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
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
|
Network Working Group R. Housley
Request for Comments: 3370 RSA Laboratories
Obsoletes: 2630, 3211 August 2002
Category: Standards Track
Cryptographic Message Syntax (CMS) Algorithms
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes the conventions for using several
cryptographic algorithms with the Cryptographic Message Syntax (CMS).
The CMS is used to digitally sign, digest, authenticate, or encrypt
arbitrary message contents.
Table of Contents
1 Introduction ............................................... 2
1.1 Changes Since RFC 2630 ..................................... 2
1.2 Terminology ................................................ 2
2 Message Digest Algorithms .................................. 3
2.1 SHA-1 ...................................................... 3
2.2 MD5 ........................................................ 3
3 Signature Algorithms ....................................... 4
3.1 DSA ........................................................ 4
3.2 RSA ........................................................ 5
4 Key Management Algorithms .................................. 6
4.1 Key Agreement Algorithms ................................... 6
4.1.1 X9.42 Ephemeral-Static Diffie-Hellman ...................... 7
4.1.2 X9.42 Static-Static Diffie-Hellman ......................... 8
4.2 Key Transport Algorithms ................................... 9
4.2.1 RSA (PKCS #1 v1.5) ......................................... 10
4.3 Symmetric Key-Encryption Key Algorithms .................... 10
4.3.1 Triple-DES Key Wrap ........................................ 11
4.3.2 RC2 Key Wrap ............................................... 12
4.4 Key Derivation Algorithms .................................. 12
Housley Standards Track [Page 1]
^L
RFC 3370 CMS Algorithms August 2002
4.4.1 PBKDF2 ..................................................... 13
5 Content Encryption Algorithms .............................. 13
5.1 Triple-DES CBC ............................................. 14
5.2 RC2 CBC .................................................... 14
6 Message Authentication Code (MAC) Algorithms ............... 15
6.1 HMAC with SHA-1 ............................................ 15
7 ASN.1 Module ............................................... 16
8 References ................................................. 18
9 Security Considerations .................................... 20
10 Acknowledgments ............................................ 22
11 Author's Address ........................................... 23
12 Full Copyright Statement ................................... 24
1 Introduction
The Cryptographic Message Syntax (CMS) [CMS] is used to digitally
sign, digest, authenticate, or encrypt arbitrary message contents.
This companion specification describes the use of common
cryptographic algorithms with the CMS. Implementations of the CMS
may support these algorithms; implementations of the CMS may also
support other algorithms as well. However, if an implementation
chooses to support one of the algorithms discussed in this document,
then the implementation MUST do so as described in this document.
The CMS values are generated using ASN.1 [X.208-88], using BER-
encoding [X.209-88]. Algorithm identifiers (which include ASN.1
object identifiers) identify cryptographic algorithms, and some
algorithms require additional parameters. When needed, parameters
are specified with an ASN.1 structure. The algorithm identifier for
each algorithm is specified, and when needed, the parameter structure
is specified. The fields in the CMS employed by each algorithm are
identified.
1.1 Changes Since RFC 2630
This document obsoletes section 12 of RFC 2630 [OLDCMS]. RFC 3369
[CMS] obsoletes the rest of RFC 2630. Separation of the protocol and
algorithm specifications allows each one to be updated without
impacting the other. However, the conventions for using additional
algorithms with the CMS are likely to be specified in separate
documents.
1.2 Terminology
In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD,
SHOULD NOT, RECOMMENDED, and MAY are to be interpreted as described
in [STDWORDS].
Housley Standards Track [Page 2]
^L
RFC 3370 CMS Algorithms August 2002
2 Message Digest Algorithms
This section specifies the conventions employed by CMS
implementations that support SHA-1 or MD5.
Digest algorithm identifiers are located in the SignedData
digestAlgorithms field, the SignerInfo digestAlgorithm field, the
DigestedData digestAlgorithm field, and the AuthenticatedData
digestAlgorithm field.
Digest values are located in the DigestedData digest field and the
Message Digest authenticated attribute. In addition, digest values
are input to signature algorithms.
2.1 SHA-1
The SHA-1 message digest algorithm is defined in FIPS Pub 180-1
[SHA1]. The algorithm identifier for SHA-1 is:
sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
oiw(14) secsig(3) algorithm(2) 26 }
There are two possible encodings for the SHA-1 AlgorithmIdentifier
parameters field. The two alternatives arise from the fact that when
the 1988 syntax for AlgorithmIdentifier was translated into the 1997
syntax, the OPTIONAL associated with the AlgorithmIdentifier
parameters got lost. Later the OPTIONAL was recovered via a defect
report, but by then many people thought that algorithm parameters
were mandatory. Because of this history some implementations encode
parameters as a NULL element and others omit them entirely. The
correct encoding is to omit the parameters field; however,
implementations MUST also handle a SHA-1 AlgorithmIdentifier
parameters field which contains a NULL.
The AlgorithmIdentifier parameters field is OPTIONAL. If present,
the parameters field MUST contain a NULL. Implementations MUST
accept SHA-1 AlgorithmIdentifiers with absent parameters.
