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1 Internet Draft T Dean
2 draft-ietf-smime-domsec-01.txt W Ottaway
3 November 17, 1998 DERA
4 Expires in six months
6 Domain Security Services using S/MIME
8 Status of this memo
10 This document is an Internet-Draft. 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 To view the entire list of Internet-Drafts, please check the
21 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
22 Directories on ftp.is.co.za (Africa), ftp.nordu.net ( Northern Europe),
23 munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or
24 ftp.isi.edu(US West Coast).
26 Abstract
28 This document describes how the S/MIME protocol can be processed and
29 generated by a number of components of a messaging 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 do not have
38 encryption or signing capabilities at the desktop, but wish to
39 interoperate 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, Dave
52 Kemp and Jim Schaad.
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 feasible or practical to
63 provide end-to-end ('desktop-to-desktop') security services,
64 particularly between different security domains. An organisation which
65 is considering providing end-to-end security services will typically
66 have to deal with 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) Cross-certification problems: There may not be any cross-certificate
78 links between two organisations. Or an organisation may be sensitive
79 about parts of its PKI and unwilling to expose them to outside
80 access. For either of these reasons, end-to-end encryption and
81 signature validation 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 5) Cost: providing the necessary key management infrastructure and other
89 items such as hardware tokens for all users may be too expensive.
91 One solution to these problems is to provide message security services
92 at the level of a domain or an organisation. This document specifies how
93 these 'domain security services' can be provided using the S/MIME
94 protocol. Domain security services may replace or complement mechanisms
95 at the desktop. For example, a domain may decide to provide desktop-to-
96 desktop signatures but domain-to-domain encryption services. Or it may
97 allow desktop-to-desktop services for intra-domain use, but enforce
98 domain-based services for communication with other domains.
100 Messages can be processed and generated by a number of components of a
101 messaging system, such as message transfer agents, guards and gateways.
102 Any of these agents may provide domain security services.
104 The term 'Third Party' as used in this document means any entity in a
105 messaging system other than the originator and final recipient(s) that
106 processes messages. This includes Message Transfer Agents (MTAs),
107 domain mail servers, guards and firewalls operating at security
108 boundaries, and gateways that translate between different protocol
109 formats. A third party may sign, encrypt, decrypt, and check signatures
110 on a message.
112 Throughout this draft the terms MAY, MUST, MUST NOT and SHOULD NOT are
113 used in capital letters. This conforms to the definitions in [2].
115 2. Overview of Domain Security Services
117 In a distributed system, a message is sent from an originator to a set
118 of recipients that may be in the same or different security domains.
119 This section first defines what is meant by a security domain. It then
120 gives an informal overview of the security services that are provided by
121 S/MIME between different security domains. These services are provided
122 by a combination of mechanisms in the sender's and recipient's domains.
123 Later sections describe definitively how these services map onto
124 elements of the S/MIME protocol.
126 2.1 Definition of a Security Domain
128 A 'security domain' is defined as a collection of hardware and personnel
129 operating under a single security authority and performing a common
130 business function. Members of a security domain will of necessity share
131 a high degree of mutual trust, due to their shared aims and objectives.
132 A security domain is typically protected from direct outside attack by
133 physical measures and from indirect (electronic) attack by a combination
134 of firewalls and guards at network boundaries. The interface between two
135 security domains is termed a 'security boundary'. One example of a
136 security domain is an organisational network ('Intranet').
138 2.2 Signing by a third party
140 A third-party may sign messages for one or more of the following reasons:
142 1. When messages need to be reformatted inside the message transfer
143 system. Message reformatting is needed at gateways between X.400 and
144 SMTP-MIME domains, or on conversion between HTTP-MIME and SMTP-MIME
145 message representations. The third party signature is needed because
146 the reformatting process renders the originator's signature
147 unverifiable by the recipient(s).
149 2. To bridge between two domains that have incompatible or disconnected
150 signature systems, such as when there are no cross-certificate links
151 between their Public Key Infrastructures (PKIs). The third party
152 signature is needed because the originator's signature is not
153 directly verifiable by the recipient(s). It is typically created at
154 the boundary between the domains.
156 3. When end users do not have signing capabilities at the desktop.
158 A third party may wish to convey the signature semantics to the
159 recipient(s) when creating its digital signature. This document
160 specifies three signature types to convey these semantics, as follows.
162 A third party may sign a message, and optionally add additional
163 attributes to it. An example is the addition of the 'Equivalent Label'
164 attribute defined in ESS [4]. In this case the 'additional attributes'
165 signature is used.
