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
|
Transport Layer Security Working D. Taylor
Group Forge Research Pty Ltd
Internet-Draft November 29, 2002
Expires: May 30, 2003
Using SRP for TLS Authentication
draft-ietf-tls-srp-04
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on May 30, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This memo presents a technique for using the SRP [2] (Secure Remote
Password) protocol as an authentication method for the TLS
[1](Transport Layer Security) protocol.
Taylor Expires May 30, 2003 [Page 1]
Internet-Draft Using SRP for TLS Authentication November 2002
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. SRP Authentication in TLS . . . . . . . . . . . . . . . . . 4
2.1 Modifications to the TLS Handshake Sequence . . . . . . . . 4
2.1.1 Message Sequence . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2 Session Re-use . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 SRP Verifier Message Digest Selection . . . . . . . . . . . 5
2.3 Changes to the Handshake Message Contents . . . . . . . . . 5
2.3.1 Client hello . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3.2 Server certificate . . . . . . . . . . . . . . . . . . . . . 5
2.3.3 Server key exchange . . . . . . . . . . . . . . . . . . . . 5
2.3.4 Client key exchange . . . . . . . . . . . . . . . . . . . . 6
2.4 Calculating the Pre-master Secret . . . . . . . . . . . . . 6
2.5 Cipher Suite Definitions . . . . . . . . . . . . . . . . . . 6
2.6 New Message Structures . . . . . . . . . . . . . . . . . . . 7
2.6.1 ExtensionType . . . . . . . . . . . . . . . . . . . . . . . 7
2.6.2 Client Hello . . . . . . . . . . . . . . . . . . . . . . . . 7
2.6.3 Server Key Exchange . . . . . . . . . . . . . . . . . . . . 7
2.6.4 Client Key Exchange . . . . . . . . . . . . . . . . . . . . 9
3. Security Considerations . . . . . . . . . . . . . . . . . . 10
References . . . . . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . 11
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
Full Copyright Statement . . . . . . . . . . . . . . . . . . 13
Taylor Expires May 30, 2003 [Page 2]
Internet-Draft Using SRP for TLS Authentication November 2002
1. Introduction
At the time of writing, TLS uses public key certificiates with RSA/
DSA digital signatures, or Kerberos, for authentication.
These authentication methods do not seem well suited to the
applications now being adapted to use TLS (IMAP [4], FTP [6], or
TELNET [7], for example). Given these protocols (and others like
them) are designed to use the user name and password method of
authentication, being able to safely use user names and passwords to
authenticate the TLS connection provides a much easier route to
additional security than implementing a public key infrastructure in
certain situations.
SRP is an authentication method that allows the use of user names and
passwords over unencrypted channels without revealing the password to
an eavesdropper. SRP also supplies a shared secret at the end of the
authetication sequence that can be used to generate encryption keys.
This document describes the use of the SRP authentication method for
TLS.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
Taylor Expires May 30, 2003 [Page 3]
Internet-Draft Using SRP for TLS Authentication November 2002
2. SRP Authentication in TLS
2.1 Modifications to the TLS Handshake Sequence
The advent of SRP-6 [3] allows the SRP protocol to be implemented
using the standard sequence of handshake messages defined in [1].
The parameters to various messages are given in the following
diagram.
2.1.1 Message Sequence
Handshake Message Flow for SRP Authentication
Client Server
| |
Client Hello (I) ------------------------> |
| <---------------------------- Server Hello
| <---------------------------- Certificate*
| <---------------------------- Server Key Exchange (N, g, s, B)
| <---------------------------- Server Hello Done
Client Key Exchange (A) -----------------> |
[Change cipher spec] |
Finished --------------------------------> |
| [Change cipher spec]
| <---------------------------- Finished
| |
Application Data <--------------> Application Data
* Indicates optional or situation-dependent messages that are not
always sent.
The identifiers given after each message name refer to the SRP
variables included in that message. The variables I, N, g, s, A, and
B are defined in [3].
An extended client hello message, as defined in [8], is used to send
the client identifier (the user name).
Servers MAY add an SRP extension to the server hello message. For
the cipher suites defined in this document no information is carried
in the SRP extension in the server hello message. The option to add
an SRP extension to the server hello message is given in case it is
required in future.
2.1.2 Session Re-use
The short handshake mechanism for re-using sessions for new
Taylor Expires May 30, 2003 [Page 4]
Internet-Draft Using SRP for TLS Authentication November 2002
connections, and renegotiating keys for existing connections will
still work with the SRP authentication mechanism and handshake.
When a client attemps to re-use a session that uses SRP
authentication, it MUST include the SRP extension carrying the user
name (I) in the client hello message, in case the server cannot or
will not allow re-use of the session, meaning a full handshake
sequence is required.
