File: rfc3079.txt

package info (click to toggle)
doc-rfc 20181229-2
  • links: PTS, VCS
  • area: non-free
  • in suites: buster
  • size: 570,944 kB
  • sloc: xml: 285,646; sh: 107; python: 90; perl: 42; makefile: 14
file content (1179 lines) | stat: -rw-r--r-- 38,905 bytes parent folder | download | duplicates (5)
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
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179






Network Working Group                                            G. Zorn
Request for Comments: 3079                                 cisco Systems
Category: Informational                                       March 2001


 Deriving Keys for use with Microsoft Point-to-Point Encryption (MPPE)

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   The Point-to-Point Protocol (PPP) provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.

   The PPP Compression Control Protocol provides a method to negotiate
   and utilize compression protocols over PPP encapsulated links.

   Microsoft Point to Point Encryption (MPPE) is a means of representing
   PPP packets in an encrypted form.  MPPE uses the RSA RC4 algorithm to
   provide data confidentiality.  The length of the session key to be
   used for initializing encryption tables can be negotiated.  MPPE
   currently supports 40-bit, 56-bit and 128-bit session keys.  MPPE
   session keys are changed frequently; the exact frequency depends upon
   the options negotiated, but may be every packet.  MPPE is negotiated
   within option 18 in the Compression Control Protocol.

   This document describes the method used to derive initial MPPE
   session keys from a variety of credential types.  It is expected that
   this memo will be updated whenever Microsoft defines a new key
   derivation method for MPPE, since its primary purpose is to provide
   an open, easily accessible reference for third-parties wishing to
   interoperate with Microsoft products.

   MPPE itself (including the protocol used to negotiate its use, the
   details of the encryption method used and the algorithm used to
   change session keys during a session) is described in RFC 3078.







Zorn                         Informational                      [Page 1]

RFC 3079                  MPPE Key Derivation                 March 2001


Table of Contents

   1.  Specification of Requirements ............................... 2
   2.  Deriving Session Keys from MS-CHAP Credentials .............. 2
   2.1.  Generating 40-bit Session Keys ............................ 3
   2.2.  Generating 56-bit Session Keys ............................ 3
   2.3.  Generating 128-bit Session Keys ........................... 4
   2.4.  Key Derivation Functions .................................. 5
   2.5.  Sample Key Derivations .................................... 6
   2.5.1.  Sample 40-bit Key Derivation ............................ 6
   2.5.2.  Sample 56-bit Key Derivation ............................ 6
   2.5.3.  Sample 128-bit Key Derivation ........................... 7
   3.  Deriving Session Keys from MS-CHAP-2 Credentials ............ 7
   3.1.  Generating 40-bit Session Keys ............................ 8
   3.2.  Generating 56-bit Session Keys ............................ 9
   3.3.  Generating 128-bit Session Keys ...........................10
   3.4.  Key Derivation Functions ..................................11
   3.5.  Sample Key Derivations ....................................13
   3.5.1.  Sample 40-bit Key Derivation ............................13
   3.5.2.  Sample 56-bit Key Derivation ............................14
   3.5.3.  Sample 128-bit Key Derivation ...........................15
   4.  Deriving MPPE Session Keys from TLS Session Keys ............16
   4.1.  Generating 40-bit Session Keys ............................16
   4.2.  Generating 56-bit Session Keys ............................17
   4.3.  Generating 128-bit Session Keys ...........................17
   5.  Security Considerations .....................................18
   5.1.  MS-CHAP Credentials .......................................18
   5.2.  EAP-TLS Credentials .......................................19
   6.  References ..................................................19
   7.  Acknowledgements ............................................20
   8.  Author's Address ............................................20
   9.  Full Copyright Statement ....................................21

1.  Specification of Requirements

   In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
   "recommended", "SHOULD", and "SHOULD NOT" are to be interpreted as
   described in [6].

2.  Deriving Session Keys from MS-CHAP Credentials

   The Microsoft Challenge-Handshake Authentication Protocol (MS-CHAP-1)
   [2] is a Microsoft-proprietary PPP [1] authentication protocol,
   providing the functionality to which LAN-based users are accustomed
   while integrating the encryption and hashing algorithms used on
   Windows networks.





Zorn                         Informational                      [Page 2]

RFC 3079                  MPPE Key Derivation                 March 2001


   The following sections detail the methods used to derive initial
   session keys (40-, 56- and 128-bit) from MS-CHAP-1 credentials.

   Implementation Note

      The initial session key in both directions is derived from the
      credentials of the peer that initiated the call and the challenge
      used (if any) is the challenge from the first authentication.
      This is true for both unilateral and bilateral authentication, as
      well as for each link in a multilink bundle.  In the multi-chassis
      multilink case, implementations are responsible for ensuring that
      the correct keys are generated on all participating machines.

