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1 Internet Draft Editor: Blake Ramsdell,
2 draft-ietf-smime-cert-08.txt Worldtalk
3 April 23, 1999
4 Expires October 23, 1999
6 S/MIME Version 3 Certificate Handling
8 Status of this memo
10 This document is an Internet-Draft and is in full conformance with all
11 provisions of Section 10 of RFC2026.
13 Internet-Drafts are working documents of the Internet Engineering Task
14 Force (IETF), its areas, and its working groups. Note that other
15 groups may also distribute working documents as Internet-Drafts.
17 Internet-Drafts are draft documents valid for a maximum of six months
18 and may be updated, replaced, or obsoleted by other documents at any
19 time. It is inappropriate to use Internet-Drafts as reference
20 material or to cite them other than as "work in progress."
22 The list of current Internet-Drafts can be accessed at
23 http://www.ietf.org/ietf/1id-abstracts.txt
25 The list of Internet-Draft Shadow Directories can be accessed at
26 http://www.ietf.org/shadow.html.
28 1. Overview
30 S/MIME (Secure/Multipurpose Internet Mail Extensions), described in
31 [SMIME-MSG], provides a method to send and receive secure MIME
32 messages. Before using a public key to provide security services, the
33 S/MIME agent MUST certify that the public key is valid. S/MIME agents
34 MUST use PKIX certificates to validate public keys as described in the
35 Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL
36 Profile [KEYM]. S/MIME agents MUST meet the certificate processing
37 requirements documented in this document in addition to those stated
38 in [KEYM].
40 This specification is compatible with the Cryptographic Message Syntax
41 [CMS] in that it uses the data types defined by CMS. It also inherits
42 all the varieties of architectures for certificate-based key
43 management supported by CMS.
45 1.1 Definitions
47 For the purposes of this draft, the following definitions apply.
49 ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680-689.
51 Attribute Certificate (AC): An X.509 AC is a separate structure from a
52 subject's public key X.509 Certificate. A subject may have multiple
53 X.509 ACs associated with each of its public key X.509 Certificates.
54 Each X.509 AC binds one or more Attributes with one of the subject's
55 public key X.509 Certificates. The X.509 AC syntax is defined in
56 [X.509]
58 BER: Basic Encoding Rules for ASN.1, as defined in ITU-T X.690.
60 Certificate: A type that binds an entity's distinguished name to a
61 public key with a digital signature. This type is defined in the
62 Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL
63 Profile [KEYM]. This type also contains the distinguished name of the
64 certificate issuer (the signer), an issuer-specific serial number, the
65 issuer's signature algorithm identifier, a validity period, and
66 extensions also defined in that document.
68 Certificate Revocation List (CRL): A type that contains information
69 about certificates whose validity an issuer has prematurely revoked.
70 The information consists of an issuer name, the time of issue, the
71 next scheduled time of issue, a list of certificate serial numbers and
72 their associated revocation times, and extensions as defined in
73 [KEYM]. The CRL is signed by the issuer. The type intended by this
74 specification is the one defined in [KEYM].
76 DER: Distinguished Encoding Rules for ASN.1, as defined in ITU-T
77 X.690.
79 Receiving agent: software that interprets and processes S/MIME CMS
80 objects, MIME body parts that contain CMS objects, or both.
82 Sending agent: software that creates S/MIME CMS objects, MIME body
83 parts that contain CMS objects, or both.
85 S/MIME agent: user software that is a receiving agent, a sending
86 agent, or both.
88 1.2 Compatibility with Prior Practice of S/MIME
90 S/MIME version 3 agents should attempt to have the greatest
91 interoperability possible with S/MIME version 2 agents. S/MIME version
92 2 is described in RFC 2311 through RFC 2315, inclusive. RFC 2311 also
93 has historical information about the development of S/MIME.
95 1.3 Terminology
97 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
98 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
99 document are to be interpreted as described in [MUSTSHOULD].
101 1.4 Discussion of This Draft
103 This draft is being discussed on the "ietf-smime" mailing list.
104 To subscribe, send a message to:
105 ietf-smime-request@imc.org
106 with the single word
107 subscribe
108 in the body of the message. There is a Web site for the mailing list
109 at .
111 2. CMS Options
113 The CMS message format allows for a wide variety of options in content
114 and algorithm support. This section puts forth a number of support
115 requirements and recommendations in order to achieve a base level of
116 interoperability among all S/MIME implementations. Most of the CMS
117 format for S/MIME messages is defined in [SMIME-MSG].