If the server does agree to re-use an existing session the server
MUST ignore the information in the SRP extension of the client hello
message, except for its inclusion in the finished message hashes.
This is to ensure attackers cannot replace the authenticated identity
without supplying the proper authentication information.
2.2 SRP Verifier Message Digest Selection
Implementations conforming to this document MUST use the SHA-1
message digest with the SRP algorithm.
2.3 Changes to the Handshake Message Contents
This section describes the changes to the TLS handshake message
contents when SRP is being used for authentication. The definitions
of the new message contents and the on-the-wire changes are given in
Section 2.6.
2.3.1 Client hello
The user name is appended to the standard client hello message using
the hello message extension mechanism defined in [8].
2.3.2 Server certificate
The server MUST send a certificate if it agrees to an SRP cipher
suite that requires the server to provide additional authentication
in the form of a digital signature. See Section 2.5 for details of
which ciphersuites defined in this document require a server
certificate to be sent.
Because the server's certificate is only used for generating a
digital signature in SRP cipher suites, the certificate sent MUST
contain a public key that can be used for generating digital
signatures.
2.3.3 Server key exchange
The server key exchange message contains the prime (N), the generator
Taylor Expires May 30, 2003 [Page 5]
Internet-Draft Using SRP for TLS Authentication November 2002
(g), and the salt value (s) read from the SRP password file based on
the value of (I) received in the client hello extension. The server
key exchange message also contains the server's public key (B).
If the server has sent a certificate message, the server key exchange
message MUST be signed.
2.3.4 Client key exchange
The client key exchange message carries the client's public key (A).
2.4 Calculating the Pre-master Secret
The shared secret resulting from the SRP calculations (S) (defined in
[2]) is used as the pre-master secret.
The finished messages perform the same function as the client and
server evidence messages (M1 and M2) specified in [2]. If either the
client or the server calculate an incorrect value, the finished
messages will not be understood, and the connection will be dropped
as specified in [1].
2.5 Cipher Suite Definitions
The following cipher suites are added by this draft. The usage of
AES ciphersuites is as defined in [5].
CipherSuite TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x50 };
CipherSuite TLS_SRP_SHA_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x51 };
CipherSuite TLS_SRP_SHA_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x52 };
CipherSuite TLS_SRP_SHA_WITH_AES_128_CBC_SHA = { 0x00,0x53 };
CipherSuite TLS_SRP_SHA_RSA_WITH_AES_128_CBC_SHA = { 0x00,0x54 };
CipherSuite TLS_SRP_SHA_DSS_WITH_AES_128_CBC_SHA = { 0x00,0x55 };
CipherSuite TLS_SRP_SHA_WITH_AES_256_CBC_SHA = { 0x00,0x56 };
CipherSuite TLS_SRP_SHA_RSA_WITH_AES_256_CBC_SHA = { 0x00,0x57 };
CipherSuite TLS_SRP_SHA_DSS_WITH_AES_256_CBC_SHA = { 0x00,0x58 };
Cipher suites that do not include a digitial signature algorithm
identifier assume the server is authenticated by its possesion of the
SRP database.
Taylor Expires May 30, 2003 [Page 6]
Internet-Draft Using SRP for TLS Authentication November 2002
Cipher suites that begin with TLS_SRP_SHA_RSA or TLS_SRP_SHA_DSS
require the server to send a certificate message containing a
certificate with the specified type of public key, and to sign the
server key exchange message using a matching private key.
Implementations conforming to this specification MUST implement the
TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA ciphersuite, SHOULD implement the
TLS_SRP_SHA_WITH_AES_128_CBC_SHA and TLS_SRP_SHA_WITH_AES_256_CBC_SHA
ciphersuites, and MAY implement the remaining ciphersuites.
2.6 New Message Structures
This section shows the structure of the messages passed during a
handshake that uses SRP for authentication. The representation
language used is the same as that used in [1].
2.6.1 ExtensionType
A new value, "srp(6)", has been added to the enumerated
ExtensionType, defined in [8]. This value MUST be used as the
extension number for the SRP extension.
2.6.2 Client Hello
The user name (I) is encoded in an SRPExtension structure, and sent
in an extended client hello message, using an extension of type
"srp".
enum { client, server } ClientOrServerExtension;
struct {
select(ClientOrServerExtension) {
case client:
opaque srp_I<1..2^8-1>;
case server:
/* empty struct */
}
} SRPExtension;
2.6.3 Server Key Exchange
When the value of KeyExchangeAlgorithm is set to "srp", the server's
SRP parameters are sent in the server key exchange message, encoded
in a ServerSRPParams structure.
