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(See Section 2.2 of https://www.ietf.org/id-info/checklist for how to handle the case when there are no actions for IANA.) ** There are 442 instances of too long lines in the document, the longest one being 3 characters in excess of 72. ** There are 82 instances of lines with control characters in the document. == There are 2 instances of lines with non-RFC6890-compliant IPv4 addresses in the document. If these are example addresses, they should be changed. ** The document seems to lack a both a reference to RFC 2119 and the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords. RFC 2119 keyword, line 109: '...entInfo eContent MUST be absent. If th...' RFC 2119 keyword, line 111: '...entInfo eContent MUST contain the resu...' RFC 2119 keyword, line 133: '...contentType MUST be id-data. The Envel...' RFC 2119 keyword, line 162: '...receiving agents MUST be able to inter...' RFC 2119 keyword, line 243: '...request MUST be in the inside signatur...' 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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Missing reference section? 'MSP4' on line 2286 looks like a reference -- Missing reference section? 'MSG' on line 2326 looks like a reference -- Missing reference section? 'CERT' on line 2326 looks like a reference -- Missing reference section? 'SMIME2' on line 2297 looks like a reference -- Missing reference section? 'ASN1-1988' on line 2268 looks like a reference -- Missing reference section? 'MUSTSHOULD' on line 2329 looks like a reference -- Missing reference section? 'ESS' on line 2331 looks like a reference -- Missing reference section? 'CMS' on line 2277 looks like a reference -- Missing reference section? '0' on line 2237 looks like a reference -- Missing reference section? '1' on line 2238 looks like a reference -- Missing reference section? 'MTSABS' on line 2289 looks like a reference -- Missing reference section? '2' on line 2239 looks like a reference -- Missing reference section? 'UNIVERSAL 12' on line 2089 looks like a reference -- Missing reference section? 'UTF8' on line 2300 looks like a reference -- Missing reference section? 'ASN1-1994' on line 2271 looks like a reference -- Missing reference section? 'SMIME3' on line 2326 looks like a reference Summary: 11 errors (**), 0 flaws (~~), 9 warnings (==), 18 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Internet Draft Editor: Paul Hoffman 2 draft-ietf-smime-ess-10.txt Internet Mail Consortium 3 November 12, 1998 4 Expires in six months 6 Enhanced Security Services for S/MIME 8 Status of this memo 10 This document is an Internet-Draft. Internet-Drafts are working documents 11 of the Internet Engineering Task Force (IETF), its areas, and its working 12 groups. Note that other groups may also distribute working documents as 13 Internet-Drafts. 15 Internet-Drafts are draft documents valid for a maximum of six months and 16 may be updated, replaced, or obsoleted by other documents at any time. It 17 is inappropriate to use Internet-Drafts as reference material or to cite 18 them other than as "work in progress." 20 To learn the current status of any Internet-Draft, please check the 21 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow 22 Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au 23 (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West 24 Coast). 26 1. Introduction 28 This document describes three optional security service extensions for 29 S/MIME. These services provide functionality that is similar to the Message 30 Security Protocol [MSP4], but are useful in many other environments, 31 particularly business and finance. The services are: 32 - signed receipts 33 - security labels 34 - secure mailing lists 36 The services described here are extensions to S/MIME version 3 ([MSG] and 37 [CERT]), and some of them can also be added to S/MIME version 2 [SMIME2]. 38 The extensions described here will not cause an S/MIME version 3 recipient 39 to be unable to read messages from an S/MIME version 2 sender. However, 40 some of the extensions will cause messages created by an S/MIME version 3 41 sender to be unreadable by an S/MIME version 2 recipient. 43 This document describes both the procedures and the attributes needed for 44 the three services. Note that some of the attributes described in this 45 document are quite useful in other contexts and should be considered when 46 extending S/MIME or other CMS applications. 48 The format of the messages are described in ASN.1:1988 [ASN1-1988]. 50 The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 51 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY', and 'OPTIONAL' in this 52 document are to be interpreted as described in [MUSTSHOULD]. 54 This draft is being discussed on the 'ietf-smime' mailing list. To 55 subscribe, send a message to: 56 ietf-smime-request@imc.org 57 with the single word 58 subscribe 59 in the body of the message. There is a Web site for the mailing list at 60 . 62 1.1 Triple Wrapping 64 Some of the features of each service use the concept of a "triple wrapped" 65 message. A triple wrapped message is one that has been signed, then 66 encrypted, then signed again. The signers of the inner and outer signatures 67 may be different entities or the same entity. Note that the S/MIME 68 specification does not limit the number of nested encapsulations, so there 69 may be more than three wrappings. 71 1.1.1 Purpose of Triple Wrapping 73 Not all messages need to be triple wrapped. Triple wrapping is used when a 74 message must be signed, then encrypted, and then have signed attributes 75 bound to the encrypted body. Outer attributes may be added or removed by 76 the message originator or intermediate agents, and may be signed by 77 intermediate agents or the final recipient. 79 The inside signature is used for content integrity, non-repudiation with 80 proof of origin, and binding attributes (such as a security label) to the 81 original content. These attributes go from the originator to the recipient, 82 regardless of the number of intermediate entities such as mail list agents 83 that process the message. The signed attributes can be used for access 84 control to the inner body. Requests for signed receipts by the originator 85 are carried in the inside signature as well. 87 The encrypted body provides confidentiality, including confidentiality of 88 the attributes that are carried in the inside signature. 90 The outside signature provides authentication and integrity for information 91 that is processed hop-by-hop, where each hop is an intermediate entity such 92 as a mail list agent. The outer signature binds attributes (such as a 93 security label) to the encrypted body. These attributes can be used for 94 access control and routing decisions. 96 1.1.2 Steps for Triple Wrapping 98 The steps to create a triple wrapped message are: 100 1. Start with a message body, called the "original content". 102 2. Encapsulate the original content with the appropriate MIME Content-type 103 headers, such as "Content-type: text/plain". An exception to this MIME 104 encapsulation rule is that a signed receipt is not put in MIME headers. 106 3. Sign the result of step 2 (the inner MIME headers and the original 107 content). The SignedData encapContentInfo eContentType object identifier 108 MUST be id-data. If the structure you create in step 4 is multipart/signed, 109 then the SignedData encapContentInfo eContent MUST be absent. If the 110 structure you create in step 4 is application/pkcs7-mime, then the 111 SignedData encapContentInfo eContent MUST contain the result of step 2 112 above. The SignedData structure is encapsulated by a ContentInfo SEQUENCE 113 with a contentType of id-signedData. 115 4. Add an appropriate MIME construct to the signed message from step 3 as 116 defined in [MSG]. The resulting message is called the "inside signature". 118 - If you are signing using multipart/signed, the MIME construct added 119 consists of a Content-type of multipart/signed with parameters, the 120 boundary, the result of step 2 above, the boundary, a Content-type of 121 application/pkcs7-signature, optional MIME headers (such as 122 Content-transfer-encoding and Content-disposition), and a body part that 123 is the result of step 3 above. 125 - If you are instead signing using application/pkcs7-mime, the MIME 126 construct added consists of a Content-type of application/pkcs7-mime 127 with parameters, optional MIME headers (such as 128 Content-transfer-encoding and Content-disposition), and the result of 129 step 3 above. 131 5. Encrypt the result of step 4 as a single block, turning it into an 132 application/pkcs7-mime object. The EnvelopedData encryptedContentInfo 133 contentType MUST be id-data. The EnvelopedData structure is encapsulated by 134 a ContentInfo SEQUENCE with a contentType of id-envelopedData. This is 135 called the "encrypted body". 137 6. Add the appropriate MIME headers: a Content-type of 138 application/pkcs7-mime with parameters, and optional MIME headers such as 139 Content-transfer-encoding and Content-disposition. 141 7. Using the same logic as in step 3 above, sign the result of step 6 (the 142 MIME headers and the encrypted body) as a single block 144 8. Using the same logic as in step 4 above, add an appropriate MIME 145 construct to the signed message from step 7. The resulting message is 146 called the "outside signature", and is also the triple wrapped message. 148 1.2 Format of a Triple Wrapped Message 150 A triple wrapped message has many layers of encapsulation. The structure 151 differs based on the choice of format for the signed portions of the 152 message. Because of the way that MIME encapsulates data, the layers do not 153 appear in order, and the notion of "layers" becomes vague. 155 There is no need to use the multipart/signed format in an inner signature 156 because it is known that the recipient is able to process S/MIME messages 157 (because they decrypted the middle wrapper). A sending agent might choose 158 to use the multipart/signed format in the outer layer so that a non-S/MIME 159 agent could see that the next inner layer is encrypted; however, this is 160 not of great value, since all it shows the recipient is that the rest of 161 the message is unreadable. Because many sending agents always use 162 multipart/signed structures, all receiving agents MUST be able to interpret 163 either multipart/signed or application/pkcs7-mime signature structures. 165 The format of a triple wrapped message that uses multipart/signed for both 166 signatures is: 168 [step 8] Content-type: multipart/signed; 169 [step 8] protocol="application/pkcs7-signature"; 170 [step 8] boundary=outerboundary 171 [step 8] 172 [step 8] --outerboundary 173 [step 6] Content-type: application/pkcs7-mime; ) 174 [step 6] smime-type=enveloped-data ) 175 [step 6] ) 176 [step 4] Content-type: multipart/signed; | ) 177 [step 4] protocol="application/pkcs7-signature"; | ) 178 [step 4] boundary=innerboundary | ) 179 [step 4] | ) 180 [step 4] --innerboundary | ) 181 [step 2] Content-type: text/plain % | ) 182 [step 2] % | ) 183 [step 1] Original content % | ) 184 [step 4] | ) 185 [step 4] --innerboundary | ) 186 [step 4] Content-type: application/pkcs7-signature | ) 187 [step 4] | ) 188 [step 3] inner SignedData block (eContent is missing) | ) 189 [step 4] | ) 190 [step 4] --innerboundary-- | ) 191 [step 8] 192 [step 8] --outerboundary 193 [step 8] Content-type: application/pkcs7-signature 194 [step 8] 195 [step 7] outer SignedData block (eContent is missing) 196 [step 8] 197 [step 8] --outerboundary-- 199 % = These lines are what the inner signature is computed over. 200 | = These lines are what is encrypted in step 5. This encrypted result 201 is opaque and is a part of an EnvelopedData block. 202 ) = These lines are what the outer signature is computed over. 204 The format of a triple wrapped message that uses application/pkcs7-mime for 205 the both signatures is: 207 [step 8] Content-type: application/pkcs7-mime; 208 [step 8] smime-type=signed-data 209 [step 8] 210 [step 7] outer SignedData block (eContent is present) O 211 [step 6] Content-type: application/pkcs7-mime; ) O 212 [step 6] smime-type=enveloped-data; ) O 213 [step 6] ) O 214 [step 4] Content-type: application/pkcs7-mime; | ) O 215 [step 4] smime-type=signed-data | ) O 216 [step 4] | ) O 217 [step 3] inner SignedData block (eContent is present) I | ) O 218 [step 2] Content-type: text/plain I | ) O 219 [step 2] I | ) O 220 [step 1] Original content I | ) O 222 I = These lines are the inner SignedData block, which is opaque and 223 contains the ASN.1 encoded result of step 2 as well as control 224 information. 225 | = These lines are what is encrypted in step 5. This encrypted result 226 is opaque and is a part of an EnvelopedData block. 227 ) = These lines are what the outer signature is computed over. 228 O = These lines are the outer SignedData block, which is opaque and 229 contains the ASN.1 encoded result of step 6 as well as control 230 information. 232 1.3 Security Services and Triple Wrapping 234 The three security services described in this document are used with triple 235 wrapped messages in different ways. This section briefly describes the 236 relationship of each service with triple wrapping; the other sections of 237 the document go into greater detail. 239 1.3.1 Signed Receipts and Triple Wrapping 241 A signed receipt may be requested in any SignedData object. However, if a 242 signed receipt is requested for a triple wrapped message, the receipt 243 request MUST be in the inside signature, not in the outside signature. A 244 secure mailing list agent may change the receipt policy in the outside 245 signature of a triple wrapped message when that message is processed by the 246 mailing list. 248 Note: the signed receipts and receipt requests described in this draft 249 differ from those described in the work done by the IETF Receipt 250 Notification Working Group. The output of that Working Group, when 251 finished, is not expected to work well with triple wrapped messages as 252 described in this document. 254 1.3.2 Security Labels and Triple Wrapping 256 A security label may be included in the signed attributes of any SignedData 257 object. A security label attribute may be included in either the inner 258 signature, outer signature, or both. 260 The inner security label is used for access control decisions related to 261 the plaintext original content. The inner signature provides authentication 262 and cryptographically protects the integrity of the original signer's 263 security label that is in the inside body. This strategy facilitates the 264 forwarding of messages because the original signer's security label is 265 included in the SignedData block which can be forwarded to a third party 266 that can verify the inner signature which will cover the inner security 267 label. The confidentiality security service can be applied to the inner 268 security label by encrypting the entire inner SignedData block within an 269 EnvelopedData block. 