Implementations MUST accept SHA-1 AlgorithmIdentifiers with NULL
parameters. Implementations SHOULD generate SHA-1
AlgorithmIdentifiers with absent parameters.
2.2 MD5
The MD5 digest algorithm is defined in RFC 1321 [MD5]. The algorithm
identifier for MD5 is:
md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) digestAlgorithm(2) 5 }
Housley Standards Track [Page 3]
^L
RFC 3370 CMS Algorithms August 2002
The AlgorithmIdentifier parameters field MUST be present, and the
parameters field MUST contain NULL. Implementations MAY accept the
MD5 AlgorithmIdentifiers with absent parameters as well as NULL
parameters.
3 Signature Algorithms
This section specifies the conventions employed by CMS
implementations that support DSA or RSA (PKCS #1 v1.5).
Signature algorithm identifiers are located in the SignerInfo
signatureAlgorithm field of SignedData. Also, signature algorithm
identifiers are located in the SignerInfo signatureAlgorithm field of
countersignature attributes.
Signature values are located in the SignerInfo signature field of
SignedData. Also, signature values are located in the SignerInfo
signature field of countersignature attributes.
3.1 DSA
The DSA signature algorithm is defined in FIPS Pub 186 [DSS]. DSA
MUST be used with the SHA-1 message digest algorithm.
The algorithm identifier for DSA subject public keys in certificates
is:
id-dsa OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) x9-57 (10040) x9cm(4) 1 }
DSA signature validation requires three parameters, commonly called
p, q, and g. When the id-dsa algorithm identifier is used, the
AlgorithmIdentifier parameters field is optional. If present, the
AlgorithmIdentifier parameters field MUST contain the three DSA
parameter values encoded using the Dss-Parms type. If absent, the
subject DSA public key uses the same DSA parameters as the
certificate issuer.
Dss-Parms ::= SEQUENCE {
p INTEGER,
q INTEGER,
g INTEGER }
When the id-dsa algorithm identifier is used, the DSA public key,
commonly called Y, MUST be encoded as an INTEGER. The output of this
encoding is carried in the certificate subject public key.
Dss-Pub-Key ::= INTEGER -- Y
Housley Standards Track [Page 4]
^L
RFC 3370 CMS Algorithms August 2002
The algorithm identifier for DSA with SHA-1 signature values is:
id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) x9-57 (10040) x9cm(4) 3 }
When the id-dsa-with-sha1 algorithm identifier is used, the
AlgorithmIdentifier parameters field MUST be absent.
When signing, the DSA algorithm generates two values, commonly called
r and s. To transfer these two values as one signature, they MUST be
encoded using the Dss-Sig-Value type:
Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER }
3.2 RSA
The RSA (PKCS #1 v1.5) signature algorithm is defined in RFC 2437
[NEWPKCS#1]. RFC 2437 specifies the use of the RSA signature
algorithm with the SHA-1 and MD5 message digest algorithms.
The algorithm identifier for RSA subject public keys in certificates
is:
rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }
When the rsaEncryption algorithm identifier is used, the
AlgorithmIdentifier parameters field MUST contain NULL.
When the rsaEncryption algorithm identifier is used, the RSA public
key, which is composed of a modulus and a public exponent, MUST be
encoded using the RSAPublicKey type. The output of this encoding is
carried in the certificate subject public key.
RSAPublicKey ::= SEQUENCE {
modulus INTEGER, -- n
publicExponent INTEGER } -- e
CMS implementations that include the RSA (PKCS #1 v1.5) signature
algorithm MUST also implement the SHA-1 message digest algorithm.
Such implementations SHOULD also support the MD5 message digest
algorithm.
Housley Standards Track [Page 5]
^L
RFC 3370 CMS Algorithms August 2002
The rsaEncryption algorithm identifier is used to identify RSA (PKCS
#1 v1.5) signature values regardless of the message digest algorithm
employed. CMS implementations that include the RSA (PKCS #1 v1.5)
signature algorithm MUST support the rsaEncryption signature value
algorithm identifier, and CMS implementations MAY support RSA (PKCS
#1 v1.5) signature value algorithm identifiers that specify both the
RSA (PKCS #1 v1.5) signature algorithm and the message digest
algorithm.
The algorithm identifier for RSA (PKCS #1 v1.5) with SHA-1 signature
values is:
sha1WithRSAEncryption OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 5 }
The algorithm identifier for RSA (PKCS #1 v1.5) with MD5 signature
values is:
md5WithRSAEncryption OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 4 }
When the rsaEncryption, sha1WithRSAEncryption, or
md5WithRSAEncryption signature value algorithm identifiers are used,
the AlgorithmIdentifier parameters field MUST be NULL.
When signing, the RSA algorithm generates a single value, and that
value is used directly as the signature value.
4 Key Management Algorithms
CMS accommodates the following general key management techniques: key
agreement, key transport, previously distributed symmetric key-
encryption keys, and passwords.