2.1.  Generating 40-bit Session Keys

   MPPE uses a derivative of the peer's LAN Manager password as the 40-
   bit session key used for initializing the RC4 encryption tables.

   The first step is to obfuscate the peer's password using the
   LmPasswordHash() function (described in [2]).  The first 8 octets of
   the result are used as the basis for the session key generated in the
   following way:

/*
* PasswordHash is the basis for the session key
* SessionKey is a copy of PasswordHash and is the generative session key
* 8 is the length (in octets) of the key to be generated.
*
*/
Get_Key(PasswordHash, SessionKey, 8)

/*
* The effective length of the key is reduced to 40 bits by
* replacing the first three bytes as follows:
*/
SessionKey[0] = 0xd1 ;
SessionKey[1] = 0x26 ;
SessionKey[2] = 0x9e ;

2.2.  Generating 56-bit Session Keys

   MPPE uses a derivative of the peer's LAN Manager password as the 56-
   bit session key used for initializing the RC4 encryption tables.

   The first step is to obfuscate the peer's password using the
   LmPasswordHash() function (described in [2]).  The first 8 octets of
   the result are used as the basis for the session key generated in the
   following way:



Zorn                         Informational                      [Page 3]

RFC 3079                  MPPE Key Derivation                 March 2001


/*
* PasswordHash is the basis for the session key
* SessionKey is a copy of PasswordHash and is the generative session key
* 8 is the length (in octets) of the key to be generated.
*
*/
Get_Key(PasswordHash, SessionKey, 8)

/*
* The effective length of the key is reduced to 56 bits by
* replacing the first byte as follows:
*/
SessionKey[0] = 0xd1 ;

2.3.  Generating 128-bit Session Keys

   MPPE uses a derivative of the peer's Windows NT password as the 128-
   bit session key used for initializing encryption tables.

   The first step is to obfuscate the peer's password using
   NtPasswordHash() function as described in [2].  The first 16 octets
   of the result are then hashed again using the MD4 algorithm.  The
   first 16 octets of the second hash are used as the basis for the
   session key generated in the following way:

/*
* Challenge (as described in [9]) is sent by the PPP authenticator
* during authentication and is 8 octets long.
* NtPasswordHashHash is the basis for the session key.
* On return, InitialSessionKey contains the initial session
* key to be used.
*/
Get_Start_Key(Challenge, NtPasswordHashHash, InitialSessionKey)

/*
* CurrentSessionKey is a copy of InitialSessionKey
* and is the generative session key.
* Length (in octets) of the key to generate is 16.
*
*/
Get_Key(InitialSessionKey, CurrentSessionKey, 16)










Zorn                         Informational                      [Page 4]

RFC 3079                  MPPE Key Derivation                 March 2001


2.4.  Key Derivation Functions

   The following procedures are used to derive the session key.

/*
 * Pads used in key derivation
 */

SHApad1[40] =
   {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};

SHApad2[40] =
   {0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2,
    0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2,
    0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2,
    0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2};

/*
 * SHAInit(), SHAUpdate() and SHAFinal() functions are an
 * implementation of Secure Hash Algorithm (SHA-1) [7]. These are
 * available in public domain or can be licensed from
 * RSA Data Security, Inc.
 *
 * 1) InitialSessionKey is 8 octets long for 56- and 40-bit
 *    session keys, 16 octets long for 128 bit session keys.
 * 2) CurrentSessionKey is same as InitialSessionKey when this
 *    routine is called for the first time for the session.
 */

Get_Key(
IN     InitialSessionKey,
IN/OUT CurrentSessionKey
IN     LengthOfDesiredKey )
{
   SHAInit(Context)
   SHAUpdate(Context, InitialSessionKey, LengthOfDesiredKey)
   SHAUpdate(Context, SHAPad1, 40)
   SHAUpdate(Context, CurrentSessionKey, LengthOfDesiredKey)
   SHAUpdate(Context, SHAPad2, 40)
   SHAFinal(Context, Digest)
   memcpy(CurrentSessionKey, Digest, LengthOfDesiredKey)
}

Get_Start_Key(
IN  Challenge,



Zorn                         Informational                      [Page 5]

RFC 3079                  MPPE Key Derivation                 March 2001


IN  NtPasswordHashHash,
OUT InitialSessionKey)
{
   SHAInit(Context)
   SHAUpdate(Context, NtPasswordHashHash, 16)
   SHAUpdate(Context, NtPasswordHashHash, 16)
   SHAUpdate(Context, Challenge, 8)
   SHAFinal(Context, Digest)
   memcpy(InitialSessionKey, Digest, 16)
}

2.5.  Sample Key Derivations

   The following sections illustrate 40-, 56- and 128-bit key
   derivations.  All intermediate values are in hexadecimal.