119 2.1 CertificateRevocationLists
121 Receiving agents MUST support the Certificate Revocation List (CRL)
122 format defined in [KEYM]. If sending agents include CRLs in outgoing
123 messages, the CRL format defined in [KEYM] MUST be used.
125 All agents MUST be capable of performing revocation checks using CRLs
126 as specified in [KEYM]. All agents MUST perform revocation status
127 checking in accordance with [KEYM]. Receiving agents MUST recognize
128 CRLs in received S/MIME messages.
130 Agents SHOULD store CRLs received in messages for use in processing
131 later messages.
133 Agents MUST handle multiple valid Certificate Authority (CA)
134 certificates containing the same subject name and the same public keys
135 but with overlapping validity intervals.
137 2.2 CertificateChoices
139 Receiving agents MUST support PKIX v1 and PKIX v3 certificates. See
140 [KEYM] for details about the profile for certificate formats. End
141 entity certificates MAY include an Internet mail address, as described
142 in section 3.1.
144 Receiving agents SHOULD support X.509 attribute certificates.
146 2.2.1 Historical Note About CMS Certificates
148 The CMS message format supports a choice of certificate formats for
149 public key content types: PKIX, PKCS #6 Extended Certificates and
150 X.509 Attribute Certificates. The PKCS #6 format is not in widespread
151 use. In addition, PKIX certificate extensions address much of the same
152 functionality and flexibility as was intended in the PKCS #6. Thus,
153 sending and receiving agents MUST NOT use PKCS #6 extended
154 certificates.
156 2.3 CertificateSet
158 Receiving agents MUST be able to handle an arbitrary number of
159 certificates of arbitrary relationship to the message sender and to
160 each other in arbitrary order. In many cases, the certificates
161 included in a signed message may represent a chain of certification
162 from the sender to a particular root. There may be, however,
163 situations where the certificates in a signed message may be unrelated
164 and included for convenience.
166 Sending agents SHOULD include any certificates for the user's public
167 key(s) and associated issuer certificates. This increases the
168 likelihood that the intended recipient can establish trust in the
169 originator's public key(s). This is especially important when sending
170 a message to recipients that may not have access to the sender's
171 public key through any other means or when sending a signed message to
172 a new recipient. The inclusion of certificates in outgoing messages
173 can be omitted if S/MIME objects are sent within a group of
174 correspondents that has established access to each other's
175 certificates by some other means such as a shared directory or manual
176 certificate distribution. Receiving S/MIME agents SHOULD be able to
177 handle messages without certificates using a database or directory
178 lookup scheme.
180 A sending agent SHOULD include at least one chain of certificates up
181 to, but not including, a Certificate Authority (CA) that it believes
182 that the recipient may trust as authoritative. A receiving agent
183 SHOULD be able to handle an arbitrarily large number of certificates
184 and chains.
186 Agents MAY send CA certificates, that is, certificates that are self-
187 signed and can be considered the "root" of other chains. Note that
188 receiving agents SHOULD NOT simply trust any self-signed certificates
189 as valid CAs, but SHOULD use some other mechanism to determine if this
190 is a CA that should be trusted. Also note that in the case of DSA
191 certificates the parameters may be located in the root certificate.
192 This would require that the recipient possess the root certificate in
193 order to perform a signature verification, and is a valid example of a
194 case where transmitting the root certificate may be required.
196 Receiving agents MUST support chaining based on the distinguished name
197 fields. Other methods of building certificate chains may be supported
198 but are not currently recommended.
200 Receiving agents SHOULD support the decoding of X.509 attribute
201 certificates included in CMS objects. All other issues regarding the
202 generation and use of X.509 attribute certificates are outside of the
203 scope of this specification.
205 3. Using Distinguished Names for Internet Mail
207 End-entity certificates MAY contain an Internet mail address as
208 described in [RFC-822]. The address must be an "addr-spec" as defined
209 in Section 6.1 of that specification. The email address SHOULD be in
210 the subjectAltName extension, and SHOULD NOT be in the subject
211 distinguished name.
213 Receiving agents MUST recognize email addresses in the subjectAltName
214 field. Receiving agents MUST recognize email addresses in the
215 Distinguished Name field in the PKCS #9 emailAddress attribute.
217 Sending agents SHOULD make the address in the From or Sender header in
218 a mail message match an Internet mail address in the signer's
219 certificate. Receiving agents MUST check that the address in the From
220 or Sender header of a mail message matches an Internet mail address in
221 the signer's certificate, if mail addresses are present in the
222 certificate. A receiving agent SHOULD provide some explicit alternate
223 processing of the message if this comparison fails, which may be to
224 display a message that shows the recipient the addresses in the
225 certificate or other certificate details.
