idnits 2.17.1
draft-ietf-smime-ess-07.txt:
-(454): Line appears to be too long, but this could be caused by non-ascii characters in UTF-8 encoding
-(459): Line appears to be too long, but this could be caused by non-ascii characters in UTF-8 encoding
-(898): Line appears to be too long, but this could be caused by non-ascii characters in UTF-8 encoding
-(961): Line appears to be too long, but this could be caused by non-ascii characters in UTF-8 encoding
-(1115): Line appears to be too long, but this could be caused by non-ascii characters in UTF-8 encoding
-(1745): Line appears to be too long, but this could be caused by non-ascii characters in UTF-8 encoding
-(1909): Line appears to be too long, but this could be caused by non-ascii characters in UTF-8 encoding
Checking boilerplate required by RFC 5378 and the IETF Trust (see
https://trustee.ietf.org/license-info):
----------------------------------------------------------------------------
** Cannot find the required boilerplate sections (Copyright, IPR, etc.) in
this document.
Expected boilerplate is as follows today (2024-04-26) according to
https://trustee.ietf.org/license-info :
IETF Trust Legal Provisions of 28-dec-2009, Section 6.a:
This Internet-Draft is submitted in full conformance with the provisions
of BCP 78 and BCP 79.
IETF Trust Legal Provisions of 28-dec-2009, Section 6.b(i), paragraph 2:
Copyright (c) 2024 IETF Trust and the persons identified as the document
authors. All rights reserved.
IETF Trust Legal Provisions of 28-dec-2009, Section 6.b(i), paragraph 3:
This document is subject to BCP 78 and the IETF Trust's Legal Provisions
Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this
document must include Simplified BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided
without warranty as described in the Simplified BSD License.
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
Internet-Drafts being working documents.
** 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.
== There are 22 instances of lines with non-ascii characters in the
document.
== 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 2058 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 386 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 100: '...entInfo eContent MUST be absent. If th...'
RFC 2119 keyword, line 102: '...entInfo eContent MUST contain the resu...'
RFC 2119 keyword, line 125: '...contentType MUST be id-data. The Envel...'
RFC 2119 keyword, line 154: '...receiving agents MUST be able to inter...'
RFC 2119 keyword, line 235: '...request MUST be in the inside signatur...'
(126 more instances...)
Miscellaneous warnings:
----------------------------------------------------------------------------
== Line 1730 has weird spacing: '... | none inst...'
== Line 1732 has weird spacing: '... | none none...'
== Line 1733 has weird spacing: '... | none inst...'
== Line 1734 has weird spacing: '... | none inst...'
== Line 1735 has weird spacing: '... | none inst...'
-- 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 (August 5, 1998) is 9396 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 1980 looks like a reference
-- Missing reference section? 'SMIME3' on line 1994 looks like a reference
-- Missing reference section? 'SMIME2' on line 1991 looks like a reference
-- Missing reference section? 'ASN1-1988' on line 1971 looks like a
reference
-- Missing reference section? 'ESS' on line 302 looks like a reference
-- Missing reference section? 'CMS' on line 1977 looks like a reference
-- Missing reference section? 'MSG' on line 306 looks like a reference
-- Missing reference section? '0' on line 1963 looks like a reference
-- Missing reference section? '1' on line 1964 looks like a reference
-- Missing reference section? 'MTSABS' on line 1983 looks like a reference
-- Missing reference section? '2' on line 1965 looks like a reference
-- Missing reference section? 'UNIVERSAL 12' on line 1819 looks like a
reference
-- Missing reference section? 'UTF8' on line 1998 looks like a reference
-- Missing reference section? 'ASN1-1994' on line 1974 looks like a
reference
Summary: 11 errors (**), 0 flaws (~~), 9 warnings (==), 16 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-07.txt Internet Mail Consortium
3 August 5, 1998
4 Expires in six months
6 Enhanced Security Services for S/MIME
8 Status of this memo
10 This document is an Internet-Draft. Internet-Drafts are working documents
11 of the Internet Engineering Task Force (IETF), its areas, and its working
12 groups. Note that other groups may also distribute working documents as
13 Internet-Drafts.
15 Internet-Drafts are draft documents valid for a maximum of six months and
16 may be updated, replaced, or obsoleted by other documents at any time. It
17 is inappropriate to use Internet-Drafts as reference material or to cite
18 them other than as "work in progress."