2.5.1.  Sample 40-bit Key Derivation


   Initial Values
      Password = "clientPass"

   Step 1: LmPasswordHash(Password, PasswordHash)
      PasswordHash = 76 a1 52 93 60 96 d7 83 0e 23 90 22 74 04 af d2

   Step 2: Copy PasswordHash to SessionKey
      SessionKey = 76 a1 52 93 60 96 d7 83 0e 23 90 22 74 04 af d2

   Step 3: GetKey(PasswordHash, SessionKey, 8)
      SessionKey = d8 08 01 53 8c ec 4a 08

   Step 4: Reduce the effective key length to 40 bits
      SessionKey = d1 26 9e 53 8c ec 4a 08

2.5.2.  Sample 56-bit Key Derivation

   Initial Values
      Password = "clientPass"

   Step 1: LmPasswordHash(Password, PasswordHash)
      PasswordHash = 76 a1 52 93 60 96 d7 83 0e 23 90 22 74 04 af d2

   Step 2: Copy PasswordHash to SessionKey
      SessionKey = 76 a1 52 93 60 96 d7 83 0e 23 90 22 74 04 af d2

   Step 3: GetKey(PasswordHash, SessionKey, 8)
      SessionKey = d8 08 01 53 8c ec 4a 08




Zorn                         Informational                      [Page 6]

RFC 3079                  MPPE Key Derivation                 March 2001


   Step 4: Reduce the effective key length to 56 bits
      SessionKey = d1 08 01 53 8c ec 4a 08

2.5.3.  Sample 128-bit Key Derivation

Initial Values
   Password = "clientPass"
   Challenge = 10 2d b5 df 08 5d 30 41

Step 1: NtPasswordHash(Password, PasswordHash)
   PasswordHash = 44 eb ba 8d 53 12 b8 d6 11 47 44 11 f5 69 89 ae

Step 2: PasswordHashHash = MD4(PasswordHash)
   PasswordHashHash = 41 c0 0c 58 4b d2 d9 1c 40 17 a2 a1 2f a5 9f 3f

Step 3: GetStartKey(Challenge, PasswordHashHash, InitialSessionKey)
   InitialSessionKey = a8 94 78 50 cf c0 ac ca d1 78 9f b6 2d dc dd b0

Step 4: Copy InitialSessionKey to CurrentSessionKey
   CurrentSessionKey = a8 94 78 50 cf c0 ac c1 d1 78 9f b6 2d dc dd b0

Step 5: GetKey(InitialSessionKey, CurrentSessionKey, 16)
   CurrentSessionKey = 59 d1 59 bc 09 f7 6f 1d a2 a8 6a 28 ff ec 0b 1e

3.  Deriving Session Keys from MS-CHAP-2 Credentials

   Version 2 of the Microsoft Challenge-Handshake Authentication
   Protocol (MS-CHAP-2) [8] is a Microsoft-proprietary PPP
   authentication protocol, providing the functionality to which LAN-
   based users are accustomed while integrating the encryption and
   hashing algorithms used on Windows networks.

   The following sections detail the methods used to derive initial
   session keys from MS-CHAP-2 credentials.  40-, 56- and 128-bit keys
   are all derived using the same algorithm from the authenticating
   peer's Windows NT password.  The only difference is in the length of
   the keys and their effective strength: 40- and 56-bit keys are 8
   octets in length, while 128-bit keys are 16 octets long.  Separate
   keys are derived for the send and receive directions of the session.

   Implementation Note

      The initial session keys in both directions are derived from the
      credentials of the peer that initiated the call and the challenges
      used are those from the first authentication.  This is true as
      well for each link in a multilink bundle.  In the multi-chassis
      multilink case, implementations are responsible for ensuring that
      the correct keys are generated on all participating machines.



Zorn                         Informational                      [Page 7]

RFC 3079                  MPPE Key Derivation                 March 2001


3.1.  Generating 40-bit Session Keys

   When used in conjunction with MS-CHAP-2 authentication, the initial
   MPPE session keys are derived from the peer's Windows NT password.

   The first step is to obfuscate the peer's password using
   NtPasswordHash() function as described in [8].

      NtPasswordHash(Password, PasswordHash)

   The first 16 octets of the result are then hashed again using the MD4
   algorithm.