4.1 Key Agreement Algorithms
This section specifies the conventions employed by CMS
implementations that support key agreement using X9.42 Ephemeral-
Static Diffie-Hellman (X9.42 E-S D-H) and X9.42 Static-Static
Diffie-Hellman (X9.42 S-S D-H).
When a key agreement algorithm is used, a key-encryption algorithm is
also needed. Therefore, when key agreement is supported, a key-
encryption algorithm MUST be provided for each content-encryption
algorithm. The key wrap algorithms for Triple-DES and RC2 are
described in RFC 3217 [WRAP].
Housley Standards Track [Page 6]
^L
RFC 3370 CMS Algorithms August 2002
For key agreement of RC2 key-encryption keys, 128 bits MUST be
generated as input to the key expansion process used to compute the
RC2 effective key [RC2].
Key agreement algorithm identifiers are located in the EnvelopedData
RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm and
AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
keyEncryptionAlgorithm fields.
Key wrap algorithm identifiers are located in the KeyWrapAlgorithm
parameters within the EnvelopedData RecipientInfos
KeyAgreeRecipientInfo keyEncryptionAlgorithm and AuthenticatedData
RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm fields.
Wrapped content-encryption keys are located in the EnvelopedData
RecipientInfos KeyAgreeRecipientInfo RecipientEncryptedKeys
encryptedKey field. Wrapped message-authentication keys are located
in the AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
RecipientEncryptedKeys encryptedKey field.
4.1.1 X9.42 Ephemeral-Static Diffie-Hellman
Ephemeral-Static Diffie-Hellman key agreement is defined in RFC 2631
[DH-X9.42]. When using Ephemeral-Static Diffie-Hellman, the
EnvelopedData RecipientInfos KeyAgreeRecipientInfo field is used as
follows:
version MUST be 3.
originator MUST be the originatorKey alternative. The
originatorKey algorithm field MUST contain the dh-public-number
object identifier with absent parameters. The originatorKey
publicKey field MUST contain the sender's ephemeral public key.
The dh-public-number object identifier is:
dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) ansi-x942(10046) number-type(2) 1 }
ukm may be present or absent. CMS implementations MUST support
ukm being absent, and CMS implementations SHOULD support ukm being
present. When present, the ukm is used to ensure that a different
key-encryption key is generated when the ephemeral private key
might be used more than once.
Housley Standards Track [Page 7]
^L
RFC 3370 CMS Algorithms August 2002
keyEncryptionAlgorithm MUST be the id-alg-ESDH algorithm
identifier. The algorithm identifier parameter field for id-alg-
ESDH is KeyWrapAlgorithm, and this parameter MUST be present. The
KeyWrapAlgorithm denotes the symmetric encryption algorithm used
to encrypt the content-encryption key with the pairwise key-
encryption key generated using the X9.42 Ephemeral-Static Diffie-
Hellman key agreement algorithm. Triple-DES and RC2 key wrap
algorithms are described in RFC 3217 [WRAP]. The id-alg-ESDH
algorithm identifier and parameter syntax is:
id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
alg(3) 5 }
KeyWrapAlgorithm ::= AlgorithmIdentifier
recipientEncryptedKeys contains an identifier and an encrypted key
for each recipient. The RecipientEncryptedKey
KeyAgreeRecipientIdentifier MUST contain either the
issuerAndSerialNumber identifying the recipient's certificate or
the RecipientKeyIdentifier containing the subject key identifier
from the recipient's certificate. In both cases, the recipient's
certificate contains the recipient's static public key.
RecipientEncryptedKey EncryptedKey MUST contain the
content-encryption key encrypted with the X9.42 Ephemeral-Static
Diffie-Hellman generated pairwise key-encryption key using the
algorithm specified by the KeyWrapAlgorithm.
4.1.2 X9.42 Static-Static Diffie-Hellman
Static-Static Diffie-Hellman key agreement is defined in RFC 2631
[DH-X9.42]. When using Static-Static Diffie-Hellman, the
EnvelopedData RecipientInfos KeyAgreeRecipientInfo and
AuthenticatedData RecipientInfos KeyAgreeRecipientInfo fields are
used as follows:
version MUST be 3.
originator MUST be either the issuerAndSerialNumber or
subjectKeyIdentifier alternative. In both cases, the originator's
certificate contains the sender's static public key. RFC 3279
[CERTALGS] specifies the AlgorithmIdentifier parameters syntax and
values that are included in the originator's certificate. The
originator's certificate subject public key information field MUST
contain the dh-public-number object identifier:
dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) ansi-x942(10046) number-type(2) 1 }
Housley Standards Track [Page 8]
^L
RFC 3370 CMS Algorithms August 2002
ukm MUST be present. The ukm ensures that a different key-
encryption key is generated for each message between the same
sender and recipient.
