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'5' -- Possible downref: Normative reference to a draft: ref. '6' Summary: 8 errors (**), 0 flaws (~~), 7 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Draft T Dean 2 draft-ietf-smime-domsec-00.txt W Ottaway 3 July 15, 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 Kemp 52 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 MUST, MUST NOT and SHOULD NOT are used 113 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 Domain Signature 140 A domain signature is a S/MIME SignerInfo [3] created on behalf of a 141 collection of users in a domain. A domain signature can be used when 142 end users do not have signing capabilities at the desktop. It can also 143 be used to bridge between two domains that have incompatible or 144 disconnected signature systems, such as when there are no cross- 145 certificate links between their Public Key Infrastructures (PKIs). In 146 this case a process which creates a Domain Signature first checks the 147 originator's signature using the source Domain's PKI. It then re-signs 148 the message using its key and certificate from the destination Domain's 149 PKI, which can then be verified by the recipient(s). 151 A domain signature can also be used in situations when messages need to 152 be reformatted inside the message transfer system. Message reformatting 153 is needed at gateways between X.400 and SMTP-MIME domains, or on 154 conversion between HTTP-MIME and SMTP-MIME message representations. In 155 both cases the recipient is unable to verify the originator's signature. 156 Therefore, a Domain Signature is generated at the reformatting point to 157 indicate that the originator's signature has been authenticated. 159 2.3 Domain Encryption 161 Domain encryption is S/MIME encryption performed on behalf of a 162 collection of users in a domain. Domain encryption can be used to 163 protect information between domains, for example, when two 'Intranets' 164 are connected using the Internet. It can also be used when end users do 165 not have encryption capabilities at the desktop, or when two domains 166 employ incompatible encryption schemes. In the latter case messages 167 from the originator's domain are re-encrypted using an algorithm, key, 168 and certificate which can be decrypted by the recipient(s). 170 2.4 Additional Attributes 172 A third party in a domain may attach additional signed attributes to a 173 message. This is for one of the following reasons: 175 1) To add domain-wide attributes such as a domain security label of a 176 'system-high' domain. 178 2) To add mapped attributes, for processing by recipients in a domain 179 different from that of the originator. An example is the addition of 180 the 'Equivalent Label' attribute defined in ESS [4]. 182 2.5 Message Review and Release 184 A third party may review messages before they are released from a 185 domain. This is used when organisational policy or good security 186 practice require that messages be reviewed before they are released to 187 external recipients. Having reviewed a message, a review and release 188 signature is added to it. The review and release signature may be 189 checked by a firewall at the domain boundary, to ensure that only 190 reviewed messages are released. 192 Details of the review process are outside the scope of this document, 193 but may involve scanning message contents for viruses or other malicious 194 code, searching for prohibited words or other content, or checking for 195 the presence or absence of certain security labels. Review and release 196 may be performed by a human reviewer or an automated process. 198 3. Mapping of Domain Security Services to the S/MIME Protocol 200 This section describes the S/MIME Protocol elements that are used to 201 provide the security services described above. ESS [4] introduces the 202 concept of triple-wrapped messages that are first signed, then 203 encrypted, then signed again. This document also uses this concept of 204 triple-wrapping. In addition, this document also uses the concept of 205 'signature encapsulation'. 'Signature encapsulation' denotes a complete 206 signed message that is wrapped in a second signature, the second 207 signature covering both the content and the first (inner) signature. 