      PasswordHashHash = md4(PasswordHash)

   The first 16 octets of this second hash are used together with the
   NT- Response field from the MS-CHAP-2 Response packet [8] as the
   basis for the master session key:

      GetMasterKey(PasswordHashHash, NtResponse, MasterKey)

   Once the master key has been generated, it is used to derive two 40-
   bit session keys, one for sending and one for receiving:

      GetAsymmetricStartKey(MasterKey, MasterSendKey, 8, TRUE, TRUE)
      GetAsymmetricStartKey(MasterKey, MasterReceiveKey, 8, FALSE, TRUE)

   The master session keys are never used to encrypt or decrypt data;
   they are only used in the derivation of transient session keys.  The
   initial transient session keys are obtained by calling the function
   GetNewKeyFromSHA() (described in [3]):

GetNewKeyFromSHA(MasterSendKey, MasterSendKey, 8, SendSessionKey)
GetNewKeyFromSHA(MasterReceiveKey, MasterReceiveKey, 8,
                                               ReceiveSessionKey)

   Next, the effective strength of both keys is reduced by setting the
   first three octets to known constants:

      SendSessionKey[0] = ReceiveSessionKey[0] = 0xd1
      SendSessionKey[1] = ReceiveSessionKey[1] = 0x26
      SendSessionKey[2] = ReceiveSessionKey[2] = 0x9e

   Finally, the RC4 tables are initialized using the new session keys:

      rc4_key(SendRC4key, 8, SendSessionKey)
      rc4_key(ReceiveRC4key, 8, ReceiveSessionKey)




Zorn                         Informational                      [Page 8]

RFC 3079                  MPPE Key Derivation                 March 2001


3.2.  Generating 56-bit Session Keys

   When used in conjunction with MS-CHAP-2 authentication, the initial
   MPPE session keys are derived from the peer's Windows NT password.

   The first step is to obfuscate the peer's password using
   NtPasswordHash() function as described in [8].

      NtPasswordHash(Password, PasswordHash)

   The first 16 octets of the result are then hashed again using the MD4
   algorithm.

      PasswordHashHash = md4(PasswordHash)

   The first 16 octets of this second hash are used together with the
   NT-Response field from the MS-CHAP-2 Response packet [8] as the basis
   for the master session key:

      GetMasterKey(PasswordHashHash, NtResponse, MasterKey)

   Once the master key has been generated, it is used to derive two
   56-bit session keys, one for sending and one for receiving:

      GetAsymmetricStartKey(MasterKey, MasterSendKey, 8, TRUE, TRUE)
      GetAsymmetricStartKey(MasterKey, MasterReceiveKey, 8, FALSE, TRUE)

   The master session keys are never used to encrypt or decrypt data;
   they are only used in the derivation of transient session keys.  The
   initial transient session keys are obtained by calling the function
   GetNewKeyFromSHA() (described in [3]):

GetNewKeyFromSHA(MasterSendKey, MasterSendKey, 8, SendSessionKey)
GetNewKeyFromSHA(MasterReceiveKey, MasterReceiveKey, 8,
                                               ReceiveSessionKey)

   Next, the effective strength of both keys is reduced by setting the
   first octet to a known constant:

      SendSessionKey[0] = ReceiveSessionKey[0] = 0xd1

   Finally, the RC4 tables are initialized using the new session keys:

      rc4_key(SendRC4key, 8, SendSessionKey)
      rc4_key(ReceiveRC4key, 8, ReceiveSessionKey)






Zorn                         Informational                      [Page 9]

RFC 3079                  MPPE Key Derivation                 March 2001


3.3.  Generating 128-bit Session Keys

   When used in conjunction with MS-CHAP-2 authentication, the initial
   MPPE session keys are derived from the peer's Windows NT password.

   The first step is to obfuscate the peer's password using
   NtPasswordHash() function as described in [8].

      NtPasswordHash(Password, PasswordHash)

   The first 16 octets of the result are then hashed again using the MD4
   algorithm.

      PasswordHashHash = md4(PasswordHash)

   The first 16 octets of this second hash are used together with the
   NT-Response field from the MS-CHAP-2 Response packet [8] as the basis
   for the master session key:

      GetMasterKey(PasswordHashHash, NtResponse, MasterKey)

   Once the master key has been generated, it is used to derive two
   128-bit master session keys, one for sending and one for receiving:

GetAsymmetricStartKey(MasterKey, MasterSendKey, 16, TRUE, TRUE)
GetAsymmetricStartKey(MasterKey, MasterReceiveKey, 16, FALSE, TRUE)

   The master session keys are never used to encrypt or decrypt data;
   they are only used in the derivation of transient session keys.  The
   initial transient session keys are obtained by calling the function
   GetNewKeyFromSHA() (described in [3]):

GetNewKeyFromSHA(MasterSendKey, MasterSendKey, 16, SendSessionKey)
GetNewKeyFromSHA(MasterReceiveKey, MasterReceiveKey, 16,
                                                ReceiveSessionKey)

   Finally, the RC4 tables are initialized using the new session keys:

      rc4_key(SendRC4key, 16, SendSessionKey)
      rc4_key(ReceiveRC4key, 16, ReceiveSessionKey)











Zorn                         Informational                     [Page 10]

RFC 3079                  MPPE Key Derivation                 March 2001


3.4.  Key Derivation Functions

   The following procedures are used to derive the session key.