167 A third party may wish to declare that it is acting as a proxy on
168 behalf of an originator in a domain. In this case the 'Domain'
169 Signature is used.
171 A third party may review messages before they are released from a
172 domain. This is used when organisational policy or good security
173 practice require that messages be reviewed before they are released to
174 external recipients. Having reviewed a message, a 'review and release'
175 signature is added to it. The review and release signature may be
176 checked by a firewall at the domain boundary, to ensure that only
177 reviewed messages are released.
179 2.3 Domain Encryption
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. In the latter case messages
187 from the originator's domain are re-encrypted using an algorithm, key,
188 and certificate which can be decrypted by the recipient(s).
190 3. Mapping of Domain Security Services to the S/MIME Protocol
192 This section describes the S/MIME Protocol elements that are used to
193 provide the security services described above. ESS [4] introduces the
194 concept of triple-wrapped messages that are first signed, then
195 encrypted, then signed again. This document also uses this concept of
196 triple-wrapping. In addition, this document also uses the concept of
197 'signature encapsulation'. 'Signature encapsulation' denotes a complete
198 signed message that is wrapped in a second signature, the second
199 signature covering both the content and the first (inner) signature.
200 Signature Encapsulation may be performed on the inner or the outer
201 signature of a triple-wrapped message. The term 'parallel signatures'
202 means two or more signatures calculated over the same content. This
203 capability is described in CMS [3], where a set of one or more
204 SignerInfos can be attached to signed data.
206 3.1 Signature Types
208 An authenticated attribute is used to indicate the type of signature.
209 The ASN.1 [5] notation of this attribute is:-
211 SignatureType ::= SEQUENCE OF OBJECT IDENTIFIER
213 id-signatureType OBJECT IDENTIFIER ::= { iso (1) member-body (2)
214 us (840) rsadsi (113549) }
216 If present, the SignatureType attribute MUST be an authenticated
217 attribute, as defined in [3]. If the SignatureType attribute is absent
218 the recipient SHOULD NOT make any assumptions about the type of
219 signature.
221 This section specifies the following types of signature:
223 1) Originator Signature
224 2) Domain Signature
225 3) Additional Attributes Signature
226 4) Review and Release Signature
228 Each of these signature types is generated and processed exactly as
229 described in [3]. They are distinguished by the presence of the
230 following values in the SignatureType authenticated attribute:
232 id-sigtype-originator-sig OBJECT IDENTIFIER ::= { id-signatureType 1}
233 id-sigtype-domain-sig OBJECT IDENTIFIER ::= { id-signatureType 2 }
234 id-sigtype-add-attrib-sig OBJECT IDENTIFIER ::= { id-signatureType 3}
235 id-sigtype-review-release-sig OBJECT IDENTIFIER ::= { id-signatureType
236 4}
238 These signature types may encapsulate other signatures, or any other
239 type of content, or may be added in parallel to other signatures as
240 documented in [3].
242 A SignerInfo MUST NOT include multiple instances of SignatureType. An
243 authenticated attribute representing a SignatureType MAY include
244 multiple instances of different SignatureType values as an
245 AttributeValue of attrValues [3], as long as the SignatureType
246 'additional attributes' is not present.
248 The following sections describe the conditions under which each of
249 these types of signature may be generated, and how they are processed.
251 3.1.1 Originator Signature
253 The 'originator signature' is used to indicate that the signer is the
254 originator of the message and its contents. The 'originator signature'
255 is indicated by the presence of the value id-sigtype-originator-sig in
256 the 'signature type' authenticated attribute. There MUST be only one
257 'originator signature' signature present in a S/MIME encoding.
259 3.1.2 Domain Signature
261 A 'domain signature' is a proxy signature generated on a user's behalf in
262 a domain. A 'domain signature' on a message authenticates the fact that
263 the message has originated in that domain. Before signing, a process
264 generating a 'domain signature' MUST first satisfy itself of the
265 authenticity of the message originator. This is achieved by one of two
266 methods. Either the 'originator's signature' is checked, if S/MIME
267 signatures are used inside a domain. Or if not, some mechanism external
268 to S/MIME is used, such as the physical address of the originating
269 client or an authenticated IP link.
271 If the originator's authenticity is successfully verified by one of the
272 above methods and all other signatures present are valid, a 'domain
273 signature' may be added to a message in one of the following ways:
275 1) An unsigned message is wrapped in a SignedData, and a SignerInfo is
276 attached containing the 'domain signature'. The originator's
277 information is included as part of a header field in the encapsulated
278 message.
280 2) Signature Encapsulation is used to wrap the original signed message
281 with a 'domain signature'.
283 3) The original signed message has a 'domain signature' added in
284 parallel.
