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1 Internet-Draft T Dean
2 draft-ietf-smime-domsec-03.txt W Ottaway
3 Expires 19th April 2000 DERA
5 Domain Security Services using S/MIME
7 Status of this memo
9 This document is an Internet-Draft and is in full conformance with all
10 provisions of section 10 of RFC2026. Internet-Drafts are working
11 documents of the Internet Engineering Task Force (IETF), its areas, and
12 its working groups. Note that other groups may also distribute working
13 documents as Internet-Drafts.
15 Internet-Drafts are draft documents valid for a maximum of six months
16 and may be updated, replaced, or obsoleted by other documents at any
17 time. It is inappropriate to use Internet-Drafts as reference material
18 or to cite them other than as "work in progress."
20 The list of current Internet-Drafts can be accessed at
21 http://www.ietf.org/ietf/1id-abstracts.txt
23 The list of Internet-Draft Shadow Directories can be accessed at
24 http://www.ietf.org/shadow.html.
26 Abstract
28 This document describes how the S/MIME protocol can be processed and
29 generated by a number of components of a communication system, such as
30 message transfer agents, guards and gateways to deliver security
31 services. These services are collectively referred to as 'Domain
32 Security Services'. The mechanisms described in this document are
33 designed to solve a number of interoperability problems and technical
34 limitations that arise when different security domains wish to
35 communicate securely - for example when two domains use incompatible
36 messaging technologies such as X.400 and SMTP/MIME. This document is
37 also applicable to organisations and enterprises that have internal PKIs
38 which are not accessible by the outside world, but wish to interoperate
39 securely using the S/MIME protocol.
41 This draft is being discussed on the 'ietf-smime' mailing list. To
42 subscribe, send a message to:
43 ietf-smime-request@imc.org
44 with the single word
45 subscribe
46 in the body of the message. There is a Web site for the mailing list at
47 .
49 Acknowledgements
51 Significant comments were made by Trevor Freeman, Russ Housley,
52 Dave Kemp, Jim Schaad, Greg Colla and Michael Zolotarev.
54 1. Introduction
56 The S/MIME [1] series of standards define a data encapsulation format
57 for the provision of a number of security services including data
58 integrity, confidentiality, and authentication. S/MIME is designed for
59 use by messaging clients to deliver security services to distributed
60 messaging applications.
62 There are many circumstances when it is not desirable or practical to
63 provide end-to-end (desktop-to-desktop) security services, particularly
64 between different security domains. An organisation that is considering
65 providing end-to-end security services will typically have to deal with
66 some if not all of the following issues:
68 1) Heterogeneous Message Access Methods: Users are accessing mail using
69 mechanisms which re-format messages, such as using Web browsers.
70 Message reformatting in the Message Store makes end-to-end encryption
71 and signature validation impossible.
73 2) Message screening and audit: Server-based mechanisms such as
74 searching for prohibited words or other content, virus scanning, and
75 audit, are incompatible with end-to-end encryption.
77 3) PKI deployment issues: There may not be any certificate paths between
78 two organisations. Or an organisation may be sensitive about aspects
79 of its PKI and unwilling to expose them to outside access. For either
80 of these reasons, direct end-to-end signature validation and
81 encryption are impossible.
83 4) Heterogeneous Message transports: One organisation using X.400 wishes
84 to communicate with another using SMTP. Message reformatting at
85 gateways makes end-to-end encryption and signature validation
86 impossible.
88 This document describes an approach to solving these problems by
89 providing message security services at the level of a domain or an
90 organisation. This document specifies how these 'domain security
91 services' can be provided using the S/MIME protocol. Domain security
92 services may replace or complement mechanisms at the desktop. For
93 example, a domain may decide to provide desktop-to-desktop signatures
94 but domain-to-domain encryption services. Or it may allow desktop-to-
95 desktop services for intra-domain use, but enforce domain-based services
96 for communication with other domains.