/*
 * Pads used in key derivation
 */

SHSpad1[40] =
   {0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00};

SHSpad2[40] =
   {0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2,
    0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2,
    0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2,
    0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2, 0xf2};

/*
 * "Magic" constants used in key derivations
 */

Magic1[27] =
   {0x54, 0x68, 0x69, 0x73, 0x20, 0x69, 0x73, 0x20, 0x74,
    0x68, 0x65, 0x20, 0x4d, 0x50, 0x50, 0x45, 0x20, 0x4d,
    0x61, 0x73, 0x74, 0x65, 0x72, 0x20, 0x4b, 0x65, 0x79};

Magic2[84] =
   {0x4f, 0x6e, 0x20, 0x74, 0x68, 0x65, 0x20, 0x63, 0x6c, 0x69,
    0x65, 0x6e, 0x74, 0x20, 0x73, 0x69, 0x64, 0x65, 0x2c, 0x20,
    0x74, 0x68, 0x69, 0x73, 0x20, 0x69, 0x73, 0x20, 0x74, 0x68,
    0x65, 0x20, 0x73, 0x65, 0x6e, 0x64, 0x20, 0x6b, 0x65, 0x79,
    0x3b, 0x20, 0x6f, 0x6e, 0x20, 0x74, 0x68, 0x65, 0x20, 0x73,
    0x65, 0x72, 0x76, 0x65, 0x72, 0x20, 0x73, 0x69, 0x64, 0x65,
    0x2c, 0x20, 0x69, 0x74, 0x20, 0x69, 0x73, 0x20, 0x74, 0x68,
    0x65, 0x20, 0x72, 0x65, 0x63, 0x65, 0x69, 0x76, 0x65, 0x20,
    0x6b, 0x65, 0x79, 0x2e};

Magic3[84] =
   {0x4f, 0x6e, 0x20, 0x74, 0x68, 0x65, 0x20, 0x63, 0x6c, 0x69,
    0x65, 0x6e, 0x74, 0x20, 0x73, 0x69, 0x64, 0x65, 0x2c, 0x20,
    0x74, 0x68, 0x69, 0x73, 0x20, 0x69, 0x73, 0x20, 0x74, 0x68,
    0x65, 0x20, 0x72, 0x65, 0x63, 0x65, 0x69, 0x76, 0x65, 0x20,
    0x6b, 0x65, 0x79, 0x3b, 0x20, 0x6f, 0x6e, 0x20, 0x74, 0x68,
    0x65, 0x20, 0x73, 0x65, 0x72, 0x76, 0x65, 0x72, 0x20, 0x73,
    0x69, 0x64, 0x65, 0x2c, 0x20, 0x69, 0x74, 0x20, 0x69, 0x73,



Zorn                         Informational                     [Page 11]

RFC 3079                  MPPE Key Derivation                 March 2001


    0x20, 0x74, 0x68, 0x65, 0x20, 0x73, 0x65, 0x6e, 0x64, 0x20,
    0x6b, 0x65, 0x79, 0x2e};


   GetMasterKey(
   IN  16-octet  PasswordHashHash,
   IN  24-octet  NTResponse,
   OUT 16-octet  MasterKey )
   {
      20-octet Digest

      ZeroMemory(Digest, sizeof(Digest));

      /*
       * SHSInit(), SHSUpdate() and SHSFinal()
       * are an implementation of the Secure Hash Standard [7].
       */

      SHSInit(Context);
      SHSUpdate(Context, PasswordHashHash, 16);
      SHSUpdate(Context, NTResponse, 24);
      SHSUpdate(Context, Magic1, 27);
      SHSFinal(Context, Digest);

      MoveMemory(MasterKey, Digest, 16);
   }

   VOID
   GetAsymetricStartKey(
   IN   16-octet      MasterKey,
   OUT  8-to-16 octet SessionKey,
   IN   INTEGER       SessionKeyLength,
   IN   BOOLEAN       IsSend,
   IN   BOOLEAN       IsServer )
   {

      20-octet Digest;

      ZeroMemory(Digest, 20);

      if (IsSend) {
         if (IsServer) {
            s = Magic3
         } else {
            s = Magic2
         }
      } else {
         if (IsServer) {



Zorn                         Informational                     [Page 12]

RFC 3079                  MPPE Key Derivation                 March 2001


            s = Magic2
         } else {
            s = Magic3
         }
      }

      /*
       * SHSInit(), SHSUpdate() and SHSFinal()
       * are an implementation of the Secure Hash Standard [7].
       */

      SHSInit(Context);
      SHSUpdate(Context, MasterKey, 16);
      SHSUpdate(Context, SHSpad1, 40);
      SHSUpdate(Context, s, 84);
      SHSUpdate(Context, SHSpad2, 40);
      SHSFinal(Context, Digest);

      MoveMemory(SessionKey, Digest, SessionKeyLength);
   }

3.5.  Sample Key Derivations

   The following sections illustrate 40-, 56- and 128-bit key
   derivations. All intermediate values are in hexadecimal.

