idnits 2.17.1 draft-ietf-smime-ess-11.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- ** Looks like you're using RFC 2026 boilerplate. This must be updated to follow RFC 3978/3979, as updated by RFC 4748. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- ** Missing expiration date. The document expiration date should appear on the first and last page. ** The document seems to lack a 1id_guidelines paragraph about the list of current Internet-Drafts. ** The document seems to lack a 1id_guidelines paragraph about the list of Shadow Directories. ** The document is more than 15 pages and seems to lack a Table of Contents. == No 'Intended status' indicated for this document; assuming Proposed Standard == The page length should not exceed 58 lines per page, but there was 1 longer page, the longest (page 1) being 2467 lines Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- ** The document seems to lack an Abstract section. ** The document seems to lack an IANA Considerations section. (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 468 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 114: '...entInfo eContent MUST be absent. If th...' RFC 2119 keyword, line 116: '...entInfo eContent MUST contain the resu...' RFC 2119 keyword, line 138: '...contentType MUST be id-data. The Envel...' RFC 2119 keyword, line 167: '...receiving agents MUST be able to inter...' RFC 2119 keyword, line 248: '...request MUST be in the inside signatur...' (146 more instances...) Miscellaneous warnings: ---------------------------------------------------------------------------- == Line 1772 has weird spacing: '... | none inst...' == Line 1774 has weird spacing: '... | none none...' == Line 1775 has weird spacing: '... | none inst...' == Line 1776 has weird spacing: '... | none inst...' == Line 1777 has weird spacing: '... | none inst...' ** The document contains RFC2119-like boilerplate, but doesn't seem to mention RFC 2119. The boilerplate contains a reference [MUSTSHOULD], but that reference does not seem to mention RFC 2119 either. -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (February 26, 1999) is 9192 days in the past. Is this intentional? 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 2366 looks like a reference -- Missing reference section? 'MSG' on line 2360 looks like a reference -- Missing reference section? 'CERT' on line 2354 looks like a reference -- Missing reference section? 'SMIME2' on line 2377 looks like a reference -- Missing reference section? 'ASN1-1988' on line 2348 looks like a reference -- Missing reference section? 'MUSTSHOULD' on line 2363 looks like a reference -- Missing reference section? 'ESS' on line 325 looks like a reference -- Missing reference section? 'CMS' on line 2357 looks like a reference -- Missing reference section? '0' on line 2317 looks like a reference -- Missing reference section? '1' on line 2318 looks like a reference -- Missing reference section? 'MTSABS' on line 2369 looks like a reference -- Missing reference section? '2' on line 2319 looks like a reference -- Missing reference section? 'SMIME3' on line 2079 looks like a reference -- Missing reference section? 'UNIVERSAL 12' on line 2168 looks like a reference -- Missing reference section? 'UTF8' on line 2380 looks like a reference -- Missing reference section? 'ASN1-1994' on line 2351 looks like a reference Summary: 11 errors (**), 0 flaws (~~), 8 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-11.txt Internet Mail Consortium 3 February 26, 1999 4 Expires in six months 6 Enhanced Security Services for S/MIME 8 Status of this memo 10 Internet-Drafts are working documents of the Internet Engineering 11 Task Force (IETF), its areas, and its working groups. Note that 12 other groups may also distribute working documents as 13 Internet-Drafts. 15 Internet-Drafts are draft documents valid for a maximum of six 16 months and may be updated, replaced, or obsoleted by other 17 documents at any time. It is inappropriate to use Internet- 18 Drafts as reference material or to cite them other than as 19 "work in progress." 21 To view the list Internet-Draft Shadow Directories, see 22 http://www.ietf.org/shadow.html. 24 This document is an Internet-Draft and is in full conformance 25 with all provisions of Section 10 of RFC2026. 27 1. Introduction 29 This document describes four optional security service extensions for 30 S/MIME. The services are: 31 - signed receipts 32 - security labels 33 - secure mailing lists 34 - signing certificates 35 The first three of these services provide functionality that is similar to 36 the Message Security Protocol [MSP4], but are useful in many other 37 environments, particularly business and finance. Signing certificates are 38 useful in any environment where certificates might be transmitted with 39 signed messages. 41 The services described here are extensions to S/MIME version 3 ([MSG] and 42 [CERT]), and some of them can also be added to S/MIME version 2 [SMIME2]. 43 The extensions described here will not cause an S/MIME version 3 recipient 44 to be unable to read messages from an S/MIME version 2 sender. However, 45 some of the extensions will cause messages created by an S/MIME version 3 46 sender to be unreadable by an S/MIME version 2 recipient. 48 This document describes both the procedures and the attributes needed for 49 the three services. Note that some of the attributes described in this 50 document are quite useful in other contexts and should be considered when 51 extending S/MIME or other CMS applications. 53 The format of the messages are described in ASN.1:1988 [ASN1-1988]. 55 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 56 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 57 document are to be interpreted as described in [MUSTSHOULD]. 59 This draft is being discussed on the "ietf-smime" mailing list. To 60 subscribe, send a message to: 61 ietf-smime-request@imc.org 62 with the single word 63 subscribe 64 in the body of the message. There is a Web site for the mailing list at 65 . 67 1.1 Triple Wrapping 69 Some of the features of each service use the concept of a "triple wrapped" 70 message. A triple wrapped message is one that has been signed, then 71 encrypted, then signed again. The signers of the inner and outer signatures 72 may be different entities or the same entity. Note that the S/MIME 73 specification does not limit the number of nested encapsulations, so there 74 may be more than three wrappings. 76 1.1.1 Purpose of Triple Wrapping 78 Not all messages need to be triple wrapped. Triple wrapping is used when a 79 message must be signed, then encrypted, and then have signed attributes 80 bound to the encrypted body. Outer attributes may be added or removed by 81 the message originator or intermediate agents, and may be signed by 82 intermediate agents or the final recipient. 84 The inside signature is used for content integrity, non-repudiation with 85 proof of origin, and binding attributes (such as a security label) to the 86 original content. These attributes go from the originator to the recipient, 87 regardless of the number of intermediate entities such as mail list agents 88 that process the message. The signed attributes can be used for access 89 control to the inner body. Requests for signed receipts by the originator 90 are carried in the inside signature as well. 92 The encrypted body provides confidentiality, including confidentiality of 93 the attributes that are carried in the inside signature. 95 The outside signature provides authentication and integrity for information 96 that is processed hop-by-hop, where each hop is an intermediate entity such 97 as a mail list agent. The outer signature binds attributes (such as a 98 security label) to the encrypted body. These attributes can be used for 99 access control and routing decisions. 101 1.1.2 Steps for Triple Wrapping 103 The steps to create a triple wrapped message are: 105 1. Start with a message body, called the "original content". 107 2. Encapsulate the original content with the appropriate MIME Content-type 108 headers, such as "Content-type: text/plain". An exception to this MIME 109 encapsulation rule is that a signed receipt is not put in MIME headers. 111 3. Sign the result of step 2 (the inner MIME headers and the original 112 content). The SignedData encapContentInfo eContentType object identifier 113 MUST be id-data. If the structure you create in step 4 is multipart/signed, 114 then the SignedData encapContentInfo eContent MUST be absent. If the 115 structure you create in step 4 is application/pkcs7-mime, then the 116 SignedData encapContentInfo eContent MUST contain the result of step 2 117 above. The SignedData structure is encapsulated by a ContentInfo SEQUENCE 118 with a contentType of id-signedData. 120 4. Add an appropriate MIME construct to the signed message from step 3 as 121 defined in [MSG]. The resulting message is called the "inside signature". 123 - If you are signing using multipart/signed, the MIME construct added 124 consists of a Content-type of multipart/signed with parameters, the 125 boundary, the result of step 2 above, the boundary, a Content-type of 126 application/pkcs7-signature, optional MIME headers (such as 127 Content-transfer-encoding and Content-disposition), and a body part that 128 is the result of step 3 above. 130 - If you are instead signing using application/pkcs7-mime, the MIME 131 construct added consists of a Content-type of application/pkcs7-mime 132 with parameters, optional MIME headers (such as 133 Content-transfer-encoding and Content-disposition), and the result of 134 step 3 above. 136 5. Encrypt the result of step 4 as a single block, turning it into an 137 application/pkcs7-mime object. The EnvelopedData encryptedContentInfo 138 contentType MUST be id-data. The EnvelopedData structure is encapsulated by 139 a ContentInfo SEQUENCE with a contentType of id-envelopedData. This is 140 called the "encrypted body". 142 6. Add the appropriate MIME headers: a Content-type of 143 application/pkcs7-mime with parameters, and optional MIME headers such as 144 Content-transfer-encoding and Content-disposition. 146 7. Using the same logic as in step 3 above, sign the result of step 6 (the 147 MIME headers and the encrypted body) as a single block 149 8. Using the same logic as in step 4 above, add an appropriate MIME 150 construct to the signed message from step 7. The resulting message is 151 called the "outside signature", and is also the triple wrapped message. 153 1.2 Format of a Triple Wrapped Message 155 A triple wrapped message has many layers of encapsulation. The structure 156 differs based on the choice of format for the signed portions of the 157 message. Because of the way that MIME encapsulates data, the layers do not 158 appear in order, and the notion of "layers" becomes vague. 160 There is no need to use the multipart/signed format in an inner signature 161 because it is known that the recipient is able to process S/MIME messages 162 (because they decrypted the middle wrapper). A sending agent might choose 163 to use the multipart/signed format in the outer layer so that a non-S/MIME 164 agent could see that the next inner layer is encrypted; however, this is 165 not of great value, since all it shows the recipient is that the rest of 166 the message is unreadable. Because many sending agents always use 167 multipart/signed structures, all receiving agents MUST be able to interpret 168 either multipart/signed or application/pkcs7-mime signature structures. 170 The format of a triple wrapped message that uses multipart/signed for both 171 signatures is: 173 [step 8] Content-type: multipart/signed; 174 [step 8] protocol="application/pkcs7-signature"; 175 [step 8] boundary=outerboundary 176 [step 8] 177 [step 8] --outerboundary 178 [step 6] Content-type: application/pkcs7-mime; ) 179 [step 6] smime-type=enveloped-data ) 180 [step 6] ) 181 [step 4] Content-type: multipart/signed; | ) 182 [step 4] protocol="application/pkcs7-signature"; | ) 183 [step 4] boundary=innerboundary | ) 184 [step 4] | ) 185 [step 4] --innerboundary | ) 186 [step 2] Content-type: text/plain % | ) 187 [step 2] % | ) 188 [step 1] Original content % | ) 189 [step 4] | ) 190 [step 4] --innerboundary | ) 191 [step 4] Content-type: application/pkcs7-signature | ) 192 [step 4] | ) 193 [step 3] inner SignedData block (eContent is missing) | ) 194 [step 4] | ) 195 [step 4] --innerboundary-- | ) 196 [step 8] 197 [step 8] --outerboundary 198 [step 8] Content-type: application/pkcs7-signature 199 [step 8] 200 [step 7] outer SignedData block (eContent is missing) 201 [step 8] 202 [step 8] --outerboundary-- 204 % = These lines are what the inner signature is computed over. 205 | = These lines are what is encrypted in step 5. This encrypted result 206 is opaque and is a part of an EnvelopedData block. 207 ) = These lines are what the outer signature is computed over. 209 The format of a triple wrapped message that uses application/pkcs7-mime for 210 the both signatures is: 212 [step 8] Content-type: application/pkcs7-mime; 213 [step 8] smime-type=signed-data 214 [step 8] 215 [step 7] outer SignedData block (eContent is present) O 216 [step 6] Content-type: application/pkcs7-mime; ) O 217 [step 6] smime-type=enveloped-data; ) O 218 [step 6] ) O 219 [step 4] Content-type: application/pkcs7-mime; | ) O 220 [step 4] smime-type=signed-data | ) O 221 [step 4] | ) O 222 [step 3] inner SignedData block (eContent is present) I | ) O 223 [step 2] Content-type: text/plain I | ) O 224 [step 2] I | ) O 225 [step 1] Original content I | ) O 227 I = These lines are the inner SignedData block, which is opaque and 228 contains the ASN.1 encoded result of step 2 as well as control 229 information. 230 | = These lines are what is encrypted in step 5. This encrypted result 231 is opaque and is a part of an EnvelopedData block. 232 ) = These lines are what the outer signature is computed over. 233 O = These lines are the outer SignedData block, which is opaque and 234 contains the ASN.1 encoded result of step 6 as well as control 235 information. 237 1.3 Security Services and Triple Wrapping 239 The three security services described in this document are used with triple 240 wrapped messages in different ways. This section briefly describes the 241 relationship of each service with triple wrapping; the other sections of 242 the document go into greater detail. 244 1.3.1 Signed Receipts and Triple Wrapping 246 A signed receipt may be requested in any SignedData object. However, if a 247 signed receipt is requested for a triple wrapped message, the receipt 248 request MUST be in the inside signature, not in the outside signature. A 249 secure mailing list agent may change the receipt policy in the outside 250 signature of a triple wrapped message when that message is processed by the 251 mailing list. 253 Note: the signed receipts and receipt requests described in this draft 254 differ from those described in the work done by the IETF Receipt 255 Notification Working Group. The output of that Working Group, when 256 finished, is not expected to work well with triple wrapped messages as 257 described in this document. 