98 Whether or not a domain based service is inherently better or worse than
99 desktop based solutions is an open question. Some experts believe that
100 only end-to-end solutions can be truly made secure, while others believe
101 that the benefits offered by such things as content checking at domain
102 boundaries offers considerable increase in practical security for many
103 real systems. The additional service of allowing signature checking at
104 several points on a communications path is also an extra benefit in many
105 situations. This debate is outside the scope of this document. What is
106 offered here is a set of tools that integrators can tailor in different
107 ways to meet different needs in different circumstances.
109 Message transfer agents (MTAs), guards, firewalls and protocol
110 translation gateways all provide domain security services. As with
111 desktop based solutions, these components must be resilient against a
112 wide variety of attacks intended to subvert the security services.
113 Therefore, careful consideration should be given to security of these
114 components, to make sure that their siting and configuration minimises
115 the possibility of attack.
117 Throughout this draft the terms MAY, MUST, MUST NOT and SHOULD are used
118 in capital letters. This conforms to the definitions in [2].
120 2. Overview of Domain Security Services
122 This section gives an informal overview of the security services that
123 are provided by S/MIME between different security domains. These
124 services are provided by a combination of mechanisms in the sender's and
125 recipient's domains.
127 Later sections describe definitively how these services map onto
128 elements of the S/MIME protocol.
130 The following security mechanisms are specified in this document:
132 1. Domain signature
133 2. Review signature
134 3. Additional attributes signature
135 4. Domain encryption and decryption
137 The term 'security domain' as used in this document is defined as a
138 collection of hardware and personnel operating under a single security
139 authority and performing a common business function. Members of a
140 security domain will of necessity share a high degree of mutual trust,
141 due to their shared aims and objectives.
143 A security domain is typically protected from direct outside attack by
144 physical measures and from indirect (electronic) attack by a combination
145 of firewalls and guards at network boundaries. The interface between two
146 security domains is termed a 'security boundary'. One example of a
147 security domain is an organisational network ('Intranet').
149 2.1 Domain Signature
151 A Domain signature is an S/MIME signature generated on behalf of a set
152 of users in a domain. A Domain signature can be used to authenticate
153 information sent between domains, for example, when two 'Intranets' are
154 connected using the Internet. It can be used when two domains employ
155 incompatible signature schemes internally or when there are no
156 certification links between their PKIs. In both cases messages from the
157 originator's domain are signed over the original message and signature
158 (if present) using an algorithm, key, and certificate which can be
159 processed by the recipient(s). A domain signature is sometimes referred
160 to as an "organisational signature".
162 2.2 Review Signature
164 A third party may review messages before they are forwarded to the final
165 recipient(s) who may be in the same or a different security domain.
166 Organisational policy and good security practice often require that
167 messages be reviewed before they are released to external recipients.
168 Having reviewed a message, an S/MIME signature is added to it - a review
169 signature. An agent MAY check the review signature at the domain
170 boundary, to ensure that only reviewed messages are released.
172 2.3 Additional Attributes Signature
174 A third party MAY add additional attributes to a signed message. An
175 S/MIME signature is used for this purpose - an additional attributes
176 signature. An example of an additional attribute is the 'Equivalent
177 Label' attribute defined in ESS [3].
179 2.4 Domain Encryption and Decryption
181 Domain encryption is S/MIME encryption performed on behalf of a
182 collection of users in a domain. Domain encryption can be used to
183 protect information between domains, for example, when two 'Intranets'
184 are connected using the Internet. It can also be used when end users do
185 not have encryption capabilities at the desktop, or when two domains
186 employ incompatible encryption schemes internally. In the latter case
187 messages from the originator's domain are re-encrypted using an
188 algorithm, key, and certificate which can be decrypted by the
189 recipient(s) or an entity in their domain.
