summaryrefslogtreecommitdiffhomepage
path: root/paramiko/kex_gss.py
blob: ba24c0a0fea2a3514da466193e2dcad191a4e29c (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
# 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.,
# 59 Temple Place, Suite 330, Boston, MA  02111-1307  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 *  # noqa
from paramiko import util
from paramiko.message import Message
from paramiko.py3compat import byte_chr, byte_mask, byte_ord
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(object):
    """
    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.transport.gss_kex_used = True
        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 paket 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)
        raise SSHException('GSS KexGroup1 asked to handle packet type %d'
                           % 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)
        self.transport._set_K_H(K, sha1(str(hm)).digest())
        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,
                                      self.transport.session_id)
        else:
            self.kexgss.ssh_check_mic(mic_token,
                                      self.transport.session_id)
        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._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:\nMajor Status: %s\nMinor Status: %s\
                            \nError Message: %s\n") % (str(maj_status),
                                                       str(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 = 0xFFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E088A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3DC2007CB8A163BF0598DA48361C55D39A69163FA8FD24CF5F83655D23DCA3AD961C62F356208552BB9ED529077096966D670C354E4ABC9804F1746C08CA18217C32905E462E36CE3BE39E772C180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF6955817183995497CEA956AE515D2261898FA051015728E5A8AACAA68FFFFFFFFFFFFFFFF  # noqa
    G = 2
    NAME = "gss-group14-sha1-toWM5Slw5Ew8Mqkay+al2g=="


class KexGSSGex(object):
    """
    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
        """
        self.transport.gss_kex_used = True
        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 paket 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)
        raise SSHException('KexGex asked to handle packet type %d' % 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 (%d <= %d <= %d bits)' % (
                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 '
                '(%d bits)' % bitlen)
        self.transport._log(DEBUG, 'Got server p (%d bits)' % 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._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,
                                      self.transport.session_id)
        else:
            self.kexgss.ssh_check_mic(mic_token,
                                      self.transport.session_id)
        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:\nMajor Status: %s\nMinor Status: %s\
                            \nError Message: %s\n") % (str(maj_status),
                                                       str(min_status),
                                                       err_msg)


class NullHostKey(object):
    """
    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