227 All subject and issuer names MUST be populated (i.e. not an empty
228 SEQUENCE) in S/MIME-compliant PKIX certificates, except that the
229 subject DN in a user's (i.e. end-entity) certificate MAY be an empty
230 SEQUENCE in which case the subjectAltName extension will include the
231 subject's identifier and MUST be marked as critical.
233 4. Certificate Processing
235 A receiving agent needs to provide some certificate retrieval
236 mechanism in order to gain access to certificates for recipients of
237 digital envelopes. There are many ways to implement certificate
238 retrieval mechanisms. X.500 directory service is an excellent example
239 of a certificate retrieval-only mechanism that is compatible with
240 classic X.500 Distinguished Names. The PKIX Working Group is
241 investigating other mechanisms such as directory servers. Another
242 method under consideration by the IETF is to provide certificate
243 retrieval services as part of the existing Domain Name System (DNS).
244 Until such mechanisms are widely used, their utility may be limited by
245 the small number of correspondent's certificates that can be
246 retrieved. At a minimum, for initial S/MIME deployment, a user agent
247 could automatically generate a message to an intended recipient
248 requesting that recipient's certificate in a signed return message.
250 Receiving and sending agents SHOULD also provide a mechanism to allow
251 a user to "store and protect" certificates for correspondents in such
252 a way so as to guarantee their later retrieval. In many environments,
253 it may be desirable to link the certificate retrieval/storage
254 mechanisms together in some sort of certificate database. In its
255 simplest form, a certificate database would be local to a particular
256 user and would function in a similar way as a "address book" that
257 stores a user's frequent correspondents. In this way, the certificate
258 retrieval mechanism would be limited to the certificates that a user
259 has stored (presumably from incoming messages). A comprehensive
260 certificate retrieval/storage solution may combine two or more
261 mechanisms to allow the greatest flexibility and utility to the user.
262 For instance, a secure Internet mail agent may resort
263 to checking a centralized certificate retrieval mechanism for a
264 certificate if it can not be found in a user's local certificate
265 storage/retrieval database.
267 Receiving and sending agents SHOULD provide a mechanism for the import
268 and export of certificates, using a CMS certs-only message. This
269 allows for import and export of full certificate chains as opposed to
270 just a single certificate. This is described in [SMIME-MSG].
272 4.1 Certificate Revocation Lists
274 In general, it is always better to get the latest CRL information from
275 a CA than to get information stored away from incoming messages. A
276 receiving agent SHOULD have access to some certificate-revocation list
277 (CRL) retrieval mechanism in order to gain access to certificate-
278 revocation information when validating certificate chains. A receiving
279 or sending agent SHOULD also provide a mechanism to allow a user to
280 store incoming certificate-revocation information for correspondents
281 in such a way so as to guarantee its later retrieval.
283 Receiving and sending agents SHOULD retrieve and utilize CRL
284 information every time a certificate is verified as part of a
285 certificate chain validation even if the certificate was already
286 verified in the past. However, in many instances (such as off-line
287 verification) access to the latest CRL information may be difficult or
288 impossible. The use of CRL information, therefore, may be dictated by
289 the value of the information that is protected. The value of the CRL
290 information in a particular context is beyond the scope of this draft
291 but may be governed by the policies associated with particular
292 certificate hierarchies.
294 All agents MUST be capable of performing revocation checks using CRLs
295 as specified in [KEYM]. All agents MUST perform revocation status
296 checking in accordance with [KEYM]. Receiving agents MUST recognize
297 CRLs in received S/MIME messages.
299 4.2 Certificate Chain Validation
301 In creating a user agent for secure messaging, certificate, CRL, and
302 certificate chain validation SHOULD be highly automated while still
303 acting in the best interests of the user. Certificate, CRL, and chain
304 validation MUST be performed as per [KEYM] when validating a
305 correspondent's public key. This is necessary before using a public
306 key to provide security services such as: verifying a signature;
307 encrypting a content-encryption key (ex: RSA); or forming a pairwise
308 symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a
309 content-encryption key.