20 To learn the current status of any Internet-Draft, please check the
21 "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
22 Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au
23 (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West
24 Coast).
26 1. Introduction
28 This document describes three optional security service extensions for
29 S/MIME. These services provide functionality that is similar to the Message
30 Security Protocol [MSP4], but are useful in many other environments,
31 particularly business and finance. The services are:
32 - signed receipts
33 - security labels
34 - secure mailing lists
36 The services described here are extensions to S/MIME version 3 [SMIME3],
37 and some of them can also be added to S/MIME version 2 [SMIME2]. The
38 extensions described here will not cause an S/MIME version 3 recipient to
39 be unable to read messages from an S/MIME version 2 sender. However, some
40 of the extensions will cause messages created by an S/MIME version 3 sender
41 to be unreadable by an S/MIME version 2 recipient.
43 The format of the messages are described in ASN.1:1988 [ASN1-1988].
45 This draft is being discussed on the 'ietf-smime' mailing list. To
46 subscribe, send a message to:
47 ietf-smime-request@imc.org
48 with the single word
49 subscribe
50 in the body of the message. There is a Web site for the mailing list at
51 .
53 1.1 Triple Wrapping
55 Some of the features of each service use the concept of a "triple wrapped"
56 message. A triple wrapped message is one that has been signed, then
57 encrypted, then signed again. The signers of the inner and outer signatures
58 may be different entities or the same entity. Note that the S/MIME
59 specification does not limit the number of nested encapsulations, so there
60 may be more than three wrappings.
62 1.1.1 Purpose of Triple Wrapping
64 Not all messages need to be triple wrapped. Triple wrapping is used when a
65 message must be signed, then encrypted, and then have signed attributes
66 bound to the encrypted body. Outer attributes may be added or removed by
67 the message originator or intermediate agents, and may be signed by
68 intermediate agents or the final recipient.
70 The inside signature is used for content integrity, non-repudiation with
71 proof of origin, and binding attributes (such as a security label) to the
72 original content. These attributes go from the originator to the recipient,
73 regardless of the number of intermediate entities such as mail list agents
74 that process the message. The signed attributes can be used for access
75 control to the inner body. Requests for signed receipts by the originator
76 are carried in the inside signature as well.
78 The encrypted body provides confidentiality, including confidentiality of
79 the attributes that are carried in the inside signature.
81 The outside signature provides authentication and integrity for information
82 that is processed hop-by-hop, where each hop is an intermediate entity such
83 as a mail list agent. The outer signature binds attributes (such as a
84 security label) to the encrypted body. These attributes can be used for
85 access control and routing decisions.
87 1.1.2 Steps for Triple Wrapping
89 The steps to create a triple wrapped message are:
91 1. Start with a message body, called the "original content".
93 2. Encapsulate the original content with the appropriate MIME Content-type
94 headers, such as "Content-type: text/plain". An exception to this MIME
95 encapsulation rule is that a signed receipt is not put in MIME headers.
97 3. Sign the result of step 2 (the inner MIME headers and the original
98 content). The SignedData encapContentInfo eContentType object identifier
99 MUST be id-data. If the structure you create in step 4 is multipart/signed,
100 then the SignedData encapContentInfo eContent MUST be absent. If the
101 structure you create in step 4 is application/pkcs7-mime, then the
102 SignedData encapContentInfo eContent MUST contain the result of step 2
103 above. The SignedData structure is encapsulated by a ContentInfo SEQUENCE
104 with a contentType of id-signedData.