271 A security label may also be included in the signed attributes of the outer 272 SignedData block which will include the sensitivities of the encrypted 273 message. The outer security label is used for access control and routing 274 decisions related to the encrypted message. Note that a security label 275 attribute can only be used in an signedAttributes block. An 276 eSSSecurityLabel attribute MUST NOT be used in an EnvelopedData or unsigned 277 attributes. 279 1.3.3 Secure Mailing Lists and Triple Wrapping 281 Secure mail list message processing depends on the structure of S/MIME 282 layers present in the message sent to the mail list agent. The mail list 283 agent never changes the data that was hashed to form the inner signature, 284 if such a signature is present. If an outer signature is present, then the 285 agent will modify the data that was hashed to form that outer signature. In 286 all cases, the agent adds or updates an mlExpansionHistory attribute to 287 document the agent's processing, and ultimately adds or replaces the outer 288 signature on the message to be distributed. 290 1.3.4 Placement of Attributes 292 Certain attributes should be placed in the inner or outer SignedData 293 message; some attributes can be in either. Further, some attributes must be 294 signed, while signing is optional for others, and some attributes must not 295 be signed. The following table summarizes the recommendation of this 296 profile. In the OID column, [ESS] indicates that the attribute is defined 297 in this document. 299 | |Inner or | 300 Attribute |OID |outer |Signed 301 ------------------|----------------------------- |----------|-------- 302 contentHints |id-aa-contentHint [ESS] |either |MAY 303 contentIdentifier |id-aa-contentIdentifier [ESS] |either |MAY 304 contentReference |id-aa-contentReference [ESS] |either |MUST 305 contentType |id-contentType [CMS] |either |MUST 306 counterSignature |id-countersignature [CMS] |either |MUST NOT 307 equivalentLabel |id-aa-equivalentLabels [ESS] |either |MUST 308 eSSSecurityLabel |id-aa-securityLabel [ESS] |either |MUST 309 messageDigest |id-messageDigest [CMS] |either |MUST 310 msgSigDigest |id-aa-msgSigDigest [ESS] |inner only|MUST 311 mlExpansionHistory|id-aa-mlExpandHistory [ESS] |outer only|MUST 312 receiptRequest |id-aa-receiptRequest [ESS] |inner only|MUST 313 signingCertificate|id-aa-signingCertificate [ESS]|either |MUST 314 signingTime |id-signingTime [CMS] |either |MUST 315 smimeCapabilities |sMIMECapabilities [MSG] |either |MUST 316 sMIMEEncryption- 317 KeyPreference |id-aa-encrypKeyPref [MSG] |either |MUST 319 CMS defines signedAttrs as a SET OF Attributes and defines 320 unsignedAttributes as a SET OF Attributes. ESS defines the contentHints, 321 contentIdentifier, eSSecurityLabel, msgSigDigest, mlExpansionHistory, 322 receiptRequest, contentReference and equivalentLabels attribute types. A 323 signerInfo MUST NOT include multiple instances of any of the attribute 324 types defined in ESS. Later sections of ESS specify further restrictions 325 that apply to the receiptRequest, mlExpansionHistory and eSSecurityLabel 326 attribute types. 328 CMS defines the syntax for the signed and unsigned attributes as 329 "attrValues SET OF AttributeValue". For all of the attribute types defined 330 in ESS, if the attribute type is present in a signerInfo, then it MUST only 331 include a single instance of AttributeValue. In other words, there MUST NOT 332 be zero or multiple instances of AttributeValue present in the attrValues 333 SET OF AttributeValue. 335 If a counterSignature attribute is present, then it MUST be included in the 336 unsigned attributes. It MUST NOT be included in the signed attributes. The 337 only attributes that are allowed in a counterSignature attribute are 338 counterSignature, messageDigest, signingTime, and signingCertificate. 340 Note that the inner and outer signatures are usually those of different 341 senders. Because of this, the same attribute in the two signatures could 342 lead to very different consequences. 344 The macValue attribute defined in [CMS] is only used in authenticatedData, 345 never in signedData. 347 ContentIdentifier is an attribute (OCTET STRING) used to carry a unique 348 identifier assigned to the message. 350 1.4 Required and Optional Attributes 352 Some security gateways sign messages that pass through them. If the message 353 is any type other than a signedData type, the gateway has only one way to 354 sign the message: by wrapping it with a signedData block and MIME headers. 355 If the message to be signed by the gateway is a signedData message already, 356 the gateway can sign the message by inserting a signerInfo into the 357 signedData block. 359 The main advantage of a gateway adding a signerInfo instead of wrapping the 360 message in a new signature is that the message doesn't grow as much as if 361 the gateway wrapped the message. The main disadvantage is that the gateway 362 must check for the presence of certain attributes in the other signerInfos 363 and duplicate those attributes. 365 If a gateway or other processor adds a signerInfo to an existing signedData 366 block, it MUST copy the mlExpansionHistory and eSSSecurityLabel attributes 367 from other signerInfos. This helps ensure that the recipient will process 368 those attributes in a signerInfo that it can verify. 370 Note that someone may in the future define an attribute that must be 371 present in each signerInfo of a signedData block in order for the signature 372 to be processed. If that happens, a gateway that inserts signerInfos and 373 doesn't copy that attribute will cause every message with that attribute to 374 fail when processed by the recipient. For this reason, it is safer to wrap 375 messages with new signatures than to insert signerInfos. 377 1.5 Object Identifiers 379 The object identifiers for many of the objects described in this draft are 380 found in [CMS], [MSG], and [CERT]. Other object identifiers used in S/MIME 381 can be found in the registry kept at 382 . When this draft moves to 383 standards track within the IETF, it is intended that the IANA will maintain 384 this registry. 386 2. Signed Receipts 388 Returning a signed receipt provides to the originator proof of delivery of 389 a message, and allows the originator to demonstrate to a third party that 390 the recipient was able to verify the signature of the original message. 391 This receipt is bound to the original message through the signature; 392 consequently, this service may be requested only if a message is signed. 393 The receipt sender may optionally also encrypt a receipt to provide 394 confidentiality between the receipt sender and the receipt recipient. 396 2.1 Signed Receipt Concepts 398 The originator of a message may request a signed receipt from the message's 399 recipients. The request is indicated by adding a receiptRequest attribute 400 to the signedAttributes field of the SignerInfo object for which the 401 receipt is requested. The receiving user agent software SHOULD 402 automatically create a signed receipt when requested to do so, and return 403 the receipt in accordance with mailing list expansion options, local 404 security policies, and configuration options. 406 Because receipts involve the interaction of two parties, the terminology 407 can sometimes be confusing. In this section, the "sender" is the agent that 408 sent the original message that included a request for a receipt. The 409 "receiver" is the party that received that message and generated the 410 receipt. 412 The steps in a typical transaction are: 414 1. Sender creates a signed message including a receipt request attribute 415 (Section 2.2). 417 2. Sender transmits the resulting message to the recipient or recipients. 419 3. Recipient receives message and determines if there is a valid signature 420 and receipt request in the message (Section 2.3). 422 4. Recipient creates a signed receipt (Section 2.4). 424 5. Recipient transmits the resulting signed receipt message to the sender 425 (Section 2.5). 427 6. Sender receives the message and validates that it contains a signed 428 receipt for the original message (Section 2.6). This validation relies on 429 the sender having retained either a copy of the original message or 430 information extracted from the original message. 432 The ASN.1 syntax for the receipt request is given in Section 2.7; the ASN.1 433 syntax for the receipt is given in Section 2.8. 435 Note that a sending agent SHOULD remember when it has sent a receipt so 436 that it can avoid re-sending a receipt each time it processes the message. 438 2.2 Receipt Request Creation 440 Multi-layer S/MIME messages may contain multiple SignedData layers. 441 However, receipts may be requested only for the innermost SignedData layer 442 in a multi-layer S/MIME message, such as a triple wrapped message. Only one 443 receiptRequest attribute can be included in the signedAttributes of a 444 SignerInfo. 446 A ReceiptRequest attribute MUST NOT be included in the attributes of a 447 SignerInfo in a SignedData object that encapsulates a Receipt content. In 448 other words, the receiving agent MUST NOT request a signed receipt for a 449 signed receipt. 451 A sender requests receipts by placing a receiptRequest attribute in the 452 signed attributes of a signerInfo as follows: 454 1. A receiptRequest data structure is created. 456 2. A signed content identifier for the message is created and assigned to 457 the signedContentIdentifier field. The signedContentIdentifier is used to 458 associate the signed receipt with the message requesting the signed 459 receipt. 461 3. The entities requested to return a signed receipt are noted in the 462 receiptsFrom field. 464 4. The message originator MUST populate the receiptsTo field with a 465 GeneralNames for each entity to whom the recipient should send the signed 466 receipt.�If the message originator wants the recipient to send the signed 467 receipt to the originator, then the originator MUST include a GeneralNames 468 for itself in the receiptsTo field.�GeneralNames is a SEQUENCE OF 469 GeneralName.�receiptsTo is a SEQUENCE OF GeneralNames in which each 470 GeneralNames represents an entity.�There may be multiple GeneralName 471 instances in each GeneralNames.�At a minimum, the message originator MUST 472 populate each entity's GeneralNames with the address to which the signed 473 receipt should be sent.�Optionally, the message originator MAY also 474 populate each entity's GeneralNames with other GeneralName instances (such 475 as directoryName). 477 5. The completed receiptRequest attribute is placed in the signedAttributes 478 field of the SignerInfo object. 480 2.2.1 Multiple Receipt Requests 482 There can be multiple SignerInfos within a SignedData object, and each 483 SignerInfo may include signedAttributes. Therefore, a single SignedData 484 object may include multiple SignerInfos, each SignerInfo having a 485 receiptRequest attribute. For example, an originator can send a signed 486 message with two SignerInfos, one containing a DSS signature, the other 487 containing an RSA signature. 489 Each recipient SHOULD return only one signed receipt. 491 Not all of the SignerInfos need to include receipt requests, but in all of 492 the SignerInfos that do contain receipt requests, the receipt requests MUST 493 be identical. 495 2.2.2 Information Needed to Validate Signed Receipts 497 The sending agent MUST retain one or both of the following items to support 498 the validation of signed receipts returned by the recipients. 500 - the original signedData object requesting the signed receipt 502 - the message signature digest value used to generate the original 503 signedData signerInfo signature value and the digest value of the 504 Receipt content containing values included in the original signedData 505 object. If signed receipts are requested from multiple recipients, then 506 retaining these digest values is a performance enhancement because the 507 sending agent can reuse the saved values when verifying each returned 508 signed receipt. 510 2.3 Receipt Request Processing 512 A receiptRequest is associated only with the SignerInfo object to which the 513 receipt request attribute is directly attached. Receiving software SHOULD 514 examine the signedAttributes field of each of the SignerInfos for which it 515 verifies a signature in the innermost signedData object to determine if a 516 receipt is requested. This may result in the receiving agent processing 517 multiple receiptRequest attributes included in a single SignedData object, 518 such as requests made from different people who signed the object in 519 parallel. 521 Before processing a receiptRequest signedAttribute, the receiving agent 522 MUST verify the signature of the SignerInfo which covers the receiptRequest 523 attribute. A recipient MUST NOT process a receiptRequest attribute that has 524 not been verified. Because all receiptRequest attributes in a SignedData 525 object must be identical, the receiving application fully processes (as 526 described in the following paragraphs) the first receiptRequest attribute 527 that it encounters in a SignerInfo that it verifies, and it then ensures 528 that all other receiptRequest attributes in signerInfos that it verifies 529 are identical to the first one encountered. If there are verified 530 ReceiptRequest attributes which are not the same, then the processing 531 software MUST NOT return any signed receipt. A signed receipt SHOULD be 532 returned if any signerInfo containing a receiptRequest attribute can be 533 validated, even if other signerInfos containing the same receiptRequest 534 attribute cannot be validated because they are signed using an algorithm 535 not supported by the receiving agent. 537 If a receiptRequest attribute is absent from the signed attributes, then a 538 signed receipt has not been requested from any of the message recipients 539 and MUST NOT be created. If a receiptRequest attribute is present in the 540 signed attributes, then a signed receipt has been requested from some or 541 all of the message recipients. Note that in some cases, a receiving agent 542 might receive two almost-identical messages, one with a receipt request and 543 the other without one. In this case, the receiving agent SHOULD send a 544 signed receipt for the message that requests a signed receipt. 