191 3. Mapping of the Signature Services to the S/MIME Protocol
193 This section describes the S/MIME Protocol elements that are used to
194 provide the security services described above. ESS [3] introduces the
195 concept of triple-wrapped messages that are first signed, then
196 encrypted, then signed again. This document also uses this concept of
197 triple-wrapping. In addition, this document also uses the concept of
198 'signature encapsulation'. 'Signature encapsulation' denotes a complete
199 signed message that is wrapped in a second signature, the second
200 signature covering both the content and the first (inner) signature.
202 Signature Encapsulation MAY be performed on the inner and/or the outer
203 signature of a triple-wrapped message. The term 'parallel signatures'
204 means two or more signatures calculated over the same content. This
205 capability is described in CMS [3], where a set of one or more
206 SignerInfos can be attached to signed data.
208 For example, the originator signs a message which is then encapsulated
209 with an 'additional attributes' signature. This is then encrypted. A
210 reviewer then signs this encrypted data, which is then encapsulated by
211 a domain signature.
213 3.1 Naming conventions and Signature Types
215 An entity receiving an S/MIME signed message would normally expect the
216 signature to be that of the originator of the message. However, the
217 message security services defined in this draft require the recipient to
218 be able accept messages signed by other entities and the originator.
219 When other entities sign the message the name in the certificate will
220 not match the message senders name. An S/MIME implementation would flag
221 an error if there were a mismatch between the name in the certificate
222 and the message sender's name. (This check prevents a number of types of
223 masquerade attack.)
225 To resolve this incompatibility, this document defines a naming
226 convention that specifies the form that the signing agents name SHOULD
227 take. Adherence to this naming convention avoids the problems of
228 uncontrolled naming and the possible masquerade attacks that this would
229 produce.
231 As an assistance to implementation, a signed attribute is defined to be
232 included in the S/MIME signature - the 'signature type' attribute. On
233 receiving a message containing this attribute, the naming convention
234 checks are invoked.
236 Implementations conforming to this standard MUST support the naming
237 convention for signature generation and verification. Implementations
238 conforming to this standard MUST recognise the signature type attribute
239 for signature verification. Implementations conforming to this standard
240 SHOULD support the signature type attribute for signature generation;
241 however, this is not mandated.
243 3.1.1 Naming conventions
245 The following naming conventions are specified for agents generating
246 signatures specified in this document:
248 * For a domain signature, an agent generating this signature MUST be
249 named 'domain-signing-authority'
251 * For a review signature, an agent generating this signature MUST be
252 named 'review-authority'.
254 * For an additional attributes signature, an agent generating this
255 signature MUST be named 'attribute-authority'.
257 This name shall appear in the 'common name (CN)' component of the
258 subject field in the X.509 certificate. Additionally, if the
259 certificate contains an SMTP e-mail address, this name shall appear in
260 the end entity component of the address - on the left-hand side of the
261 '@' symbol.
263 In the case of a domain signature, an additional naming rule is
264 defined: the 'name mapping rule'. The name mapping rule states that
265 for a domain signing authority, the domain component of its name MUST be
266 the same as, or an ascendant of, the domain name of the message
267 originator(s) that it is representing. The domain component is defined
268 as follows:
270 * In the case of an X.500 distinguished subject name of an X.509
271 certificate, the domain component is the country, organisation,
272 organisational unit, state, and locality components of the
273 distinguished name.
275 * If the certificate contains an SMTP e-mail address, the domain
276 component is defined to be the SMTP address component on the right-
277 hand side of the '@' symbol.
279 For example, a domain signing authority acting on behalf of John Doe of
280 the Acme corporation, whose distinguished name is 'cn=John Doe,
281 ou=marketing,o=acme,c=us' and whose e-mail address is
282 John.Doe@marketing.acme.com, could have a certificate containing a
283 distinguished name of 'cn=domain-signing-authority, o=acme,c=us' and an
284 e-mail address of 'domain-signing-authority@acme.com'.
286 Any message received where the domain component of the domain signing
287 agents name does not match, or is not an ascendant of, the originator's
288 domain name MUST be rejected.