259 1.3.2 Security Labels and Triple Wrapping 261 A security label may be included in the signed attributes of any SignedData 262 object. A security label attribute may be included in either the inner 263 signature, outer signature, or both. 265 The inner security label is used for access control decisions related to 266 the plaintext original content. The inner signature provides authentication 267 and cryptographically protects the integrity of the original signer's 268 security label that is in the inside body. This strategy facilitates the 269 forwarding of messages because the original signer's security label is 270 included in the SignedData block which can be forwarded to a third party 271 that can verify the inner signature which will cover the inner security 272 label. The confidentiality security service can be applied to the inner 273 security label by encrypting the entire inner SignedData block within an 274 EnvelopedData block. 276 A security label may also be included in the signed attributes of the outer 277 SignedData block which will include the sensitivities of the encrypted 278 message. The outer security label is used for access control and routing 279 decisions related to the encrypted message. Note that a security label 280 attribute can only be used in an signedAttributes block. An 281 eSSSecurityLabel attribute MUST NOT be used in an EnvelopedData or unsigned 282 attributes. 284 1.3.3 Secure Mailing Lists and Triple Wrapping 286 Secure mail list message processing depends on the structure of S/MIME 287 layers present in the message sent to the mail list agent. The mail list 288 agent never changes the data that was hashed to form the inner signature, 289 if such a signature is present. If an outer signature is present, then the 290 agent will modify the data that was hashed to form that outer signature. In 291 all cases, the agent adds or updates an mlExpansionHistory attribute to 292 document the agent's processing, and ultimately adds or replaces the outer 293 signature on the message to be distributed. 295 1.3.4 Placement of Attributes 297 Certain attributes should be placed in the inner or outer SignedData 298 message; some attributes can be in either. Further, some attributes must be 299 signed, while signing is optional for others, and some attributes must not 300 be signed. ESS defines several types of attributes. ContentHints and 301 ContentIdentifier MAY appear in any list of attributes. contentReference, 302 equivalentLabel, eSSSecurityLabel and mlExpansionHistory MUST be carried in 303 a SignedAttributes or AuthAttributes type, and MUST NOT be carried in a 304 UnsignedAttributes, UnauthAttributes or UnprotectedAttributes type. 305 msgSigDigest, receiptRequest and signingCertificate MUST be carried in a 306 SignedAttributes, and MUST NOT be carried in a AuthAttributes, 307 UnsignedAttributes, UnauthAttributes or UnprotectedAttributes type. 309 The following table summarizes the recommendation of this profile. In the 310 OID column, [ESS] indicates that the attribute is defined in this document. 312 | |Inner or | 313 Attribute |OID |outer |Signed 314 ------------------|----------------------------- |----------|-------- 315 contentHints |id-aa-contentHint [ESS] |either |MAY 316 contentIdentifier |id-aa-contentIdentifier [ESS] |either |MAY 317 contentReference |id-aa-contentReference [ESS] |either |MUST 318 contentType |id-contentType [CMS] |either |MUST 319 counterSignature |id-countersignature [CMS] |either |MUST NOT 320 equivalentLabel |id-aa-equivalentLabels [ESS] |either |MUST 321 eSSSecurityLabel |id-aa-securityLabel [ESS] |either |MUST 322 messageDigest |id-messageDigest [CMS] |either |MUST 323 msgSigDigest |id-aa-msgSigDigest [ESS] |inner only|MUST 324 mlExpansionHistory|id-aa-mlExpandHistory [ESS] |outer only|MUST 325 receiptRequest |id-aa-receiptRequest [ESS] |inner only|MUST 326 signingCertificate|id-aa-signingCertificate [ESS]|either |MUST 327 signingTime |id-signingTime [CMS] |either |MUST 328 smimeCapabilities |sMIMECapabilities [MSG] |either |MUST 329 sMIMEEncryption- 330 KeyPreference |id-aa-encrypKeyPref [MSG] |either |MUST 332 CMS defines signedAttrs as a SET OF Attribute and defines unsignedAttrs as 333 a SET OF Attribute. ESS defines the contentHints, contentIdentifier, 334 eSSecurityLabel, msgSigDigest, mlExpansionHistory, receiptRequest, 335 contentReference, equivalentLabels and signingCertificate attribute types. 336 A signerInfo MUST NOT include multiple instances of any of the attribute 337 types defined in ESS. Later sections of ESS specify further restrictions 338 that apply to the receiptRequest, mlExpansionHistory and eSSecurityLabel 339 attribute types. 341 CMS defines the syntax for the signed and unsigned attributes as 342 "attrValues SET OF AttributeValue". For all of the attribute types defined 343 in ESS, if the attribute type is present in a signerInfo, then it MUST only 344 include a single instance of AttributeValue. In other words, there MUST NOT 345 be zero, or multiple, instances of AttributeValue present in the attrValues 346 SET OF AttributeValue. 348 If a counterSignature attribute is present, then it MUST be included in the 349 unsigned attributes. It MUST NOT be included in the signed attributes. The 350 only attributes that are allowed in a counterSignature attribute are 351 counterSignature, messageDigest, signingTime, and signingCertificate. 353 Note that the inner and outer signatures are usually those of different 354 senders. Because of this, the same attribute in the two signatures could 355 lead to very different consequences. 357 ContentIdentifier is an attribute (OCTET STRING) used to carry a unique 358 identifier assigned to the message. 360 1.4 Required and Optional Attributes 362 Some security gateways sign messages that pass through them. If the message 363 is any type other than a signedData type, the gateway has only one way to 364 sign the message: by wrapping it with a signedData block and MIME headers. 365 If the message to be signed by the gateway is a signedData message already, 366 the gateway can sign the message by inserting a signerInfo into the 367 signedData block. 369 The main advantage of a gateway adding a signerInfo instead of wrapping the 370 message in a new signature is that the message doesn't grow as much as if 371 the gateway wrapped the message. The main disadvantage is that the gateway 372 must check for the presence of certain attributes in the other signerInfos 373 and either omit or copy those attributes. 375 If a gateway or other processor adds a signerInfo to an existing signedData 376 block, it MUST copy the mlExpansionHistory and eSSSecurityLabel attributes 377 from other signerInfos. This helps ensure that the recipient will process 378 those attributes in a signerInfo that it can verify. 380 Note that someone may in the future define an attribute that must be 381 present in each signerInfo of a signedData block in order for the signature 382 to be processed. If that happens, a gateway that inserts signerInfos and 383 doesn't copy that attribute will cause every message with that attribute to 384 fail when processed by the recipient. For this reason, it is safer to wrap 385 messages with new signatures than to insert signerInfos. 387 1.5 Object Identifiers 389 The object identifiers for many of the objects described in this draft are 390 found in [CMS], [MSG], and [CERT]. Other object identifiers used in S/MIME 391 can be found in the registry kept at 392 . When this draft moves to 393 standards track within the IETF, it is intended that the IANA will maintain 394 this registry. 396 2. Signed Receipts 398 Returning a signed receipt provides to the originator proof of delivery of 399 a message, and allows the originator to demonstrate to a third party that 400 the recipient was able to verify the signature of the original message. 401 This receipt is bound to the original message through the signature; 402 consequently, this service may be requested only if a message is signed. 403 The receipt sender may optionally also encrypt a receipt to provide 404 confidentiality between the receipt sender and the receipt recipient. 406 2.1 Signed Receipt Concepts 408 The originator of a message may request a signed receipt from the message's 409 recipients. The request is indicated by adding a receiptRequest attribute 410 to the signedAttributes field of the SignerInfo object for which the 411 receipt is requested. The receiving user agent software SHOULD 412 automatically create a signed receipt when requested to do so, and return 413 the receipt in accordance with mailing list expansion options, local 414 security policies, and configuration options. 416 Because receipts involve the interaction of two parties, the terminology 417 can sometimes be confusing. In this section, the "sender" is the agent that 418 sent the original message that included a request for a receipt. The 419 "receiver" is the party that received that message and generated the 420 receipt. 422 The steps in a typical transaction are: 424 1. Sender creates a signed message including a receipt request attribute 425 (Section 2.2). 427 2. Sender transmits the resulting message to the recipient or recipients. 429 3. Recipient receives message and determines if there is a valid signature 430 and receipt request in the message (Section 2.3). 432 4. Recipient creates a signed receipt (Section 2.4). 434 5. Recipient transmits the resulting signed receipt message to the sender 435 (Section 2.5). 437 6. Sender receives the message and validates that it contains a signed 438 receipt for the original message (Section 2.6). This validation relies on 439 the sender having retained either a copy of the original message or 440 information extracted from the original message. 442 The ASN.1 syntax for the receipt request is given in Section 2.7; the ASN.1 443 syntax for the receipt is given in Section 2.8. 445 Note that a sending agent SHOULD remember when it has sent a receipt so 446 that it can avoid re-sending a receipt each time it processes the message. 448 A receipt request can indicate that receipts be sent to many places, not 449 just to the sender (in fact, the receipt request might indicate that the 450 receipts should not even go to the sender). In order to verify a receipt, 451 the recipient of the receipt must be the originator or a recipient of the 452 original message. Thus, the sender SHOULD NOT request that receipts be sent 453 to anyone who does not have an exact copy of the message. 455 2.2 Receipt Request Creation 457 Multi-layer S/MIME messages may contain multiple SignedData layers. 458 However, receipts may be requested only for the innermost SignedData layer 459 in a multi-layer S/MIME message, such as a triple wrapped message. Only one 460 receiptRequest attribute can be included in the signedAttributes of a 461 SignerInfo. 463 A ReceiptRequest attribute MUST NOT be included in the attributes of a 464 SignerInfo in a SignedData object that encapsulates a Receipt content. In 465 other words, the receiving agent MUST NOT request a signed receipt for a 466 signed receipt. 468 A sender requests receipts by placing a receiptRequest attribute in the 469 signed attributes of a signerInfo as follows: 471 1. A receiptRequest data structure is created. 473 2. A signed content identifier for the message is created and assigned to 474 the signedContentIdentifier field. The signedContentIdentifier is used to 475 associate the signed receipt with the message requesting the signed 476 receipt. 478 3. The entities requested to return a signed receipt are noted in the 479 receiptsFrom field. 481 4. The message originator MUST populate the receiptsTo field with a 482 GeneralNames for each entity to whom the recipient should send the signed 483 receipt. If the message originator wants the recipient to send the signed 484 receipt to the originator, then the originator MUST include a GeneralNames 485 for itself in the receiptsTo field. GeneralNames is a SEQUENCE OF 486 GeneralName. receiptsTo is a SEQUENCE OF GeneralNames in which each 487 GeneralNames represents an entity. There may be multiple GeneralName 488 instances in each GeneralNames. At a minimum, the message originator MUST 489 populate each entity's GeneralNames with the address to which the signed 490 receipt should be sent. Optionally, the message originator MAY also 491 populate each entity's GeneralNames with other GeneralName instances (such 492 as directoryName). 494 5. The completed receiptRequest attribute is placed in the signedAttributes 495 field of the SignerInfo object. 497 2.2.1 Multiple Receipt Requests 499 There can be multiple SignerInfos within a SignedData object, and each 500 SignerInfo may include signedAttributes. Therefore, a single SignedData 501 object may include multiple SignerInfos, each SignerInfo having a 502 receiptRequest attribute. For example, an originator can send a signed 503 message with two SignerInfos, one containing a DSS signature, the other 504 containing an RSA signature. 506 Each recipient SHOULD return only one signed receipt. 508 Not all of the SignerInfos need to include receipt requests, but in all of 509 the SignerInfos that do contain receipt requests, the receipt requests MUST 510 be identical. 512 2.2.2 Information Needed to Validate Signed Receipts 514 The sending agent MUST retain one or both of the following items to support 515 the validation of signed receipts returned by the recipients. 517 - the original signedData object requesting the signed receipt 519 - the message signature digest value used to generate the original 520 signedData signerInfo signature value and the digest value of the 521 Receipt content containing values included in the original signedData 522 object. If signed receipts are requested from multiple recipients, then 523 retaining these digest values is a performance enhancement because the 524 sending agent can reuse the saved values when verifying each returned 525 signed receipt. 527 2.3 Receipt Request Processing 529 A receiptRequest is associated only with the SignerInfo object to which the 530 receipt request attribute is directly attached. Receiving software SHOULD 531 examine the signedAttributes field of each of the SignerInfos for which it 532 verifies a signature in the innermost signedData object to determine if a 533 receipt is requested. This may result in the receiving agent processing 534 multiple receiptRequest attributes included in a single SignedData object, 535 such as requests made from different people who signed the object in 536 parallel. 538 Before processing a receiptRequest signedAttribute, the receiving agent 539 MUST verify the signature of the SignerInfo which covers the receiptRequest 540 attribute. A recipient MUST NOT process a receiptRequest attribute that has 541 not been verified. Because all receiptRequest attributes in a SignedData 542 object must be identical, the receiving application fully processes (as 543 described in the following paragraphs) the first receiptRequest attribute 544 that it encounters in a SignerInfo that it verifies, and it then ensures 545 that all other receiptRequest attributes in signerInfos that it verifies 546 are identical to the first one encountered. If there are verified 547 ReceiptRequest attributes which are not the same, then the processing 548 software MUST NOT return any signed receipt. A signed receipt SHOULD be 549 returned if any signerInfo containing a receiptRequest attribute can be 550 validated, even if other signerInfos containing the same receiptRequest 551 attribute cannot be validated because they are signed using an algorithm 552 not supported by the receiving agent. 