546 If a receiptRequest attribute is present in the signed attributes, the 547 following process SHOULD be used to determine if a message recipient has 548 been requested to return a signed receipt. 550 1. If an mlExpansionHistory attribute is present in the outermost 551 signedData block, do one of the following two steps, based on the absence 552 or presence of mlReceiptPolicy: 554 1.1. If an mlReceiptPolicy value is absent from the last MLData 555 element, a Mail List receipt policy has not been specified and the 556 processing software SHOULD examine the receiptRequest attribute value 557 to determine if a receipt should be created and returned. 559 1.2. If an mlReceiptPolicy value is present in the last MLData element, 560 do one of the following two steps, based on the value of 561 mlReceiptPolicy: 563 1.2.1. If the mlReceiptPolicy value is none, then the receipt 564 policy of the Mail List supersedes the originator's request for a 565 signed receipt and a signed receipt MUST NOT be created. 567 1.2.2. If the mlReceiptPolicy value is insteadOf or inAdditionTo, 568 the processing software SHOULD examine the receiptsFrom value from 569 the receiptRequest attribute to determine if a receipt should be 570 created and returned. If a receipt is created, the insteadOf and 571 inAdditionTo fields identify entities that SHOULD be sent the 572 receipt instead of or in addition to the originator. 574 2. If the receiptsFrom value of the receiptRequest attribute is 575 allOrFirstTier, do one of the following two steps based on the value of 576 allOrFirstTier. 578 2.1. If the value of allOrFirstTier is allReceipts, then a signed 579 receipt SHOULD be created. 581 2.2. If the value of allOrFirstTier is firstTierRecipients, do one of 582 the following two steps based on the presence of an mlExpansionHistory 583 attribute in an outer signedData block: 585 2.2.1. If an mlExpansionHistory attribute is present, then this 586 recipient is not a first tier recipient and a signed receipt MUST 587 NOT be created. 589 2.2.2. If an mlExpansionHistory attribute is not present, then a 590 signed receipt SHOULD be created. 592 3. If the receiptsFrom value of the receiptRequest attribute is a 593 receiptList: 595 3.1. If receiptList contains one of the GeneralNames of the recipient, 596 then a signed receipt SHOULD be created. 598 3.2. If receiptList does not contain one of the GeneralNames of the 599 recipient, then a signed receipt MUST NOT be created. 601 A flow chart for the above steps to be executed for each signerInfo for 602 which the receiving agent verifies the signature would be: 604 0. Receipt Request attribute present? 605 YES -> 1. 606 NO -> STOP 607 1. Has mlExpansionHistory in outer signedData? 608 YES -> 1.1. 609 NO -> 2. 610 1.1. mlReceiptPolicy absent? 611 YES -> 2. 612 NO -> 1.2. 613 1.2. Pick based on value of mlReceiptPolicy. 614 none -> 1.2.1. 615 insteadOf or inAdditionTo -> 1.2.2. 616 1.2.1. STOP. 617 1.2.2. Examine receiptsFrom to determine if a receipt should be created, 618 create it if required, send it to recipients designated by 619 mlReceiptPolicy, then -> STOP. 620 2. Is value of receiptsFrom allOrFirstTier? 621 YES -> Pick based on value of allOrFirstTier. 622 allReceipts -> 2.1. 623 firstTierRecipients -> 2.2. 624 NO -> 3. 625 2.1. Create a receipt, then -> STOP. 626 2.2. Has mlExpansionHistory in the outer signedData block? 627 YES -> 2.2.1. 628 NO -> 2.2.2. 629 2.2.1. STOP. 630 2.2.2. Create a receipt, then -> STOP. 631 3. Is receiptsFrom value of receiptRequest a receiptList? 632 YES -> 3.1. 633 NO -> STOP. 634 3.1. Does receiptList contain the recipient? 635 YES -> Create a receipt, then -> STOP. 636 NO -> 3.2. 637 3.2. STOP. 639 2.4 Signed Receipt Creation 641 A signed receipt is a signedData object encapsulating a Receipt content 642 (also called a "signedData/Receipt"). Signed receipts are created as 643 follows: 645 1. The signature of the original signedData signerInfo that includes the 646 receiptRequest signed attribute MUST be successfully verified before 647 creating the signedData/Receipt. 649 1.1. The content of the original signedData object is digested as 650 described in [CMS]. The resulting digest value is then compared with 651 the value of the messageDigest attribute included in the 652 signedAttributes of the original signedData signerInfo. If these digest 653 values are different, then the signature verification process fails and 654 the signedData/Receipt MUST NOT be created. 656 1.2. The ASN.1 DER encoded signedAttributes (including messageDigest, 657 receiptRequest and, possibly, other signed attributes) in the original 658 signedData signerInfo are digested as described in [CMS]. The resulting 659 digest value, called msgSigDigest, is then used to verify the signature 660 of the original signedData signerInfo. If the signature verification 661 fails, then the signedData/Receipt MUST NOT be created. 663 2. A Receipt structure is created. 665 2.1. The value of the Receipt version field is set to 1. 667 2.2. The object identifier from the contentType attribute included in 668 the original signedData signerInfo that includes the receiptRequest 669 attribute is copied into the Receipt contentType. 671 2.3. The original signedData signerInfo receiptRequest 672 signedContentIdentifier is copied into the Receipt 673 signedContentIdentifier. 675 2.4. The signature value from the original signedData signerInfo that 676 includes the receiptRequest attribute is copied into the Receipt 677 originatorSignatureValue. 679 3. The Receipt structure is ASN.1 DER encoded to produce a data stream, D1. 681 4. D1 is digested. The resulting digest value is included as the 682 messageDigest attribute in the signedAttributes of the signerInfo which 683 will eventually contain the signedData/Receipt signature value. 685 5. The digest value (msgSigDigest) calculated in Step 1 to verify the 686 signature of the original signedData signerInfo is included as the 687 msgSigDigest attribute in the signedAttributes of the signerInfo which will 688 eventually contain the signedData/Receipt signature value. 690 6. A contentType attribute including the id-ct-receipt object identifier 691 MUST be created and added to the signed attributes of the signerInfo which 692 will eventually contain the signedData/Receipt signature value. 694 7. A signingTime attribute indicating the time that the signedData/Receipt 695 is signed SHOULD be created and added to the signed attributes of the 696 signerInfo which will eventually contain the signedData/Receipt signature 697 value. Other attributes (except receiptRequest) may be added to the 698 signedAttributes of the signerInfo. 700 8. The signedAttributes (messageDigest, msgSigDigest, contentType and, 701 possibly, others) of the signerInfo are ASN.1 DER encoded and digested as 702 described in [CMS]. The resulting digest value is used to calculate the 703 signature value which is then included in the signedData/Receipt 704 signerInfo. 706 9. The ASN.1 DER encoded Receipt content MUST be directly encoded within 707 the signedData encapContentInfo eContent OCTET STRING defined in [CMS]. The 708 id-ct-receipt object identifier MUST be included in the signedData 709 encapContentInfo eContentType. This results in a single ASN.1 encoded 710 object composed of a signedData including the Receipt content. The Data 711 content type MUST NOT be used. The Receipt content MUST NOT be encapsulated 712 in a MIME header or any other header prior to being encoded as part of the 713 signedData object. 715 10. The signedData/Receipt is then put in an application/pkcs7-mime MIME 716 wrapper with the smime-type parameter set to "signed-receipt". This will 717 allow for identification of signed receipts without having to crack the 718 ASN.1 body. The smime-type parameter would still be set as normal in any 719 layer wrapped around this message. 721 11. If the signedData/Receipt is to be encrypted within an envelopedData 722 object, then an outer signedData object MUST be created that encapsulates 723 the envelopedData object, and a contentHints attribute with contentType set 724 to the id-ct-receipt object identifier MUST be included in the outer 725 signedData SignerInfo signedAttributes. When a receiving agent processes 726 the outer signedData object, the presence of the id-ct-receipt OID in the 727 contentHints contentType indicates that a signedData/Receipt is encrypted 728 within the envelopedData object encapsulated by the outer signedData. 730 All sending agents that support the generation of ESS signed receipts MUST 731 provide the ability to send encrypted signed receipts (that is, a 732 signedData/Receipt encapsulated within an envelopedData). The sending agent 733 MAY send an encrypted signed receipt in response to an 734 envelopedData-encapsulated signedData requesting a signed receipt. It is a 735 matter of local policy regarding whether or not the signed receipt should 736 be encrypted. The ESS signed receipt includes the message digest value 737 calculated for the original signedData object that requested the signed 738 receipt. If the original signedData object was sent encrypted within an 739 envelopedData object and the ESS signed receipt is sent unencrypted, then 740 the message digest value calculated for the original encrypted signedData 741 object is sent unencrypted. The responder should consider this when 742 deciding whether or not to encrypt the ESS signed receipt. 744 2.4.1 MLExpansionHistory Attributes and Receipts 746 An MLExpansionHistory attribute MUST NOT be included in the attributes of a 747 SignerInfo in a SignedData object that encapsulates a Receipt content. This 748 is true because when a SignedData/Receipt is sent to an MLA for 749 distribution, then the MLA must always encapsulate the received 750 SignedData/Receipt in an outer SignedData in which the MLA will include the 751 MLExpansionHistory attribute. The MLA cannot change the signedAttributes of 752 the received SignedData/Receipt object, so it can't add the 753 MLExpansionHistory to the SignedData/Receipt. 755 2.5 Determining the Recipients of the Signed Receipt 757 If a signed receipt was created by the process described in the sections 758 above, then the software MUST use the following process to determine to 759 whom the signed receipt should be sent. 761 1. The receiptsTo field must be present in the receiptRequest attribute. 762 The software initiates the sequence of recipients with the value(s) of 763 receiptsTo. 765 2. If the MlExpansionHistory attribute is present in the outer SignedData 766 block, and the last MLData contains an MLReceiptPolicy value of insteadOf, 767 then the software replaces the sequence of recipients with the value(s) of 768 insteadOf. 770 3. If the MlExpansionHistory attribute is present in the outer SignedData 771 block and the last MLData contains an MLReceiptPolicy value of 772 inAdditionTo, then the software adds the value(s) of inAdditionTo to the 773 sequence of recipients. 775 2.6. Signed Receipt Validation 777 A signed receipt is communicated as a single ASN.1 encoded object composed 778 of a signedData object directly including a Receipt content. It is 779 identified by the presence of the id-ct-receipt object identifier in the 780 encapContentInfo eContentType value of the signedData object including the 781 Receipt content. 783 A signedData/Receipt is validated as follows: 785 1. ASN.1 decode the signedData object including the Receipt content. 787 2. Extract the contentType, signedContentIdentifier, and 788 originatorSignatureValue from the decoded Receipt structure to identify the 789 original signedData signerInfo that requested the signedData/Receipt. 791 3. Acquire the message signature digest value calculated by the sender to 792 generate the signature value included in the original signedData signerInfo 793 that requested the signedData/Receipt. 795 3.1. If the sender-calculated message signature digest value has been 796 saved locally by the sender, it must be located and retrieved. 798 3.2. If it has not been saved, then it must be re-calculated based on 799 the original signedData content and signedAttributes as described in 800 [CMS]. 802 4. The message signature digest value calculated by the sender is then 803 compared with the value of the msgSigDigest signedAttribute included in the 804 signedData/Receipt signerInfo. If these digest values are identical, then 805 that proves that the message signature digest value calculated by the 806 recipient based on the received original signedData object is the same as 807 that calculated by the sender. This proves that the recipient received 808 exactly the same original signedData content and signedAttributes as sent 809 by the sender because that is the only way that the recipient could have 810 calculated the same message signature digest value as calculated by the 811 sender. If the digest values are different, then the signedData/Receipt 812 signature verification process fails. 814 5. Acquire the digest value calculated by the sender for the Receipt 815 content constructed by the sender (including the contentType, 816 signedContentIdentifier, and signature value that were included in the 817 original signedData signerInfo that requested the signedData/Receipt). 819 5.1. If the sender-calculated Receipt content digest value has been 820 saved locally by the sender, it must be located and retrieved. 822 5.2. If it has not been saved, then it must be re-calculated. As 823 described in section 2.4 above, step 2, create a Receipt structure 824 including the contentType, signedContentIdentifier and signature value 825 that were included in the original signedData signerInfo that requested 826 the signed receipt. The Receipt structure is then ASN.1 DER encoded to 827 produce a data stream which is then digested to produce the Receipt 828 content digest value. 830 6. The Receipt content digest value calculated by the sender is then 831 compared with the value of the messageDigest signedAttribute included in 832 the signedData/Receipt signerInfo. If these digest values are identical, 833 then that proves that the values included in the Receipt content by the 834 recipient are identical to those that were included in the original 835 signedData signerInfo that requested the signedData/Receipt. This proves 836 that the recipient received the original signedData signed by the sender, 837 because that is the only way that the recipient could have obtained the 838 original signedData signerInfo signature value for inclusion in the Receipt 839 content. If the digest values are different, then the signedData/Receipt 840 signature verification process fails. 