286 When a 'domain signature' is applied the mlExpansionHistory and
287 eSSSecurityLabel attributes MUST be copied from other signerInfos as
288 stated in [4].
290 If the originator's authenticity is not successfully verified, a 'domain
291 signature' MUST NOT be generated.
293 On reception, the 'domain signature' may be used to verify the
294 authenticity of a message. If there is a SignerInfo with the signature
295 type 'originator', its certificate should be used to identify the
296 originator. This information can then be displayed to the recipient.
297 Alternatively, if a 'domain signature' has encapsulated a complete
298 MIME-encoded message, the originator information (SMTP 'From' field)
299 contained within it denotes the originator of the message. If neither
300 of these cases is true no assumptions can be made about the originator.
302 A domain signer can be assumed to have verified any signatures that it
303 encapsulates. Therefore, it is not necessary to verify these signatures
304 before treating the message as authentic. However, this standard does
305 not preclude a recipient from attempting to verify any other signatures
306 that are present.
308 The 'domain signature' is indicated by the presence of the value Id-at-
309 sigtype-domain-sig in the 'signature type' authenticated attribute.
310 There MAY be multiple 'domain signature' signatures in a S/MIME encoding.
312 3.1.3 Additional Attributes Signature
314 The 'additional attributes' signature type indicates that the SignerInfo
315 contains additional attributes that are associated with the message.
316 All attributes in the applicable SignerInfo MUST be treated as
317 additional attributes. Successful verification of an 'additional
318 attributes' signature means only that the attributes are authentically
319 bound to the message. A recipient MUST NOT assume that its successful
320 verification also authenticates the message originator. The authenticity
321 of the message originator should be verified by checking the signature of
322 the appropriate type, if present.
324 A signer may include any of the attributes listed in [3] or this document
325 when generating an 'additional attributes' signature. The following
326 attributes have a special meaning, when present in an 'additional
327 attributes' signature:
329 1) Equivalent Label: label values in this attribute are to be treated as
330 equivalent to the security label contained in an encapsulated
331 SignerInfo, if present.
333 2) Security Label: the label value indicates the aggregate sensitivity
334 of the inner message content plus any encapsulated signedData and
335 envelopedData containers. The label on the original data is indicated
336 by the value in the originator's signature, if present.
338 An 'additional attributes' signature is indicated by the presence of the
339 value Id-at-sigtype-add-attrib-sig in the 'signature type' authenticated
340 attribute. No other Object Identifiers may be included in the sequence
341 of OIDs if this value is present. An 'additional attributes' signature
342 may be added in parallel with other signatures in a SET OF SignerInfos.
343 There MAY be multiple 'additional attributes' signatures in a S/MIME
344 encoding.
346 3.1.4 Review and Release
348 The 'review and release' signature indicates that the signer has
349 reviewed the message. Successful verification of a 'review and release'
350 signature means only that the signer has approved the message for
351 release from a domain. A device on a domain boundary such as a Mail
352 Guard or firewall may be configured to check review and release
353 signatures. A recipient MUST NOT assume that its successful verification
354 also authenticates the message originator. The authenticity of the
355 message originator should be verified by checking the signature
356 identified as the originators, if present.
358 A 'review and release' signature is indicated by the presence of the
359 value Id-at-sigtype-review-release-sig in the 'signature type'
360 authenticated attribute. There MAY be multiple 'review and release'
361 signatures in a S/MIME encoding.
363 3.2 Domain Encryption and Decryption
365 Domain encryption is encryption performed by a third party on behalf of
366 a set of originators in a domain. Domain decryption is decryption
367 performed by a third party on behalf of a set of recipients in a domain.
368 These processes may be performed in combination, as shown below.
370 --------------------------------------------------------------------
371 | | Recipient Decryption | Domain Decryption |
372 |------------------------|----------------------|--------------------|
373 | Originator Encryption | Case(a) | Case(c) |
374 | Domain Encryption | Case(b) | Case(d) |
375 --------------------------------------------------------------------
377 Case (a), encryption of messages by the originator for decryption by the
378 final recipient(s), is described in CMS [3]. In Cases (b) and (d),
379 encryption is performed not by the originator but by a third party in
380 the sending domain. In Cases (c) and (d), decryption is performed not
381 by the recipient(s) but by a third party in the destination domain.
383 A client implementation that conforms to this standard MUST support
384 cases (a) and (c) for transmission, and cases (a) and (b) for reception.
385 A Domain Encryption implementation that conforms to this standard MUST
386 support cases (b) and (d), for transmission, and cases (c) and (d) for
387 reception.