106 4. Add an appropriate MIME construct to the signed message from step 3 as
107 defined in [SMIME3]. The resulting message is called the "inside
108 signature".
110 - If you are signing using multipart/signed, the MIME construct added
111 consists of a Content-type of multipart/signed with parameters, the
112 boundary, the result of step 2 above, the boundary, a Content-type of
113 application/pkcs7-signature, optional MIME headers (such as
114 Content-transfer-encoding and Content-disposition), and a body part that
115 is the result of step 3 above.
117 - If you are instead signing using application/pkcs7-mime, the MIME
118 construct added consists of a Content-type of application/pkcs7-mime
119 with parameters, optional MIME headers (such as
120 Content-transfer-encoding and Content-disposition), and the result of
121 step 3 above.
123 5. Encrypt the result of step 4 as a single block, turning it into an
124 application/pkcs7-mime object. The EnvelopedData encryptedContentInfo
125 contentType MUST be id-data. The EnvelopedData structure is encapsulated by
126 a ContentInfo SEQUENCE with a contentType of id-envelopedData. This is
127 called the "encrypted body".
129 6. Add the appropriate MIME headers: a Content-type of
130 application/pkcs7-mime with parameters, and optional MIME headers such as
131 Content-transfer-encoding and Content-disposition.
133 7. Using the same logic as in step 3 above, sign the result of step 6 (the
134 MIME headers and the encrypted body) as a single block
136 8. Using the same logic as in step 4 above, add an appropriate MIME
137 construct to the signed message from step 7. The resulting message is
138 called the "outside signature", and is also the triple wrapped message.
140 1.2 Format of a Triple Wrapped Message
142 A triple wrapped message has many layers of encapsulation. The structure
143 differs based on the choice of format for the signed portions of the
144 message. Because of the way that MIME encapsulates data, the layers do not
145 appear in order, and the notion of "layers" becomes vague.
147 There is no need to use the multipart/signed format in an inner signature
148 because it is known that the recipient is able to process S/MIME messages
149 (because they decrypted the middle wrapper). A sending agent might choose
150 to use the multipart/signed format in the outer layer so that a non-S/MIME
151 agent could see that the next inner layer is encrypted; however, this is
152 not of great value, since all it shows the recipient is that the rest of
153 the message is unreadable. Because many sending agents always use
154 multipart/signed structures, all receiving agents MUST be able to interpret
155 either multipart/signed or application/pkcs7-mime signature structures.
157 The format of a triple wrapped message that uses multipart/signed for both
158 signatures is:
160 [step 8] Content-type: multipart/signed;
161 [step 8] protocol="application/pkcs7-signature";
162 [step 8] boundary=outerboundary
163 [step 8]
164 [step 8] --outerboundary
165 [step 6] Content-type: application/pkcs7-mime; )
166 [step 6] smime-type=enveloped-data )
167 [step 6] )
168 [step 4] Content-type: multipart/signed; | )
169 [step 4] protocol="application/pkcs7-signature"; | )
170 [step 4] boundary=innerboundary | )
171 [step 4] | )
172 [step 4] --innerboundary | )
173 [step 2] Content-type: text/plain % | )
174 [step 2] % | )
175 [step 1] Original content % | )
176 [step 4] | )
177 [step 4] --innerboundary | )
178 [step 4] Content-type: application/pkcs7-signature | )
179 [step 4] | )
180 [step 3] inner SignedData block (eContent is missing) | )
181 [step 4] | )
182 [step 4] --innerboundary-- | )
183 [step 8]
184 [step 8] --outerboundary
185 [step 8] Content-type: application/pkcs7-signature
186 [step 8]
187 [step 7] outer SignedData block (eContent is missing)
188 [step 8]
189 [step 8] --outerboundary--
191 % = These lines are what the inner signature is computed over.
192 | = These lines are what is encrypted in step 5. This encrypted result
193 is opaque and is a part of an EnvelopedData block.
194 ) = These lines are what the outer signature is computed over.
