods: {}'.format(authlist),
            ):
                self._log(DEBUG, msg)
            self.transport.saved_exception = BadAuthenticationType(
                'Bad authentication type', authlist
            )
        else:
            self._log(
                INFO,
                'Authentication ({}) failed.'.format(self.auth_method)
            )
        self.authenticated = False
        self.username = None
        if self.auth_event is not None:
            self.auth_event.set()

    def _parse_userauth_banner(self, m):
        banner = m.get_string()
        self.banner = banner
        self._log(INFO, 'Auth banner: {}'.format(banner))
        # who cares.

    def _parse_userauth_info_request(self, m):
        if self.auth_method != 'keyboard-interactive':
            raise SSHException('Illegal info request from server')
        title = m.get_text()
        instructions = m.get_text()
        m.get_binary()  # lang
        prompts = m.get_int()
        prompt_list = []
        for i in range(prompts):
            prompt_list.append((m.get_text(), m.get_boolean()))
        response_list = self.interactive_handler(
            title, instructions, prompt_list)

        m = Message()
        m.add_byte(cMSG_USERAUTH_INFO_RESPONSE)
        m.add_int(len(response_list))
        for r in response_list:
            m.add_string(r)
        self.transport._send_message(m)

    def _parse_userauth_info_response(self, m):
        if not self.transport.server_mode:
            raise SSHException('Illegal info response from server')
        n = m.get_int()
        responses = []
        for i in range(n):
            responses.append(m.get_text())
        result = self.transport.server_object.check_auth_interactive_response(
            responses)
        if isinstance(result, InteractiveQuery):
            # make interactive query instead of response
            self._interactive_query(result)
            return
        self._send_auth_result(
            self.auth_username, 'keyboard-interactive', result)

    def _handle_local_gss_failure(self, e):
        self.transport.saved_exception = e
        self._log(DEBUG, "GSSAPI failure: {}".format(e))
        self._log(INFO, 'Authentication ({}) failed.'.format(
            self.auth_method))
        self.authenticated = False
        self.username = None
        if self.auth_event is not None:
            self.auth_event.set()
        return

    # TODO: do the same to the other tables, in Transport.
    # TODO 3.0: MAY make sense to make these tables into actual
    # classes/instances that can be fed a mode bool or whatever. Or,
    # alternately (both?) make the message types small classes or enums that
    # embed this info within themselves (which could also then tidy up the
    # current 'integer -> human readable short string' stuff in common.py).
    # TODO: if we do that, also expose 'em publicly.

    # Messages which should be handled _by_ servers (sent by clients)
    _server_handler_table = {
        MSG_SERVICE_REQUEST: _parse_service_request,
        MSG_USERAUTH_REQUEST: _parse_userauth_request,
        MSG_USERAUTH_INFO_RESPONSE: _parse_userauth_info_response,
    }

    # Messages which should be handled _by_ clients (sent by servers)
    _client_handler_table = {
        MSG_SERVICE_ACCEPT: _parse_service_accept,
        MSG_USERAUTH_SUCCESS: _parse_userauth_success,
        MSG_USERAUTH_FAILURE: _parse_userauth_failure,
        MSG_USERAUTH_BANNER: _parse_userauth_banner,
        MSG_USERAUTH_INFO_REQUEST: _parse_userauth_info_request,
    }

    # NOTE: prior to the fix for #1283, this was a static dict instead of a
    # property. Should be backwards compatible in most/all cases.
    @property
    def _handler_table(self):
        if self.transport.server_mode:
            return self._server_handler_table
        else:
            return self._client_handler_table


class GssapiWithMicAuthHandler(object):
    """A specialized Auth handler for gssapi-with-mic

    During the GSSAPI token exchange we need a modified dispatch table,
    because the packet type numbers are not unique.
    """

    method = "gssapi-with-mic"

    def __init__(self, delegate, sshgss):
        self._delegate = delegate
        self.sshgss = sshgss

    def abort(self):
        self._restore_delegate_auth_handler()
        return self._delegate.abort()

    @property
    def transport(self):
        return self._delegate.transport

    @property
    def _send_auth_result(self):
        return self._delegate._send_auth_result

    @property
    def auth_username(self):
        return self._delegate.auth_username

    @property
    def gss_host(self):
        return self._delegate.gss_host

    def _restore_delegate_auth_handler(self):
        self.transport.auth_handler = self._delegate

    def _parse_userauth_gssapi_token(self, m):
        client_token = m.get_string()
        # use the client token as input to establish a secure
        # context.
        sshgss = self.sshgss
        try:
            token = sshgss.ssh_accept_sec_context(self.gss_host,
                                                  client_token,
                                                  self.auth_username)
        except Exception as e:
            self.transport.saved_exception = e
            result = AUTH_FAILED
            self._restore_delegate_auth_handler()
            self._send_auth_result(self.auth_username, self.method, result)
            raise
        if token is not None:
            m = Message()
            m.add_byte(cMSG_USERAUTH_GSSAPI_TOKEN)
            m.add_string(token)
            self.transport._expected_packet = (MSG_USERAUTH_GSSAPI_TOKEN,
                                               MSG_USERAUTH_GSSAPI_MIC,
                                               MSG_USERAUTH_REQUEST)
            self.transport._send_message(m)

    def _parse_userauth_gssapi_mic(self, m):
        mic_token = m.get_string()
        sshgss = self.sshgss
        username = self.auth_username
        self._restore_delegate_auth_handler()
        try:
            sshgss.ssh_check_mic(mic_token,
                                 self.transport.session_id,
                                 username)
        except Exception as e:
            self.transport.saved_exception = e
            result = AUTH_FAILED
            self._send_auth_result(username, self.method, result)
            raise
        # TODO: Implement client credential saving.
        # The OpenSSH server is able to create a TGT with the delegated
        # client credentials, but this is not supported by GSS-API.
        result = AUTH_SUCCESSFUL
        self.transport.server_object.check_auth_gssapi_with_mic(username,
                                                                result)
        # okay, send result
        self._send_auth_result(username, self.method, result)

    def _parse_service_request(self, m):
        self._restore_delegate_auth_handler()
        return self._delegate._parse_service_request(m)

    def _parse_userauth_request(self, m):
        self._restore_delegate_auth_handler()
        return self._delegate._parse_userauth_request(m)

    __handler_table = {
        MSG_SERVICE_REQUEST: _parse_service_request,
        MSG_USERAUTH_REQUEST: _parse_userauth_request,
        MSG_USERAUTH_GSSAPI_TOKEN: _parse_userauth_gssapi_token,
        MSG_USERAUTH_GSSAPI_MIC: _parse_userauth_gssapi_mic,
    }

    @property
    def _handler_table(self):
        # TODO: determine if we can cut this up like we did for the primary
        # AuthHandler class.
        return self.__handler_table