842 7. The ASN.1 DER encoded signedAttributes of the signedData/Receipt 843 signerInfo are digested as described in [CMS]. 845 8. The resulting digest value is then used to verify the signature value 846 included in the signedData/Receipt signerInfo. If the signature 847 verification is successful, then that proves the integrity of the 848 signedData/receipt signerInfo signedAttributes and authenticates the 849 identity of the signer of the signedData/Receipt signerInfo. Note that the 850 signedAttributes include the recipient-calculated Receipt content digest 851 value (messageDigest attribute) and recipient-calculated message signature 852 digest value (msgSigDigest attribute). Therefore, the aforementioned 853 comparison of the sender-generated and recipient-generated digest values 854 combined with the successful signedData/Receipt signature verification 855 proves that the recipient received the exact original signedData content 856 and signedAttributes (proven by msgSigDigest attribute) that were signed by 857 the sender of the original signedData object (proven by messageDigest 858 attribute). If the signature verification fails, then the 859 signedData/Receipt signature verification process fails. 861 The signature verification process for each signature algorithm that is 862 used in conjunction with the CMS protocol is specific to the algorithm. 863 These processes are described in documents specific to the algorithms. 865 2.7 Receipt Request Syntax 867 A receiptRequest attribute value has ASN.1 type ReceiptRequest. Use the 868 receiptRequest attribute only within the signed attributes associated with 869 a signed message. 871 ReceiptRequest ::= SEQUENCE { 872 signedContentIdentifier ContentIdentifier, 873 receiptsFrom ReceiptsFrom, 874 receiptsTo SEQUENCE SIZE (1..ub-receiptsTo)) OF GeneralNames } 876 ub-receiptsTo INTEGER ::= 16 878 id-aa-receiptRequest OBJECT IDENTIFIER ::= { iso(1) member-body(2) 879 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 1} 881 ContentIdentifier ::= OCTET STRING 883 id-aa-contentIdentifier OBJECT IDENTIFIER ::= { iso(1) member-body(2) 884 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 7} 886 A signedContentIdentifier MUST be created by the message originator when 887 creating a receipt request. To ensure global uniqueness, the minimal 888 signedContentIdentifier SHOULD contain a concatenation of user-specific 889 identification information (such as a user name or public keying material 890 identification information), a GeneralizedTime string, and a random number. 892 The receiptsFrom field is used by the originator to specify the recipients 893 requested to return a signed receipt. A CHOICE is provided to allow 894 specification of: 895 - receipts from all recipients are requested 896 - receipts from first tier (recipients that did not receive the 897 message as members of a mailing list) recipients are requested 898 - receipts from a specific list of recipients are requested 900 ReceiptsFrom ::= CHOICE { 901 allOrFirstTier [0] AllOrFirstTier, 902 -- formerly "allOrNone [0]AllOrNone" 903 receiptList [1] SEQUENCE OF GeneralNames } 905 AllOrFirstTier ::= INTEGER { -- Formerly AllOrNone 906 allReceipts (0), 907 firstTierRecipients (1) } 909 The receiptsTo field is used by the originator to identify the user(s) to 910 whom the identified recipient should send signed receipts. The message 911 originator MUST populate the receiptsTo field with a GeneralNames for each 912 entity to whom the recipient should send the signed receipt.�If the message 913 originator wants the recipient to send the signed receipt to the 914 originator, then the originator MUST include a GeneralNames for itself in 915 the receiptsTo field. 917 2.8 Receipt Syntax 919 Receipts are represented using a new content type, Receipt. The Receipt 920 content type shall have ASN.1 type Receipt. Receipts must be encapsulated 921 within a SignedData message. 923 Receipt ::= SEQUENCE { 924 version Version, -- Version is imported from [CMS] 925 contentType ContentType, 926 signedContentIdentifier ContentIdentifier, 927 originatorSignatureValue OCTET STRING } 929 id-ct-receipt OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) 930 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-ct(1) 1} 932 The version field defines the syntax version number, which is 1 for this 933 version of the standard. 935 2.9 Content Hints 937 Many applications find it useful to have information that describes the 938 innermost signed content of a multi-layer message available on the 939 outermost signature layer. The contentHints attribute provides such 940 information. 942 Content-hints attribute values have ASN.1 type contentHints. 944 ContentHints ::= SEQUENCE { 945 � contentDescription UTF8String (SIZE (1..MAX)) OPTIONAL, 946 � contentType ContentType } 948 id-aa-contentHint OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) 949 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 4} 951 The contentDescription field may be used to provide information that the 952 recipient may use to select protected messages for processing, such as a 953 message subject. If this field is set, then the attribute is expected to 954 appear on the signedData object enclosing an envelopedData object and not 955 on the inner signedData object. The (SIZE (1..MAX)) construct constrains 956 the sequence to have at least one entry. MAX indicates the upper bound is 957 unspecified. Implementations are free to choose an upper bound that suits 958 their environment. 960 Messages which contain a signedData object wrapped around an envelopedData 961 object, thus masking the inner content type of the message, SHOULD include 962 a contentHints attribute, except for the case of the data content type. 963 Specific message content types may either force or preclude the inclusion 964 of the contentHints attribute. For example, when a signedData/Receipt is 965 encrypted within an envelopedData object, an outer signedData object MUST 966 be created that encapsulates the envelopedData object and a contentHints 967 attribute with contentType set to the id-ct-receipt object identifier MUST 968 be included in the outer signedData SignerInfo signedAttributes. 970 2.10� Message Signature Digest Attribute 972 The msgSigDigest attribute can only be used in the signed attributes of a 973 signed receipt.�It contains the digest of the ASN.1 DER encoded 974 signedAttributes included in the original signedData that requested the 975 signed receipt.�Only one msgSigDigest attribute can appear in an signed 976 attributes set. It is defined as follows: 978 msgSigDigest ::= OCTET STRING 980 id-aa-msgSigDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2) 981 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 5} 983 2.11 Signed Content Reference Attribute 985 The contentReference attribute is a link from one SignedData to another. It 986 may be used to link a reply to the original message to which it refers, or 987 to incorporate by reference one SignedData into another. The first 988 SignedData MUST include a contentIdentifier signed attribute, which SHOULD 989 be constructed as specified in section 2.7. The second SignedData links to 990 the first by including a ContentReference signed attribute containing the 991 content type, content identifier, and signature value from the first 992 SignedData. 994 ContentReference ::= SEQUENCE { 995 contentType ContentType, 996 signedContentIdentifier ContentIdentifier, 997 originatorSignatureValue OCTET STRING } 999 id-aa-contentReference OBJECT IDENTIFIER ::= { iso(1) member-body(2) 1000 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 10 } 1002 3. Security Labels 1004 This section describes the syntax to be used for security labels that can 1005 optionally be associated with S/MIME encapsulated data. A security label is 1006 a set of security information regarding the sensitivity of the content that 1007 is protected by S/MIME encapsulation. 1009 "Authorization" is the act of granting rights and/or privileges to users 1010 permitting them access to an object. "Access control" is a means of 1011 enforcing these authorizations. The sensitivity information in a security 1012 label can be compared with a user's authorizations to determine if the user 1013 is allowed to access the content that is protected by S/MIME encapsulation. 1015 Security labels may be used for other purposes such as a source of routing 1016 information. The labels are often priority based ("secret", "confidential", 1017 "restricted", and so on) or role-based, describing which kind of people can 1018 see the information ("patient's health-care team", "medical billing 1019 agents", "unrestricted", and so on). 1021 3.1 Security Label Processing Rules 1023 A sending agent may include a security label attribute in the signed 1024 attributes of a signedData object. A receiving agent examines the security 1025 label on a received message and determines whether or not the recipient is 1026 allowed to see the contents of the message. 1028 3.1.1 Adding Security Labels 1030 A sending agent that is using security labels MUST put the security label 1031 attribute in the signedAttributes field of a SignerInfo block. The security 1032 label attribute MUST NOT be included in the unsigned attributes. Integrity 1033 and authentication security services MUST be applied to the security label, 1034 therefore it MUST be included as an signed attribute, if used. This causes 1035 the security label attribute to be part of the data that is hashed to form 1036 the SignerInfo signature value. A SignerInfo block MUST NOT have more than 1037 one security label signed attribute. 1039 When there are multiple SignedData blocks applied to a message, a security 1040 label attribute may be included in either the inner signature, outer 1041 signature, or both. A security label signed attribute may be included in a 1042 signedAttributes field within the inner SignedData block. The inner 1043 security label will include the sensitivities of the original content and 1044 will be used for access control decisions related to the plaintext 1045 encapsulated content. The inner signature provides authentication of the 1046 inner security label and cryptographically protects the original signer's 1047 inner security label of the original content. 1049 When the originator signs the plaintext content and signed attributes, the 1050 inner security label is bound to the plaintext content. An intermediate 1051 entity cannot change the inner security label without invalidating the 1052 inner signature. The confidentiality security service can be applied to the 1053 inner security label by encrypting the entire inner signedData object 1054 within an EnvelopedData block. 1056 A security label signed attribute may also be included in a 1057 signedAttributes field within the outer SignedData block. The outer 1058 security label will include the sensitivities of the encrypted message and 1059 will be used for access control decisions related to the encrypted message 1060 and for routing decisions. The outer signature provides authentication of 1061 the outer security label (as well as for the encapsulated content which may 1062 include nested S/MIME messages). 1064 There can be multiple SignerInfos within a SignedData object, and each 1065 SignerInfo may include signedAttributes. Therefore, a single SignedData 1066 object may include multiple eSSSecurityLabels, each SignerInfo having an 1067 eSSSecurityLabel attribute. For example, an originator can send a signed 1068 message with two SignerInfos, one containing a DSS signature, the other 1069 containing an RSA signature. If any of the SignerInfos included in a 1070 SignedData object include an eSSSecurityLabel attribute, then all of the 1071 SignerInfos in that SignedData object MUST include an eSSSecurityLabel 1072 attribute and the value of each MUST be identical. 1074 3.1.2 Processing Security Labels 1076 Before processing an eSSSecurityLabel signedAttribute, the receiving agent 1077 MUST verify the signature of the SignerInfo which covers the 1078 eSSSecurityLabel attribute. A recipient MUST NOT process an 1079 eSSSecurityLabel attribute that has not been verified. 1081 A receiving agent MUST process the eSSSecurityLabel attribute, if present, 1082 in each SignerInfo in the SignedData object for which it verifies the 1083 signature. This may result in the receiving agent processing multiple 1084 eSSSecurityLabels included in a single SignedData object. Because all 1085 eSSSecurityLabels in a SignedData object must be identical, the receiving 1086 agent processes (such as performing access control) on the first 1087 eSSSecurityLabel that it encounters in a SignerInfo that it verifies, and 1088 then ensures that all other eSSSecurityLabels in signerInfos that it 1089 verifies are identical to the first one encountered. If the 1090 eSSSecurityLabels in the signerInfos that it verifies are not all 1091 identical, then the receiving agent MUST warn the user of this condition. 1093 Receiving agents SHOULD have a local policy regarding whether or not to 1094 show the inner content of a signedData object that includes an 1095 eSSSecurityLabel security-policy-identifier that the processing software 1096 does not recognize. If the receiving agent does not recognize the 1097 eSSSecurityLabel security-policy-identifier value, then it SHOULD stop 1098 processing the message and indicate an error. 1100 3.2 Syntax of eSSSecurityLabel 1102 The eSSSecurityLabel syntax is derived directly from [MTSABS] ASN.1 module. 1103 (The MTSAbstractService module begins with "DEFINITIONS IMPLICIT TAGS 1104 ::=".) Further, the eSSSecurityLabel syntax is compatible with that used in 1105 [MSP4]. 