554 If a receiptRequest attribute is absent from the signed attributes, then a 555 signed receipt has not been requested from any of the message recipients 556 and MUST NOT be created. If a receiptRequest attribute is present in the 557 signed attributes, then a signed receipt has been requested from some or 558 all of the message recipients. Note that in some cases, a receiving agent 559 might receive two almost-identical messages, one with a receipt request and 560 the other without one. In this case, the receiving agent SHOULD send a 561 signed receipt for the message that requests a signed receipt. 563 If a receiptRequest attribute is present in the signed attributes, the 564 following process SHOULD be used to determine if a message recipient has 565 been requested to return a signed receipt. 567 1. If an mlExpansionHistory attribute is present in the outermost 568 signedData block, do one of the following two steps, based on the absence 569 or presence of mlReceiptPolicy: 571 1.1. If an mlReceiptPolicy value is absent from the last MLData 572 element, a Mail List receipt policy has not been specified and the 573 processing software SHOULD examine the receiptRequest attribute value 574 to determine if a receipt should be created and returned. 576 1.2. If an mlReceiptPolicy value is present in the last MLData element, 577 do one of the following two steps, based on the value of 578 mlReceiptPolicy: 580 1.2.1. If the mlReceiptPolicy value is none, then the receipt 581 policy of the Mail List supersedes the originator's request for a 582 signed receipt and a signed receipt MUST NOT be created. 584 1.2.2. If the mlReceiptPolicy value is insteadOf or inAdditionTo, 585 the processing software SHOULD examine the receiptsFrom value from 586 the receiptRequest attribute to determine if a receipt should be 587 created and returned. If a receipt is created, the insteadOf and 588 inAdditionTo fields identify entities that SHOULD be sent the 589 receipt instead of or in addition to the originator. 591 2. If the receiptsFrom value of the receiptRequest attribute is 592 allOrFirstTier, do one of the following two steps based on the value of 593 allOrFirstTier. 595 2.1. If the value of allOrFirstTier is allReceipts, then a signed 596 receipt SHOULD be created. 598 2.2. If the value of allOrFirstTier is firstTierRecipients, do one of 599 the following two steps based on the presence of an mlExpansionHistory 600 attribute in an outer signedData block: 602 2.2.1. If an mlExpansionHistory attribute is present, then this 603 recipient is not a first tier recipient and a signed receipt MUST 604 NOT be created. 606 2.2.2. If an mlExpansionHistory attribute is not present, then a 607 signed receipt SHOULD be created. 609 3. If the receiptsFrom value of the receiptRequest attribute is a 610 receiptList: 612 3.1. If receiptList contains one of the GeneralNames of the recipient, 613 then a signed receipt SHOULD be created. 615 3.2. If receiptList does not contain one of the GeneralNames of the 616 recipient, then a signed receipt MUST NOT be created. 618 A flow chart for the above steps to be executed for each signerInfo for 619 which the receiving agent verifies the signature would be: 621 0. Receipt Request attribute present? 622 YES -> 1. 623 NO -> STOP 624 1. Has mlExpansionHistory in outer signedData? 625 YES -> 1.1. 626 NO -> 2. 627 1.1. mlReceiptPolicy absent? 628 YES -> 2. 629 NO -> 1.2. 630 1.2. Pick based on value of mlReceiptPolicy. 631 none -> 1.2.1. 632 insteadOf or inAdditionTo -> 1.2.2. 633 1.2.1. STOP. 634 1.2.2. Examine receiptsFrom to determine if a receipt should be created, 635 create it if required, send it to recipients designated by 636 mlReceiptPolicy, then -> STOP. 637 2. Is value of receiptsFrom allOrFirstTier? 638 YES -> Pick based on value of allOrFirstTier. 639 allReceipts -> 2.1. 640 firstTierRecipients -> 2.2. 641 NO -> 3. 642 2.1. Create a receipt, then -> STOP. 643 2.2. Has mlExpansionHistory in the outer signedData block? 644 YES -> 2.2.1. 645 NO -> 2.2.2. 646 2.2.1. STOP. 647 2.2.2. Create a receipt, then -> STOP. 648 3. Is receiptsFrom value of receiptRequest a receiptList? 649 YES -> 3.1. 650 NO -> STOP. 651 3.1. Does receiptList contain the recipient? 652 YES -> Create a receipt, then -> STOP. 653 NO -> 3.2. 654 3.2. STOP. 656 2.4 Signed Receipt Creation 658 A signed receipt is a signedData object encapsulating a Receipt content 659 (also called a "signedData/Receipt"). Signed receipts are created as 660 follows: 662 1. The signature of the original signedData signerInfo that includes the 663 receiptRequest signed attribute MUST be successfully verified before 664 creating the signedData/Receipt. 666 1.1. The content of the original signedData object is digested as 667 described in [CMS]. The resulting digest value is then compared with 668 the value of the messageDigest attribute included in the 669 signedAttributes of the original signedData signerInfo. If these digest 670 values are different, then the signature verification process fails and 671 the signedData/Receipt MUST NOT be created. 673 1.2. The ASN.1 DER encoded signedAttributes (including messageDigest, 674 receiptRequest and, possibly, other signed attributes) in the original 675 signedData signerInfo are digested as described in [CMS]. The resulting 676 digest value, called msgSigDigest, is then used to verify the signature 677 of the original signedData signerInfo. If the signature verification 678 fails, then the signedData/Receipt MUST NOT be created. 680 2. A Receipt structure is created. 682 2.1. The value of the Receipt version field is set to 1. 684 2.2. The object identifier from the contentType attribute included in 685 the original signedData signerInfo that includes the receiptRequest 686 attribute is copied into the Receipt contentType. 688 2.3. The original signedData signerInfo receiptRequest 689 signedContentIdentifier is copied into the Receipt 690 signedContentIdentifier. 692 2.4. The signature value from the original signedData signerInfo that 693 includes the receiptRequest attribute is copied into the Receipt 694 originatorSignatureValue. 696 3. The Receipt structure is ASN.1 DER encoded to produce a data stream, D1. 698 4. D1 is digested. The resulting digest value is included as the 699 messageDigest attribute in the signedAttributes of the signerInfo which 700 will eventually contain the signedData/Receipt signature value. 702 5. The digest value (msgSigDigest) calculated in Step 1 to verify the 703 signature of the original signedData signerInfo is included as the 704 msgSigDigest attribute in the signedAttributes of the signerInfo which will 705 eventually contain the signedData/Receipt signature value. 707 6. A contentType attribute including the id-ct-receipt object identifier 708 MUST be created and added to the signed attributes of the signerInfo which 709 will eventually contain the signedData/Receipt signature value. 711 7. A signingTime attribute indicating the time that the signedData/Receipt 712 is signed SHOULD be created and added to the signed attributes of the 713 signerInfo which will eventually contain the signedData/Receipt signature 714 value. Other attributes (except receiptRequest) may be added to the 715 signedAttributes of the signerInfo. 717 8. The signedAttributes (messageDigest, msgSigDigest, contentType and, 718 possibly, others) of the signerInfo are ASN.1 DER encoded and digested as 719 described in [CMS]. The resulting digest value is used to calculate the 720 signature value which is then included in the signedData/Receipt 721 signerInfo. 723 9. The ASN.1 DER encoded Receipt content MUST be directly encoded within 724 the signedData encapContentInfo eContent OCTET STRING defined in [CMS]. The 725 id-ct-receipt object identifier MUST be included in the signedData 726 encapContentInfo eContentType. This results in a single ASN.1 encoded 727 object composed of a signedData including the Receipt content. The Data 728 content type MUST NOT be used. The Receipt content MUST NOT be encapsulated 729 in a MIME header or any other header prior to being encoded as part of the 730 signedData object. 732 10. The signedData/Receipt is then put in an application/pkcs7-mime MIME 733 wrapper with the smime-type parameter set to "signed-receipt". This will 734 allow for identification of signed receipts without having to crack the 735 ASN.1 body. The smime-type parameter would still be set as normal in any 736 layer wrapped around this message. 738 11. If the signedData/Receipt is to be encrypted within an envelopedData 739 object, then an outer signedData object MUST be created that encapsulates 740 the envelopedData object, and a contentHints attribute with contentType set 741 to the id-ct-receipt object identifier MUST be included in the outer 742 signedData SignerInfo signedAttributes. When a receiving agent processes 743 the outer signedData object, the presence of the id-ct-receipt OID in the 744 contentHints contentType indicates that a signedData/Receipt is encrypted 745 within the envelopedData object encapsulated by the outer signedData. 747 All sending agents that support the generation of ESS signed receipts MUST 748 provide the ability to send encrypted signed receipts (that is, a 749 signedData/Receipt encapsulated within an envelopedData). The sending agent 750 MAY send an encrypted signed receipt in response to an 751 envelopedData-encapsulated signedData requesting a signed receipt. It is a 752 matter of local policy regarding whether or not the signed receipt should 753 be encrypted. The ESS signed receipt includes the message digest value 754 calculated for the original signedData object that requested the signed 755 receipt. If the original signedData object was sent encrypted within an 756 envelopedData object and the ESS signed receipt is sent unencrypted, then 757 the message digest value calculated for the original encrypted signedData 758 object is sent unencrypted. The responder should consider this when 759 deciding whether or not to encrypt the ESS signed receipt. 761 2.4.1 MLExpansionHistory Attributes and Receipts 763 An MLExpansionHistory attribute MUST NOT be included in the attributes of a 764 SignerInfo in a SignedData object that encapsulates a Receipt content. This 765 is true because when a SignedData/Receipt is sent to an MLA for 766 distribution, then the MLA must always encapsulate the received 767 SignedData/Receipt in an outer SignedData in which the MLA will include the 768 MLExpansionHistory attribute. The MLA cannot change the signedAttributes of 769 the received SignedData/Receipt object, so it can't add the 770 MLExpansionHistory to the SignedData/Receipt. 772 2.5 Determining the Recipients of the Signed Receipt 774 If a signed receipt was created by the process described in the sections 775 above, then the software MUST use the following process to determine to 776 whom the signed receipt should be sent. 778 1. The receiptsTo field must be present in the receiptRequest attribute. 779 The software initiates the sequence of recipients with the value(s) of 780 receiptsTo. 782 2. If the MlExpansionHistory attribute is present in the outer SignedData 783 block, and the last MLData contains an MLReceiptPolicy value of insteadOf, 784 then the software replaces the sequence of recipients with the value(s) of 785 insteadOf. 787 3. If the MlExpansionHistory attribute is present in the outer SignedData 788 block and the last MLData contains an MLReceiptPolicy value of 789 inAdditionTo, then the software adds the value(s) of inAdditionTo to the 790 sequence of recipients. 792 2.6. Signed Receipt Validation 794 A signed receipt is communicated as a single ASN.1 encoded object composed 795 of a signedData object directly including a Receipt content. It is 796 identified by the presence of the id-ct-receipt object identifier in the 797 encapContentInfo eContentType value of the signedData object including the 798 Receipt content. 800 Although receipients are not supposed to send more than one signed receipt, 801 receiving agents SHOULD be able to accept multiple signed receipts from a 802 recipient. 804 A signedData/Receipt is validated as follows: 806 1. ASN.1 decode the signedData object including the Receipt content. 808 2. Extract the contentType, signedContentIdentifier, and 809 originatorSignatureValue from the decoded Receipt structure to identify the 810 original signedData signerInfo that requested the signedData/Receipt. 812 3. Acquire the message signature digest value calculated by the sender to 813 generate the signature value included in the original signedData signerInfo 814 that requested the signedData/Receipt. 816 3.1. If the sender-calculated message signature digest value has been 817 saved locally by the sender, it must be located and retrieved. 819 3.2. If it has not been saved, then it must be re-calculated based on 820 the original signedData content and signedAttributes as described in 821 [CMS]. 823 4. The message signature digest value calculated by the sender is then 824 compared with the value of the msgSigDigest signedAttribute included in the 825 signedData/Receipt signerInfo. If these digest values are identical, then 826 that proves that the message signature digest value calculated by the 827 recipient based on the received original signedData object is the same as 828 that calculated by the sender. This proves that the recipient received 829 exactly the same original signedData content and signedAttributes as sent 830 by the sender because that is the only way that the recipient could have 831 calculated the same message signature digest value as calculated by the 832 sender. If the digest values are different, then the signedData/Receipt 833 signature verification process fails. 835 5. Acquire the digest value calculated by the sender for the Receipt 836 content constructed by the sender (including the contentType, 837 signedContentIdentifier, and signature value that were included in the 838 original signedData signerInfo that requested the signedData/Receipt). 840 5.1. If the sender-calculated Receipt content digest value has been 841 saved locally by the sender, it must be located and retrieved. 843 5.2. If it has not been saved, then it must be re-calculated. As 844 described in section 2.4 above, step 2, create a Receipt structure 845 including the contentType, signedContentIdentifier and signature value 846 that were included in the original signedData signerInfo that requested 847 the signed receipt. The Receipt structure is then ASN.1 DER encoded to 848 produce a data stream which is then digested to produce the Receipt 849 content digest value. 851 6. The Receipt content digest value calculated by the sender is then 852 compared with the value of the messageDigest signedAttribute included in 853 the signedData/Receipt signerInfo. If these digest values are identical, 854 then that proves that the values included in the Receipt content by the 855 recipient are identical to those that were included in the original 856 signedData signerInfo that requested the signedData/Receipt. This proves 857 that the recipient received the original signedData signed by the sender, 858 because that is the only way that the recipient could have obtained the 859 original signedData signerInfo signature value for inclusion in the Receipt 860 content. If the digest values are different, then the signedData/Receipt 861 signature verification process fails. 