196 The format of a triple wrapped message that uses application/pkcs7-mime for
197 the both signatures is:
199 [step 8] Content-type: application/pkcs7-mime;
200 [step 8] smime-type=signed-data
201 [step 8]
202 [step 7] outer SignedData block (eContent is present) O
203 [step 6] Content-type: application/pkcs7-mime; ) O
204 [step 6] smime-type=enveloped-data; ) O
205 [step 6] ) O
206 [step 4] Content-type: application/pkcs7-mime; | ) O
207 [step 4] smime-type=signed-data | ) O
208 [step 4] | ) O
209 [step 3] inner SignedData block (eContent is present) I | ) O
210 [step 2] Content-type: text/plain I | ) O
211 [step 2] I | ) O
212 [step 1] Original content I | ) O
214 I = These lines are the inner SignedData block, which is opaque and
215 contains the ASN.1 encoded result of step 2 as well as control
216 information.
217 | = These lines are what is encrypted in step 5. This encrypted result
218 is opaque and is a part of an EnvelopedData block.
219 ) = These lines are what the outer signature is computed over.
220 O = These lines are the outer SignedData block, which is opaque and
221 contains the ASN.1 encoded result of step 6 as well as control
222 information.
224 1.3 Security Services and Triple Wrapping
226 The three security services described in this document are used with triple
227 wrapped messages in different ways. This section briefly describes the
228 relationship of each service with triple wrapping; the other sections of
229 the document go into greater detail.
231 1.3.1 Signed Receipts and Triple Wrapping
233 A signed receipt may be requested in any SignedData object. However, if a
234 signed receipt is requested for a triple wrapped message, the receipt
235 request MUST be in the inside signature, not in the outside signature. A
236 secure mailing list agent may change the receipt policy in the outside
237 signature of a triple wrapped message when that message is processed by the
238 mailing list.
240 Note: the signed receipts and receipt requests described in this draft
241 differ from those described in the work done by the IETF Receipt
242 Notification Working Group. The output of that Working Group, when
243 finished, is not expected to work well with triple wrapped messages as
244 described in this document.
246 1.3.2 Security Labels and Triple Wrapping
248 A security label may be included in the signed attributes of any SignedData
249 object. A security label attribute may be included in either the inner
250 signature, outer signature, or both.
252 The inner security label is used for access control decisions related to
253 the plaintext original content. The inner signature provides authentication
254 and cryptographically protects the original signer's security label that is
255 on the inside body. This strategy facilitates the forwarding of messages
256 because the original signer's security label is included in the SignedData
257 block which can be forwarded to a third party that can verify the inner
258 signature which will cover the inner security label. The confidentiality
259 security service can be applied to the inner security label by encrypting
260 the entire inner SignedData block within an EnvelopedData block.
262 A security label may also be included in the signed attributes of the outer
263 SignedData block which will include the sensitivities of the encrypted
264 message. The outer security label is used for access control and routing
265 decisions related to the encrypted message. Note that a security label
266 attribute can only be used in an signedAttributes block. An
267 eSSSecurityLabel attribute MUST NOT be used in an EnvelopedData or unsigned
268 attributes.
270 1.3.3 Secure Mailing Lists and Triple Wrapping
272 Secure mail list message processing depends on the structure of S/MIME
273 layers present in the message sent to the mail list agent. The agent never
274 changes the data that was hashed to form the inner signature, if such a
275 signature is present. If an outer signature is present, then the agent will
276 modify the data that was hashed to form that outer signature. In all cases,
277 the agent adds or updates an mlExpansionHistory attribute to document the
278 agent's processing, and ultimately adds or replaces the outer signature on
279 the message to be distributed.
281 1.3.4 Placement of Attributes
283 Certain attributes should be placed in the inner or outer SignedData
284 message; some attributes can be in either. Further, some attributes must be
285 signed, while signing is optional for others, and some attributes must not
286 be signed. The following table summarizes the recommendation of this
287 profile.