311 Certificates and CRLs are made available to the chain validation
312 procedure in two ways: a) incoming messages, and b) certificate and
313 CRL retrieval mechanisms. Certificates and CRLs in incoming messages
314 are not required to be in any particular order nor are they required
315 to be in any way related to the sender or recipient of the message
316 (although in most cases they will be related to the sender). Incoming
317 certificates and CRLs SHOULD be cached for use in chain validation and
318 optionally stored for later use. This temporary certificate and CRL
319 cache SHOULD be used to augment any other certificate and CRL
320 retrieval mechanisms for chain validation on incoming signed messages.
322 4.3 Certificate and CRL Signing Algorithms
324 Certificates and Certificate-Revocation Lists (CRLs) are signed by the
325 certificate issuer. A receiving agent MUST be capable of verifying the
326 signatures on certificates and CRLs made with id-dsa-with-sha1 [DSS].
328 A receiving agent SHOULD be capable of verifying the signatures on
329 certificates and CRLs made with md2WithRSAEncryption,
330 md5WithRSAEncryption and sha-1WithRSAEncryption signature algorithms
331 with key sizes from 512 bits to 2048 bits described in [PKCS#1V2].
333 4.4 PKIX Certificate Extensions
335 PKIX describes an extensible framework in which the basic certificate
336 information can be extended and how such extensions can be used to
337 control the process of issuing and validating certificates. The PKIX
338 Working Group has ongoing efforts to identify and create extensions
339 which have value in particular certification environments. Further,
340 there are active efforts underway to issue PKIX certificates for
341 business purposes. This document identifies the minumum required set
342 of certificate extensions which have the greatest value in the S/MIME
343 environment. The syntax and semantics of all the identified extensions
344 are defined in [KEYM].
346 Sending and receiving agents MUST correctly handle the Basic
347 Constraints Certificate Extension, the Key Usage Certificate
348 Extension, authorityKeyID, subjectKeyID, and the subjectAltNames when
349 they appear in end-user certificates. Some mechanism SHOULD exist to
350 handle the defined certificate extensions when they appear in
351 intermediate or CA certificates.
353 Certificates issued for the S/MIME environment SHOULD NOT contain any
354 critical extensions (extensions that have the critical field set to
355 TRUE) other than those listed here. These extensions SHOULD be marked
356 as non-critical unless the proper handling of the extension is deemed
357 critical to the correct interpretation of the associated certificate.
358 Other extensions may be included, but those extensions SHOULD NOT be
359 marked as critical.
361 Interpretation and syntax for all extensions MUST follow [KEYM],
362 unless otherwise specified here.
364 4.4.1 Basic Constraints Certificate Extension
366 The basic constraints extension serves to delimit the role and
367 position of an issuing authority or end-entity certificate plays in a
368 chain of certificates.
370 For example, certificates issued to CAs and subordinate CAs contain a
371 basic constraint extension that identifies them as issuing authority
372 certificates. End-entity certificates contain an extension that
373 constrains the certificate from being an issuing authority
374 certificate.
376 Certificates SHOULD contain a basicConstraints extension in CA
377 certificates, and SHOULD NOT contain that extension in end entity
378 certificates.
380 4.4.2 Key Usage Certificate Extension
382 The key usage extension serves to limit the technical purposes for
383 which a public key listed in a valid certificate may be used. Issuing
384 authority certificates may contain a key usage extension that
385 restricts the key to signing certificates, certificate revocation
386 lists and other data.
388 For example, a certification authority may create subordinate issuer
389 certificates which contain a keyUsage extension which specifies that
390 the corresponding public key can be used to sign end user certs and
391 sign CRLs.
393 If a key usage extension is included in a PKIX certificate, then it
394 MUST be marked as critical.
396 4.4.2.1 Key Usage in Diffie-Hellman Key Exchange Certificates
398 For Diffie-Hellman key exchange certificates (certificates in which
399 the subject public key algorithm is dhpublicnumber), if the keyUsage
400 keyAgreement bit is set to 1 AND if the public key is to be used to
401 form a pairwise key to decrypt data, then the S/MIME agent MUST only
402 use the public key if the keyUsage encipherOnly bit is set to 0. If
403 the keyUsage keyAgreement bit is set to 1 AND if the key is to be used
404 to form a pairwise key to encrypt data, then the S/MIME agent MUST
405 only use the public key if the keyUsage decipherOnly bit is set to 0.