863 7. The ASN.1 DER encoded signedAttributes of the signedData/Receipt 864 signerInfo are digested as described in [CMS]. 866 8. The resulting digest value is then used to verify the signature value 867 included in the signedData/Receipt signerInfo. If the signature 868 verification is successful, then that proves the integrity of the 869 signedData/receipt signerInfo signedAttributes and authenticates the 870 identity of the signer of the signedData/Receipt signerInfo. Note that the 871 signedAttributes include the recipient-calculated Receipt content digest 872 value (messageDigest attribute) and recipient-calculated message signature 873 digest value (msgSigDigest attribute). Therefore, the aforementioned 874 comparison of the sender-generated and recipient-generated digest values 875 combined with the successful signedData/Receipt signature verification 876 proves that the recipient received the exact original signedData content 877 and signedAttributes (proven by msgSigDigest attribute) that were signed by 878 the sender of the original signedData object (proven by messageDigest 879 attribute). If the signature verification fails, then the 880 signedData/Receipt signature verification process fails. 882 The signature verification process for each signature algorithm that is 883 used in conjunction with the CMS protocol is specific to the algorithm. 884 These processes are described in documents specific to the algorithms. 886 2.7 Receipt Request Syntax 888 A receiptRequest attribute value has ASN.1 type ReceiptRequest. Use the 889 receiptRequest attribute only within the signed attributes associated with 890 a signed message. 892 ReceiptRequest ::= SEQUENCE { 893 signedContentIdentifier ContentIdentifier, 894 receiptsFrom ReceiptsFrom, 895 receiptsTo SEQUENCE SIZE (1..ub-receiptsTo)) OF GeneralNames } 897 ub-receiptsTo INTEGER ::= 16 899 id-aa-receiptRequest OBJECT IDENTIFIER ::= { iso(1) member-body(2) 900 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 1} 902 ContentIdentifier ::= OCTET STRING 904 id-aa-contentIdentifier OBJECT IDENTIFIER ::= { iso(1) member-body(2) 905 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 7} 907 A signedContentIdentifier MUST be created by the message originator when 908 creating a receipt request. To ensure global uniqueness, the minimal 909 signedContentIdentifier SHOULD contain a concatenation of user-specific 910 identification information (such as a user name or public keying material 911 identification information), a GeneralizedTime string, and a random number. 913 The receiptsFrom field is used by the originator to specify the recipients 914 requested to return a signed receipt. A CHOICE is provided to allow 915 specification of: 916 - receipts from all recipients are requested 917 - receipts from first tier (recipients that did not receive the 918 message as members of a mailing list) recipients are requested 919 - receipts from a specific list of recipients are requested 921 ReceiptsFrom ::= CHOICE { 922 allOrFirstTier [0] AllOrFirstTier, 923 -- formerly "allOrNone [0]AllOrNone" 924 receiptList [1] SEQUENCE OF GeneralNames } 926 AllOrFirstTier ::= INTEGER { -- Formerly AllOrNone 927 allReceipts (0), 928 firstTierRecipients (1) } 930 The receiptsTo field is used by the originator to identify the user(s) to 931 whom the identified recipient should send signed receipts. The message 932 originator MUST populate the receiptsTo field with a GeneralNames for each 933 entity to whom the recipient should send the signed receipt. If the message 934 originator wants the recipient to send the signed receipt to the 935 originator, then the originator MUST include a GeneralNames for itself in 936 the receiptsTo field. 938 2.8 Receipt Syntax 940 Receipts are represented using a new content type, Receipt. The Receipt 941 content type shall have ASN.1 type Receipt. Receipts must be encapsulated 942 within a SignedData message. 944 Receipt ::= SEQUENCE { 945 version ESSVersion, 946 contentType ContentType, 947 signedContentIdentifier ContentIdentifier, 948 originatorSignatureValue OCTET STRING } 950 id-ct-receipt OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) 951 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-ct(1) 1} 953 ESSVersion ::= INTEGER { v1(1) } 955 The version field defines the syntax version number, which is 1 for this 956 version of the standard. 958 2.9 Content Hints 960 Many applications find it useful to have information that describes the 961 innermost signed content of a multi-layer message available on the 962 outermost signature layer. The contentHints attribute provides such 963 information. 965 Content-hints attribute values have ASN.1 type contentHints. 967 ContentHints ::= SEQUENCE { 968 contentDescription UTF8String (SIZE (1..MAX)) OPTIONAL, 969 contentType ContentType } 971 id-aa-contentHint OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) 972 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 4} 974 The contentDescription field may be used to provide information that the 975 recipient may use to select protected messages for processing, such as a 976 message subject. If this field is set, then the attribute is expected to 977 appear on the signedData object enclosing an envelopedData object and not 978 on the inner signedData object. The (SIZE (1..MAX)) construct constrains 979 the sequence to have at least one entry. MAX indicates the upper bound is 980 unspecified. Implementations are free to choose an upper bound that suits 981 their environment. 983 Messages which contain a signedData object wrapped around an envelopedData 984 object, thus masking the inner content type of the message, SHOULD include 985 a contentHints attribute, except for the case of the data content type. 986 Specific message content types may either force or preclude the inclusion 987 of the contentHints attribute. For example, when a signedData/Receipt is 988 encrypted within an envelopedData object, an outer signedData object MUST 989 be created that encapsulates the envelopedData object and a contentHints 990 attribute with contentType set to the id-ct-receipt object identifier MUST 991 be included in the outer signedData SignerInfo signedAttributes. 993 2.10 Message Signature Digest Attribute 995 The msgSigDigest attribute can only be used in the signed attributes of a 996 signed receipt. It contains the digest of the ASN.1 DER encoded 997 signedAttributes included in the original signedData that requested the 998 signed receipt. Only one msgSigDigest attribute can appear in an signed 999 attributes set. It is defined as follows: 1001 msgSigDigest ::= OCTET STRING 1003 id-aa-msgSigDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2) 1004 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 5} 1006 2.11 Signed Content Reference Attribute 1008 The contentReference attribute is a link from one SignedData to another. It 1009 may be used to link a reply to the original message to which it refers, or 1010 to incorporate by reference one SignedData into another. The first 1011 SignedData MUST include a contentIdentifier signed attribute, which SHOULD 1012 be constructed as specified in section 2.7. The second SignedData links to 1013 the first by including a ContentReference signed attribute containing the 1014 content type, content identifier, and signature value from the first 1015 SignedData. 1017 ContentReference ::= SEQUENCE { 1018 contentType ContentType, 1019 signedContentIdentifier ContentIdentifier, 1020 originatorSignatureValue OCTET STRING } 1022 id-aa-contentReference OBJECT IDENTIFIER ::= { iso(1) member-body(2) 1023 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 10 } 1025 3. Security Labels 1027 This section describes the syntax to be used for security labels that can 1028 optionally be associated with S/MIME encapsulated data. A security label is 1029 a set of security information regarding the sensitivity of the content that 1030 is protected by S/MIME encapsulation. 1032 "Authorization" is the act of granting rights and/or privileges to users 1033 permitting them access to an object. "Access control" is a means of 1034 enforcing these authorizations. The sensitivity information in a security 1035 label can be compared with a user's authorizations to determine if the user 1036 is allowed to access the content that is protected by S/MIME encapsulation. 1038 Security labels may be used for other purposes such as a source of routing 1039 information. The labels often describe ranked levels ("secret", 1040 "confidential", "restricted", and so on) or are role-based, describing 1041 which kind of people can see the information ("patient's health-care team", 1042 "medical billing agents", "unrestricted", and so on). 1044 3.1 Security Label Processing Rules 1046 A sending agent may include a security label attribute in the signed 1047 attributes of a signedData object. A receiving agent examines the security 1048 label on a received message and determines whether or not the recipient is 1049 allowed to see the contents of the message. 1051 3.1.1 Adding Security Labels 1053 A sending agent that is using security labels MUST put the security label 1054 attribute in the signedAttributes field of a SignerInfo block. The security 1055 label attribute MUST NOT be included in the unsigned attributes. Integrity 1056 and authentication security services MUST be applied to the security label, 1057 therefore it MUST be included as an signed attribute, if used. This causes 1058 the security label attribute to be part of the data that is hashed to form 1059 the SignerInfo signature value. A SignerInfo block MUST NOT have more than 1060 one security label signed attribute. 1062 When there are multiple SignedData blocks applied to a message, a security 1063 label attribute may be included in either the inner signature, outer 1064 signature, or both. A security label signed attribute may be included in a 1065 signedAttributes field within the inner SignedData block. The inner 1066 security label will include the sensitivities of the original content and 1067 will be used for access control decisions related to the plaintext 1068 encapsulated content. The inner signature provides authentication of the 1069 inner security label and cryptographically protects the original signer's 1070 inner security label of the original content. 1072 When the originator signs the plaintext content and signed attributes, the 1073 inner security label is bound to the plaintext content. An intermediate 1074 entity cannot change the inner security label without invalidating the 1075 inner signature. The confidentiality security service can be applied to the 1076 inner security label by encrypting the entire inner signedData object 1077 within an EnvelopedData block. 1079 A security label signed attribute may also be included in a 1080 signedAttributes field within the outer SignedData block. The outer 1081 security label will include the sensitivities of the encrypted message and 1082 will be used for access control decisions related to the encrypted message 1083 and for routing decisions. The outer signature provides authentication of 1084 the outer security label (as well as for the encapsulated content which may 1085 include nested S/MIME messages). 1087 There can be multiple SignerInfos within a SignedData object, and each 1088 SignerInfo may include signedAttributes. Therefore, a single SignedData 1089 object may include multiple eSSSecurityLabels, each SignerInfo having an 1090 eSSSecurityLabel attribute. For example, an originator can send a signed 1091 message with two SignerInfos, one containing a DSS signature, the other 1092 containing an RSA signature. If any of the SignerInfos included in a 1093 SignedData object include an eSSSecurityLabel attribute, then all of the 1094 SignerInfos in that SignedData object MUST include an eSSSecurityLabel 1095 attribute and the value of each MUST be identical. 1097 3.1.2 Processing Security Labels 1099 Before processing an eSSSecurityLabel signedAttribute, the receiving agent 1100 MUST verify the signature of the SignerInfo which covers the 1101 eSSSecurityLabel attribute. A recipient MUST NOT process an 1102 eSSSecurityLabel attribute that has not been verified. 1104 A receiving agent MUST process the eSSSecurityLabel attribute, if present, 1105 in each SignerInfo in the SignedData object for which it verifies the 1106 signature. This may result in the receiving agent processing multiple 1107 eSSSecurityLabels included in a single SignedData object. Because all 1108 eSSSecurityLabels in a SignedData object must be identical, the receiving 1109 agent processes (such as performing access control) on the first 1110 eSSSecurityLabel that it encounters in a SignerInfo that it verifies, and 1111 then ensures that all other eSSSecurityLabels in signerInfos that it 1112 verifies are identical to the first one encountered. If the 1113 eSSSecurityLabels in the signerInfos that it verifies are not all 1114 identical, then the receiving agent MUST warn the user of this condition. 1116 Receiving agents SHOULD have a local policy regarding whether or not to 1117 show the inner content of a signedData object that includes an 1118 eSSSecurityLabel security-policy-identifier that the processing software 1119 does not recognize. If the receiving agent does not recognize the 1120 eSSSecurityLabel security-policy-identifier value, then it SHOULD stop 1121 processing the message and indicate an error. 1123 3.2 Syntax of eSSSecurityLabel 1125 The eSSSecurityLabel syntax is derived directly from [MTSABS] ASN.1 module. 1126 (The MTSAbstractService module begins with "DEFINITIONS IMPLICIT TAGS 1127 ::=".) Further, the eSSSecurityLabel syntax is compatible with that used in 1128 [MSP4]. 1130 ESSSecurityLabel ::= SET { 1131 security-policy-identifier SecurityPolicyIdentifier, 1132 security-classification SecurityClassification OPTIONAL, 1133 privacy-mark ESSPrivacyMark OPTIONAL, 1134 security-categories SecurityCategories OPTIONAL } 1136 id-aa-securityLabel OBJECT IDENTIFIER ::= { iso(1) member-body(2) 1137 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 2} 1139 SecurityPolicyIdentifier ::= OBJECT IDENTIFIER 1141 SecurityClassification ::= INTEGER { 1142 unmarked (0), 1143 unclassified (1), 1144 restricted (2), 1145 confidential (3), 1146 secret (4), 1147 top-secret (5) } (0..ub-integer-options) 1149 ub-integer-options INTEGER ::= 256 1151 ESSPrivacyMark ::= CHOICE { 1152 pString PrintableString (SIZE (1..ub-privacy-mark-length)), 1153 utf8String UTF8String (SIZE (1..MAX)) 1154 } 1156 ub-privacy-mark-length INTEGER ::= 128 1158 SecurityCategories ::= SET SIZE (1..ub-security-categories) OF 1159 SecurityCategory 1161 ub-security-categories INTEGER ::= 64 1163 SecurityCategory ::= SEQUENCE { 1164 type [0] OBJECT IDENTIFIER, 1165 value [1] ANY DEFINED BY type -- defined by type 1166 } 1168 --Note: The aforementioned SecurityCategory syntax produces identical 1169 --hex encodings as the following SecurityCategory syntax that is 1170 --documented in the X.411 specification: 1171 -- 1172 --SecurityCategory ::= SEQUENCE { 1173 -- type [0] SECURITY-CATEGORY, 1174 -- value [1] ANY DEFINED BY type } 1175 -- 1176 --SECURITY-CATEGORY MACRO ::= 1177 --BEGIN 1178 --TYPE NOTATION ::= type | empty 1179 --VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER) 1180 --END 1182 3.3 Security Label Components 1184 This section gives more detail on the the various components of the 1185 eSSSecurityLabel syntax. 