290 This naming rule prevents agents from one organisation masquerading as
291 domain signing authorities on behalf of another. For the other types of
292 signature defined in this document, no such named mapping rule is
293 defined.
295 Implementations conforming to this standard MUST support this name
296 mapping convention as a minimum. Implementations MAY choose to
297 supplement this convention with other locally defined conventions.
298 However, these MUST be agreed between sender and recipient domains prior
299 to secure exchange of messages.
301 On verifying the signature, a receiving agent MUST ensure that the
302 naming convention has been adhered to. Any message that violates the
303 convention shall be rejected as invalid.
305 3.1.2 Signature type attribute
307 An S/MIME authenticated attribute is also used to indicate the type of
308 signature. This should be used in conjunction with the naming
309 conventions specified in the previous section. When an S/MIME signed
310 message containing the signature type attribute is received it triggers
311 the software to verify that the correct naming convention has been used.
313 The ASN.1 [4] notation of this attribute is: -
315 SignatureType ::= SEQUENCE OF OBJECT IDENTIFIER
317 id-signatureType OBJECT IDENTIFIER ::= { iso (1) member-body (2)
318 us (840) rsadsi (113549) }
320 If present, the SignatureType attribute MUST be an authenticated
321 attribute, as defined in [5]. If the SignatureType attribute is absent
322 the recipient SHOULD assume that the signature is that of the message
323 originator.
325 Each of the signature types defined here are generated and processed
326 exactly as described in [5]. They are distinguished by the presence of
327 the following values in the SignatureType authenticated attribute:
329 id-sigtype-domain-sig OBJECT IDENTIFIER ::= { id-signatureType 2 } for a
330 domain signature.
332 id-sigtype-add-attrib-sig OBJECT IDENTIFIER ::= { id-signatureType 3}
333 for an additional attributes signature.
335 id-sigtype-review OBJECT IDENTIFIER ::= { id-signatureType 4} for a
336 review signature.
338 For completeness, an attribute type is also specified for an originator
339 signature. However, this signature type is optional. It is defined as
340 follows:
342 id-sigtype-originator-sig OBJECT IDENTIFIER ::= { id-signatureType 1}
343 for an originator's signature.
345 The originator signature MUST NOT encapsulate other signatures. The
346 other signature types specified in this document MAY encapsulate other
347 signatures. All the signature types MAY be added in parallel to other
348 signatures as documented in [5].
350 A SignerInfo MUST NOT include multiple instances of SignatureType. An
351 authenticated attribute representing a SignatureType MAY include
352 multiple instances of different SignatureType values as an
353 AttributeValue of attrValues [5], as long as the SignatureType
354 'additional attributes' is not present.
356 The following sections describe the conditions under which each of these
357 types of signature may be generated, and how they are processed.
359 3.2 Domain Signature Generation and Verification
361 A 'domain signature' is a proxy signature generated on a user's behalf
362 in the user's domain. The signature MUST adhere to the naming
363 conventions in 3.1.1, including the name mapping convention. A 'domain
364 signature' on a message authenticates the fact that the message has
365 originated in that domain. Before signing, a process generating a
366 'domain signature' MUST first satisfy itself of the authenticity of the
367 message originator. This is achieved by one of two methods. Either the
368 'originator's signature' is checked, if S/MIME signatures are used
369 inside a domain. Or if not, some mechanism external to S/MIME is used,
370 such as the physical address of the originating client or an
371 authenticated IP link.
373 If the originator's authenticity is successfully verified by one of the
374 above methods and all other signatures present are valid, a 'domain
375 signature' MAY be added to a message in one of the following ways:
377 1) An unsigned message has a null signature added to it (i.e. the
378 message is wrapped in a signedData that has no signerInfo attached),
379 and then a 'domain signature' is added as defined in methods 2) or 3)
380 below. The originator's information is included as part of a header
381 field in the encapsulated message.
383 2) Signature Encapsulation is used to wrap the original signed message
384 with a 'domain signature'.