289 | |Inner or |
290 Attribute |OID |outer |Signed
291 ------------------|-----------------------------|----------|--------
292 contentHints |id-aa-contentHint [ESS] |either |MAY
293 contentIdentifier |id-aa-contentIdentifier [ESS]|either |MAY
294 contentReference |id-aa-contentReference [ESS] |either |MUST
295 contentType |id-contentType [CMS] |either |MUST
296 counterSignature |id-countersignature [CMS] |either |MUST NOT
297 equivalentLabel |id-aa-equivalentLabels [ESS] |either |MUST
298 eSSSecurityLabel |id-aa-securityLabel [ESS] |either |MUST
299 messageDigest |id-messageDigest [CMS] |either |MUST
300 msgSigDigest |id-aa-msgSigDigest [ESS] |inner only|MUST
301 mlExpansionHistory|id-aa-mlExpandHistory [ESS] |outer only|MUST
302 receiptRequest |id-aa-receiptRequest [ESS] |inner only|MUST
303 signingTime |id-signingTime [CMS] |either |MUST
304 smimeCapabilities |sMIMECapabilities [MSG] |either |MUST
305 sMIMEEncryption-
306 KeyPreference |id-aa-encrypKeyPref [MSG] |either |MUST
308 CMS defines signedAttrs as a SET OF Attributes and defines
309 unsignedAttributes as a SET OF Attributes. ESS defines the contentHints,
310 contentIdentifier, eSSecurityLabel, msgSigDigest, mlExpansionHistory,
311 receiptRequest, contentReference and equivalentLabels attribute types. A
312 signerInfo MUST NOT include multiple instances of any of the attribute
313 types defined in ESS. Later sections of ESS specify further restrictions
314 that apply to the receiptRequest, mlExpansionHistory and eSSecurityLabel
315 attribute types.
317 CMS defines the syntax for the signed and unsigned attributes as
318 "attrValues SET OF AttributeValue". For all of the attribute types defined
319 in ESS, if the attribute type is present in a signerInfo, then it MUST only
320 include a single instance of AttributeValue. In other words, there MUST NOT
321 be zero or multiple instances of AttributeValue present in the attrValues
322 SET OF AttributeValue.
324 If a counterSignature attribute is present, then it MUST be included in the
325 unsigned attributes. It MUST NOT be included in the signed attributes. The
326 only attributes that are allowed in a counterSignature attribute are
327 counterSignature, messageDigest, signingTime, and signingCertificate.
329 Note that the inner and outer signatures are usually for different senders.
330 The same attribute in the two signatures could lead to very different
331 consequences.
333 The macValue attribute is only used in authenticatedData, never in
334 signedData.
336 ContentIdentifier is an attribute (OCTET STRING) used to carry a unique
337 identifier assigned to the message.
339 1.4 Required and Optional Attributes
341 Some security gateways sign messages that pass through them. If the message
342 is any type other than a signedData type, the gateway has only one way to
343 sign the message: by wrapping it with a signedData block and MIME headers.
344 If the message to be signed by the gateway is a signedData message already,
345 the gateway can sign the message by inserting a signerInfo into the
346 signedData block.
348 The main advantage of a gateway adding a signerInfo instead of wrapping the
349 message in a new signature is that the message doesn't grow as much as if
350 the gateway wrapped the message. The main disadvantage is that the gateway
351 must check for the presence of certain attributes in the other signerInfos
352 and duplicate those attributes.
354 If a gateway or other processor adds a signerInfo to an existing signedData
355 block, it MUST copy the mlExpansionHistory and eSSSecurityLabel attributes
356 from other signerInfos. This helps ensure that the recipient will process
357 those attributes in a signerInfo that it can verify.
359 Note that someone may in the future define an attribute that must be
360 present in each signerInfo of a signedData block in order for the signature
361 to be processed. If that happens, a gateway that inserts signerInfos and
362 doesn't copy that attribute will cause every message with that attribute to
363 fail when processed by the recipient. For this reason, it is safer to wrap
364 messages with new signatures than to insert signerInfos.