407 4.4.3 Subject Alternative Name Extension
409 The subject alternative name extension is used in S/MIME as the
410 preferred means to convey the RFC-822 email address(es) that
411 correspond to the entity for this certificate. Any RFC-822 email
412 addresses present MUST be encoded using the rfc822Name CHOICE of the
413 GeneralName type. Since the SubjectAltName type is a SEQUENCE OF
414 GeneralName, multiple RFC-822 email addresses MAY be present.
416 5. Security Considerations
418 All of the security issues faced by any cryptographic application must
419 be faced by a S/MIME agent. Among these issues are protecting the
420 user's private key, preventing various attacks, and helping the user
421 avoid mistakes such as inadvertently encrypting a message for the
422 wrong recipient. The entire list of security considerations is beyond
423 the scope of this document, but some significant concerns are listed
424 here.
426 When processing certificates, there are many situations where the
427 processing might fail. Because the processing may be done by a user
428 agent, a security gateway, or other program, there is no single way to
429 handle such failures. Just because the methods to handle the failures
430 has not been listed, however, the reader should not assume that they
431 are not important. The opposite is true: if a certificate is not
432 provably valid and associated with the message, the processing
433 software should take immediate and noticable steps to inform the end
434 user about it.
436 Some of the many places where signature and certificate checking might
437 fail include:
438 - no Internet mail addresses in a certificate match the sender of a
439 message
440 - no certificate chain leads to a trusted CA
441 - no ability to check the CRL for a certificate
442 - an invalid CRL was received
443 - the CRL being checked is expired
444 - the certificate is expired
445 - the certificate has been revoked
446 There are certainly other instances where a certificate may be
447 invalid, and it is the responsibility of the processing software to
448 check them all thoroughly, and to decide what to do if the check
449 fails.
451 A. References
453 [CERTV2] "S/MIME Version 2 Certificate Handling", RFC 2312
455 [CMS] "Cryptographic Message Syntax", Internet Draft draft-housley-
456 smime-cms
458 [DSS] NIST FIPS PUB 186, "Digital Signature Standard", 18 May 1994.
460 [KEYM] "Internet X.509 Public Key Infrastructure Certificate and CRL
461 Profile", Internet-Draft draft-ietf-pkix-ipki-part1
463 [MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement
464 Levels", RFC 2119
466 [PKCS#1V2], "PKCS #1: RSA Cryptography Specifications Version 2.0",
467 RFC 2437
469 [RFC-822], "Standard For The Format Of ARPA Internet Text Messages",
470 RFC 822.
472 [SMIME-MSG] "S/MIME Version 3 Message Specification ", Internet Draft
473 draft-ietf-smime-msg
475 [X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594-1:1997,
476 Information technology - Open Systems Interconnection - The Directory:
477 Overview of concepts, models and services
479 [X.501] ITU-T Recommendation X.501 (1997) | ISO/IEC 9594-2:1997,
480 Information technology - Open Systems Interconnection - The Directory:
481 Models
483 [X.509] ITU-T Recommendation X.509 (1997) | ISO/IEC 9594-8:1997,
484 Information technology - Open Systems Interconnection - The Directory:
485 Authentication framework
487 [X.520] ITU-T Recommendation X.520 (1997) | ISO/IEC 9594-6:1997,
488 Information technology - Open Systems Interconnection - The Directory:
489 Selected attribute types.
491 B. Acknowledgements
493 Many thanks go out to the other authors of the S/MIME v2 RFC: Steve
494 Dusse, Paul Hoffman and Jeff Weinstein. Without v2, there wouldn't be
495 a v3.
497 A number of the members of the S/MIME Working Group have also worked
498 very hard and contributed to this document. Any list of people is
499 doomed to omission and for that I apologize. In alphabetical order,
500 the following people stand out in my mind due to the fact that they
501 made direct contributions to this document.
503 Bill Flanigan
504 Elliott Ginsburg
505 Paul Hoffman
506 Russ Housley
507 Michael Myers
508 John Pawling
509 Denis Pinkas
510 Jim Schaad
512 C. Changes from last draft
514 Changed section 1.3 to contain correct reference language for
515 MUSTSHOULD (Thomas Narten)
516 Changed section D to update author's address (Blake Ramsdell)
517 Changed in section B to actual list (Blake Ramsdell)
519 D. Editor�s address
521 Blake Ramsdell
522 Worldtalk
523 17720 NE 65th St Ste 201
524 Redmond, WA 98052
525 +1 425 376 0225
526 blaker@deming.com