208 Signature Encapsulation may be performed on the inner or the outer 209 signature of a triple-wrapped message. The term 'parallel signatures' 210 means two or more signatures calculated over the same content. This 211 capability is described in CMS [3], where a set of one or more 212 SignerInfos can be attached to signed data. 214 3.1 Signature Types 216 An authenticated attribute is used to indicate the type of signature. 217 The ASN.1 [5] notation of this attribute is:- 219 SignatureType ::= SEQUENCE OF OBJECT IDENTIFIER 221 id-signatureType OBJECT IDENTIFIER ::= { iso (1) member-body (2) 222 us (840) rsadsi (113549) } 224 If present, the SignatureType attribute MUST be an authenticated 225 attribute, as defined in CMS [3]; it MUST NOT be an unauthenticated 226 attribute. If the SignatureType attribute is absent the recipient 227 SHOULD NOT make any assumptions about the type of signature. 229 A SignerInfo MUST NOT include multiple instances of SignatureType. An 230 authenticated attribute representing a SignatureType MUST only include 231 a single instance of SignatureType as an AttributeValue of attrValues 232 [3]. 234 This document specifies the following types of signature: 236 1) Originator Signature 237 2) Domain Signature 238 3) Additional Attributes Signature 239 4) Review and Release Signature 241 Each of these signature types is generated and processed exactly as 242 described in CMS [3]. They are distinguished by the presence of the 243 following values in the SignatureType authenticated attribute: 245 id-sigtype-originator-sig OBJECT IDENTIFIER ::= { id-signatureType 1} 246 id-sigtype-domain-sig OBJECT IDENTIFIER ::= { id-signatureType 2 } 247 id-sigtype-add-attrib-sig OBJECT IDENTIFIER ::= { id-signatureType 3} 248 id-sigtype-review-release-sig OBJECT IDENTIFIER ::= { id-signatureType 249 4} 251 Any of these signature types may encapsulate another signature of the 252 same or a different type, or any other type of content, as documented in 253 CMS [3]. The following sections describe the conditions under which 254 each of these types of signature may be generated, and how they are 255 processed. 257 3.1.1 Originator Signature 259 The 'originator signature' is used to indicate that the signer is the 260 originator of the message and its contents. The 'originator signature' 261 is indicated by the presence of the value id-sigtype-originator-sig in 262 the 'signature type' authenticated attribute. No other Object 263 Identifiers may be included in the sequence of OIDs if this value is 264 present. 266 3.1.2 Domain Signature 268 A 'domain signature' is generated on behalf of a collection of users in 269 a domain. A 'domain signature' on a message authenticates the fact that 270 the message has originated in that domain. Before signing, a process 271 generating a 'domain signature' MUST first satisfy itself of the 272 authenticity of the message originator. This is achieved by one of two 273 methods. Either the 'originator's signature' is checked, if S/MIME 274 signatures are used inside a domain. Or if not, some mechanism external 275 to S/MIME is used, such as the physical address of the originating 276 client or an authenticated IP link. 278 If the originator's authenticity is successfully verified by one of the 279 above methods, a 'domain signature' may be added to a message in one of 280 the following ways: 282 1) An unsigned message is wrapped in a SignedData, and a SignerInfo is 283 attached containing the 'domain signature'. The originator's 284 information is included as part of a header field in the encapsulated 285 message. 287 2) Signature Encapsulation is used to wrap the original signed message 288 with a 'domain signature'. As stated above, the originator's 289 signature MUST first be checked by the domain signer. 291 When a 'domain signature' is applied the mlExpansionHistory and 292 eSSSecurityLabel attributes MUST be copied from other signerInfos as 293 stated in [4]. 295 If the originator's authenticity is not successfully verified, a 'domain 296 signature' MUST NOT be generated. 298 On reception, the 'domain signature' may be used to verify the 299 authenticity of a message. If it encapsulates a SignedData, the 300 certificate(s) of the inner SignerInfo should be used to identify the 301 originator. This information can then be displayed to the recipient. A 302 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. No 310 other Object Identifiers may be included in the sequence of OIDs if this 311 value is present. 