keyEncryptionAlgorithm MUST be the id-alg-SSDH algorithm
identifier. The algorithm identifier parameter field for id-alg-
SSDH is KeyWrapAlgorihtm, and this parameter MUST be present. The
KeyWrapAlgorithm denotes the symmetric encryption algorithm used
to encrypt the content-encryption key with the pairwise key-
encryption key generated using the X9.42 Static-Static Diffie-
Hellman key agreement algorithm. Triple-DES and RC2 key wrap
algorithms are described in RFC 3217 [WRAP]. The id-alg-SSDH
algorithm identifier and parameter syntax is:
id-alg-SSDH OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
alg(3) 10 }
KeyWrapAlgorithm ::= AlgorithmIdentifier
recipientEncryptedKeys contains an identifier and an encrypted key
for each recipient. The RecipientEncryptedKey
KeyAgreeRecipientIdentifier MUST contain either the
issuerAndSerialNumber identifying the recipient's certificate or
the RecipientKeyIdentifier containing the subject key identifier
from the recipient's certificate. In both cases, the recipient's
certificate contains the recipient's static public key.
RecipientEncryptedKey EncryptedKey MUST contain the content-
encryption key encrypted with the X9.42 Static-Static Diffie-
Hellman generated pairwise key-encryption key using the algorithm
specified by the KeyWrapAlgortihm.
4.2 Key Transport Algorithms
This section specifies the conventions employed by CMS
implementations that support key transport using RSA (PKCS #1 v1.5).
Key transport algorithm identifiers are located in the EnvelopedData
RecipientInfos KeyTransRecipientInfo keyEncryptionAlgorithm field.
Key transport encrypted content-encryption keys are located in the
EnvelopedData RecipientInfos KeyTransRecipientInfo encryptedKey
field.
Housley Standards Track [Page 9]
^L
RFC 3370 CMS Algorithms August 2002
4.2.1 RSA (PKCS #1 v1.5)
The RSA key transport algorithm is the RSA encryption scheme defined
in RFC 2313 [PKCS#1], block type is 02, where the message to be
encrypted is the content-encryption key. The algorithm identifier
for RSA (PKCS #1 v1.5) is:
rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }
The AlgorithmIdentifier parameters field MUST be present, and the
parameters field MUST contain NULL.
When using a Triple-DES content-encryption key, CMS implementations
MUST adjust the parity bits for each DES key comprising the Triple-
DES key prior to RSA encryption.
The use of RSA (PKCS #1 v1.5) encryption, as defined in RFC 2313
[PKCS#1], to provide confidentiality has a known vulnerability. The
vulnerability is primarily relevant to usage in interactive
applications rather than to store-and-forward environments. Further
information and proposed countermeasures are discussed in the
Security Considerations section of this document and RFC 3218 [MMA].
Note that the same RSA encryption scheme is also defined in RFC 2437
[NEWPKCS#1]. Within RFC 2437, this RSA encryption scheme is called
RSAES-PKCS1-v1_5.
4.3 Symmetric Key-Encryption Key Algorithms
This section specifies the conventions employed by CMS
implementations that support symmetric key-encryption key management
using Triple-DES or RC2 key-encryption keys. When RC2 is supported,
RC2 128-bit keys MUST be used as key-encryption keys, and they MUST
be used with the RC2ParameterVersion parameter set to 58. A CMS
implementation MAY support mixed key-encryption and content-
encryptionalgorithms. For example, a 40-bit RC2 content-encryption
key MAY be wrapped with a 168-bit Triple-DES key-encryption key or
with a 128-bit RC2 key-encryption key.
Key wrap algorithm identifiers are located in the EnvelopedData
RecipientInfos KEKRecipientInfo keyEncryptionAlgorithm and
AuthenticatedData RecipientInfos KEKRecipientInfo
keyEncryptionAlgorithm fields.
Housley Standards Track [Page 10]
^L
RFC 3370 CMS Algorithms August 2002
Wrapped content-encryption keys are located in the EnvelopedData
RecipientInfos KEKRecipientInfo encryptedKey field. Wrapped
message-authentication keys are located in the AuthenticatedData
RecipientInfos KEKRecipientInfo encryptedKey field.
The output of a key agreement algorithm is a key-encryption key, and
this key-encryption key is used to encrypt the content-encryption
key. To support key agreement, key wrap algorithm identifiers are
located in the KeyWrapAlgorithm parameter of the EnvelopedData
RecipientInfos KeyAgreeRecipientInfo keyEncryptionAlgorithm and
AuthenticatedData RecipientInfos KeyAgreeRecipientInfo
keyEncryptionAlgorithm fields. However, only key agreement
algorithms that inherently provide authentication ought to be used
with AuthenticatedData. Wrapped content-encryption keys are located
in the EnvelopedData RecipientInfos KeyAgreeRecipientInfo
RecipientEncryptedKeys encryptedKey field, wrapped message-
authentication keys are located in the AuthenticatedData
RecipientInfos KeyAgreeRecipientInfo RecipientEncryptedKeys
encryptedKey field.
4.3.1 Triple-DES Key Wrap
A CMS implementation MAY support mixed key-encryption and content-
encryption algorithms. For example, a 128-bit RC2 content-encryption
key MAY be wrapped with a 168-bit Triple-DES key-encryption key.