1107 ESSSecurityLabel ::= SET { 1108 security-policy-identifier SecurityPolicyIdentifier, 1109 security-classification SecurityClassification OPTIONAL, 1110 privacy-mark ESSPrivacyMark OPTIONAL, 1111 security-categories SecurityCategories OPTIONAL } 1113 id-aa-securityLabel OBJECT IDENTIFIER ::= { iso(1) member-body(2) 1114 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 2} 1116 SecurityPolicyIdentifier ::= OBJECT IDENTIFIER 1118 SecurityClassification ::= INTEGER { 1119 unmarked (0), 1120 unclassified (1), 1121 restricted (2), 1122 confidential (3), 1123 secret (4), 1124 top-secret (5) } (0..ub-integer-options) 1126 ub-integer-options INTEGER ::= 256 1128 ESSPrivacyMark ::= CHOICE { 1129 ��� pString����� PrintableString (SIZE (1..ub-privacy-mark-length)), 1130 ��� utf8String�� UTF8String (SIZE (1..MAX)) 1131 } 1133 ub-privacy-mark-length INTEGER ::= 128 1135 SecurityCategories ::= SET SIZE (1..ub-security-categories) OF 1136 SecurityCategory 1138 ub-security-categories INTEGER ::= 64 1140 SecurityCategory ::= SEQUENCE { 1141 type [0] OBJECT IDENTIFIER, 1142 value [1] ANY DEFINED BY type -- defined by type 1143 } 1145 --Note: The aforementioned SecurityCategory syntax produces identical 1146 --hex encodings as the following SecurityCategory syntax that is 1147 --documented in the X.411 specification: 1148 -- 1149 --SecurityCategory ::= SEQUENCE { 1150 -- type [0] SECURITY-CATEGORY, 1151 -- value [1] ANY DEFINED BY type } 1152 -- 1153 --SECURITY-CATEGORY MACRO ::= 1154 --BEGIN 1155 --TYPE NOTATION ::= type | empty 1156 --VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER) 1157 --END 1159 3.3 Security Label Components 1161 This section gives more detail on the the various components of the 1162 eSSSecurityLabel syntax. 1164 3.3.1 Security Policy Identifier 1166 A security policy is a set of criteria for the provision of security 1167 services. The eSSSecurityLabel security-policy-identifier is used to 1168 identify the security policy in force to which the security label relates. 1169 It indicates the semantics of the other security label components. 1171 3.3.2 Security Classification 1173 This specification defines the use of the Security Classification field 1174 exactly as is specified in the X.411 Recommendation, which states in part: 1176 If present, a security-classification may have one of a hierarchical 1177 list of values. The basic security-classification hierarchy is defined 1178 in this Recommendation, but the use of these values is defined by the 1179 security-policy in force. Additional values of security-classification, 1180 and their position in the hierarchy, may also be defined by a 1181 security-policy as a local matter or by bilateral agreement. The basic 1182 security-classification hierarchy is, in ascending order: unmarked, 1183 unclassified, restricted, confidential, secret, top-secret. 1185 This means that the security policy in force (identified by the 1186 eSSSecurityLabel security-policy-identifier) defines the 1187 SecurityClassification integer values and their meanings. 1189 An organization can develop its own security policy that defines the 1190 SecurityClassification INTEGER values and their meanings. However, the 1191 general interpretation of the X.411 specification is that the values of 0 1192 through 5 are reserved for the "basic hierarchy" values of unmarked, 1193 unclassified, restricted, confidential, secret, and top-secret. Note that 1194 X.411 does not provide the rules for how these values are used to label 1195 data and how access control is performed using these values. 1197 There is no universal definition of the rules for using these "basic 1198 hierarchy" values. Each organization (or group of organizations) will 1199 define a security policy which documents how the "basic hierarchy" values 1200 are used (if at all) and how access control is enforced (if at all) within 1201 their domain. 1203 Therefore, the security-classification value MUST be accompanied by a 1204 security-policy-identifier value to define the rules for its use. For 1205 example, a company's "secret" classification may convey a different meaning 1206 than the US Government "secret" classification. In summary, a security 1207 policy SHOULD NOT use integers 0 through 5 for other than their X.411 1208 meanings, and SHOULD instead use other values in a hierarchical fashion. 1210 Note that the set of valid security-classification values MUST be 1211 hierarchical, but these values do not necessarily need to be in ascending 1212 numerical order. Further, the values do not need to be contiguous. 1214 For example, in the Defense Message System 1.0 security policy, the 1215 security-classification value of 11 indicates Sensitive-But-Unclassified 1216 and 5 indicates top-secret. The hierarchy of sensitivity ranks top-secret 1217 as more sensitive than Sensitive-But-Unclassified even though the numerical 1218 value of top-secret is less than Sensitive-But-Unclassified. 1220 (Of course, if security-classification values are both hierarchical and in 1221 ascending order, a casual reader of the security policy is more likely to 1222 understand it.) 1224 An example of a security policy that does not use any of the X.411 values 1225 might be: 1226 10 -- anyone 1227 15 -- Morgan Corporation and its contractors 1228 20 -- Morgan Corporation employees 1229 25 -- Morgan Corporation board of directors 1231 An example of a security policy that uses part of the X.411 hierarchy might 1232 be: 1233 0 -- unmarked 1234 1 -- unclassified, can be read by everyone 1235 2 -- restricted to Timberwolf Productions staff 1236 6 -- can only be read to Timberwolf Productions executives 1238 3.3.3 Privacy Mark 1240 If present, the eSSSecurityLabel privacy-mark is not used for access 1241 control. The content of the eSSSecurityLabel privacy-mark may be defined by 1242 the security policy in force (identified by the eSSSecurityLabel 1243 security-policy-identifier) which may define a list of values to be used. 1244 Alternately, the value may be determined by the originator of the 1245 security-label. 1247 3.3.4 Security Categories 1249 If present, the eSSSecurityLabel security-categories provide further 1250 granularity for the sensitivity of the message. The security policy in 1251 force (identified by the eSSSecurityLabel security-policy-identifier) is 1252 used to indicate the syntaxes that are allowed to be present in the 1253 eSSSecurityLabel security-categories. Alternately, the security-categories 1254 and their values may be defined by bilateral agreement. 1256 3.4 Equivalent Security Labels 1258 Because organizations are allowed to define their own security policies, 1259 many different security policies will exist. Some organizations may wish to 1260 create equivalencies between their security policies with the security 1261 policies of other organizations. For example, the Acme Company and the 1262 Widget Corporation may reach a bilateral agreement that the "Acme private" 1263 security-classification value is equivalent to the "Widget sensitive" 1264 security-classification value. 1266 Receiving agents MUST NOT process an equivalentLabels attribute in a 1267 message if the agent does not trust the signer of that attribute to 1268 translate the original eSSSecurityLabel values to the security policy 1269 included in the equivalentLabels attribute. Receiving agents have the 1270 option to process equivalentLabels attributes but do not have to. It is 1271 acceptable for a receiving agent to only process eSSSecurityLabels. All 1272 receiving agents SHOULD recognize equivalentLabels attributes even if they 1273 do not process them. 1275 3.4.1 Creating Equivalent Labels 1277 The EquivalentLabels signed attribute is defined as: 1279 EquivalentLabels ::= SEQUENCE OF ESSSecurityLabel 1281 id-aa-equivalentLabels OBJECT IDENTIFIER ::= { iso(1) member-body(2) 1282 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 9} 1284 As stated earlier, the ESSSecurityLabel contains the sensitivity values 1285 selected by the original signer of the signedData. If an ESSSecurityLabel 1286 is present in a signerInfo, all signerInfos in the signedData MUST contain 1287 an ESSSecurityLabel and they MUST all be identical. In addition to an 1288 ESSSecurityLabel, a signerInfo MAY also include an equivalentLabels signed 1289 attribute. If present, the equivalentLabels attribute MUST include one or 1290 more security labels that are believed by the signer to be semantically 1291 equivalent to the ESSSecurityLabel attribute included in the same 1292 signerInfo. 1294 All security-policy object identifiers MUST be unique in the set of 1295 ESSSecurityLabel and EquivalentLabels security labels. Before using an 1296 EquivalentLabels attribute, a receiving agent MUST ensure that all 1297 security-policy OIDs are unique in the security label or labels included in 1298 the EquivalentLabels. Once the receiving agent selects the security label 1299 (within the EquivalentLabels) to be used for processing, then the 1300 security-policy OID of the selected EquivalentLabels security label MUST be 1301 compared with the ESSSecurityLabel security-policy OID to ensure that they 1302 are unique. 1304 In the case that an ESSSecurityLabel attribute is not included in a 1305 signerInfo, then an EquivalentLabels attribute may still be included. For 1306 example, in the Acme security policy, the absence of an ESSSecurityLabel 1307 could be defined to equate to a security label composed of the Acme 1308 security-policy OID and the "unmarked" security-classification. 1310 Note that equivalentLabels MUST NOT be used to convey security labels that 1311 are semantically different from the ESSSecurityLabel included in the 1312 signerInfos in the signedData. If an entity needs to apply a security label 1313 that is semantically different from the ESSSecurityLabel, then it MUST 1314 include the sematically different security label in an outer signedData 1315 object that encapsulates the signedData object that includes the 1316 ESSSecurityLabel. 1318 If present, the equivalentLabels attribute MUST be an signed attribute; it 1319 MUST NOT be an unsigned attribute. [CMS] defines signedAttributes as a SET 1320 OF Attribute. A signerInfo MUST NOT include multiple instances of the 1321 equivalentLabels attribute. CMS defines the ASN.1 syntax for the signed 1322 attributes to include attrValues SET OF AttributeValue. A equivalentLabels 1323 attribute MUST only include a single instance of AttributeValue. There MUST 1324 NOT be zero or multiple instances of AttributeValue present in the 1325 attrValues SET OF AttributeValue. 1327 3.4.2 Processing Equivalent Labels 1329 A receiving agent SHOULD process the ESSSecurityLabel before processing any 1330 EquivalentLabels. If the policy in the ESSSecurityLabel is understood by 1331 the receiving agent, it MUST process that label and MUST ignore all 1332 EquivalentLabels. 1334 When processing an EquivalentLabels attribute, the receiving agent MUST 1335 validate the signature on the EquivalentLabels attribute. A receiving agent 1336 MUST NOT act on an equivalentLabels attribute for which the signature could 1337 not be validated, and MUST NOT act on an equivalentLabels attribute unless 1338 that attribute is signed by an entity trusted to translate the original 1339 eSSSecurityLabel values to the security policy included in the 1340 equivalentLabels attribute. Determining who is allowed to specify 1341 equivalence mappings is a local policy. If a message has more than one 1342 EquivalentLabels attribute, the receiving agent SHOULD process the first 1343 one that it reads and validates that contains the security policy of 1344 interest to the receiving agent. 1346 4. Mail List Management 1348 Sending agents must create recipient-specific data structures for each 1349 recipient of an encrypted message. This process can impair performance for 1350 messages sent to a large number of recipients. Thus, Mail List Agents 1351 (MLAs) that can take a single message and perform the recipient-specific 1352 encryption for every recipient are often desired. 1354 An MLA appears to the message originator as a normal message recipient, but 1355 the MLA acts as a message expansion point for a Mail List (ML). The sender 1356 of a message directs the message to the MLA, which then redistributes the 1357 message to the members of the ML. This process offloads the per-recipient 1358 processing from individual user agents and allows for more efficient 1359 management of large MLs. MLs are true message recipients served by MLAs 1360 that provide cryptographic and expansion services for the mailing list. 1362 In addition to cryptographic handling of messages, secure mailing lists 1363 also have to prevent mail loops. A mail loop is where one mailing list is a 1364 member of a second mailing list, and the second mailing list is a member of 1365 the first. A message will go from one list to the other in a 1366 rapidly-cascading succession of mail that will be distributed to all other 1367 members of both lists. 1369 To prevent mail loops, MLAs use the mlExpansionHistory attribute of the 1370 outer signature of a triple wrapped message. The mlExpansionHistory 1371 attribute is essentially a list of every MLA that has processed the 1372 message. If an MLA sees its own unique entity identifier in the list, it 1373 knows that a loop has been formed, and does not send the message to the 1374 list again. 1376 4.1 Mail List Expansion 1378 Mail list expansion processing is noted in the value of the 1379 mlExpansionHistory attribute, located in the signed attributes of the MLA's 1380 SignerInfo block. The MLA creates or updates the signed mlExpansionHistory 1381 attribute value each time the MLA expands and signs a message for members 1382 of a mail list. 1384 The MLA MUST add an MLData record containing the MLA's identification 1385 information, date and time of expansion, and optional receipt policy to the 1386 end of the mail list expansion history sequence. If the mlExpansionHistory 1387 attribute is absent, then the MLA MUST add the attribute and the current 1388 expansion becomes the first element of the sequence. If the 1389 mlExpansionHistory attribute is present, then the MLA MUST add the current 1390 expansion information to the end of the existing MLExpansionHistory 1391 sequence. Only one mlExpansionHistory attribute can be included in the 1392 signedAttributes of a SignerInfo. 1394 Note that if the mlExpansionHistory attribute is absent, then the recipient 1395 is a first tier message recipient. 1397 There can be multiple SignerInfos within a SignedData object, and each 1398 SignerInfo may include signedAttributes. Therefore, a single SignedData 1399 object may include multiple SignerInfos, each SignerInfo having a 1400 mlExpansionHistory attribute. For example, an MLA can send a signed message 1401 with two SignerInfos, one containing a DSS signature, the other containing 1402 an RSA signature. 1404 If an MLA creates a SignerInfo that includes an mlExpansionHistory 1405 attribute, then all of the SignerInfos created by the MLA for that 1406 SignedData object MUST include an mlExpansionHistory attribute, and the 1407 value of each MUST be identical. Note that other agents might later add 1408 SignerInfo attributes to the SignedData block, and those additional 1409 SignerInfos might not include mlExpansionHistory attributes. 