386 3) The original signed message has a 'domain signature' added in
387 parallel.
389 An entity generating a domain signature MUST do so using a certificate
390 containing a subject name that follows the naming convention specified
391 in 3.1.1.
393 When a 'domain signature' is applied the mlExpansionHistory and
394 eSSSecurityLabel attributes MUST be copied from other signerInfos as
395 stated in [3].
397 If the originator's authenticity is not successfully verified or all
398 the signatures present are not valid, a 'domain signature' MUST NOT be
399 generated.
401 On reception, the 'domain signature' SHOULD be used to verify the
402 authenticity of a message. A check MUST be made to ensure that both the
403 naming convention and the name mapping convention have been used as
404 specified in this standard.
406 A recipient MAY assume that successful verification of the domain
407 signature also authenticates the message originator.
409 If there is an originator signature present, the name in that
410 certificate SHOULD be used to identify the originator. This information
411 can then be displayed to the recipient.
413 Alternatively, if a 'domain signature' has encapsulated a complete
414 MIME-encoded message, the originator information (SMTP 'From' field)
415 contained within it denotes the originator of the message.
417 If neither of these cases is true no assumptions can be made about the
418 originator.
420 A domain signer can be assumed to have verified any signatures that it
421 encapsulates. Therefore, it is not necessary to verify these signatures
422 before treating the message as authentic. However, this standard does
423 not preclude a recipient from attempting to verify any other signatures
424 that are present.
426 The 'domain signature' is indicated by the presence of the value
427 Id-at-sigtype-domain-sig in a 'signature type' authenticated attribute.
429 There MAY be multiple 'domain signature' signatures in an S/MIME
430 encoding.
432 3.3 Additional Attributes Signature generation and verification
434 The 'additional attributes' signature type indicates that the
435 SignerInfo contains additional attributes that are associated with the
436 message.
438 All attributes in the applicable SignerInfo MUST be treated as
439 additional attributes. Successful verification of an 'additional
440 attributes' signature means only that the attributes are authentically
441 bound to the message. A recipient MUST NOT assume that its successful
442 verification also authenticates the message originator.
444 An entity generating an 'additional attributes' signature MUST do so
445 using a certificate containing a subject name that follows the naming
446 convention specified in 3.1.1. On reception, a check MUST be made to
447 ensure that the naming convention has been used.
449 A signer MAY include any of the attributes listed in [5] or in this
450 document when generating an 'additional attributes' signature. The
451 following attributes have a special meaning, when present in an
452 'additional attributes' signature:
454 1) Equivalent Label: label values in this attribute are to be treated as
455 equivalent to the security label contained in an encapsulated
456 SignerInfo, if present.
458 2) Security Label: the label value indicates the aggregate sensitivity
459 of the inner message content plus any encapsulated signedData and
460 envelopedData containers. The label on the original data is indicated
461 by the value in the originator's signature, if present.
463 An 'additional attributes' signature is indicated by the presence of the
464 value Id-at-sigtype-add-attrib-sig in a 'signature type' authenticated
465 attribute. No other Object Identifiers MAY be included in the sequence
466 of OIDs if this value is present. An 'additional attributes' signature
467 MAY be added in parallel with other signatures in a SET OF SignerInfos.
469 There MAY be multiple 'additional attributes' signatures in an S/MIME
470 encoding.
472 3.4 Review Signature generation and verification
474 The review signature indicates that the signer has reviewed the message.
475 Successful verification of a review signature means only that the signer
476 has approved the message for onward transmission to the recipient(s).
477 When the recipient is in another domain, a device on a domain boundary
478 such as a Mail Guard or firewall may be configured to check review
479 signatures. A recipient MUST NOT assume that its successful verification
480 also authenticates the message originator.
482 An entity generating a signed review signature MUST do so using a
483 certificate containing a subject name that follows the naming convention
484 specified in 3.1.1. On reception, a check MUST be made to ensure that
485 the naming convention has been used.