Triple-DES key encryption has the algorithm identifier:
id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }
The AlgorithmIdentifier parameter field MUST be NULL.
The key wrap algorithm used to encrypt a Triple-DES content-
encryption key with a Triple-DES key-encryption key is specified in
section 3.1 of RFC 3217 [WRAP]. The corresponding key unwrap
algorithm is specified in section 3.2 of RFC 3217 [WRAP].
Out-of-band distribution of the Triple-DES key-encryption key used to
encrypt the Triple-DES content-encryption key is beyond the scope of
this document.
Housley Standards Track [Page 11]
^L
RFC 3370 CMS Algorithms August 2002
4.3.2 RC2 Key Wrap
A CMS implementation MAY support mixed key-encryption and content-
encryption algorithms. For example, a 128-bit RC2 content-encryption
key MAY be wrapped with a 168-bit Triple-DES key-encryption key.
Similarly, a 40-bit RC2 content-encryption key MAY be wrapped with a
128-bit RC2 key-encryption key.
RC2 key encryption has the algorithm identifier:
id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }
The AlgorithmIdentifier parameter field MUST be RC2wrapParameter:
RC2wrapParameter ::= RC2ParameterVersion
RC2ParameterVersion ::= INTEGER
The RC2 effective-key-bits (key size) greater than 32 and less than
256 is encoded in the RC2ParameterVersion. For the effective-key-
bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
and 58 respectively. These values are not simply the RC2 key length.
Note that the value 160 must be encoded as two octets (00 A0),
because the one octet (A0) encoding represents a negative number.
RC2 128-bit keys MUST be used as key-encryption keys, and they MUST
be used with the RC2ParameterVersion parameter set to 58.
The key wrap algorithm used to encrypt a RC2 content-encryption key
with a RC2 key-encryption key is specified in section 4.1 of RFC 3217
[WRAP]. The corresponding key unwrap algorithm is specified 4.2 of
RFC 3217 [WRAP].
Out-of-band distribution of the RC2 key-encryption key used to
encrypt the RC2 content-encryption key is beyond of the scope of this
document.
4.4 Key Derivation Algorithms
This section specifies the conventions employed by CMS
implementations that support password-based key management using
PBKDF2.
Key derivation algorithms are used to convert a password into a key-
encryption key as part of the password-based key management
technique.
Housley Standards Track [Page 12]
^L
RFC 3370 CMS Algorithms August 2002
Key derivation algorithm identifiers are located in the EnvelopedData
RecipientInfos PasswordRecipientInfo keyDerivationAlgorithm and
AuthenticatedData RecipientInfos PasswordRecipientInfo
keyDerivationAlgorithm fields.
The key-encryption key that is derived from the password is used to
encrypt the content-encryption key.
The content-encryption keys encrypted with password-derived key-
encryption keys are located in the EnvelopedData RecipientInfos
PasswordRecipientInfo encryptedKey field. The message-authentication
keys encrypted with password-derived key-encryption keys are located
in the AuthenticatedData RecipientInfos PasswordRecipientInfo
encryptedKey field.
4.4.1 PBKDF2
The PBKDF2 key derivation algorithm is specified in RFC 2898
[PKCS#5]. The KeyDerivationAlgorithmIdentifer identifies the key-
derivation algorithm, and any associated parameters used to derive
the key-encryption key from the user-supplied password. The
algorithm identifier for the PBKDF2 key derivation algorithm is:
id-PBKDF2 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-5(5) 12 }
The AlgorithmIdentifier parameter field MUST be PBKDF2-params:
PBKDF2-params ::= SEQUENCE {
salt CHOICE {
specified OCTET STRING,
otherSource AlgorithmIdentifier },
iterationCount INTEGER (1..MAX),
keyLength INTEGER (1..MAX) OPTIONAL,
prf AlgorithmIdentifier
DEFAULT { algorithm hMAC-SHA1, parameters NULL } }
Within the PBKDF2-params, the salt MUST use the specified OCTET
STRING.
5 Content Encryption Algorithms
This section specifies the conventions employed by CMS
implementations that support content encryption using Three-Key
Triple-DES in CBC mode, Two-Key Triple-DES in CBC mode, or RC2 in CBC
mode.
Housley Standards Track [Page 13]
^L
RFC 3370 CMS Algorithms August 2002
Content encryption algorithm identifiers are located in the
EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm and the
EncryptedData EncryptedContentInfo contentEncryptionAlgorithm fields.
Content encryption algorithms are used to encipher the content
located in the EnvelopedData EncryptedContentInfo encryptedContent
field and the EncryptedData EncryptedContentInfo encryptedContent
field.
5.1 Triple-DES CBC
The Triple-DES algorithm is described in ANSI X9.52 [3DES]. The
Triple-DES is composed from three sequential DES [DES] operations:
encrypt, decrypt, and encrypt. Three-Key Triple-DES uses a different
key for each DES operation. Two-Key Triple-DES uses one key for the
two encrypt operations and a different key for the decrypt operation.