1187 3.3.1 Security Policy Identifier 1189 A security policy is a set of criteria for the provision of security 1190 services. The eSSSecurityLabel security-policy-identifier is used to 1191 identify the security policy in force to which the security label relates. 1192 It indicates the semantics of the other security label components. 1194 3.3.2 Security Classification 1196 This specification defines the use of the Security Classification field 1197 exactly as is specified in the X.411 Recommendation, which states in part: 1199 If present, a security-classification may have one of a hierarchical 1200 list of values. The basic security-classification hierarchy is defined 1201 in this Recommendation, but the use of these values is defined by the 1202 security-policy in force. Additional values of security-classification, 1203 and their position in the hierarchy, may also be defined by a 1204 security-policy as a local matter or by bilateral agreement. The basic 1205 security-classification hierarchy is, in ascending order: unmarked, 1206 unclassified, restricted, confidential, secret, top-secret. 1208 This means that the security policy in force (identified by the 1209 eSSSecurityLabel security-policy-identifier) defines the 1210 SecurityClassification integer values and their meanings. 1212 An organization can develop its own security policy that defines the 1213 SecurityClassification INTEGER values and their meanings. However, the 1214 general interpretation of the X.411 specification is that the values of 0 1215 through 5 are reserved for the "basic hierarchy" values of unmarked, 1216 unclassified, restricted, confidential, secret, and top-secret. Note that 1217 X.411 does not provide the rules for how these values are used to label 1218 data and how access control is performed using these values. 1220 There is no universal definition of the rules for using these "basic 1221 hierarchy" values. Each organization (or group of organizations) will 1222 define a security policy which documents how the "basic hierarchy" values 1223 are used (if at all) and how access control is enforced (if at all) within 1224 their domain. 1226 Therefore, the security-classification value MUST be accompanied by a 1227 security-policy-identifier value to define the rules for its use. For 1228 example, a company's "secret" classification may convey a different meaning 1229 than the US Government "secret" classification. In summary, a security 1230 policy SHOULD NOT use integers 0 through 5 for other than their X.411 1231 meanings, and SHOULD instead use other values in a hierarchical fashion. 1233 Note that the set of valid security-classification values MUST be 1234 hierarchical, but these values do not necessarily need to be in ascending 1235 numerical order. Further, the values do not need to be contiguous. 1237 For example, in the Defense Message System 1.0 security policy, the 1238 security-classification value of 11 indicates Sensitive-But-Unclassified 1239 and 5 indicates top-secret. The hierarchy of sensitivity ranks top-secret 1240 as more sensitive than Sensitive-But-Unclassified even though the numerical 1241 value of top-secret is less than Sensitive-But-Unclassified. 1243 (Of course, if security-classification values are both hierarchical and in 1244 ascending order, a casual reader of the security policy is more likely to 1245 understand it.) 1247 An example of a security policy that does not use any of the X.411 values 1248 might be: 1249 10 -- anyone 1250 15 -- Morgan Corporation and its contractors 1251 20 -- Morgan Corporation employees 1252 25 -- Morgan Corporation board of directors 1254 An example of a security policy that uses part of the X.411 hierarchy might 1255 be: 1256 0 -- unmarked 1257 1 -- unclassified, can be read by everyone 1258 2 -- restricted to Timberwolf Productions staff 1259 6 -- can only be read to Timberwolf Productions executives 1261 3.3.3 Privacy Mark 1263 If present, the eSSSecurityLabel privacy-mark is not used for access 1264 control. The content of the eSSSecurityLabel privacy-mark may be defined by 1265 the security policy in force (identified by the eSSSecurityLabel 1266 security-policy-identifier) which may define a list of values to be used. 1267 Alternately, the value may be determined by the originator of the 1268 security-label. 1270 3.3.4 Security Categories 1272 If present, the eSSSecurityLabel security-categories provide further 1273 granularity for the sensitivity of the message. The security policy in 1274 force (identified by the eSSSecurityLabel security-policy-identifier) is 1275 used to indicate the syntaxes that are allowed to be present in the 1276 eSSSecurityLabel security-categories. Alternately, the security-categories 1277 and their values may be defined by bilateral agreement. 1279 3.4 Equivalent Security Labels 1281 Because organizations are allowed to define their own security policies, 1282 many different security policies will exist. Some organizations may wish to 1283 create equivalencies between their security policies with the security 1284 policies of other organizations. For example, the Acme Company and the 1285 Widget Corporation may reach a bilateral agreement that the "Acme private" 1286 security-classification value is equivalent to the "Widget sensitive" 1287 security-classification value. 1289 Receiving agents MUST NOT process an equivalentLabels attribute in a 1290 message if the agent does not trust the signer of that attribute to 1291 translate the original eSSSecurityLabel values to the security policy 1292 included in the equivalentLabels attribute. Receiving agents have the 1293 option to process equivalentLabels attributes but do not have to. It is 1294 acceptable for a receiving agent to only process eSSSecurityLabels. All 1295 receiving agents SHOULD recognize equivalentLabels attributes even if they 1296 do not process them. 1298 3.4.1 Creating Equivalent Labels 1300 The EquivalentLabels signed attribute is defined as: 1302 EquivalentLabels ::= SEQUENCE OF ESSSecurityLabel 1304 id-aa-equivalentLabels OBJECT IDENTIFIER ::= { iso(1) member-body(2) 1305 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 9} 1307 As stated earlier, the ESSSecurityLabel contains the sensitivity values 1308 selected by the original signer of the signedData. If an ESSSecurityLabel 1309 is present in a signerInfo, all signerInfos in the signedData MUST contain 1310 an ESSSecurityLabel and they MUST all be identical. In addition to an 1311 ESSSecurityLabel, a signerInfo MAY also include an equivalentLabels signed 1312 attribute. If present, the equivalentLabels attribute MUST include one or 1313 more security labels that are believed by the signer to be semantically 1314 equivalent to the ESSSecurityLabel attribute included in the same 1315 signerInfo. 1317 All security-policy object identifiers MUST be unique in the set of 1318 ESSSecurityLabel and EquivalentLabels security labels. Before using an 1319 EquivalentLabels attribute, a receiving agent MUST ensure that all 1320 security-policy OIDs are unique in the security label or labels included in 1321 the EquivalentLabels. Once the receiving agent selects the security label 1322 (within the EquivalentLabels) to be used for processing, then the 1323 security-policy OID of the selected EquivalentLabels security label MUST be 1324 compared with the ESSSecurityLabel security-policy OID to ensure that they 1325 are unique. 1327 In the case that an ESSSecurityLabel attribute is not included in a 1328 signerInfo, then an EquivalentLabels attribute may still be included. For 1329 example, in the Acme security policy, the absence of an ESSSecurityLabel 1330 could be defined to equate to a security label composed of the Acme 1331 security-policy OID and the "unmarked" security-classification. 1333 Note that equivalentLabels MUST NOT be used to convey security labels that 1334 are semantically different from the ESSSecurityLabel included in the 1335 signerInfos in the signedData. If an entity needs to apply a security label 1336 that is semantically different from the ESSSecurityLabel, then it MUST 1337 include the sematically different security label in an outer signedData 1338 object that encapsulates the signedData object that includes the 1339 ESSSecurityLabel. 1341 If present, the equivalentLabels attribute MUST be an signed attribute; it 1342 MUST NOT be an unsigned attribute. [CMS] defines signedAttributes as a SET 1343 OF Attribute. A signerInfo MUST NOT include multiple instances of the 1344 equivalentLabels attribute. CMS defines the ASN.1 syntax for the signed 1345 attributes to include attrValues SET OF AttributeValue. A equivalentLabels 1346 attribute MUST only include a single instance of AttributeValue. There MUST 1347 NOT be zero or multiple instances of AttributeValue present in the 1348 attrValues SET OF AttributeValue. 1350 3.4.2 Processing Equivalent Labels 1352 A receiving agent SHOULD process the ESSSecurityLabel before processing any 1353 EquivalentLabels. If the policy in the ESSSecurityLabel is understood by 1354 the receiving agent, it MUST process that label and MUST ignore all 1355 EquivalentLabels. 1357 When processing an EquivalentLabels attribute, the receiving agent MUST 1358 validate the signature on the EquivalentLabels attribute. A receiving agent 1359 MUST NOT act on an equivalentLabels attribute for which the signature could 1360 not be validated, and MUST NOT act on an equivalentLabels attribute unless 1361 that attribute is signed by an entity trusted to translate the original 1362 eSSSecurityLabel values to the security policy included in the 1363 equivalentLabels attribute. Determining who is allowed to specify 1364 equivalence mappings is a local policy. If a message has more than one 1365 EquivalentLabels attribute, the receiving agent SHOULD process the first 1366 one that it reads and validates that contains the security policy of 1367 interest to the receiving agent. 1369 4. Mail List Management 1371 Sending agents must create recipient-specific data structures for each 1372 recipient of an encrypted message. This process can impair performance for 1373 messages sent to a large number of recipients. Thus, Mail List Agents 1374 (MLAs) that can take a single message and perform the recipient-specific 1375 encryption for every recipient are often desired. 1377 An MLA appears to the message originator as a normal message recipient, but 1378 the MLA acts as a message expansion point for a Mail List (ML). The sender 1379 of a message directs the message to the MLA, which then redistributes the 1380 message to the members of the ML. This process offloads the per-recipient 1381 processing from individual user agents and allows for more efficient 1382 management of large MLs. MLs are true message recipients served by MLAs 1383 that provide cryptographic and expansion services for the mailing list. 1385 In addition to cryptographic handling of messages, secure mailing lists 1386 also have to prevent mail loops. A mail loop is where one mailing list is a 1387 member of a second mailing list, and the second mailing list is a member of 1388 the first. A message will go from one list to the other in a 1389 rapidly-cascading succession of mail that will be distributed to all other 1390 members of both lists. 1392 To prevent mail loops, MLAs use the mlExpansionHistory attribute of the 1393 outer signature of a triple wrapped message. The mlExpansionHistory 1394 attribute is essentially a list of every MLA that has processed the 1395 message. If an MLA sees its own unique entity identifier in the list, it 1396 knows that a loop has been formed, and does not send the message to the 1397 list again. 1399 4.1 Mail List Expansion 1401 Mail list expansion processing is noted in the value of the 1402 mlExpansionHistory attribute, located in the signed attributes of the MLA's 1403 SignerInfo block. The MLA creates or updates the signed mlExpansionHistory 1404 attribute value each time the MLA expands and signs a message for members 1405 of a mail list. 1407 The MLA MUST add an MLData record containing the MLA's identification 1408 information, date and time of expansion, and optional receipt policy to the 1409 end of the mail list expansion history sequence. If the mlExpansionHistory 1410 attribute is absent, then the MLA MUST add the attribute and the current 1411 expansion becomes the first element of the sequence. If the 1412 mlExpansionHistory attribute is present, then the MLA MUST add the current 1413 expansion information to the end of the existing MLExpansionHistory 1414 sequence. Only one mlExpansionHistory attribute can be included in the 1415 signedAttributes of a SignerInfo. 1417 Note that if the mlExpansionHistory attribute is absent, then the recipient 1418 is a first tier message recipient. 1420 There can be multiple SignerInfos within a SignedData object, and each 1421 SignerInfo may include signedAttributes. Therefore, a single SignedData 1422 object may include multiple SignerInfos, each SignerInfo having a 1423 mlExpansionHistory attribute. For example, an MLA can send a signed message 1424 with two SignerInfos, one containing a DSS signature, the other containing 1425 an RSA signature. 1427 If an MLA creates a SignerInfo that includes an mlExpansionHistory 1428 attribute, then all of the SignerInfos created by the MLA for that 1429 SignedData object MUST include an mlExpansionHistory attribute, and the 1430 value of each MUST be identical. Note that other agents might later add 1431 SignerInfo attributes to the SignedData block, and those additional 1432 SignerInfos might not include mlExpansionHistory attributes. 1434 A recipient MUST verify the signature of the SignerInfo which covers the 1435 mlExpansionHistory attribute before processing the mlExpansionHistory, and 1436 MUST NOT process the mlExpansionHistory attribute unless the signature over 1437 it has been verified. If a SignedData object has more than one SignerInfo 1438 that has an mlExpansionHistory attribute, the recipient MUST compare the 1439 mlExpansionHistory attributes in all the SignerInfos that it has verified, 1440 and MUST NOT process the mlExpansionHistory attribute unless every verified 1441 mlExpansionHistory attribute in the SignedData block is identical. If the 1442 mlExpansionHistory attributes in the verified signerInfos are not all 1443 identical, then the receiving agent MUST stop processing the message and 1444 SHOULD notify the user or MLA administrator of this error condition. In the 1445 mlExpansionHistory processing, SignerInfos that do not have an 1446 mlExpansionHistory attribute are ignored. 1448 4.1.1 Detecting Mail List Expansion Loops 1450 Prior to expanding a message, the MLA examines the value of any existing 1451 mail list expansion history attribute to detect an expansion loop. An 1452 expansion loop exists when a message expanded by a specific MLA for a 1453 specific mail list is redelivered to the same MLA for the same mail list. 1455 Expansion loops are detected by examining the mailListIdentifier field of 1456 each MLData entry found in the mail list expansion history. If an MLA finds 1457 its own identification information, then the MLA must discontinue expansion 1458 processing and should provide warning of an expansion loop to a human mail 1459 list administrator. The mail list administrator is responsible for 1460 correcting the loop condition. 