487 A review signature is indicated by the presence of the value
488 Id-at-sigtype-review-sig in a 'signature type' authenticated attribute.
490 There MAY be multiple review signatures in an S/MIME encoding.
492 3.5 Originator Signature
494 The 'originator signature' is used to indicate that the signer is the
495 originator of the message and its contents. It is included in this
496 document for completeness only. An originator signature is indicated
497 either by the absence of the signature type attribute, or by the
498 presence of the value id-sigtype-originator-sig in a 'signature type'
499 authenticated attribute. There MUST be only one 'originator signature'
500 signature present in an S/MIME encoding and it MUST be one of the inner
501 most signatures.
503 4. Encryption and Decryption
505 Domain encryption is encryption performed by a third party on behalf of
506 a set of originators in a domain. Domain decryption is decryption
507 performed by a third party on behalf of a set of recipients in a domain.
509 Depending on security policy, messages may be encrypted for decryption
510 by the final recipient and by a domain decryption agent in the
511 originator's and/or the recipient's domain. This is achieved by
512 generating a RecipientInfo for each type of agent that is transmitted
513 along with the encrypted message.
515 The processes of domain encryption and decryption may be performed in
516 combination, as shown below.
518 --------------------------------------------------------------------
519 | | Recipient Decryption | Domain Decryption |
520 |------------------------|----------------------|--------------------|
521 | Originator Encryption | Case(a) | Case(c) |
522 | Domain Encryption | Case(b) | Case(d) |
523 --------------------------------------------------------------------
525 Case (a), encryption of messages by the originator for decryption by the
526 final recipient(s), is described in CMS [5]. In Cases (b) and (d),
527 encryption is performed not by the originator but by a third party in
528 the sending domain. In Cases (c) and (d), decryption is performed not by
529 the recipient(s) but by a third party in the destination domain.
531 A client implementation that conforms to this standard MUST support
532 cases (a) and (c) for transmission, and cases (a) and (b) for reception.
534 A Domain Encryption implementation that conforms to this standard MUST
535 support cases (b) and (d), for transmission, and cases (c) and (d) for
536 reception.
538 The process of encryption and decryption is documented in CMS [5]. The
539 only additional requirement introduced by domain encryption and
540 decryption is for greater flexibility in the management of keys, as
541 described in the following subsections. As with signatures, a naming
542 convention and name mapping convention are used to locate the correct
543 key.
545 The mechanisms described below are applicable both to key agreement and
546 key transport systems, as documented in CMS [5]. The phrase 'encryption
547 key' is used as a collective term to cover the key management keys used
548 by both techniques.
550 4.1 Domain Encryption Naming Conventions
552 A domain encryption agent MUST be named 'domain-confidentiality-
553 authority'. Also a domain decryption agent MUST be named 'domain-
554 confidentiality-authority'. This name MUST appear in the 'common name
555 (CN)' component of the subject field in the X.509 certificate.
556 Additionally, if the certificate contains an SMTP e-mail address, this
557 name MUST appear in the end entity component of the address - on the
558 left-hand side of the '@' symbol.
560 Along with this naming convention, an additional naming rule is defined:
561 the 'name mapping rule'. The name mapping rule states that for an
562 encryption agent, the domain component of its name MUST be the same as,
563 or an ascendant of, the domain name of the set of entities that it
564 represents. The domain component is defined as follows:
566 * In the case of an X.500 distinguished name of an X.509 certificate,
567 the domain component is the country, organisation, organisational
568 unit, state, and locality components of the distinguished name.
570 * If the certificate contains an SMTP e-mail address, the domain
571 component is defined to be the SMTP address component on the right-
572 hand side of the '@' symbol.
574 For example, an encryption authority acting on behalf of John Doe of the
575 Acme corporation, whose distinguished name is 'cn=John Doe,ou=marketing,
576 o=acme,c=us' and whose e-mail address is John.Doe@marketing.acme.com,
577 could have a certificate containing a distinguished name of
578 'cn=domain-confidentiality-authority, o=acme,c=us' and an e-mail address
579 of 'domain-confidentiality-authority@acme.com'. The key associated with
580 this certificate would be used for encrypting messages for John Doe.