3.5.1.  Sample 40-bit Key Derivation

Initial Values
   UserName = "User"
            =  55 73 65 72

   Password = "clientPass"
            = 63 00 6C 00 69 00 65 00 6E 00
              74 00 50 00 61 00 73 00 73 00

   AuthenticatorChallenge = 5B 5D 7C 7D 7B 3F 2F 3E 3C 2C
                            60 21 32 26 26 28
   PeerChallenge = 21 40 23 24 25 5E 26 2A 28 29 5F 2B 3A 33 7C 7E

   Challenge = D0 2E 43 86 BC E9 12 26

   NT-Response =
   82 30 9E CD 8D 70 8B 5E A0 8F AA 39 81 CD 83 54 42 33
   11 4A 3D 85 D6 DF

Step 1: NtPasswordHash(Password, PasswordHash)
   PasswordHash = 44 EB BA 8D 53 12 B8 D6 11 47 44 11 F5 69 89 AE



Zorn                         Informational                     [Page 13]

RFC 3079                  MPPE Key Derivation                 March 2001


Step 2: PasswordHashHash = MD4(PasswordHash)
   PasswordHashHash = 41 C0 0C 58 4B D2 D9 1C 40 17 A2 A1 2F A5 9F 3F

Step 3: Derive the master key (GetMasterKey())
   MasterKey = FD EC E3 71 7A 8C 83 8C B3 88 E5 27 AE 3C DD 31

Step 4: Derive the master send session key (GetAsymmetricStartKey())
   SendStartKey40 = 8B 7C DC 14 9B 99 3A 1B

Step 5: Derive the initial send session key (GetNewKeyFromSHA())
   SendSessionKey40 = D1 26 9E C4 9F A6 2E 3E

Sample Encrypted Message
   rc4(SendSessionKey40, "test message") = 92 91 37 91 7E 58 03 D6
                                           68 D7 58 98

3.5.2.  Sample 56-bit Key Derivation

Initial Values
   UserName = "User"
            =  55 73 65 72

   Password = "clientPass"
            = 63 00 6C 00 69 00 65 00 6E 00 74 00 50
              00 61 00 73 00 73 00

   AuthenticatorChallenge = 5B 5D 7C 7D 7B 3F 2F 3E 3C 2C
                            60 21 32 26 26 28
   PeerChallenge = 21 40 23 24 25 5E 26 2A 28 29 5F 2B 3A 33 7C 7E

   Challenge = D0 2E 43 86 BC E9 12 26

   NT-Response =
   82 30 9E CD 8D 70 8B 5E A0 8F AA 39 81 CD 83 54 42 33
   11 4A 3D 85 D6 DF

Step 1: NtPasswordHash(Password, PasswordHash)
   PasswordHash = 44 EB BA 8D 53 12 B8 D6 11 47 44 11 F5 69 89 AE

Step 2: PasswordHashHash = MD4(PasswordHash)
   PasswordHashHash = 41 C0 0C 58 4B D2 D9 1C 40 17 A2 A1 2F A5 9F 3F

Step 3: Derive the master key (GetMasterKey())
   MasterKey = FD EC E3 71 7A 8C 83 8C B3 88 E5 27 AE 3C DD 31

Step 4: Derive the master send session key (GetAsymmetricStartKey())
   SendStartKey56 = 8B 7C DC 14 9B 99 3A 1B




Zorn                         Informational                     [Page 14]

RFC 3079                  MPPE Key Derivation                 March 2001


Step 5: Derive the initial send session key (GetNewKeyFromSHA())
   SendSessionKey56 = D1 5C 00 C4 9F A6 2E 3E

Sample Encrypted Message
   rc4(SendSessionKey40, "test message") = 3F 10 68 33 FA 44 8D
                                           A8 42 BC 57 58

3.5.3.  Sample 128-bit Key Derivation

Initial Values
   UserName = "User"
            =  55 73 65 72

   Password = "clientPass"
            = 63 00 6C 00 69 00 65 00 6E 00
              74 00 50 00 61 00 73 00 73 00

   AuthenticatorChallenge = 5B 5D 7C 7D 7B 3F 2F 3E 3C 2C
                            60 21 32 26 26 28

   PeerChallenge = 21 40 23 24 25 5E 26 2A 28 29 5F 2B 3A 33 7C 7E

   Challenge = D0 2E 43 86 BC E9 12 26

   NT-Response =
   82 30 9E CD 8D 70 8B 5E A0 8F AA 39 81 CD 83 54 42 33
   11 4A 3D 85 D6 DF

Step 1: NtPasswordHash(Password, PasswordHash)
   PasswordHash = 44 EB BA 8D 53 12 B8 D6 11 47 44 11 F5 69 89 AE