1462 4.2 Mail List Agent Processing 1464 The first few paragraphs of this section provide a high-level description 1465 of MLA processing. The rest of the section provides a detailed description 1466 of MLA processing. 1468 MLA message processing depends on the structure of the S/MIME layers in the 1469 message sent to the MLA for expansion. In addition to sending triple 1470 wrapped messages to an MLA, an entity can send other types of messages to 1471 an MLA, such as: 1472 - a single wrapped signedData or envelopedData message 1473 - a double wrapped message (such as signed and enveloped, enveloped and 1474 signed, or signed and signed, and so on) 1475 - a quadruple-wrapped message (such as if a well-formed triple wrapped 1476 message was sent through a gateway that added an outer SignedData layer) 1478 In all cases, the MLA MUST parse all layers of the received message to 1479 determine if there are any signedData layers that include an 1480 eSSSecurityLabel signedAttribute. This may include decrypting an 1481 EnvelopedData layer to determine if an encapsulated SignedData layer 1482 includes an eSSSecurityLabel attribute. The MLA MUST fully process each 1483 eSSSecurityLabel attribute found in the various signedData layers, 1484 including performing access control checks, before distributing the message 1485 to the ML members. The details of the access control checks are beyond the 1486 scope of this document. The MLA MUST verify the signature of the signerInfo 1487 including the eSSSecurityLabel attribute before using it. 1489 In all cases, the MLA MUST sign the message to be sent to the ML members in 1490 a new "outer" signedData layer. The MLA MUST add or update an 1491 mlExpansionHistory attribute in the "outer" signedData that it creates to 1492 document MLA processing. If there was an "outer" signedData layer included 1493 in the original message received by the MLA, then the MLA-created "outer" 1494 signedData layer MUST include each signed attribute present in the 1495 original "outer" signedData layer, unless the MLA explicitly replaces an 1496 attribute (such as signingTime or mlExpansionHistory) with a new value. 1498 When an S/MIME message is received by the MLA, the MLA MUST first determine 1499 which received signedData layer, if any, is the "outer" signedData layer. 1500 To identify the received "outer" signedData layer, the MLA MUST verify the 1501 signature and fully process the signedAttributes in each of the 1502 outer signedData layers (working from the outside in) to determine if any 1503 of them either include an mlExpansionHistory attribute or encapsulate an 1504 envelopedData object. 1506 The MLA's search for the "outer" signedData layer is completed when it 1507 finds one of the following: 1508 - the "outer" signedData layer that includes an mlExpansionHistory 1509 attribute or encapsulates an envelopedData object 1510 - an envelopedData layer 1511 - the original content (that is, a layer that is neither envelopedData nor 1512 signedData). 1514 If the MLA finds an "outer" signedData layer, then the MLA MUST perform 1515 the following steps: 1517 1. Strip off all of the signedData layers that encapsulated the "outer" 1518 signedData layer 1520 2. Strip off the "outer" signedData layer itself (after remembering the 1521 included signedAttributes) 1523 3. Expand the envelopedData (if present) 1525 4. Sign the message to be sent to the ML members in a new "outer" 1526 signedData layer that includes the signedAttributes (unless explicitly 1527 replaced) from the original, received "outer" signedData layer. 1529 If the MLA finds an "outer" signedData layer that includes an 1530 mlExpansionHistory attribute AND the MLA subsequently finds an 1531 envelopedData layer buried deeper with the layers of the received message, 1532 then the MLA MUST strip off all of the signedData layers down to the 1533 envelopedData layer (including stripping off the original "outer" 1534 signedData layer) and MUST sign the expanded envelopedData in a new "outer" 1535 signedData layer that includes the signedAttributes (unless explicitly 1536 replaced) from the original, received "outer" signedData layer. 1538 If the MLA does not find an "outer" signedData layer AND does not find an 1539 envelopedData layer, then the MLA MUST sign the original, received message 1540 in a new "outer" signedData layer. If the MLA does not find an "outer" 1541 signedData AND does find an envelopedData layer then it MUST expand the 1542 envelopedData layer, if present, and sign it in a new "outer" signedData 1543 layer. 1545 4.2.1 Examples of Rule Processing 1547 The following examples help explain the rules above: 1549 1) A message (S1(Original Content)) (where S = SignedData) is sent to the 1550 MLA in which the signedData layer does not include an MLExpansionHistory 1551 attribute. The MLA verifies and fully processes the signedAttributes in S1. 1552 The MLA decides that there is not an original, received "outer" signedData 1553 layer since it finds the original content, but never finds an envelopedData 1554 and never finds an mlExpansionHistory attribute. The MLA calculates a new 1555 signedData layer, S2, resulting in the following message sent to the ML 1556 recipients: (S2(S1(Original Content))). The MLA includes an 1557 mlExpansionHistory attribute in S2. 1559 2) A message (S3(S2(S1(Original Content)))) is sent to the MLA in which 1560 none of the signedData layers includes an MLExpansionHistory attribute. The 1561 MLA verifies and fully processes the signedAttributes in S3, S2 and S1. The 1562 MLA decides that there is not an original, received "outer" signedData 1563 layer since it finds the original content, but never finds an envelopedData 1564 and never finds an mlExpansionHistory attribute. The MLA calculates a new 1565 signedData layer, S4, resulting in the following message sent to the ML 1566 recipients: (S4(S3(S2(S1(Original Content))))). The MLA includes an 1567 mlExpansionHistory attribute in S4. 1569 3) A message (E1(S1(Original Content))) (where E = envelopedData) is sent 1570 to the MLA in which S1 does not include an MLExpansionHistory attribute. 1571 The MLA decides that there is not an original, received "outer" signedData 1572 layer since it finds the E1 as the outer layer. The MLA expands the 1573 recipientInformation in E1. The MLA calculates a new signedData layer, S2, 1574 resulting in the following message sent to the ML recipients: 1575 (S2(E1(S1(Original Content)))). The MLA includes an mlExpansionHistory 1576 attribute in S2. 1578 4) A message (S2(E1(S1(Original Content)))) is sent to the MLA in which S2 1579 includes an MLExpansionHistory attribute. The MLA verifies the signature 1580 and fully processes the signedAttributes in S2. The MLA finds the 1581 mlExpansionHistory attribute in S2, so it decides that S2 is the "outer" 1582 signedData. The MLA remembers the signedAttributes included in S2 for later 1583 inclusion in the new outer signedData that it applies to the message. The 1584 MLA strips off S2. The MLA then expands the recipientInformation in E1 1585 (this invalidates the signature in S2 which is why it was stripped). The 1586 MLA calculates a new signedData layer, S3, resulting in the following 1587 message sent to the ML recipients: (S3(E1(S1(Original Content)))). The MLA 1588 includes in S3 the attributes from S2 (unless it specifically replaces an 1589 attribute value) including an updated mlExpansionHistory attribute. 1591 5) A message (S3(S2(E1(S1(Original Content))))) is sent to the MLA in which 1592 none of the signedData layers include an MLExpansionHistory attribute. The 1593 MLA verifies the signature and fully processes the signedAttributes in S3 1594 and S2. When the MLA encounters E1, then it decides that S2 is the "outer" 1595 signedData since S2 encapsulates E1. The MLA remembers the signedAttributes 1596 included in S2 for later inclusion in the new outer signedData that it 1597 applies to the message. The MLA strips off S3 and S2. The MLA then expands 1598 the recipientInformation in E1 (this invalidates the signatures in S3 and 1599 S2 which is why they were stripped). The MLA calculates a new signedData 1600 layer, S4, resulting in the following message sent to the ML recipients: 1601 (S4(E1(S1(Original Content)))). The MLA includes in S4 the attributes from 1602 S2 (unless it specifically replaces an attribute value) and includes a new 1603 mlExpansionHistory attribute. 1605 6) A message (S3(S2(E1(S1(Original Content))))) is sent to the MLA in which 1606 S3 includes an MLExpansionHistory attribute. In this case, the MLA verifies 1607 the signature and fully processes the signedAttributes in S3. The MLA finds 1608 the mlExpansionHistory in S3, so it decides that S3 is the "outer" 1609 signedData. The MLA remembers the signedAttributes included in S3 for later 1610 inclusion in the new outer signedData that it applies to the message. The 1611 MLA keeps on parsing encapsulated layers because it must determine if there 1612 are any eSSSecurityLabel attributes contained within. The MLA verifies the 1613 signature and fully processes the signedAttributes in S2. When the MLA 1614 encounters E1, then it strips off S3 and S2. The MLA then expands the 1615 recipientInformation in E1 (this invalidates the signatures in S3 and S2 1616 which is why they were stripped). The MLA calculates a new signedData 1617 layer, S4, resulting in the following message sent to the ML recipients: 1618 (S4(E1(S1(Original Content)))). The MLA includes in S4 the attributes from 1619 S3 (unless it specifically replaces an attribute value) including an 1620 updated mlExpansionHistory attribute. 1622 4.2.3 Processing Choices 1624 The processing used depends on the type of the outermost layer of the 1625 message. There are three cases for the type of the outermost data: 1626 - EnvelopedData 1627 - SignedData 1628 - data 1630 4.2.3.1 Processing for EnvelopedData 1632 1. The MLA locates its own RecipientInfo and uses the information it 1633 contains to obtain the message key. 1635 2. The MLA removes the existing recipientInfos field and replaces it with a 1636 new recipientInfos value built from RecipientInfo structures created for 1637 each member of the mailing list. The MLA also removes the existing 1638 originatorInfo field and replaces it with a new originatorInfo value built 1639 from information describing the MLA. 1641 3. The MLA encapsulates the expanded encrypted message in a SignedData 1642 block, adding an mlExpansionHistory attribute as described in the "Mail 1643 List Expansion" section to document the expansion. 1645 4. The MLA signs the new message and delivers the updated message to mail 1646 list members to complete MLA processing. 1648 4.2.3.2 Processing for SignedData 1650 MLA processing of multi-layer messages depends on the type of data in each 1651 of the layers. Step 3 below specifies that different processing will take 1652 place depending on the type of CMS message that has been signed. That is, 1653 it needs to know the type of data at the next inner layer, which may or may 1654 not be the innermost layer. 1656 1. The MLA verifies the signature value found in the outermost SignedData 1657 layer associated with the signed data. MLA processing of the message 1658 terminates if the message signature is invalid. 1660 2. If the outermost SignedData layer includes an signed mlExpansionHistory 1661 attribute, the MLA checks for an expansion loop as described in the 1662 "Detecting Mail List Expansion Loops" section, then go to step 3. If the 1663 outermost SignedData layer does not include an signed mlExpansionHistory 1664 attribute, the MLA signs the whole message (including this outermost 1665 SignedData layer that doesn't have an mlExpansionHistory attribute), and 1666 delivers the updated message to mail list members to complete MLA 1667 processing. 1669 3. Determine the type of the data that has been signed. That is, look at 1670 the type of data on the layer just below the SignedData, which may or may 1671 not be the "innermost" layer. Based on the type of data, perform either 1672 step 3.1 (EnvelopedData), step 3.2 (SignedData), or step 3.3 (all other 1673 types). 1675 3.1. If the signed data is EnvelopedData, the MLA performs expansion 1676 processing of the encrypted message as described previously. Note that 1677 this process invalidates the signature value in the outermost 1678 SignedData layer associated with the original encrypted message. 1679 Proceed to section 3.2 with the result of the expansion. 1681 3.2. If the signed data is SignedData, or is the result of expanding an 1682 EnvelopedData block in step 3.1: 1684 3.2.1. The MLA strips the existing outermost SignedData layer after 1685 remembering the value of the mlExpansionHistory and all other 1686 signed attributes in that layer, if present. 1688 3.2.2. If the signed data is EnvelopedData (from step 3.1), the MLA 1689 encapsulates the expanded encrypted message in a new outermost 1690 SignedData layer. On the other hand, if the signed data is 1691 SignedData (from step 3.2), the MLA encapsulates the signed data in 1692 a new outermost SignedData layer. 1694 3.2.3. The outermost signedData layer created by the MLA replaces 1695 the original outermost signedData layer. The MLA MUST create an 1696 signed attribute list for the new outermost signedData layer which 1697 MUST include each signed attribute present in the original 1698 outermost signedData layer, unless the MLA explicitly replaces one 1699 or more particular attributes with new value. A special case is the 1700 mlExpansionHistory attribute. The MLA MUST add an 1701 mlExpansionHistory signed attribute to the outer signedData layer 1702 as follows: 1704 3.2.3.1. If the original outermost SignedData layer included an 1705 mlExpansionHistory attribute, the attribute's value is copied 1706 and updated with the current ML expansion information as 1707 described in the "Mail List Expansion" section. 1709 3.2.3.2. If the original outermost SignedData layer did not 1710 include an mlExpansionHistory attribute, a new attribute value 1711 is created with the current ML expansion information as 1712 described in the "Mail List Expansion" section. 1714 3.3. If the signed data is not EnvelopedData or SignedData: 1716 3.3.1. The MLA encapsulates the received signedData object in an 1717 outer SignedData object, and adds an mlExpansionHistory attribute 1718 to the outer SignedData object containing the current ML expansion 1719 information as described in the "Mail List Expansion" section. 1721 4. The MLA signs the new message and delivers the updated message to mail 1722 list members to complete MLA processing. 1724 A flow chart for the above steps would be: 1726 1. Has a valid signature? 1727 YES -> 2. 1728 NO -> STOP. 1729 2. Does outermost SignedData layer contain mlExpansionHistory? 1730 YES -> Check it, then -> 3. 1731 NO -> Sign message (including outermost SignedData that 1732 doesn't have mlExpansionHistory), deliver it, STOP. 1733 3. Check type of data just below outermost SignedData. 1734 EnvelopedData -> 3.1. 1735 SignedData -> 3.2. 1736 all others -> 3.3. 