If a certificate is sent to the client the server key exchange
Taylor Expires May 30, 2003 [Page 7]
Internet-Draft Using SRP for TLS Authentication November 2002
message must be signed. The following table gives the
SignatureAlgorithm value to be used for each ciphersuite.
Ciphersuite SignatureAlgorithm
TLS_SRP_SHA_WITH_3DES_EDE_CBC_SHA anonymous
TLS_SRP_SHA_RSA_WITH_3DES_EDE_CBC_SHA rsa
TLS_SRP_SHA_DSS_WITH_3DES_EDE_CBC_SHA dsa
TLS_SRP_SHA_WITH_AES_128_CBC_SHA anonymous
TLS_SRP_SHA_RSA_WITH_AES_128_CBC_SHA rsa
TLS_SRP_SHA_DSS_WITH_AES_128_CBC_SHA dsa
TLS_SRP_SHA_WITH_AES_256_CBC_SHA anonymous
TLS_SRP_SHA_RSA_WITH_AES_256_CBC_SHA rsa
TLS_SRP_SHA_DSS_WITH_AES_256_CBC_SHA dsa
struct {
select (KeyExchangeAlgorithm) {
case diffie_hellman:
ServerDHParams params;
Signature signed_params;
case rsa:
ServerRSAParams params;
Signature signed_params;
case srp: /* new entry */
ServerSRPParams params;
Signature signed_params;
};
} ServerKeyExchange;
struct {
opaque srp_N<1..2^16-1>;
opaque srp_g<1..2^16-1>;
opaque srp_s<1..2^8-1>
opaque srp_B<1..2^16-1>;
} ServerSRPParams; /* SRP parameters */
Taylor Expires May 30, 2003 [Page 8]
Internet-Draft Using SRP for TLS Authentication November 2002
2.6.4 Client Key Exchange
When the value of KeyExchangeAlgorithm is set to "srp", the client's
ephemeral public key (A) is sent in the client key exchange message,
encoded in an ClientSRPPublic structure.
An extra value, srp, has been added to the enumerated
KeyExchangeAlgorithm, originally defined in TLS [1].
struct {
select (KeyExchangeAlgorithm) {
case rsa: EncryptedPreMasterSecret;
case diffie_hellman: ClientDiffieHellmanPublic;
case srp: ClientSRPPublic; /* new entry */
} exchange_keys;
} ClientKeyExchange;
enum { rsa, diffie_hellman, srp } KeyExchangeAlgorithm;
struct {
opaque srp_A<1..2^16-1>;
} ClientSRPPublic;
Taylor Expires May 30, 2003 [Page 9]
Internet-Draft Using SRP for TLS Authentication November 2002
3. Security Considerations
If an attacker is able to steal the SRP verifier file, the attacker
can masquerade as the real host. Filesystem based X.509 certificate
installations are vulnerable to a similar attack unless the server's
certificate is issued from a PKI that maintains revocation lists, and
the client TLS code can both contact the PKI and make use of the
revocation list.
Taylor Expires May 30, 2003 [Page 10]
Internet-Draft Using SRP for TLS Authentication November 2002
References
[1] Dierks, T. and C. Allen, "The TLS Protocol", RFC 2246, January
1999.
[2] Wu, T., "The SRP Authentication and Key Exchange System", RFC
2945, September 2000.
[3] Wu, T., "SRP-6: Improvements and Refinements to the Secure
Remote Password Protocol", October 2002.
[4] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595, June
1999.
[5] Chown, P., "Advanced Encryption Standard (AES) Ciphersuites for
Transport Layer Security (TLS)", RFC 3268, June 2002.
[6] Ford-Hutchinson, P., Carpenter, M., Hudson, T., Murray, E. and
V. Wiegand, "Securing FTP with TLS", draft-murray-auth-ftp-ssl-
09 (work in progress), April 2002.
[7] Boe, M. and J. Altman, "TLS-based Telnet Security", draft-ietf-
tn3270e-telnet-tls-06 (work in progress), April 2002.
[8] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J. and T.
Wright, "TLS Extensions", draft-ietf-tls-extensions-05 (work in
progress), July 2002.
Author's Address
David Taylor
Forge Research Pty Ltd
EMail: DavidTaylor@forge.com.au
URI: http://www.forge.com.au/
Taylor Expires May 30, 2003 [Page 11]
Internet-Draft Using SRP for TLS Authentication November 2002
Appendix A. Acknowledgements
Thanks to all on the IETF tls mailing list for ideas and analysis.
Thanks to Tom Wu for adapting the SRP protocol so it fits the
standard TLS handshake message sequence.
Taylor Expires May 30, 2003 [Page 12]
Internet-Draft Using SRP for TLS Authentication November 2002
Full Copyright Statement
Copyright (C) The Internet Society (2002). 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.
Taylor Expires May 30, 2003 [Page 13]
|