Step 2: PasswordHashHash = MD4(PasswordHash)
   PasswordHashHash = 41 C0 0C 58 4B D2 D9 1C 40 17 A2 A1 2F A5 9F 3F

Step 2: Derive the master key (GetMasterKey())
   MasterKey = FD EC E3 71 7A 8C 83 8C B3 88 E5 27 AE 3C DD 31

Step 3: Derive the send master session key (GetAsymmetricStartKey())

   SendStartKey128 = 8B 7C DC 14 9B 99 3A 1B A1 18 CB 15 3F 56 DC CB

Step 4: Derive the initial send session key (GetNewKeyFromSHA())
   SendSessionKey128 = 40 5C B2 24 7A 79 56 E6 E2 11 00 7A E2 7B 22 D4

Sample Encrypted Message
  rc4(SendSessionKey128, "test message") = 81 84 83 17 DF 68
                                           84 62 72 FB 5A BE




Zorn                         Informational                     [Page 15]

RFC 3079                  MPPE Key Derivation                 March 2001


4.  Deriving MPPE Session Keys from TLS Session Keys

   The Extensible Authentication Protocol (EAP) [10] is a PPP extension
   that provides support  for  additional  authentication methods within
   PPP.  Transport  Level  Security  (TLS) [11] provides for mutual
   authentication, integrity-protected ciphersuite negotiation and key
   exchange between two  endpoints.  EAP-TLS [12] is an EAP
   authentication type which allows the use of TLS within the PPP
   authentication framework.  The following sections describe the
   methods used to derive initial session keys from TLS session keys.
   56-, 40- and 128-bit keys are derived using the same algorithm.  The
   only difference is in the length of the keys and their effective
   strength: 56- and 40-bit keys are 8 octets in length, while 128-bit
   keys are 16 octets long.  Separate keys are derived for the send and
   receive directions of the session.

4.1.  Generating 40-bit Session Keys

   When MPPE is used in conjunction with EAP-TLS authentication, the TLS
   master secret is used as the master session key.

   The algorithm used to derive asymmetrical master session keys from
   the TLS master secret is described in [12].  The master session keys
   are never used to encrypt or decrypt data; they are only used in the
   derivation of transient session keys.

   Implementation Note

      If the asymmetrical master keys are less than 8 octets in length,
      they MUST be padded on the left with zeroes before being used to
      derive the initial transient session keys.  Conversely, if the
      asymmetrical master keys are more than 8 octets in length, they
      must be truncated to 8 octets before being used to derive the
      initial transient session keys.

   The initial transient session keys are obtained by calling the
   function GetNewKeyFromSHA() (described in [3]):

GetNewKeyFromSHA(MasterSendKey, MasterSendKey, 8, SendSessionKey)
GetNewKeyFromSHA(MasterReceiveKey, MasterReceiveKey, 8,
ReceiveSessionKey)

   Next, the effective strength of both keys is reduced by setting the
   first three octets to known constants:

      SendSessionKey[0] = ReceiveSessionKey[0] = 0xD1
      SendSessionKey[1] = ReceiveSessionKey[1] = 0x26
      SendSessionKey[2] = ReceiveSessionKey[2] = 0x9E



Zorn                         Informational                     [Page 16]

RFC 3079                  MPPE Key Derivation                 March 2001


   Finally, the RC4 tables are initialized using the new session keys:

      rc4_key(SendRC4key, 8, SendSessionKey)
      rc4_key(ReceiveRC4key, 8, ReceiveSessionKey)

4.2.  Generating 56-bit Session Keys

   When MPPE is used in conjunction with EAP-TLS authentication, the TLS
   master secret is used as the master session key.

   The algorithm used to derive asymmetrical master session keys from
   the TLS master secret is described in [12].  The master session keys
   are never used to encrypt or decrypt data; they are only used in the
   derivation of transient session keys.

   Implementation Note

      If the asymmetrical master keys are less than 8 octets in length,
      they MUST be padded on the left with zeroes before being used to
      derive the initial transient session keys.  Conversely, if the
      asymmetrical master keys are more than 8 octets in length, they
      must be truncated to 8 octets before being used to derive the
      initial transient session keys.

   The initial transient session keys are obtained by calling the
   function GetNewKeyFromSHA() (described in [3]):

GetNewKeyFromSHA(MasterSendKey, MasterSendKey, 8, SendSessionKey)
GetNewKeyFromSHA(MasterReceiveKey, MasterReceiveKey, 8,
ReceiveSessionKey)

   Next, the effective strength of both keys is reduced by setting the
   initial octet to a known constant:

      SendSessionKey[0] = ReceiveSessionKey[0] = 0xD1

   Finally, the RC4 tables are initialized using the new session keys:

      rc4_key(SendRC4key, 8, SendSessionKey)
      rc4_key(ReceiveRC4key, 8, ReceiveSessionKey)

4.3.  Generating 128-bit Session Keys

   When MPPE is used in conjunction with EAP-TLS authentication, the TLS
   master secret is used as the master session key.