313 3.1.3 Additional Attributes Signature 315 The 'additional attributes' signature type indicates that the SignerInfo 316 contains additional attributes that are associated with the message. 317 All attributes in the applicable SignerInfo MUST be treated as 318 additional attributes. Successful verification of an 'additional 319 attributes' signature means only that the attributes are authentically 320 bound to the message. A recipient MUST NOT assume that its successful 321 verification also authenticates the message originator. The authenticity 322 of the message originator should be verified by checking an encapsulated 323 signature of the appropriate type. 325 A signer may include any of the attributes listed in CMS [3] or ESS [4] 326 when generating an 'additional attributes' signature. The following 327 attributes have a special meaning, when present in an 'additional 328 attributes' signature: 330 1) Equivalent Label: label values in this attribute are to be treated as 331 equivalent to the security label contained in an encapsulated 332 SignerInfo, if present. 334 2) Security Label: the label value indicates the aggregate sensitivity 335 of the inner message content plus any encapsulated signedData and 336 envelopedData containers. The label on the original data is indicated 337 by the value in the originator's signature, if present. 339 An 'additional attributes' signature is indicated by the presence of the 340 value Id-at-sigtype-add-attrib-sig in the 'signature type' authenticated 341 attribute. No other Object Identifiers may be included in the sequence 342 of OIDs if this value is present. An 'additional attributes' signature 343 may be added in parallel with other signatures in a SET OF SignerInfos. 345 3.1.4 Review and Release 347 The 'Review And Release' signature indicates that the signer has 348 reviewed the message. Successful verification of a 'Review and Release' 349 signature means only that the signer has approved the message for 350 release from a domain. A device on a domain boundary such as a Mail 351 Guard or firewall may be configured to check review and release 352 signatures. A recipient MUST NOT assume that its successful verification 353 also authenticates the message originator. The authenticity of the 354 message originator should be verified by checking an encapsulated 355 signature of the appropriate type. 357 A 'Review and Release' signature is indicated by the presence of the 358 value Id-at-sigtype-review-release-sig in the 'signature type' 359 authenticated attribute. No other Object Identifiers may be included in 360 the sequence of OIDs if this value is present. 362 3.2 Domain Encryption and Decryption 364 Domain encryption is encryption performed by a third party on behalf of 365 a set of originators in a domain. Domain decryption is decryption 366 performed by a third party on behalf of a set of recipients in a domain. 367 These processes may be performed in combination, as shown below. 369 -------------------------------------------------------------------- 370 | | Recipient Decryption | Domain Decryption | 371 |------------------------|----------------------|--------------------| 372 | Originator Encryption | Case(a) | Case(c) | 373 | Domain Encryption | Case(b) | Case(d) | 374 -------------------------------------------------------------------- 376 Case (a), encryption of messages by the originator for decryption by the 377 final recipient(s), is described in CMS [3]. In Cases (b) and (d), 378 encryption is performed not by the originator but by a third party in 379 the sending domain. In Cases (c) and (d), decryption is performed not 380 by the recipient(s) but by a third party in the destination domain. 382 A client implementation that conforms to this standard MUST support 383 cases (a) and (c) for transmission, and cases (a) and (b) for reception. 384 A Domain Encryption implementation that conforms to this standard MUST 385 support cases (b) and (d), for transmission, and cases (c) and (d) for 386 reception. 388 The process of encryption and decryption is documented in CMS [3]. The 389 only additional requirement introduced by domain encryption and 390 decryption is for greater flexibility in the management of keys, as 391 described in the following subsections. 393 The mechanisms described below are applicable both to key agreement and 394 key transport systems, as documented in CMS. The phrase 'encryption 395 key' is used as a collective term to cover the key management keys used 396 by both techniques. 