1411 A recipient MUST verify the signature of the SignerInfo which covers the 1412 mlExpansionHistory attribute before processing the mlExpansionHistory, and 1413 MUST NOT process the mlExpansionHistory attribute unless the signature over 1414 it has been verified. If a SignedData object has more than one SignerInfo 1415 that has an mlExpansionHistory attribute, the recipient MUST compare the 1416 mlExpansionHistory attributes in all the SignerInfos that it has verified, 1417 and MUST NOT process the mlExpansionHistory attribute unless every verified 1418 mlExpansionHistory attribute in the SignedData block is identical. If the 1419 mlExpansionHistory attributes in the verified signerInfos are not all 1420 identical, then the receiving agent MUST stop processing the message and 1421 SHOULD notify the user or MLA administrator of this error condition. In the 1422 mlExpansionHistory processing, SignerInfos that do not have an 1423 mlExpansionHistory attribute are ignored. 1425 4.1.1 Detecting Mail List Expansion Loops 1427 Prior to expanding a message, the MLA examines the value of any existing 1428 mail list expansion history attribute to detect an expansion loop. An 1429 expansion loop exists when a message expanded by a specific MLA for a 1430 specific mail list is redelivered to the same MLA for the same mail list. 1432 Expansion loops are detected by examining the mailListIdentifier field of 1433 each MLData entry found in the mail list expansion history. If an MLA finds 1434 its own identification information, then the MLA must discontinue expansion 1435 processing and should provide warning of an expansion loop to a human mail 1436 list administrator. The mail list administrator is responsible for 1437 correcting the loop condition. 1439 4.2 Mail List Agent Processing 1441 The first few paragraphs of this section provide a high-level description 1442 of MLA processing. The rest of the section provides a detailed description 1443 of MLA processing. 1445 MLA message processing depends on the structure of the S/MIME layers in the 1446 message sent to the MLA for expansion. In addition to sending triple 1447 wrapped messages to an MLA, an entity can send other types of messages to 1448 an MLA, such as: 1449 - a single wrapped signedData or envelopedData message 1450 - a double wrapped message (such as signed and enveloped, enveloped and 1451 signed, or signed and signed, and so on) 1452 - a quadruple-wrapped message (such as if a well-formed triple wrapped 1453 message was sent through a gateway that added an outer SignedData layer) 1455 In all cases, the MLA MUST parse all layers of the received message to 1456 determine if there are any signedData layers that include an 1457 eSSSecurityLabel signedAttribute. This may include decrypting an 1458 EnvelopedData layer to determine if an encapsulated SignedData layer 1459 includes an eSSSecurityLabel attribute. The MLA MUST fully process each 1460 eSSSecurityLabel attribute found in the various signedData layers, 1461 including performing access control checks, before distributing the message 1462 to the ML members. The details of the access control checks are beyond the 1463 scope of this document. The MLA MUST verify the signature of the signerInfo 1464 including the eSSSecurityLabel attribute before using it. 1466 In all cases, the MLA MUST sign the message to be sent to the ML members in 1467 a new "outer" signedData layer. The MLA MUST add or update an 1468 mlExpansionHistory attribute in the "outer" signedData that it creates to 1469 document MLA processing. If there was an "outer" signedData layer included 1470 in the original message received by the MLA, then the MLA-created "outer" 1471 signedData layer MUST include each signed attribute present in the 1472 original "outer" signedData layer, unless the MLA explicitly replaces an 1473 attribute (such as signingTime or mlExpansionHistory) with a new value. 1475 When an S/MIME message is received by the MLA, the MLA MUST first determine 1476 which received signedData layer, if any, is the "outer" signedData layer. 1477 To identify the received "outer" signedData layer, the MLA MUST verify the 1478 signature and fully process the signedAttributes in each of the 1479 outer signedData layers (working from the outside in) to determine if any 1480 of them either include an mlExpansionHistory attribute or encapsulate an 1481 envelopedData object. 1483 The MLA's search for the "outer" signedData layer is completed when it 1484 finds one of the following: 1485 - the "outer" signedData layer that includes an mlExpansionHistory 1486 attribute or encapsulates an envelopedData object 1487 - an envelopedData layer 1488 - the original content (that is, a layer that is neither envelopedData nor 1489 signedData). 1491 If the MLA finds an "outer" signedData layer, then the MLA MUST perform 1492 the following steps: 1494 1. Strip off all of the signedData layers that encapsulated the "outer" 1495 signedData layer 1497 2. Strip off the "outer" signedData layer itself (after remembering the 1498 included signedAttributes) 1500 3. Expand the envelopedData (if present) 1502 4. Sign the message to be sent to the ML members in a new "outer" 1503 signedData layer that includes the signedAttributes (unless explicitly 1504 replaced) from the original, received "outer" signedData layer. 1506 If the MLA finds an "outer" signedData layer that includes an 1507 mlExpansionHistory attribute AND the MLA subsequently finds an 1508 envelopedData layer buried deeper with the layers of the received message, 1509 then the MLA MUST strip off all of the signedData layers down to the 1510 envelopedData layer (including stripping off the original "outer" 1511 signedData layer) and MUST sign the expanded envelopedData in a new "outer" 1512 signedData layer that includes the signedAttributes (unless explicitly 1513 replaced) from the original, received "outer" signedData layer. 1515 If the MLA does not find an "outer" signedData layer AND does not find an 1516 envelopedData layer, then the MLA MUST sign the original, received message 1517 in a new "outer" signedData layer. If the MLA does not find an "outer" 1518 signedData AND does find an envelopedData layer then it MUST expand the 1519 envelopedData layer, if present, and sign it in a new "outer" signedData 1520 layer. 1522 4.2.1 Examples of Rule Processing 1524 The following examples help explain the rules above: 1526 1) A message (S1(Original Content)) (where S = SignedData) is sent to the 1527 MLA in which the signedData layer does not include an MLExpansionHistory 1528 attribute. The MLA verifies and fully processes the signedAttributes in S1. 1529 The MLA decides that there is not an original, received "outer" signedData 1530 layer since it finds the original content, but never finds an envelopedData 1531 and never finds an mlExpansionHistory attribute. The MLA calculates a new 1532 signedData layer, S2, resulting in the following message sent to the ML 1533 recipients: (S2(S1(Original Content))). The MLA includes an 1534 mlExpansionHistory attribute in S2. 1536 2) A message (S3(S2(S1(Original Content)))) is sent to the MLA in which 1537 none of the signedData layers includes an MLExpansionHistory attribute. The 1538 MLA verifies and fully processes the signedAttributes in S3, S2 and S1. The 1539 MLA decides that there is not an original, received "outer" signedData 1540 layer since it finds the original content, but never finds an envelopedData 1541 and never finds an mlExpansionHistory attribute. The MLA calculates a new 1542 signedData layer, S4, resulting in the following message sent to the ML 1543 recipients: (S4(S3(S2(S1(Original Content))))). The MLA includes an 1544 mlExpansionHistory attribute in S4. 1546 3) A message (E1(S1(Original Content))) (where E = envelopedData) is sent 1547 to the MLA in which S1 does not include an MLExpansionHistory attribute. 1548 The MLA decides that there is not an original, received "outer" signedData 1549 layer since it finds the E1 as the outer layer. The MLA expands the 1550 recipientInformation in E1. The MLA calculates a new signedData layer, S2, 1551 resulting in the following message sent to the ML recipients: 1552 (S2(E1(S1(Original Content)))). The MLA includes an mlExpansionHistory 1553 attribute in S2. 1555 4) A message (S2(E1(S1(Original Content)))) is sent to the MLA in which S2 1556 includes an MLExpansionHistory attribute. The MLA verifies the signature 1557 and fully processes the signedAttributes in S2. The MLA finds the 1558 mlExpansionHistory attribute in S2, so it decides that S2 is the "outer" 1559 signedData. The MLA remembers the signedAttributes included in S2 for later 1560 inclusion in the new outer signedData that it applies to the message. The 1561 MLA strips off S2. The MLA then expands the recipientInformation in E1 1562 (this invalidates the signature in S2 which is why it was stripped). The 1563 MLA calculates a new signedData layer, S3, resulting in the following 1564 message sent to the ML recipients: (S3(E1(S1(Original Content)))). The MLA 1565 includes in S3 the attributes from S2 (unless it specifically replaces an 1566 attribute value) including an updated mlExpansionHistory attribute. 1568 5) A message (S3(S2(E1(S1(Original Content))))) is sent to the MLA in which 1569 none of the signedData layers include an MLExpansionHistory attribute. The 1570 MLA verifies the signature and fully processes the signedAttributes in S3 1571 and S2. When the MLA encounters E1, then it decides that S2 is the "outer" 1572 signedData since S2 encapsulates E1. The MLA remembers the signedAttributes 1573 included in S2 for later inclusion in the new outer signedData that it 1574 applies to the message. The MLA strips off S3 and S2. The MLA then expands 1575 the recipientInformation in E1 (this invalidates the signatures in S3 and 1576 S2 which is why they were stripped). The MLA calculates a new signedData 1577 layer, S4, resulting in the following message sent to the ML recipients: 1578 (S4(E1(S1(Original Content)))). The MLA includes in S4 the attributes from 1579 S2 (unless it specifically replaces an attribute value) and includes a new 1580 mlExpansionHistory attribute. 1582 6) A message (S3(S2(E1(S1(Original Content))))) is sent to the MLA in which 1583 S3 includes an MLExpansionHistory attribute. In this case, the MLA verifies 1584 the signature and fully processes the signedAttributes in S3. The MLA finds 1585 the mlExpansionHistory in S3, so it decides that S3 is the "outer" 1586 signedData. The MLA remembers the signedAttributes included in S3 for later 1587 inclusion in the new outer signedData that it applies to the message. The 1588 MLA keeps on parsing encapsulated layers because it must determine if there 1589 are any eSSSecurityLabel attributes contained within. The MLA verifies the 1590 signature and fully processes the signedAttributes in S2. When the MLA 1591 encounters E1, then it strips off S3 and S2. The MLA then expands the 1592 recipientInformation in E1 (this invalidates the signatures in S3 and S2 1593 which is why they were stripped). The MLA calculates a new signedData 1594 layer, S4, resulting in the following message sent to the ML recipients: 1595 (S4(E1(S1(Original Content)))). The MLA includes in S4 the attributes from 1596 S3 (unless it specifically replaces an attribute value) including an 1597 updated mlExpansionHistory attribute. 1599 4.2.3 Processing Choices 1601 The processing used depends on the type of the outermost layer of the 1602 message. There are three cases for the type of the outermost data: 1603 - EnvelopedData 1604 - SignedData 1605 - data 1607 4.2.3.1 Processing for EnvelopedData 1609 1. The MLA locates its own RecipientInfo and uses the information it 1610 contains to obtain the message key. 1612 2. The MLA removes the existing recipientInfos field and replaces it with a 1613 new recipientInfos value built from RecipientInfo structures created for 1614 each member of the mailing list. The MLA also removes the existing 1615 originatorInfo field and replaces it with a new originatorInfo value built 1616 from information describing the MLA. 1618 3. The MLA encapsulates the expanded encrypted message in a SignedData 1619 block, adding an mlExpansionHistory attribute as described in the "Mail 1620 List Expansion" section to document the expansion. 1622 4. The MLA signs the new message and delivers the updated message to mail 1623 list members to complete MLA processing. 1625 4.2.3.2 Processing for SignedData 1627 MLA processing of multi-layer messages depends on the type of data in each 1628 of the layers. Step 3 below specifies that different processing will take 1629 place depending on the type of CMS message that has been signed. That is, 1630 it needs to know the type of data at the next inner layer, which may or may 1631 not be the innermost layer. 1633 1. The MLA verifies the signature value found in the outermost SignedData 1634 layer associated with the signed data. MLA processing of the message 1635 terminates if the message signature is invalid. 1637 2. If the outermost SignedData layer includes an signed mlExpansionHistory 1638 attribute, the MLA checks for an expansion loop as described in the 1639 "Detecting Mail List Expansion Loops" section, then go to step 3. If the 1640 outermost SignedData layer does not include an signed mlExpansionHistory 1641 attribute, go directly to step 4. 1643 3. Determine the type of the data that has been signed. That is, look at 1644 the type of data on the layer just below the SignedData, which may or may 1645 not be the "innermost" layer. Based on the type of data, perform either 1646 step 3.1 (EnvelopedData), step 3.2 (SignedData), or step 3.3 (all other 1647 types). 1649 3.1. If the signed data is EnvelopedData, the MLA performs expansion 1650 processing of the encrypted message as described previously. Note that 1651 this process invalidates the signature value in the outermost 1652 SignedData layer associated with the original encrypted message. 1653 Proceed to section 3.2 with the result of the expansion. 1655 3.2. If the signed data is SignedData, or is the result of expanding an 1656 EnvelopedData block in step 3.1: 1658 3.2.1. The MLA strips the existing outermost SignedData layer after 1659 remembering the value of the mlExpansionHistory and all other 1660 signed attributes in that layer, if present. 