Zorn                         Informational                     [Page 17]

RFC 3079                  MPPE Key Derivation                 March 2001


   The algorithm used to derive asymmetrical master session keys from
   the TLS master secret is described in [12].  Note that the send key
   on one side is the receive key on the other.

   The master session keys are never used to encrypt or decrypt data;
   they are only used in the derivation of transient session keys.

   Implementation Note

      If the asymmetrical master keys are less than 16 octets in length,
      they MUST be padded on the left with zeroes before being used to
      derive the initial transient session keys.  Conversely, if the
      asymmetrical master keys are more than 16 octets in length, they
      must be truncated to 16 octets before being used to derive the
      initial transient session keys.

   The initial transient session keys are obtained by calling the
   function GetNewKeyFromSHA() (described in [3]):

GetNewKeyFromSHA(MasterSendKey, MasterSendKey, 16, SendSessionKey)
GetNewKeyFromSHA(MasterReceiveKey, MasterReceiveKey, 16,
ReceiveSessionKey)

   Finally, the RC4 tables are initialized using the new session keys:

      rc4_key(SendRC4key, 16, SendSessionKey)
      rc4_key(ReceiveRC4key, 16, ReceiveSessionKey)

5.  Security Considerations

5.1.  MS-CHAP Credentials

   Because of the way in which 40-bit keys are derived from MS-CHAP-1
   credentials, the initial 40-bit session key will be identical in all
   sessions established under the same peer credentials.  For this
   reason, and because RC4 with a 40-bit key length is believed to be a
   relatively weak cipher, peers SHOULD NOT use 40-bit keys derived from
   the LAN Manager password hash (as described above) if it can be
   avoided.

   Since the MPPE session keys are derived from user passwords (in the
   MS- CHAP-1 and MS-CHAP-2 cases), care should be taken to ensure the
   selection of strong passwords and passwords should be changed
   frequently.







Zorn                         Informational                     [Page 18]

RFC 3079                  MPPE Key Derivation                 March 2001


5.2.  EAP-TLS Credentials

   The strength of the session keys is dependent upon the security of
   the TLS protocol.

   The EAP server may be on a separate machine from the PPP
   authenticator; if this is the case, adequate care must be taken in
   the transmission of the EAP-TLS master keys to the authenticator.

6.  References

   [1]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
        1661, July 1994.

   [2]  Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions", RFC 2433,
        October 1998.

   [3]  Pall, G. and G. Zorn, "Microsoft Point-to-Point Encryption
        (MPPE) RFC 3078, March 2001.

   [4]  RC4 is a proprietary encryption algorithm available under
        license from RSA Data Security Inc.  For licensing information,
        contact:
               RSA Data Security, Inc.
               100 Marine Parkway
               Redwood City, CA 94065-1031

   [5]  Pall, G., "Microsoft Point-to-Point Compression (MPPC)
        Protocol", RFC 2118, March 1997.

   [6]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [7]  "Secure Hash Standard", Federal Information Processing Standards
        Publication 180-1, National Institute of Standards and
        Technology, April 1995.

   [8]  Zorn, G., "Microsoft PPP CHAP Extensions, Version 2", RFC 2759,
        January 2000.

   [9]  Simpson, W., "PPP Challenge Handshake Authentication Protocol
        (CHAP)", RFC 1994, August 1996.

   [10] Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication
        Protocol (EAP)", RFC 2284, March 1998.






Zorn                         Informational                     [Page 19]

RFC 3079                  MPPE Key Derivation                 March 2001


   [11] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
        2246, January 1999.

   [12] Aboba, B. and D. Simon, "PPP EAP TLS Authentication Protocol",
        RFC 2716, October 1999.

7.  Acknowledgements

   Anthony Bell, Richard B. Ward, Terence Spies and Thomas Dimitri, all
   of Microsoft Corporation, significantly contributed to the design and
   development of MPPE.

   Additional thanks to Robert Friend, Joe Davies, Jody Terrill, Archie
   Cobbs, Mark Deuser, Vijay Baliga, Brad Robel-Forrest and Jeff Haag
   for useful feedback.

   The technical portions of this memo were completed while the author
   was employed by Microsoft Corporation.

8.  Author's Address

   Questions about this memo can also be directed to:

   Glen Zorn
   cisco Systems
   500 108th Avenue N.E.
   Suite 500
   Bellevue, Washington 98004
   USA

   Phone: +1 425 438 8218
   FAX:   +1 425 438 1848
   EMail: gwz@cisco.com


















Zorn                         Informational                     [Page 20]

RFC 3079                  MPPE Key Derivation                 March 2001


9.  Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















Zorn                         Informational                     [Page 21]