The same algorithm identifiers are used for Three-Key Triple-DES and
Two-Key Triple-DES. The algorithm identifier for Triple-DES in
Cipher Block Chaining (CBC) mode is:
des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }
The AlgorithmIdentifier parameters field MUST be present, and the
parameters field must contain a CBCParameter:
CBCParameter ::= IV
IV ::= OCTET STRING -- exactly 8 octets
5.2 RC2 CBC
The RC2 algorithm is described in RFC 2268 [RC2]. The algorithm
identifier for RC2 in CBC mode is:
rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) encryptionAlgorithm(3) 2 }
The AlgorithmIdentifier parameters field MUST be present, and the
parameters field MUST contain a RC2CBCParameter:
RC2CBCParameter ::= SEQUENCE {
rc2ParameterVersion INTEGER,
iv OCTET STRING } -- exactly 8 octets
Housley Standards Track [Page 14]
^L
RFC 3370 CMS Algorithms August 2002
The RC2 effective-key-bits (key size) greater than 32 and less than
256 is encoded in the rc2ParameterVersion. For the effective-key-
bits of 40, 64, and 128, the rc2ParameterVersion values are 160, 120,
and 58 respectively. These values are not simply the RC2 key length.
Note that the value 160 must be encoded as two octets (00 A0), since
the one octet (A0) encoding represents a negative number.
6 Message Authentication Code Algorithms
This section specifies the conventions employed by CMS
implementations that support the HMAC with SHA-1 message
authentication code (MAC).
MAC algorithm identifiers are located in the AuthenticatedData
macAlgorithm field.
MAC values are located in the AuthenticatedData mac field.
6.1 HMAC with SHA-1
The HMAC with SHA-1 algorithm is described in RFC 2104 [HMAC]. The
algorithm identifier for HMAC with SHA-1 is:
hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1)
identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) 8 1 2 }
There are two possible encodings for the HMAC with SHA-1
AlgorithmIdentifier parameters field. The two alternatives arise
from the fact that when the 1988 syntax for the AlgorithmIdentifier
type was translated into the 1997 syntax, the OPTIONAL associated
with the AlgorithmIdentifier parameters got lost. Later the OPTIONAL
was recovered via a defect report, but by then many people thought
that algorithm parameters were mandatory. Because of this history
some implementations may encode parameters as a NULL while others
omit them entirely.
The AlgorithmIdentifier parameters field is OPTIONAL. If present,
the parameters field MUST contain a NULL. Implementations MUST
accept HMAC with SHA-1 AlgorithmIdentifiers with absent parameters.
Implementations MUST accept HMAC with SHA-1 AlgorithmIdentifiers with
NULL parameters. Implementations SHOULD generate HMAC with SHA-1
AlgorithmIdentifiers with absent parameters.
Housley Standards Track [Page 15]
^L
RFC 3370 CMS Algorithms August 2002
7 ASN.1 Module
CryptographicMessageSyntaxAlgorithms
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cmsalg-2001(16) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS All
-- The types and values defined in this module are exported for use
-- in the other ASN.1 modules. Other applications may use them for
-- their own purposes.
IMPORTS
-- Imports from RFC 3280 [PROFILE], Appendix A.1
AlgorithmIdentifier
FROM PKIX1Explicit88 { iso(1)
identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) mod(0)
pkix1-explicit(18) } ;
-- Algorithm Identifiers
sha-1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
oiw(14) secsig(3) algorithm(2) 26 }
md5 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) digestAlgorithm(2) 5 }
id-dsa OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
x9-57(10040) x9cm(4) 1 }
id-dsa-with-sha1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) x9-57(10040) x9cm(4) 3 }
rsaEncryption OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1 }
md5WithRSAEncryption OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 4 }
sha1WithRSAEncryption OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 5 }
dh-public-number OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) ansi-x942(10046) number-type(2) 1 }
Housley Standards Track [Page 16]
^L
RFC 3370 CMS Algorithms August 2002
id-alg-ESDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 5 }
id-alg-SSDH OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 10 }
id-alg-CMS3DESwrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 6 }
id-alg-CMSRC2wrap OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 7 }
des-ede3-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) encryptionAlgorithm(3) 7 }
rc2-cbc OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) encryptionAlgorithm(3) 2 }
hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) 8 1 2 }
id-PBKDF2 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-5(5) 12 }
-- Public Key Types
Dss-Pub-Key ::= INTEGER -- Y
RSAPublicKey ::= SEQUENCE {
modulus INTEGER, -- n
publicExponent INTEGER } -- e
DHPublicKey ::= INTEGER -- y = g^x mod p
-- Signature Value Types
Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER }
-- Algorithm Identifier Parameter Types
Dss-Parms ::= SEQUENCE {
p INTEGER,
q INTEGER,
g INTEGER }
Housley Standards Track [Page 17]
^L
RFC 3370 CMS Algorithms August 2002
DHDomainParameters ::= SEQUENCE {
p INTEGER, -- odd prime, p=jq +1
g INTEGER, -- generator, g
q INTEGER, -- factor of p-1
j INTEGER OPTIONAL, -- subgroup factor
validationParms ValidationParms OPTIONAL }
ValidationParms ::= SEQUENCE {
seed BIT STRING,
pgenCounter INTEGER }
KeyWrapAlgorithm ::= AlgorithmIdentifier
RC2wrapParameter ::= RC2ParameterVersion
RC2ParameterVersion ::= INTEGER
CBCParameter ::= IV
IV ::= OCTET STRING -- exactly 8 octets
RC2CBCParameter ::= SEQUENCE {
rc2ParameterVersion INTEGER,
iv OCTET STRING } -- exactly 8 octets
PBKDF2-params ::= SEQUENCE {
salt CHOICE {
specified OCTET STRING,
otherSource AlgorithmIdentifier },
iterationCount INTEGER (1..MAX),
keyLength INTEGER (1..MAX) OPTIONAL,
prf AlgorithmIdentifier
DEFAULT { algorithm hMAC-SHA1, parameters NULL } }
END -- of CryptographicMessageSyntaxAlgorithms
8 References
[3DES] American National Standards Institute. ANSI X9.52-1998,
Triple Data Encryption Algorithm Modes of Operation.