1737 3.1. Expand the encrypted message, then -> 3.2. 1738 3.2. -> 3.2.1. 1739 3.2.1. Strip outermost SignedData layer, note value of mlExpansionHistory 1740 and other signed attributes, then -> 3.2.2. 1741 3.2.2. Encapsulate in new signature, then -> 3.2.3. 1742 3.2.3. Create new signedData layer. Was there an old mlExpansionHistory? 1743 YES -> copy the old mlExpansionHistory values, then -> 4. 1744 NO -> create new mlExpansionHistory value, then -> 4. 1745 3.3. Encapsulate in a SignedData layer and add an mlExpansionHistory 1746 attribute, then -> 4. 1747 4. Sign message, deliver it, STOP. 1749 4.2.3.3 Processing for data 1751 1. The MLA encapsulates the message in a SignedData layer, and adds an 1752 mlExpansionHistory attribute containing the current ML expansion 1753 information as described in the "Mail List Expansion" section. 1755 2. The MLA signs the new message and delivers the updated message to mail 1756 list members to complete MLA processing. 1758 4.3 Mail List Agent Signed Receipt Policy Processing 1760 If a mailing list (B) is a member of another mailing list (A), list B often 1761 needs to propagate forward the mailing list receipt policy of A. As a 1762 general rule, a mailing list should be conservative in propagating forward 1763 the mailing list receipt policy because the ultimate recipient need only 1764 process the last item in the ML expansion history. The MLA builds the 1765 expansion history to meet this requirement. 1767 The following table describes the outcome of the union of mailing list A's 1768 policy (the rows in the table) and mailing list B's policy (the columns in 1769 the table). 1771 | B's policy 1772 A's policy | none insteadOf inAdditionTo missing 1773 ------------------------------------------------------------------------- 1774 none | none none none none 1775 insteadOf | none insteadOf(B) *1 insteadOf(A) 1776 inAdditionTo | none insteadOf(B) *2 inAdditionTo(A) 1777 missing | none insteadOf(B) inAdditionTo(B) missing 1779 *1 = insteadOf(insteadOf(A) + inAdditionTo(B)) 1780 *2 = inAdditionTo(inAdditionTo(A) + inAdditionTo(B)) 1782 4.4 Mail List Expansion History Syntax 1784 An mlExpansionHistory attribute value has ASN.1 type MLExpansionHistory. If 1785 there are more than ub-ml-expansion-history mailing lists in the sequence, 1786 the receiving agent should provide notification of the error to a human 1787 mail list administrator. The mail list administrator is responsible for 1788 correcting the overflow condition. 1790 MLExpansionHistory ::= SEQUENCE 1791 SIZE (1..ub-ml-expansion-history) OF MLData 1793 id-aa-mlExpandHistory OBJECT IDENTIFIER ::= { iso(1) member-body(2) 1794 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 3} 1796 ub-ml-expansion-history INTEGER ::= 64 1798 MLData contains the expansion history describing each MLA that has 1799 processed a message. As an MLA distributes a message to members of an ML, 1800 the MLA records its unique identifier, date and time of expansion, and 1801 receipt policy in an MLData structure. 1803 MLData ::= SEQUENCE { 1804 mailListIdentifier EntityIdentifier, 1805 expansionTime GeneralizedTime, 1806 mlReceiptPolicy MLReceiptPolicy OPTIONAL } 1808 EntityIdentifier ::= CHOICE { 1809 issuerAndSerialNumber IssuerAndSerialNumber, 1810 subjectKeyIdentifier SubjectKeyIdentifier } 1812 The receipt policy of the ML can withdraw the originator's request for the 1813 return of a signed receipt. However, if the originator of the message has 1814 not requested a signed receipt, the MLA cannot request a signed receipt. In 1815 the event that a ML's signed receipt policy supersedes the originator's 1816 request for signed receipts, such that the originator will not receive any 1817 signed receipts, then the MLA MAY inform the originator of that fact. 1819 When present, the mlReceiptPolicy specifies a receipt policy that 1820 supersedes the originator's request for signed receipts. The policy can be 1821 one of three possibilities: receipts MUST NOT be returned (none); receipts 1822 should be returned to an alternate list of recipients, instead of to the 1823 originator (insteadOf); or receipts should be returned to a list of 1824 recipients in addition to the originator (inAdditionTo). 1826 MLReceiptPolicy ::= CHOICE { 1827 none [0] NULL, 1828 insteadOf [1] SEQUENCE SIZE (1..MAX) OF GeneralNames, 1829 inAdditionTo [2] SEQUENCE SIZE (1..MAX) OF GeneralNames } 1831 5. Signing Certificate Attribute 1833 Concerns have been raised over the fact that the certificate which the 1834 signer of a CMS SignedData object desired to be bound into the verification 1835 process of the SignedData object is not cryptographically bound into the 1836 signature itself. This section addresses this issue by creating a new 1837 attribute to be placed in the signed attributes section of a SignerInfo 1838 object. 1840 This section also presents a description of a set of possible attacks 1841 dealing with the substitution of one certificate to verify the signature 1842 for the desired certificate. A set of ways for preventing or addressing 1843 these attacks is presented to deal with the simplest of the attacks. 1845 Attribute certificates can be used as part of a signature verification 1846 process. There is no way in CMS to include the list of attribute 1847 certificates to be used in the verification process. The set of attribute 1848 certificates used in the signature verification process needs to have the 1849 ability for the signer to restrict the set of certificates. This 1850 information needs to be encoded in a manner that is covered by the 1851 signature on the SignedData object. The methods in this section allows for 1852 the set of attribute certificates to be listed as part of the signing 1853 certificate attribute. 1855 Explicit certificate policies can also be used as part of a signature 1856 verification process. If a signer desires to state an explicit certificate 1857 policy that should be used when validating the signature, that policy needs 1858 to be cryptographically bound into the signing process. The methods 1859 described in this section allows for a set of certificate policy statements 1860 to be listed as part of the signing certificate attribute. 1862 5.1. Attack Descriptions 1864 At least three different attacks can be launched against a possible 1865 signature verification process by replacing the certificate or certficates 1866 used in the signature verification process. 1868 5.1.1 Substitution Attack Description 1870 The first attack deals with simple substitution of one certificate for 1871 another certificate. In this attack, the issuer and serial number in the 1872 SignerInfo is modified to refer to a new certificate. This new certificate 1873 is used during the signature verification process. 1875 The first version of this attack is a simple denial of service attack where 1876 an invalid certificate is substituted for the valid certificate. This 1877 renders the message unverifiable, as the public key in the certificate no 1878 longer matches the private key used to sign the message. 1880 The second version is a substitution of one valid certificate for the 1881 original valid certificate where the public keys in the certificates match. 1882 This allows the signature to be validated under potentially different 1883 certificate constraints than the originator of the message intended. 1885 5.1.2 Reissue of Certificate Description 1887 The second attack deals with a certificate authority (CA) re-issuing the 1888 signing certificate (or potentially one of its certificates). This attack 1889 may start becoming more frequent as Certificate Authorities reissue their 1890 own root certificates, or as certificate authorities change policies in the 1891 certificate while reissuing their root certificates. This problem also 1892 occurs when cross certificates (with potentially different restrictions) 1893 are used in the process of verifying a signature. 1895 5.1.3 Rogue Duplicate CA Description 1897 The third attack deals with a rogue entity setting up a certificate 1898 authority that attempts to duplicate the structure of an existing CA. 1899 Specifically, the rogue entity issues a new certificate with the same 1900 public keys as the signer used, but signed by the rogue entity's private 1901 key. 1903 5.2 Attack Responses 1905 This document does not attempt to solve all of the above attacks; however, 1906 a brief description of responses to each of the attacks is given in this 1907 section. 1909 5.2.1 Substitution Attack Response 1911 The denial of service attack cannot be prevented. After the certificate 1912 identifier has been modified in transit, no verification of the signature 1913 is possible. There is also no way to automatically identify the attack 1914 because it is indistinguishable from a message corruption. 1916 The substitution of a valid certificate can be responded to in two 1917 different manners. The first is to make a blanket statement that the use of 1918 the same public key in two different certificates is bad practice and has 1919 to be avoided. In practice, there is no practical way to prevent users from 1920 getting new certificates with the same public keys, and it should be 1921 assumed that they will do this. Section 5.4 provides a new attribute that 1922 can be included in the SignerInfo signed attributes. This binds the correct 1923 certificate identifier into the signature. This will convert the attack 1924 from a potentially successful one to simply a denial of service attack. 1926 5.2.2 Reissue of Certificate Response 1928 A CA should never reissue a certificate with different attributes. 1929 Certificate Authorities that do so are following poor practices and cannot 1930 be relied on. Using the hash of the certificate as the reference to the 1931 certificate prevents this attach for end-entity certificates. 1933 Preventing the attack based on reissuing of CA certificates would require a 1934 substantial change to the usage of the signingCertificate attribute 1935 presented in section 5.4. It would require that ESSCertIDs would need to be 1936 included in the attribute to represent the issuer certificates in the 1937 signer's certification path. This presents problems when the relying party 1938 is using a cross-certificate as part of its authentication process, and 1939 this certificate does not appear on the list of certificates. The problems 1940 outside of a closed PKI make the addition of this information prone to 1941 error, possibly causing the rejection of valid chains. 1943 5.2.3 Rogue Duplicate CA Response 1945 The best method of preventing this attack is to avoid trusting the rogue 1946 CA. The use of the hash to identify certificates prevents the use of 1947 end-entity certificates from the rogue authority. However the only true way 1948 to prevent this attack is to never trust the rogue CA. 1950 5.3 Related Signature Verification Context 1952 Some applications require that additional information be used as part of 1953 the signature validation process. In particular, attribute certificates and 1954 policy identifiers provide additional information about the abilities and 1955 intent of the signer. The signing certificate attribute described in 1956 Section 5.4 provides the ability to bind this context information as part 1957 of the signature. 1959 5.3.1 Attribute Certificates 1961 Some applications require that attribute certificates be validated. This 1962 validation requires that the application be able to find the correct 1963 attribute certificates to perform the verification process; however there 1964 is no list of attribute certificates in a SignerInfo object. The sender has 1965 the ability to include a set of attribute certificates in a SignedData 1966 object. The receiver has the ability to retrieve attribute certificates 1967 from a directory service. There are some circumstances where the signer may 1968 wish to limit the set of attribute certificates that may be used in 1969 verifying a signature. It is useful to be able to list the set of attribute 1970 certificates the signer wants the recipient to use in validating the 1971 signature. 1973 5.3.2 Policy Information 1975 A related aspect of the certificate binding is the issue of multiple 1976 certification paths. In some instances, the semantics of a certificate in 1977 its use with a message may depend on the Certificate Authorities and 1978 policies that apply. To address this issue, the signer may also wish to 1979 bind that context under the signature. While this could be done by either 1980 signing the complete certification path or a policy ID, only a binding for 1981 the policy ID is described here. 1983 5.4 Signing Certificate Attribute Definition 1985 The signing certificate attribute is designed to prevent the simple 1986 substitution and re-issue attacks, and to allow for a restricted set of 1987 attribute certificates to be used in verifying a signature. 1989 The definition of SigningCertificate is 1991 SigningCertificate ::= SEQUENCE { 1992 certs SEQUENCE OF ESSCertID, 1993 policies SEQUENCE OF PolicyInformation OPTIONAL 1994 } 1996 id-aa-signingCertificate OBJECT IDENTIFIER ::= { iso(1) 1997 member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 1998 smime(16) id-aa(2) 12 } 2000 The first certificate identified in the sequence of certificate identifiers 2001 MUST be the certificate used to verify the signature. The encoding of the 2002 ESSCertID for this certificate SHOULD include the issuerSerial field. If 2003 other constraints ensure that issuerAndSerialNumber will be present in the 2004 SignerInfo, the issuerSerial field MAY be omitted. The certificate 2005 identified is used during the signature verification process. If the hash 2006 of the certificate does not match the certificate used to verify the 2007 signature, the signature MUST be considered invalid. 2009 If more than one certificate is present in the sequence of ESSCertIDs, the 2010 certificates after the first one limit the set of authorization 2011 certificates that are used during signature validation. Authorization 2012 certificates can be either attribute certificates or normal certificates. 2013 The issuerSerial SHOULD be present in these certificates, unless the client 2014 who is validating the signature is expected to have easy access to all the 2015 certificates required for validation. If only the signing certificate is 2016 present in the sequence, there are no restrictions on the set of 2017 authorization certificates used in validating the signature. 2019 The sequence of policy information terms identifies those certificate 2020 policies that the signer asserts apply to the certificate, and under which 2021 the certificate should be relied upon. This value suggests a policy value 2022 to be used in the relying party's certification path validation. 2024 If present, the SigningCertificate attribute MUST be a signed attribute; it 2025 MUST NOT be an unsigned attribute. CMS defines SignedAttributes as a SET OF 2026 Attribute. A SignerInfo MUST NOT include multiple instances of the 2027 SigningCertificate attribute. CMS defines the ASN.1 syntax for the signed 2028 attributes to include attrValues SET OF AttributeValue. A 2029 SigningCertificate attribute MUST include only a single instance of 2030 AttributeValue. There MUST NOT be zero or multiple instances of 2031 AttributeValue present in the attrValues SET OF AttributeValue. 