389 The process of encryption and decryption is documented in CMS [3]. The
390 only additional requirement introduced by domain encryption and
391 decryption is for greater flexibility in the management of keys, as
392 described in the following subsections.
394 The mechanisms described below are applicable both to key agreement and
395 key transport systems, as documented in CMS. The phrase 'encryption
396 key' is used as a collective term to cover the key management keys used
397 by both techniques.
399 3.2.1 Domain Encryption Key Management
401 Domain Encryption is shown as cases (b) and (d) in the above table.
402 Domain Encryption uses a domain-wide encryption key from the sender's
403 domain. Information about this key is conveyed to the recipient by
404 one of two methods:-
406 1) Public information about this key is held in a certificate and
407 conveyed to the recipient(s) in the 'Certs' field of the
408 OriginatorInfo in the Envelope.
410 2) The recipient(s) looks up the key for the users domain by replacing
411 the originator's name within the address with the domain encryption
412 name [6].
414 For example :-
416 a) If the originator is originator@foo.com then lookup key for
417 domain-encrypting-authority@foo.com.
419 b) If the originator is c=us;a=com;p=foo;s=originator then look up
420 key for c=us;a=com;p=foo;s=domain-encrypting-authority.
422 c) If the originator is c=gb;o=foo;cn=originator then look up key for
423 c=gb;o=foo;cn=domain-encrypting-authority.
425 An implementation conforming to this standard MUST support both methods.
427 The name in the encryption certificate may not match the name in any
428 encapsulated signatures. For example, when a message is signed by the
429 originator and is encrypted by the domain. An implementation that
430 conforms to this standard MUST allow for this possibility. This includes
431 both a client and a third party implementation.
433 3.2.2 Domain Decryption Key Management
435 Domain Decryption is shown as cases (c) and (d) in the above table. In
436 these cases, the encryption process uses a domain-wide encryption key
437 for the recipient(s)' domain. The selection of this key is achieved by
438 one of two methods:
440 1) The sending process explicitly searches for a certificate containing
441 the domain encryption key of the recipient(s)' domain. This is
442 achieved by mapping the recipient(s)' name to a domain name and then
443 locating the encryption certificate containing that domain name.
444 Mapping from recipient names to Domain names, and conventions for
445 domain names are outside the scope of this standard.
447 2) All the members of the receiving domain are issued with certificates
448 containing a single key. The private component of that key is held
449 by an entity in the domain that performs the decryption process
450 on their behalf. By selecting the appropriate certificate, a sending
451 process will implicitly encrypt for decryption by the Domain
452 Decryption process.
454 An implementation that conforms to this standard MUST support mechanism
455 (1). It may also support mechanism (2). This includes both a client and
456 a third party implementation.
458 4. Security Considerations
460 Domain Security Services provide a method for digitally
461 signing data, digesting data, encrypting data, and authenticating
462 data.
464 Implementations must protect the signer's private key. Compromise of
465 the signer's private key permits masquerade.
467 Implementations must protect the key management private key and the
468 content-encryption key. Compromise of the key management private key
469 may result in the disclosure of all messages protected with that key.
470 Similarly, compromise of the content-encryption key may result in
471 disclosure of the encrypted content.
473 5. References
475 [1] Ramsdell, B., "S/MIME Version 3 Message Specification", Internet
476 Draft draft-ietf-smime-msg-04, May 1998.
478 [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
479 Levels", BCP 14, RFC 2119, March 1997.
481 [3] Housley, R., "Cryptographic Message Syntax", Internet Draft
482 draft-ietf-smime-cms-05.txt, March 1998.
484 [4] Hoffman, P., "Enhanced Security Services for S/MIME", Internet Draft
485 draft-ietf-smime-ess-06.txt, May 1998.
487 [5] International Telecommunications Union, Recommendation X.208, "Open
488 systems interconnection: specification of Abstract Syntax Notation
489 (ASN.1)", CCITT Blue Book, 1989.
491 [6] Ramsdell, B., "Role Names in X.509 Certificates", Internet Draft
492 draft-ramsdell-role-names, April 29 1998.
494 6. Authors' Addresses
496 Tim Dean
497 DERA Malvern
498 St. Andrews Road
499 Malvern
500 Worcs
501 WR14 3PS
503 Phone: +44 (0)1684 894239
504 Fax: +44 (0)1684 6113
505 Email: t.dean@eris.dera.gov.uk
507 William Ottaway
508 DERA Malvern
509 St. Andrews Road
510 Malvern
511 Worcs
512 WR14 3PS
514 Phone: +44 (0)1684 894079
515 Fax: +44 (0)1684 896113
516 Email: w.ottaway@eris.dera.gov.uk