1998.
[CERTALGS] Bassham, L., Housley, R. and W. Polk, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 3279, April 2002.
Housley Standards Track [Page 18]
^L
RFC 3370 CMS Algorithms August 2002
[CMS] Housley, R., "Cryptographic Message Syntax", RFC 3269,
August 2002.
[DES] American National Standards Institute. ANSI X3.106,
"American National Standard for Information Systems -
Data Link Encryption". 1983.
[DH-X9.42] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
2631, June 1999.
[DSS] National Institute of Standards and Technology. FIPS Pub
186: Digital Signature Standard. 19 May 1994.
[HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message
Authentication", RFC 2104, February 1997.
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[MMA] Rescorla, E., "Preventing the Million Message Attack on
CMS", RFC 3218, January 2002.
[MODES] National Institute of Standards and Technology. FIPS Pub
81: DES Modes of Operation. 2 December 1980.
[NEWPKCS#1] Kaliski, B. and J. Staddon, "PKCS #1: RSA Encryption,
Version 2.0, RFC 2437, October 1998.
[OLDCMS] Housley, R., "Cryptographic Message Syntax", RFC 2630,
June 1999.
[PKCS#1] Kaliski, B, "PKCS #1: RSA Encryption, Version 2.0", RFC
2437, October, 1998.
[PKCS#5] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification", RFC 2898, September 2000.
[PROFILE] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile", RFC 3280,
April 2002.
[RANDOM] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security, RFC 1750, December 1994.
[RC2] Rivest, R., "A Description of the RC2 (r) Encryption
Algorithm", RFC 2268, March 1998.
Housley Standards Track [Page 19]
^L
RFC 3370 CMS Algorithms August 2002
[SHA1] National Institute of Standards and Technology. FIPS Pub
180-1: Secure Hash Standard. 17 April 1995.
[STDWORDS] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[WRAP] Housley, R., "Triple-DES and RC2 Key Wrapping", RFC 3217,
December 2001.
[X.208-88] CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
[X.209-88] CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One (ASN.1).
1988.
9 Security Considerations
The CMS provides a method for digitally signing data, digesting data,
encrypting data, and authenticating data. This document identifies
the conventions for using several cryptographic algorithms for use
with the CMS.
Implementations must protect the signer's private key. Compromise of
the signer's private key permits masquerade.
Implementations must protect the key management private key, the
key-encryption key, and the content-encryption key. Compromise of
the key management private key or the key-encryption key may result
in the disclosure of all contents protected with that key.
Similarly, compromise of the content-encryption key may result in
disclosure of the associated encrypted content.
Implementations must protect the key management private key and the
message-authentication key. Compromise of the key management private
key permits masquerade of authenticated data. Similarly, compromise
of the message-authentication key may result in undetectable
modification of the authenticated content.
The key management technique employed to distribute message-
authentication keys must itself provide authentication, otherwise the
content is delivered with integrity from an unknown source. Neither
RSA [PKCS#1, NEWPKCS#1] nor Ephemeral-Static Diffie-Hellman [DH-
X9.42] provide the necessary data origin authentication. Static-
Static Diffie-Hellman [DH-X9.42] does provide the necessary data
origin authentication when both the originator and recipient public
keys are bound to appropriate identities in X.509 certificates
[PROFILE].
Housley Standards Track [Page 20]
^L
RFC 3370 CMS Algorithms August 2002
When more than two parties share the same message-authentication key,
data origin authentication is not provided. Any party that knows the
message-authentication key can compute a valid MAC, therefore the
content could originate from any one of the parties.