1662 3.2.2. If the signed data is EnvelopedData (from step 3.1), the MLA 1663 encapsulates the expanded encrypted message in a new outermost 1664 SignedData layer. On the other hand, if the signed data is 1665 SignedData (from step 3.2), the MLA encapsulates the signed data in 1666 a new outermost SignedData layer. 1668 3.2.3. The outermost signedData layer created by the MLA replaces 1669 the original outermost signedData layer.�The MLA MUST create an 1670 signed attribute list for the new outermost signedData layer which 1671 MUST include each signed attribute present in the original 1672 outermost signedData layer, unless the MLA explicitly replaces one 1673 or more particular attributes with new value.�A special case is the 1674 mlExpansionHistory attribute.�The MLA MUST add an 1675 mlExpansionHistory signed attribute to the outer signedData layer 1676 as follows: 1678 3.2.3.1. If the original outermost SignedData layer included an 1679 mlExpansionHistory attribute, the attribute's value is copied 1680 and updated with the current ML expansion information as 1681 described in the "Mail List Expansion" section. 1683 3.2.3.2. If the original outermost SignedData layer did not 1684 include an mlExpansionHistory attribute, a new attribute value 1685 is created with the current ML expansion information as 1686 described in the "Mail List Expansion" section. 1688 3.3. If the signed data is not EnvelopedData or SignedData: 1690 3.3.1. The MLA encapsulates the received signedData object in an 1691 outer SignedData object, and adds an mlExpansionHistory attribute 1692 to the outer SignedData object containing the current ML expansion 1693 information as described in the "Mail List Expansion" section. 1695 4. The MLA signs the new message and delivers the updated message to mail 1696 list members to complete MLA processing. 1698 A flow chart for the above steps would be: 1700 1. Has a valid signature? 1701 YES -> 2. 1702 NO -> STOP. 1703 2. Does outermost SignedData layer contain mlExpansionHistory? 1704 YES -> Check it, then -> 3. 1705 NO -> 4. 1706 3. Check type of data just below outermost 1707 SignedData. 1708 EnvelopedData -> 3.1. 1709 SignedData -> 3.2. 1710 all others -> 3.3. 1711 3.1. Expand the encrypted message, then -> 3.2. 1712 3.2. -> 3.2.1. 1713 3.2.1. Strip outermost SignedData layer, note value of mlExpansionHistory 1714 and other signed attributes, then -> 3.2.2. 1715 3.2.2. Encapsulate in new signature, then -> 3.2.3. 1716 3.2.3. Create new signedData layer. Was there an old mlExpansionHistory? 1717 YES -> copy the old mlExpansionHistory values, then -> 4. 1718 NO -> create new mlExpansionHistory value, then -> 4. 1719 3.3. Encapsulate in a SignedData layer and add an mlExpansionHistory 1720 attribute, then -> 4. 1721 4. Sign message, deliver it, STOP. 1723 4.2.3.3 Processing for data 1725 1. The MLA encapsulates the message in a SignedData layer, and adds an 1726 mlExpansionHistory attribute containing the current ML expansion 1727 information as described in the "Mail List Expansion" section. 1729 2. The MLA signs the new message and delivers the updated message to mail 1730 list members to complete MLA processing. 1732 4.3 Mail List Agent Signed Receipt Policy Processing 1734 If a mailing list (B) is a member of another mailing list (A), list B often 1735 needs to propagate forward the mailing list receipt policy of A. As a 1736 general rule, a mailing list should be conservative in propagating forward 1737 the mailing list receipt policy because the ultimate recipient need only 1738 process the last item in the ML expansion history. The MLA builds the 1739 expansion history to meet this requirement. 1741 The following table describes the outcome of the union of mailing list A's 1742 policy (the rows in the table) and mailing list B's policy (the columns in 1743 the table). 1745 | B's policy 1746 A's policy | none insteadOf inAdditionTo missing 1747 ------------------------------------------------------------------------- 1748 none | none none none none 1749 insteadOf | none insteadOf(B) *1 insteadOf(A) 1750 inAdditionTo | none insteadOf(B) *2 inAdditionTo(A) 1751 missing | none insteadOf(B) inAdditionTo(B) missing 1753 *1 = insteadOf(insteadOf(A) + inAdditionTo(B)) 1754 *2 = inAdditionTo(inAdditionTo(A) + inAdditionTo(B)) 1756 4.4 Mail List Expansion History Syntax 1758 An mlExpansionHistory attribute value has ASN.1 type MLExpansionHistory. If 1759 there are more than ub-ml-expansion-history mailing lists in the sequence, 1760 the receiving agent should provide notification of the error to a human 1761 mail list administrator.�The mail list administrator is responsible for 1762 correcting the overflow condition. 1764 MLExpansionHistory ::= SEQUENCE 1765 SIZE (1..ub-ml-expansion-history) OF MLData 1767 id-aa-mlExpandHistory OBJECT IDENTIFIER ::= { iso(1) member-body(2) 1768 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 3} 1770 ub-ml-expansion-history INTEGER ::= 64 1772 MLData contains the expansion history describing each MLA that has 1773 processed a message. As an MLA distributes a message to members of an ML, 1774 the MLA records its unique identifier, date and time of expansion, and 1775 receipt policy in an MLData structure. 1777 MLData ::= SEQUENCE { 1778 mailListIdentifier EntityIdentifier, 1779 -- EntityIdentifier is imported from [CMS] 1780 expansionTime GeneralizedTime, 1781 mlReceiptPolicy MLReceiptPolicy OPTIONAL } 1783 EntityIdentifier ::= CHOICE { 1784 issuerAndSerialNumber IssuerAndSerialNumber, 1785 subjectKeyIdentifier SubjectKeyIdentifier } 1787 The receipt policy of the ML can withdraw the originator's request for the 1788 return of a signed receipt. However, if the originator of the message has 1789 not requested a signed receipt, the MLA cannot request a signed receipt. In 1790 the event that a ML's signed receipt policy supersedes the originator's 1791 request for signed receipts, such that the originator will not receive any 1792 signed receipts, then the MLA MAY inform the originator of that fact. 1794 When present, the mlReceiptPolicy specifies a receipt policy that 1795 supersedes the originator's request for signed receipts. The policy can be 1796 one of three possibilities: receipts MUST NOT be returned (none); receipts 1797 should be returned to an alternate list of recipients, instead of to the 1798 originator (insteadOf); or receipts should be returned to a list of 1799 recipients in addition to the originator (inAdditionTo). 1801 MLReceiptPolicy ::= CHOICE { 1802 none [0] NULL, 1803 insteadOf [1] SEQUENCE SIZE (1..MAX) OF GeneralNames, 1804 inAdditionTo [2] SEQUENCE SIZE (1..MAX) OF GeneralNames } 1806 5. Signing Certificate Attribute 1808 Concerns have been raised over the fact that the certificate which the 1809 signer of a CMS SignedData object desired to be bound into the verification 1810 process of the SignedData object is not cryptographically bound into the 1811 signature itself. This section addresses this issue by creating a new 1812 attribute to be placed in the signed attributes section of a SignerInfo 1813 object. 1815 This section also presents a description of a set of possible attacks 1816 dealing with the substitution of one certificate to verify the signature 1817 for the desired certificate. A set of ways for preventing or addressing 1818 these attacks is presented to deal with the simplest of the attacks. 1820 Attribute certificates can be used as part of a signature verification 1821 process. There is no way in CMS to include the list of attribute 1822 certificates to be used in the verification process. The set of attribute 1823 certificates used in the signature verification process needs to have the 1824 ability for the signer to restrict the set of certificates. This 1825 information needs to be encoded in a manner that is covered by the 1826 signature on the SignedData object. The methods in this section allows for 1827 the set of attribute certificates to be listed as part of the signing 1828 certificate attribute. 1830 Explicit certificate policies can also be used as part of a signature 1831 verification process. If a signer desires to state an explicit certificate 1832 policy that should be used when validating the signature, that policy needs 1833 to be cryptographically bound into the signing process. The methods 1834 described in this section allows for a set of certificate policy statements 1835 to be listed as part of the signing certificate attribute. 1837 5.1. Attack Descriptions 1839 At least three different attacks can be launched against a possible 1840 signature verification process by changing the certificate or certficates 1841 used in the signature verification process. 1843 5.1.1 Substitution Attack Description 1845 The first attack deals with simple substitution of one certificate for 1846 another certificate. In this attack, the issuer and serial number in the 1847 SignerInfo is modified to refer to a new certificate. This new certificate 1848 is used during the signature verification process. 1850 The first version of this attack is a simple denial of service attack where 1851 an invalid certificate is substituted for the valid certificate. This 1852 renders the message unverifiable, as the public key in the certificate no 1853 longer matches the private key used to sign the message. 1855 The second version is a substitution of one valid certificate for the 1856 original valid certificate where the public keys in the certificates match. 1857 This allows the signature to be validated under potentially different 1858 certificate constraints than the originator of the message intended. 1860 5.1.2 Reissue of Certificate Description 1862 The second attack deals with a certificate authority (CA) re-issuing the 1863 signing certificate (or potentially one of its certificates). This attack 1864 may start becoming more frequent as Certificate Authorities reissue their 1865 own root certificates, or as certificate authorities change policies in the 1866 certificate while reissuing their root certificates. This problem also 1867 occurs when cross certificates (with potentially different restrictions) 1868 are used in the process of verifying a signature. 1870 5.1.3 Rogue Duplicate CA Description 1872 The third attack deals with a rogue entity setting up a certificate 1873 authority that attempts to duplicate the structure of an existing CA. 1874 Specifically, the rogue entity issues a new certificate with the same 1875 public keys as the signer used, but signed by the rogue entity's private 1876 key. 1878 5.2 Attack Responses 1880 This document does not attempt to solve all of the above attacks; however, 1881 a brief description of responses to each of the attacks is given in this 1882 section. 1884 5.2.1 Substitution Attack Response 1886 The denial of service attack cannot be prevented. After the certificate 1887 identifier has been modified in transit, no verification of the signature 1888 is possible. There is also no way to automatically identify the attack 1889 because it is indistinguishable from a message corruption. 1891 The substitution of a valid certificate can be responded to in two 1892 different manners. The first is to make a blanket statement that the use of 1893 the same public key in two different certificates is bad practice and has 1894 to be avoided. In practice, there is no practical way to prevent users from 1895 getting new certificates with the same public keys, and it should be 1896 assumed that they will do this. Section 5.4 provides a new attribute that 1897 can be included in the SignerInfo signed attributes. This binds the correct 1898 certificate identifier into the signature. This will convert the attack 1899 from a potentially successful one to simply a denial of service attack. 1901 5.2.2 Reissue of Certificate Response 1903 A CA should never reissue a certificate with different attributes. 1904 Certificate Authorities that do so are following poor practices and cannot 1905 be relied on. Using the hash of the certificate as the reference to the 1906 certificate prevents this attach for end-entity certificates. 1908 Preventing the attack based on reissuing of CA certificates would require a 1909 substantial change to the usage of the signingCertificate attribute 1910 presented in section 5.4. It would require that ESSCertIDs would need to be 1911 included in the attribute to represent the issuer certificates in the 1912 signer's certification path. This presents problems when the relying party 1913 is using a cross-certificate as part of its authentication process, and 1914 this certificate does not appear on the list of certificates. The problems 1915 outside of a closed PKI make the addition of this information prone to 1916 error, possibly causing the rejection of valid chains. 1918 5.2.3 Rogue Duplicate CA Response 1920 The best method of preventing this attack is to avoid trusting the rogue 1921 CA. The use of the hash to identify certificates prevents the use of 1922 end-entity certificates from the rogue authority. However the only true way 1923 to prevent this attack is to never trust the rogue CA. 1925 5.3 Related Signature Verification Context 1927 Some applications require that additional information be used as part of 1928 the signature validation process. In particular, attribute certificates and 1929 policy identifiers provide additional information about the abilities and 1930 intent of the signer. The signing certificate attribute described in 1931 Section 5.4 provides the ability to bind this context information as part 1932 of the signature. 1934 5.3.1 Attribute Certificates 1936 Some applications require that attribute certificates be validated. This 1937 validation requires that the application be able to find the correct 1938 attribute certificates to perform the verification process; however there 1939 is no list of attribute certificates in a SignerInfo object. The sender has 1940 the ability to include a set of attribute certificates in a SignedData 1941 object. The receiver has the ability to retrieve attribute certificates 1942 from a directory service. There are some circumstances where the signer may 1943 wish to limit the set of attribute certificates that may be used in 1944 verifying a signature. It is useful to be able to list the set of attribute 1945 certificates the signer wants the recipient to use in validating the 1946 signature. 