582 Any message received where the domain component of the domain encrypting
583 agents name does not match, or is not an ascendant of, the domain name
584 of the entities it represents MUST be rejected.
586 This naming rule prevents messages being encrypted for the wrong domain
587 decryption agent.
589 Implementations conforming to this standard MUST support this name
590 mapping convention as a minimum. Implementations may choose to
591 supplement this convention with other locally defined conventions.
592 However, these MUST be agreed between sender and recipient domains
593 prior to sending any messages.
595 4.2 Domain Encryption Key Management
597 Domain encryption is encryption performed by a third party on behalf of
598 a set of originators in a domain. Domain Encryption is shown as cases
599 (b) and (d) in the above table.
601 Domain encryption uses a domain-wide encryption key from the sender's
602 domain. Information about this key is conveyed to the recipient in the
603 RecipientInfo field as specified in CMS [5]. A domain encryption agent
604 MUST be named according to the naming convention specified in section
605 4.1. This is so that the same key can be used on reply to a domain-
606 encrypted message.
608 The domain encryption agent extracts the recipients address from the
609 message and uses this to obtain the recipients domain-confidentiality-
610 authority public key and/or the recipients public key. For example,
611 the recipients address is used as an index for a directory search. The
612 directory search MAY return the recipients certificate and/or a domain-
613 confidentiality-authority attribute that contains the location of the
614 recipient's domain decrytping agents certificate. If the directory
615 search returns no certificates then encryption can not be performed and
616 the message MUST NOT be sent. If one or both certificates are available
617 then the originator's domain encrypting agent encrypts the message for
618 the recipient and the recipient's domain decrypting agent.
620 4.3 Domain Decryption Key Management
622 Domain decryption is decryption performed by a third party on behalf of
623 a set of recipients in a domain.
625 Domain Decryption is shown as cases (c) and (d) in the above table. In
626 these cases, the encryption process has used a domain-wide encryption
627 key for the recipient(s)' domain, that has been obtained by using the
628 recipient's address (See example in section 4.2).
630 5. Security Considerations
632 Implementations MUST protect all private keys. Compromise of the
633 signer's private key permits masquerade.
635 Similarly, compromise of the content-encryption key may result in
636 disclosure of the encrypted content.
638 Compromise of key material is regarded as an even more serious issue for
639 domain security services than for an S/MIME client. This is because
640 compromise of the private key may in turn compromise the security of a
641 whole domain. Therefore, great care should be used when considering its
642 protection.
644 6. References
646 [1] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC2633,
647 June 1999.
649 [2] Bradner, S., "Key words for use in RFCs to Indicate
650 Requirement Levels", BCP 14, RFC 2119, March 1997.
652 [3] Hoffman, P., "Enhanced Security Services for S/MIME", RFC 2634,
653 June 1999.
655 [4] International Telecommunications Union, Recommendation X.208, "Open
656 systems interconnection: specification of Abstract Syntax Notation
657 (ASN.1)", CCITT Blue Book, 1989.
659 [5] Housley, R., "Cryptographic Message Syntax", RFC 2630, June 1999.
661 7. Authors' Addresses
663 Tim Dean
664 DERA Malvern
665 St. Andrews Road
666 Malvern
667 Worcs
668 WR14 3PS
670 Phone: +44 (0) 1684 894239
671 Fax: +44 (0) 1684 896660
672 Email: t.dean@eris.dera.gov.uk
674 William Ottaway
675 DERA Malvern
676 St. Andrews Road
677 Malvern
678 Worcs
679 WR14 3PS
681 Phone: +44 (0) 1684 894079
682 Fax: +44 (0) 1684 896660
683 Email: w.ottaway@eris.dera.gov.uk
685 This draft expires 19th April 2000