Implementations must randomly generate content-encryption keys,
message-authentication keys, initialization vectors (IVs), one-time
values (such as the k value when generating a DSA signature), and
padding. Also, the generation of public/private key pairs relies on
a random numbers. The use of inadequate pseudo-random number
generators (PRNGs) to generate cryptographic such values can result
in little or no security. An attacker may find it much easier to
reproduce the PRNG environment that produced the keys, searching the
resulting small set of possibilities, rather than brute force
searching the whole key space. The generation of quality random
numbers is difficult. RFC 1750 [RANDOM] offers important guidance in
this area, and Appendix 3 of FIPS Pub 186 [DSS] provides one quality
PRNG technique.
When using key agreement algorithms or previously distributed
symmetric key-encryption keys, a key-encryption key is used to
encrypt the content-encryption key. If the key-encryption and
content-encryption algorithms are different, the effective security
is determined by the weaker of the two algorithms. If, for example,
content is encrypted with 168-bit Triple-DES and the Triple-DES
content-encryption key is wrapped with a 40-bit RC2 key, then at most
40 bits of protection is provided. A trivial search to determine the
value of the 40-bit RC2 key can recover Triple-DES key, and then the
Triple-DES key can be used to decrypt the content. Therefore,
implementers must ensure that key-encryption algorithms are as strong
or stronger than content-encryption algorithms.
RFC 3217 [WRAP] specifies key wrap algorithms used to encrypt a
Triple-DES content-encryption key with a Triple-DES key-encryption
key [3DES] or to encrypt a RC2 content-encryption key with a RC2
key-encryption key [RC2]. The key wrap algorithms makes use of CBC
mode [MODES]. These key wrap algorithms have been reviewed for use
with Triple-DES and RC2. They have not been reviewed for use with
other cryptographic modes or other encryption algorithms. Therefore,
if a CMS implementation wishes to support ciphers in addition to
Triple-DES or RC2, then additional key wrap algorithms need to be
defined to support the additional ciphers.
Implementers should be aware that cryptographic algorithms become
weaker with time. As new cryptanalysis techniques are developed and
computing performance improves, the work factor to break a particular
cryptographic algorithm will reduce. Therefore, cryptographic
Housley Standards Track [Page 21]
^L
RFC 3370 CMS Algorithms August 2002
algorithm implementations should be modular allowing new algorithms
to be readily inserted. That is, implementers should be prepared to
regularly update the set of algorithms in their implementations.
Users of the CMS, particularly those employing the CMS to support
interactive applications, should be aware that RSA (PKCS #1 v1.5), as
specified in RFC 2313 [PKCS#1], is vulnerable to adaptive chosen
ciphertext attacks when applied for encryption purposes.
Exploitation of this identified vulnerability, revealing the result
of a particular RSA decryption, requires access to an oracle which
will respond to a large number of ciphertexts (based on currently
available results, hundreds of thousands or more), which are
constructed adaptively in response to previously-received replies
providing information on the successes or failures of attempted
decryption operations. As a result, the attack appears significantly
less feasible to perpetrate for store-and-forward S/MIME environments
than for directly interactive protocols. Where the CMS constructs
are applied as an intermediate encryption layer within an interactive
request-response communications environment, exploitation could be
more feasible.
An updated version of PKCS #1 has been published, PKCS #1 Version 2.0
[NEWPKCS#1]. This updated document supersedes RFC 2313. PKCS #1
Version 2.0 preserves support for the encryption padding format
defined in PKCS #1 Version 1.5 [PKCS#1], and it also defines a new
alternative. To resolve the adaptive chosen ciphertext
vulnerability, the PKCS #1 Version 2.0 specifies and recommends use
of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption
is used to provide confidentiality. Designers of protocols and
systems employing CMS for interactive environments should either
consider usage of OAEP, or should ensure that information which could
reveal the success or failure of attempted PKCS #1 Version 1.5
decryption operations is not provided. Support for OAEP will likely
be added to a future version of the CMS algorithm specification.
See RFC 3218 [MMA] for more information about thwarting the adaptive
chosen ciphertext vulnerability in PKCS #1 Version 1.5
implementations.
10 Acknowledgments
This document is the result of contributions from many professionals.
I appreciate the hard work of all members of the IETF S/MIME Working
Group. I extend a special thanks to Rich Ankney, Simon Blake-Wilson,
Tim Dean, Steve Dusse, Carl Ellison, Peter Gutmann, Bob Jueneman,
Stephen Henson, Paul Hoffman, Scott Hollenbeck, Don Johnson, Burt
Kaliski, John Linn, John Pawling, Blake Ramsdell, Francois Rousseau,
Jim Schaad, and Dave Solo for their efforts and support.
Housley Standards Track [Page 22]
^L
RFC 3370 CMS Algorithms August 2002
11 Author Address
Russell Housley
RSA Laboratories
918 Spring Knoll Drive
Herndon, VA 20170
EMail: rhousley@rsasecurity.com
Housley Standards Track [Page 23]
^L
RFC 3370 CMS Algorithms August 2002
12. 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.
Housley Standards Track [Page 24]
^L
|