1948 5.3.2 Policy Information 1950 A related aspect of the certificate binding is the issue of multiple 1951 certification paths. In some instances, the semantics of a certificate in 1952 its use with a message may depend on the Certificate Authorities and 1953 policies that apply. To address this issue, the signer may also wish to 1954 bind that context under the signature. While this could be done by either 1955 signing the complete certification path or a policy ID, only a binding for 1956 the policy ID is described here. 1958 5.4 Signing Certificate Attribute Definition 1960 The signing certificate attribute is designed to prevent the simple 1961 substitution and re-issue attacks, and to allow for a restricted set of 1962 attribute certificates to be used in verifying a signature. 1964 The definition of SigningCertificate is 1966 SigningCertificate ::= SEQUENCE { 1967 certs SEQUENCE OF ESSCertID, 1968 policies SEQUENCE OF PolicyInformation OPTIONAL 1969 } 1971 id-aa-signingCertificate OBJECT IDENTIFIER ::= { iso(1) 1972 member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 1973 smime(16) id-aa(2) 12 } 1975 The first certificate identified in the sequence of certificate identifiers 1976 MUST be the certificate used to verify the signature. The encoding of the 1977 ESSCertID for this certificate SHOULD NOT include the issuerSerial because 1978 the issuerAndSerialNumber is already present in the SignerInfo. The 1979 certificate identified is used during the signature verification process. 1980 If the hash of the certificate does not match the certificate used to 1981 verify the signature, the signature MUST be considered invalid. 1983 If more than one certificate is present in the sequence of ESSCertIDs, the 1984 certificates after the first one limit the set of attribute certificates 1985 that are used during signature validation. The issuerSerial SHOULD be 1986 present in these certificates, unless the client who is validating the 1987 signature is expected to have easy access to all the certificates required 1988 for validation. If only the signing certificate is present in the sequence. 1989 there are no restrictions on the set of attribute certificates used in 1990 validating the signature. 1992 The sequence of policy information terms identifies those certificate 1993 policies that the signer asserts apply to the certificate, and under which 1994 the certificate should be relied upon. This value suggests a policy value 1995 to be used in the relying party's certification path validation. 1997 If present, the SigningCertificate attribute MUST be a signed attribute; it 1998 MUST NOT be an unsigned attribute. CMS defines SignedAttributes as a SET OF 1999 Attribute. A SignerInfo MUST NOT include multiple instances of the 2000 SigningCertificate attribute. CMS defines the ASN.1 syntax for the signed 2001 attributes to include attrValues SET OF AttributeValue. A 2002 SigningCertificate attribute MUST include only a single instance of 2003 AttributeValue. There MUST NOT be zero or multiple instances of 2004 AttributeValue present in the attrValues SET OF AttributeValue. 2006 5.4.1 Certificate Identification 2008 The best way to identify certificates is an often-discussed issue. CMS has 2009 imposed a restriction for SignedData objects that the issuer DN must be 2010 present in all signing certificates. The issuer/serial number pair is 2011 therefore sufficient to identify the correct signing certificate. This 2012 information is already present, as part of the SignerInfo object, and 2013 duplication of this information would be unfortunate. A hash of the entire 2014 certificate serves the same function (allowing the receiver to verify that 2015 the same certificate is being used as when the message was signed), is 2016 smaller, and permits a detection of the simple substitution attacks. 2018 Attribute certificates do not have an issuer/serial number pair represented 2019 anywhere in a SignerInfo object. When an attribute certificate is not 2020 included in the SignedData object, it becomes much more difficult to get 2021 the correct set of certificates based only on a hash of the certificate. 2022 For this reason, attribute certificates are identified by the IssuerSerial 2023 object. 2025 This document defines a certificate identifier as: 2027 ESSCertID ::= SEQUENCE { 2028 certHash Hash, 2029 issuerSerial IssuerSerial OPTIONAL 2030 } 2032 Hash ::= OCTET STRING -- SHA1 hash of entire certificate 2034 IssuerSerial ::= SEQUENCE { 2035 issuer GeneralNames, 2036 serialNumber CertificateSerialNumber 2037 } 2039 When creating an ESSCertID, the certHash is computed over the entire DER 2040 encoded certificate including the signature. The issuerSerial would 2041 normally be present unless the value can be inferred from other 2042 information. 2044 When encoding IssuerSerial, serialNumber is the serial number that uniquely 2045 identifies the certificate. For non-attribute certificates, the issuer MUST 2046 contain only the issuer name from the certificate encoded in the 2047 directoryName choice of GeneralNames. For attribute certificates, the 2048 issuer MUST contain the issuer name field from the attribute certificate. 2050 6. Security Considerations 2052 This entire document discusses security. 2054 A. ASN.1 Module 2056 ExtendedSecurityServices 2057 { iso(1) member-body(2) us(840) rsadsi(113549) 2058 pkcs(1) pkcs-9(9) smime(16) modules(0) ess(2) } 2060 DEFINITIONS IMPLICIT TAGS ::= 2061 BEGIN 2063 IMPORTS 2065 -- Cryptographic Message Syntax (CMS) 2066 ContentType, IssuerAndSerialNumber, SubjectKeyIdentifier, Version 2067 FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840) 2068 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1)} 2070 -- PKIX Certificate and CRL Profile, Sec A.2 Implicitly Tagged Module, 2071 -- 1988 Syntax 2072 PolicyInformation FROM PKIX1Implicit88 {iso(1) 2073 identified-organization(3) dod(6) internet(1) security(5) 2074 mechanisms(5) pkix(7)id-mod(0) id-pkix1-implicit-88(2)} 2076 -- X.509 2077 GeneralNames, CertificateSerialNumber FROM CertificateExtensions 2078 {joint-iso-ccitt ds(5) module(1) certificateExtensions(26) 0}; 2080 -- Extended Security Services 2082 -- The construct "SEQUENCE SIZE (1..MAX) OF" appears in several ASN.1 2083 -- constructs in this module. A valid ASN.1 SEQUENCE can have zero or 2084 -- more entries. The SIZE (1..MAX) construct constrains the SEQUENCE to 2085 -- have at least one entry. MAX indicates the upper bound is unspecified. 2086 -- Implementations are free to choose an upper bound that suits their 2087 -- environment. 2089 UTF8String ::= [UNIVERSAL 12] IMPLICIT OCTET STRING 2090 -- The contents are formatted as described in [UTF8] 2092 -- Section 2.7 2094 ReceiptRequest ::= SEQUENCE { 2095 signedContentIdentifier ContentIdentifier, 2096 receiptsFrom ReceiptsFrom, 2097 receiptsTo SEQUENCE SIZE (1..ub-receiptsTo) OF GeneralNames } 2099 ub-receiptsTo INTEGER ::= 16 2101 id-aa-receiptRequest OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2102 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 1} 2104 ContentIdentifier ::= OCTET STRING 2106 id-aa-contentIdentifier OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2107 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 7} 2109 ReceiptsFrom ::= CHOICE { 2110 allOrFirstTier [0] AllOrFirstTier, 2111 -- formerly "allOrNone [0]AllOrNone" 2112 receiptList [1] SEQUENCE OF GeneralNames } 2114 AllOrFirstTier ::= INTEGER { -- Formerly AllOrNone 2115 allReceipts (0), 2116 firstTierRecipients (1) } 2118 -- Section 2.8 2120 Receipt ::= SEQUENCE { 2121 version Version, -- Version is imported from [CMS] 2122 contentType ContentType, 2123 signedContentIdentifier ContentIdentifier, 2124 originatorSignatureValue OCTET STRING } 2126 id-ct-receipt OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) 2127 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-ct(1) 1} 2129 -- Section 2.9 2131 ContentHints ::= SEQUENCE { 2132 � contentDescription UTF8String (SIZE (1..MAX)) OPTIONAL, 2133 � contentType ContentType } 2135 id-aa-contentHint OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) 2136 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 4} 2138 -- Section 2.10 2140 MsgSigDigest ::= OCTET STRING 2142 id-aa-msgSigDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2143 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 5} 2145 -- Section 2.11 2147 ContentReference ::= SEQUENCE { 2148 contentType ContentType, 2149 signedContentIdentifier ContentIdentifier, 2150 originatorSignatureValue OCTET STRING } 2152 id-aa-contentReference OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2153 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 10 } 2155 -- Section 3.2 2157 ESSSecurityLabel ::= SET { 2158 security-policy-identifier SecurityPolicyIdentifier, 2159 security-classification SecurityClassification OPTIONAL, 2160 privacy-mark ESSPrivacyMark OPTIONAL, 2161 security-categories SecurityCategories OPTIONAL } 2163 id-aa-securityLabel OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2164 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 2} 2166 SecurityPolicyIdentifier ::= OBJECT IDENTIFIER 2168 SecurityClassification ::= INTEGER { 2169 unmarked (0), 2170 unclassified (1), 2171 restricted (2), 2172 confidential (3), 2173 secret (4), 2174 top-secret (5) } (0..ub-integer-options) 2176 ub-integer-options INTEGER ::= 256 2178 ESSPrivacyMark ::= CHOICE { 2179 ��� pString����� PrintableString (SIZE (1..ub-privacy-mark-length)), 2180 ��� utf8String�� UTF8String (SIZE (1..MAX)) 2181 } 2183 ub-privacy-mark-length INTEGER ::= 128 2185 SecurityCategories ::= SET SIZE (1..ub-security-categories) OF 2186 SecurityCategory 2188 ub-security-categories INTEGER ::= 64 2190 SecurityCategory ::= SEQUENCE { 2191 type [0] OBJECT IDENTIFIER, 2192 value [1] ANY DEFINED BY type -- defined by type 2193 } 2195 --Note: The aforementioned SecurityCategory syntax produces identical 2196 --hex encodings as the following SecurityCategory syntax that is 2197 --documented in the X.411 specification: 2198 -- 2199 --SecurityCategory ::= SEQUENCE { 2200 -- type [0] SECURITY-CATEGORY, 2201 -- value [1] ANY DEFINED BY type } 2202 -- 2203 --SECURITY-CATEGORY MACRO ::= 2204 --BEGIN 2205 --TYPE NOTATION ::= type | empty 2206 --VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER) 2207 --END 2209 -- Section 3.4 2211 EquivalentLabels ::= SEQUENCE OF ESSSecurityLabel 2213 id-aa-equivalentLabels OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2214 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 9} 2216 -- Section 4.4 2218 MLExpansionHistory ::= SEQUENCE 2219 SIZE (1..ub-ml-expansion-history) OF MLData 2221 id-aa-mlExpandHistory OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2222 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 3} 2224 ub-ml-expansion-history INTEGER ::= 64 2226 MLData ::= SEQUENCE { 2227 mailListIdentifier EntityIdentifier, 2228 -- EntityIdentifier is imported from [CMS] 2229 expansionTime GeneralizedTime, 2230 mlReceiptPolicy MLReceiptPolicy OPTIONAL } 2232 EntityIdentifier ::= CHOICE { 2233 issuerAndSerialNumber IssuerAndSerialNumber, 2234 subjectKeyIdentifier SubjectKeyIdentifier } 2236 MLReceiptPolicy ::= CHOICE { 2237 none [0] NULL, 2238 insteadOf [1] SEQUENCE SIZE (1..MAX) OF GeneralNames, 2239 inAdditionTo [2] SEQUENCE SIZE (1..MAX) OF GeneralNames } 2241 -- Section 5.4 2243 SigningCertificate ::= SEQUENCE { 2244 certs SEQUENCE OF ESSCertID, 2245 policies SEQUENCE OF PolicyInformation OPTIONAL 2246 } 2248 id-aa-signingCertificate OBJECT IDENTIFIER ::= { iso(1) 2249 member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 2250 smime(16) id-aa(2) 12 } 2252 ESSCertID ::= SEQUENCE { 2253 certHash Hash, 2254 issuerSerial IssuerSerial OPTIONAL 2255 } 2257 Hash ::= OCTET STRING -- SHA1 hash of entire certificate 2259 IssuerSerial ::= SEQUENCE { 2260 issuer GeneralNames, 2261 serialNumber CertificateSerialNumber 2262 } 2264 END -- of ExtendedSecurityServices 2266 B. References 2268 [ASN1-1988] "Recommendation X.208: Specification of Abstract Syntax 2269 Notation One (ASN.1)" 2271 [ASN1-1994] "Recommendation X.680: Specification of Abstract Syntax 2272 Notation One (ASN.1)" 2274 [CERT] "S/MIME Version 3 Certificate Handling", Internet Draft 2275 draft-ietf-smime-cert-xx. 2277 [CMS] "Cryptographic Message Syntax", Internet Draft 2278 draft-ietf-smime-cms-xx. 2280 [MSG] "S/MIME Version 3 Message Specification", Internet Draft 2281 draft-ietf-smime-msg-xx. 2283 [MUSTSHOULD] "Key Words for Use in RFCs to Indicate Requirement Levels", 2284 RFC 2119. 2286 [MSP4] "Secure Data Network System (SDNS) Message Security Protocol (MSP) 2287 4.0", Specification SDN.701, Revision A, 1997-02-06. 2289 [MTSABS] "1988 International Telecommunication Union (ITU) Data 2290 Communication Networks Message Handling Systems: Message Transfer System: 2291 Abstract Service Definition and Procedures, Volume VIII, Fascicle VIII.7, 2292 Recommendation X.411"; MTSAbstractService {joint-iso-ccitt mhs-motis(6) 2293 mts(3) modules(0) mts-abstract-service(1)} 2295 [PKCS7-1.5] "PKCS #7: Cryptographic Message Syntax", RFC 2315. 2297 [SMIME2] "S/MIME Version 2 Message Specification", RFC 2311, and 2298 "S/MIME Version 2 Certificate Handling", RFC 2312. 2300 [UTF8] "UTF-8, a transformation format of ISO 10646", RFC 2279. 2302 C. Acknowledgments 2304 The first draft of this work was prepared by David Solo. John Pawling did a 2305 huge amount of very detailed revision work during the many phases of the 2306 document. 2308 Many other people have contributed hard work to this draft, including: 2309 Andrew Farrell 2310 Bancroft Scott 2311 Bengt Ackzell 2312 Bill Flanigan 2313 Blake Ramsdell 2314 Carlisle Adams 2315 David Kemp 2316 Denis Pinkas 2317 Jim Schaad 2318 Russ Housley 2319 Scott Hollenbeck 2320 Steve Dusse 2322 D. Changes from draft-ietf-smime-ess-09 to draft-ietf-smime-ess-10 2324 Numerous small clarifications throughout. 2326 Changed the [SMIME3] reference to [CERT] and [MSG]. 2328 1: Added paragraph about usefulness of attributes for other 2329 purposes. Also added reference to [MUSTSHOULD]. 2331 1.3.4: Added note about [ESS]. 2333 2.3: Added explanation at the end of the first paragraph about why 2334 you might get more than one receipt request. Also changed "conflict" 2335 to "are not the same" in the second paragraph. 2337 2.9, 3.2: Added parentheses around SIZE declarations. Also in Appendix 2338 A. 2340 4.2.3.2: Changed step 2 to say that if mlExpansionHistory isn't found, 2341 skip to step 4. Also updated the flow chart. 2343 5.4: Filled in the TBD of the id-aa-signingCertificate OID. Also in 2344 Appendix A. 2346 A: Fixed some bugs in the headers caused by typos. 2348 E. Editor's Address 2350 Paul Hoffman 2351 Internet Mail Consortium 2352 127 Segre Place 2353 Santa Cruz, CA 95060 2354 (831) 426-9827 2355 phoffman@imc.org