398 3.2.1 Domain Encryption Key Management 400 Domain Encryption is shown as cases (b) and (d) in the above table. 401 Domain Encryption uses a domain-wide encryption key from the sender's 402 domain. Information about this key is conveyed to the recipient by 403 one of two methods:- 405 1) Public information about this key is held in a certificate and 406 conveyed to the recipient(s) in the 'Certs' field of the 407 OriginatorInfo in the Envelope. 409 2) The recipient(s) looks up the key for the users domain by replacing 410 the originator's name within the address with the domain encryption 411 name [6]. 413 For example :- 415 a) If the originator is originator@foo.com then lookup key for 416 domain-encrypting-authority@foo.com. 418 b) If the originator is c=us;a=com;p=foo;s=originator then look up 419 key for c=us;a=com;p=foo;s=domain-encrypting-authority. 421 c) If the originator is c=gb;o=foo;cn=originator then look up key for 422 c=gb;o=foo;cn=domain-encrypting-authority. 424 An implementation conforming to this standard MUST support both methods. 426 The name in the encryption certificate may not match the name in any 427 encapsulated signatures. For example, when a message is signed by the 428 originator and is encrypted by the domain. An implementation that 429 conforms to this standard MUST allow for this possibility. This includes 430 both a client and a third party implementation. 432 3.2.2 Domain Decryption Key Management 434 Domain Decryption is shown as cases (c) and (d) in the above table. In 435 these cases, the encryption process uses a domain-wide encryption key 436 for the recipient(s)' domain. The selection of this key is achieved by 437 one of two methods: 439 1) The sending process explicitly searches for a certificate containing 440 the domain encryption key of the recipient(s)' domain. This is 441 achieved by mapping the recipient(s)' name to a domain name and then 442 locating the encryption certificate containing that domain name. 443 Mapping from recipient names to Domain names, and conventions for 444 domain names are outside the scope of this standard. 446 2) All the members of the receiving domain are issued with certificates 447 containing a single key. The private component of that key is held 448 by an entity in the domain that performs the decryption process 449 on their behalf. By selecting the appropriate certificate, a sending 450 process will implicitly encrypt for decryption by the Domain 451 Decryption process. 453 An implementation that conforms to this standard MUST support mechanism 454 (1). It may also support mechanism (2). This includes both a client and a 455 third party implementation. 457 4. Security Considerations 459 Domain Security Services provide a method for digitally 460 signing data, digesting data, encrypting data, and authenticating 461 data. 463 Implementations must protect the signer's private key. Compromise of 464 the signer's private key permits masquerade. 466 Implementations must protect the key management private key and the 467 content-encryption key. Compromise of the key management private key 468 may result in the disclosure of all messages protected with that key. 469 Similarly, compromise of the content-encryption key may result in 470 disclosure of the encrypted content. 472 5. References 474 [1] Ramsdell, B., "S/MIME Version 3 Message Specification", Internet 475 Draft draft-ietf-smime-msg-04, May 1998. 477 [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement 478 Levels", BCP 14, RFC 2119, March 1997. 480 [3] Housley, R., "Cryptographic Message Syntax", Internet Draft 481 draft-ietf-smime-cms-05.txt, March 1998. 483 [4] Hoffman, P., "Enhanced Security Services for S/MIME", Internet Draft 484 draft-ietf-smime-ess-06.txt, May 1998. 486 [5] International Telecommunications Union, Recommendation X.208, "Open 487 systems interconnection: specification of Abstract Syntax Notation 488 (ASN.1)", CCITT Blue Book, 1989. 490 [6] Ramsdell, B., "Role Names in X.509 Certificates", Internet Draft 491 draft-ramsdell-role-names, April 29 1998. 493 6. Authors' Addresses 495 Tim Dean 496 DERA Malvern 497 St. Andrews Road 498 Malvern 499 Worcs 500 WR14 3PS 502 Phone: +44 (0)1684 894239 503 Fax: +44 (0)1684 6113 504 Email: t.dean@eris.dera.gov.uk 506 William Ottaway 507 DERA Malvern 508 St. Andrews Road 509 Malvern 510 Worcs 511 WR14 3PS 513 Phone: +44 (0)1684 894079 514 Fax: +44 (0)1684 896113 515 Email: w.ottaway@eris.dera.gov.uk 517