2033 5.4.1 Certificate Identification 2035 The best way to identify certificates is an often-discussed issue. [CERT] 2036 has imposed a restriction for SignedData objects that the issuer DN must be 2037 present in all signing certificates. The issuer/serial number pair is 2038 therefore sufficient to identify the correct signing certificate. This 2039 information is already present, as part of the SignerInfo object, and 2040 duplication of this information would be unfortunate. A hash of the entire 2041 certificate serves the same function (allowing the receiver to verify that 2042 the same certificate is being used as when the message was signed), is 2043 smaller, and permits a detection of the simple substitution attacks. 2045 Attribute certificates do not have an issuer/serial number pair represented 2046 anywhere in a SignerInfo object. When an attribute certificate is not 2047 included in the SignedData object, it becomes much more difficult to get 2048 the correct set of certificates based only on a hash of the certificate. 2049 For this reason, attribute certificates are identified by the IssuerSerial 2050 object. 2052 This document defines a certificate identifier as: 2054 ESSCertID ::= SEQUENCE { 2055 certHash Hash, 2056 issuerSerial IssuerSerial OPTIONAL 2057 } 2059 Hash ::= OCTET STRING -- SHA1 hash of entire certificate 2061 IssuerSerial ::= SEQUENCE { 2062 issuer GeneralNames, 2063 serialNumber CertificateSerialNumber 2064 } 2066 When creating an ESSCertID, the certHash is computed over the entire DER 2067 encoded certificate including the signature. The issuerSerial would 2068 normally be present unless the value can be inferred from other 2069 information. 2071 When encoding IssuerSerial, serialNumber is the serial number that uniquely 2072 identifies the certificate. For non-attribute certificates, the issuer MUST 2073 contain only the issuer name from the certificate encoded in the 2074 directoryName choice of GeneralNames. For attribute certificates, the 2075 issuer MUST contain the issuer name field from the attribute certificate. 2077 6. Security Considerations 2079 All security considerations from [CMS] and [SMIME3] apply to applications 2080 that use procedures described in this document. 2082 As stated in Section 2.3, a recipient of a receipt request must not send 2083 back a reply if it cannot validate the signature. Similarly, if there 2084 conflicting receipt requests in a message, the recipient must not send back 2085 receipts, since an attacker may have inserted the conflicting request. 2086 Sending a signed receipt to an unvalidated sender can expose information 2087 about the recipient that it may not want to expose to unknown senders. 2089 Senders of receipts should consider encrypting the receipts to prevent a 2090 passive attacker from gleaning information in the receipts. 2092 Senders must not rely on recipients' processing software to correctly 2093 process security labels. That is, the sender cannot assume that adding a 2094 security label to a message will prevent recipients from viewing messages 2095 the sender doesn't want them to view. It is expected that there will be 2096 many S/MIME clients that will not understand security labels but will still 2097 display a labelled message to a recipient. 2099 A receiving agent that processes security labels must handle the content of 2100 the messages carefully. If the agent decides not to show the message to the 2101 intended recipient after processing the security label, the agent must take 2102 care that the recipient does not accidentally see the content at a later 2103 time. For example, if an error response sent to the originator contains the 2104 content that was hidden from the recipient, and that error response bounces 2105 back to the sender due to addressing errors, the original recipient can 2106 possibly see the content since it is unlikely that the bounce message will 2107 have the proper security labels. 2109 A man-in-the-middle attack can cause a recipient to send receipts to an 2110 attacker if that attacker has a signature that can be validated by the 2111 recipient. The attack consists of intercepting the original message and 2112 adding a mLData attribute that says that a receipt should be sent to the 2113 attacker in addition to whoever else was going to get the receipt. 2115 Mailing lists that encrypt their content may be targets for 2116 denial-of-service attacks if they do not use the mailing list management 2117 described in Section 4. Using simple RFC822 header spoofing, it is quite 2118 easy to subscribe one encrypted mailing list to another, thereby setting up 2119 an infinite loop. 2121 Mailing List Agents need to be aware that they can be used as oracles for 2122 the the adaptive chosen ciphertext attack described in [CMS]. MLAs should 2123 notify an administrator if a large number of undecryptable messages are 2124 received. 2126 When verifying a signature using certificates that come with a [CMS] 2127 message, the recipient should only verify using certificates previously 2128 known to be valid, or certificates that have come from a signed 2129 SigningCertificate attribute. Otherwise, the attacks described in Section 5 2130 can cause the receiver to possibly think a signature is valid when it is 2131 not. 2133 A. ASN.1 Module 2135 ExtendedSecurityServices 2136 { iso(1) member-body(2) us(840) rsadsi(113549) 2137 pkcs(1) pkcs-9(9) smime(16) modules(0) ess(2) } 2139 DEFINITIONS IMPLICIT TAGS ::= 2140 BEGIN 2142 IMPORTS 2144 -- Cryptographic Message Syntax (CMS) 2145 ContentType, IssuerAndSerialNumber, SubjectKeyIdentifier 2146 FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840) 2147 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1)} 2149 -- PKIX Certificate and CRL Profile, Sec A.2 Implicitly Tagged Module, 2150 -- 1988 Syntax 2151 PolicyInformation FROM PKIX1Implicit88 {iso(1) 2152 identified-organization(3) dod(6) internet(1) security(5) 2153 mechanisms(5) pkix(7)id-mod(0) id-pkix1-implicit-88(2)} 2155 -- X.509 2156 GeneralNames, CertificateSerialNumber FROM CertificateExtensions 2157 {joint-iso-ccitt ds(5) module(1) certificateExtensions(26) 0}; 2159 -- Extended Security Services 2161 -- The construct "SEQUENCE SIZE (1..MAX) OF" appears in several ASN.1 2162 -- constructs in this module. A valid ASN.1 SEQUENCE can have zero or 2163 -- more entries. The SIZE (1..MAX) construct constrains the SEQUENCE to 2164 -- have at least one entry. MAX indicates the upper bound is unspecified. 2165 -- Implementations are free to choose an upper bound that suits their 2166 -- environment. 2168 UTF8String ::= [UNIVERSAL 12] IMPLICIT OCTET STRING 2169 -- The contents are formatted as described in [UTF8] 2171 -- Section 2.7 2173 ReceiptRequest ::= SEQUENCE { 2174 signedContentIdentifier ContentIdentifier, 2175 receiptsFrom ReceiptsFrom, 2176 receiptsTo SEQUENCE SIZE (1..ub-receiptsTo) OF GeneralNames } 2178 ub-receiptsTo INTEGER ::= 16 2180 id-aa-receiptRequest OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2181 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 1} 2183 ContentIdentifier ::= OCTET STRING 2185 id-aa-contentIdentifier OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2186 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 7} 2188 ReceiptsFrom ::= CHOICE { 2189 allOrFirstTier [0] AllOrFirstTier, 2190 -- formerly "allOrNone [0]AllOrNone" 2191 receiptList [1] SEQUENCE OF GeneralNames } 2193 AllOrFirstTier ::= INTEGER { -- Formerly AllOrNone 2194 allReceipts (0), 2195 firstTierRecipients (1) } 2197 -- Section 2.8 2199 Receipt ::= SEQUENCE { 2200 version ESSVersion, 2201 contentType ContentType, 2202 signedContentIdentifier ContentIdentifier, 2203 originatorSignatureValue OCTET STRING } 2205 id-ct-receipt OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) 2206 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-ct(1) 1} 2208 ESSVersion ::= INTEGER { v1(1) } 2210 -- Section 2.9 2212 ContentHints ::= SEQUENCE { 2213 contentDescription UTF8String (SIZE (1..MAX)) OPTIONAL, 2214 contentType ContentType } 2216 id-aa-contentHint OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) 2217 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 4} 2219 -- Section 2.10 2221 MsgSigDigest ::= OCTET STRING 2223 id-aa-msgSigDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2224 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 5} 2226 -- Section 2.11 2228 ContentReference ::= SEQUENCE { 2229 contentType ContentType, 2230 signedContentIdentifier ContentIdentifier, 2231 originatorSignatureValue OCTET STRING } 2233 id-aa-contentReference OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2234 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 10 } 2236 -- Section 3.2 2238 ESSSecurityLabel ::= SET { 2239 security-policy-identifier SecurityPolicyIdentifier, 2240 security-classification SecurityClassification OPTIONAL, 2241 privacy-mark ESSPrivacyMark OPTIONAL, 2242 security-categories SecurityCategories OPTIONAL } 2244 id-aa-securityLabel OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2245 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 2} 2247 SecurityPolicyIdentifier ::= OBJECT IDENTIFIER 2249 SecurityClassification ::= INTEGER { 2250 unmarked (0), 2251 unclassified (1), 2252 restricted (2), 2253 confidential (3), 2254 secret (4), 2255 top-secret (5) } (0..ub-integer-options) 2257 ub-integer-options INTEGER ::= 256 2259 ESSPrivacyMark ::= CHOICE { 2260 pString PrintableString (SIZE (1..ub-privacy-mark-length)), 2261 utf8String UTF8String (SIZE (1..MAX)) 2262 } 2264 ub-privacy-mark-length INTEGER ::= 128 2266 SecurityCategories ::= SET SIZE (1..ub-security-categories) OF 2267 SecurityCategory 2269 ub-security-categories INTEGER ::= 64 2271 SecurityCategory ::= SEQUENCE { 2272 type [0] OBJECT IDENTIFIER, 2273 value [1] ANY DEFINED BY type -- defined by type 2274 } 2276 --Note: The aforementioned SecurityCategory syntax produces identical 2277 --hex encodings as the following SecurityCategory syntax that is 2278 --documented in the X.411 specification: 2279 -- 2280 --SecurityCategory ::= SEQUENCE { 2281 -- type [0] SECURITY-CATEGORY, 2282 -- value [1] ANY DEFINED BY type } 2283 -- 2284 --SECURITY-CATEGORY MACRO ::= 2285 --BEGIN 2286 --TYPE NOTATION ::= type | empty 2287 --VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER) 2288 --END 2290 -- Section 3.4 2292 EquivalentLabels ::= SEQUENCE OF ESSSecurityLabel 2294 id-aa-equivalentLabels OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2295 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 9} 2297 -- Section 4.4 2299 MLExpansionHistory ::= SEQUENCE 2300 SIZE (1..ub-ml-expansion-history) OF MLData 2302 id-aa-mlExpandHistory OBJECT IDENTIFIER ::= { iso(1) member-body(2) 2303 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 3} 2305 ub-ml-expansion-history INTEGER ::= 64 2307 MLData ::= SEQUENCE { 2308 mailListIdentifier EntityIdentifier, 2309 expansionTime GeneralizedTime, 2310 mlReceiptPolicy MLReceiptPolicy OPTIONAL } 2312 EntityIdentifier ::= CHOICE { 2313 issuerAndSerialNumber IssuerAndSerialNumber, 2314 subjectKeyIdentifier SubjectKeyIdentifier } 2316 MLReceiptPolicy ::= CHOICE { 2317 none [0] NULL, 2318 insteadOf [1] SEQUENCE SIZE (1..MAX) OF GeneralNames, 2319 inAdditionTo [2] SEQUENCE SIZE (1..MAX) OF GeneralNames } 2321 -- Section 5.4 2323 SigningCertificate ::= SEQUENCE { 2324 certs SEQUENCE OF ESSCertID, 2325 policies SEQUENCE OF PolicyInformation OPTIONAL 2326 } 2328 id-aa-signingCertificate OBJECT IDENTIFIER ::= { iso(1) 2329 member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 2330 smime(16) id-aa(2) 12 } 2332 ESSCertID ::= SEQUENCE { 2333 certHash Hash, 2334 issuerSerial IssuerSerial OPTIONAL 2335 } 2337 Hash ::= OCTET STRING -- SHA1 hash of entire certificate 2339 IssuerSerial ::= SEQUENCE { 2340 issuer GeneralNames, 2341 serialNumber CertificateSerialNumber 2342 } 2344 END -- of ExtendedSecurityServices 2346 B. References 2348 [ASN1-1988] "Recommendation X.208: Specification of Abstract Syntax 2349 Notation One (ASN.1)" 2351 [ASN1-1994] "Recommendation X.680: Specification of Abstract Syntax 2352 Notation One (ASN.1)" 2354 [CERT] "S/MIME Version 3 Certificate Handling", Internet Draft 2355 draft-ietf-smime-cert-xx. 2357 [CMS] "Cryptographic Message Syntax", Internet Draft 2358 draft-ietf-smime-cms-xx. 2360 [MSG] "S/MIME Version 3 Message Specification", Internet Draft 2361 draft-ietf-smime-msg-xx. 2363 [MUSTSHOULD] "Key Words for Use in RFCs to Indicate Requirement Levels", 2364 RFC 2119. 2366 [MSP4] "Secure Data Network System (SDNS) Message Security Protocol (MSP) 2367 4.0", Specification SDN.701, Revision A, 1997-02-06. 2369 [MTSABS] "1988 International Telecommunication Union (ITU) Data 2370 Communication Networks Message Handling Systems: Message Transfer System: 2371 Abstract Service Definition and Procedures, Volume VIII, Fascicle VIII.7, 2372 Recommendation X.411"; MTSAbstractService {joint-iso-ccitt mhs-motis(6) 2373 mts(3) modules(0) mts-abstract-service(1)} 2375 [PKCS7-1.5] "PKCS #7: Cryptographic Message Syntax", RFC 2315. 2377 [SMIME2] "S/MIME Version 2 Message Specification", RFC 2311, and 2378 "S/MIME Version 2 Certificate Handling", RFC 2312. 2380 [UTF8] "UTF-8, a transformation format of ISO 10646", RFC 2279. 2382 C. Acknowledgments 2384 The first draft of this work was prepared by David Solo. John Pawling did a 2385 huge amount of very detailed revision work during the many phases of the 2386 document. 2388 Many other people have contributed hard work to this draft, including: 2389 Andrew Farrell 2390 Bancroft Scott 2391 Bengt Ackzell 2392 Bill Flanigan 2393 Blake Ramsdell 2394 Carlisle Adams 2395 Darren Harter 2396 David Kemp 2397 Denis Pinkas 2398 Jim Schaad 2399 Russ Housley 2400 Scott Hollenbeck 2401 Steve Dusse 2403 D. Changes from draft-ietf-smime-ess-10 to draft-ietf-smime-ess-11 2405 1: Added wording about signing certificates (Section 5) to the 2406 introduction. 2408 1.3.4: Added the sentence at the end of the first paragraph. 2410 1.3.4: Removed the macValue description because it was removed from CMS. 2412 1.4: Second paragraph, replaced second sentence to indicate that some 2413 attributes might have to be omitted. 2415 2.1: Added paragraph at the end of this section saying that you shouldn't 2416 request that receipts be sent to people who don't have the original 2417 message. 2419 2.6: Added paragraph near beginning about accepting more than one signed 2420 receipt. 2422 2.8: Changed Version to ESSVersion and defined it as: 2424 ESSVersion ::= INTEGER { v1(1) } 2426 3: Clarified the last paragraph to indicate ranked levels. 2428 4.2.3.2: Clarified step 2 to show that, if the mlExpansionHistory was 2429 not found, you sign that whole message and stop. 2431 4.4: Removed the comment "-- EntityIdentifier is imported from [CMS]" 2433 5.1: Changed "changing" to "replacing". 2435 5.4: The paragraph beginning "The first certificate..." was changed to 2436 reflect the addition of the SKI to the SignerInfo. The paragraph beginning 2437 "If more than..." was changed to allow authorization certificates that are 2438 not attribute certificates. 2440 5.4.1: Changed "CMS" to "[CERT]" for the reference. 2442 6: Entire section is new. 2444 E. Editor's Address 2446 Paul Hoffman 2447 Internet Mail Consortium 2448 127 Segre Place 2449 Santa Cruz, CA 95060 2450 (831) 426-9827 2451 phoffman@imc.org