366 1.5 Object Identifiers
368 The object identifiers for many of the objects described in this draft are
369 found in [CMS} and [SMIME3]. Other object identifiers used in S/MIME can be
370 found in the registry kept at .
371 When this draft moves to standards track within the IETF, it is intended
372 that the IANA will maintain this registry.
374 2. Signed Receipts
376 Returning a signed receipt provides to the originator proof of delivery of
377 a message, and allows the originator to demonstrate to a third party that
378 the recipient was able to verify the signature of the original message.
379 This receipt is bound to the original message through the signature;
380 consequently, this service may be requested only if a message is signed.
381 The receipt sender may optionally also encrypt a receipt to provide
382 confidentiality between the receipt sender and the receipt recipient.
384 2.1 Signed Receipt Concepts
386 The originator of a message may request a signed receipt from the message's
387 recipients. The request is indicated by adding a receiptRequest attribute
388 to the signedAttributes field of the SignerInfo object for which the
389 receipt is requested. The receiving user agent software SHOULD
390 automatically create a signed receipt when requested to do so, and return
391 the receipt in accordance with mailing list expansion options, local
392 security policies, and configuration options.
394 Because receipts involve the interaction of two parties, the terminology
395 can sometimes be confusing. In this section, the "sender" is the agent that
396 sent the original message that included a request for a receipt. The
397 "receiver" is the party that received that message and generated the
398 receipt.
400 The steps in a typical transaction are:
402 1. Sender creates a signed message including a receipt request attribute
403 (Section 2.2).
405 2. Sender transmits the resulting message to the recipient or recipients.
407 3. Recipient receives message and determines if there is a valid signature
408 and receipt request in the message (Section 2.3).
410 4. Recipient creates a signed receipt (Section 2.4).
412 5. Recipient transmits the resulting signed receipt message to the sender
413 (Section 2.5).
415 6. Sender receives the message and validates that it contains a signed
416 receipt for the original message (Section 2.6). This validation relies on
417 the sender having retained either a copy of the original message or
418 information extracted from the original message.
420 The ASN.1 syntax for the receipt request is given in Section 2.7; the ASN.1
421 syntax for the receipt is given in Section 2.8.
423 Note that an agent SHOULD remember when it has sent a receipt so that it
424 can avoid re-sending a receipt each time it processes the message.
426 2.2 Receipt Request Creation
428 Multi-layer S/MIME messages may contain multiple SignedData layers.
429 However, receipts may be requested only for the innermost SignedData layer
430 in a multi-layer S/MIME message, such as a triple wrapped message. Only one
431 receiptRequest attribute can be included in the signedAttributes of a
432 SignerInfo.
434 A ReceiptRequest attribute MUST NOT be included in the attributes of a
435 SignerInfo in a SignedData object that encapsulates a Receipt content. In
436 other words, the user agent MUST NOT request a signed receipt for a signed
437 receipt.
439 A sender requests receipts by placing a receiptRequest attribute in the
440 signed attributes of a signerInfo as follows:
442 1. A receiptRequest data structure is created.
444 2. A signed content identifier for the message is created and assigned to
445 the signedContentIdentifier field. The signedContentIdentifier is used to
446 associate the signed receipt with the message requesting the signed
447 receipt.
449 3. The entities requested to return a signed receipt are noted in the
450 receiptsFrom field.
452 4. The message originator MUST populate the receiptsTo field with a
453 GeneralNames for each entity to whom the recipient should send the signed
454 receipt.�If the message originator wants the recipient to send the signed
455 receipt to the originator, then the originator MUST include a GeneralNames
456 for itself in the receiptsTo field.�GeneralNames is a SEQUENCE OF
457 GeneralName.�receiptsTo is a SEQUENCE OF GeneralNames in which each
458 GeneralNames represents an entity.�There may be multiple GeneralName
459 instances in each GeneralNames.�At a minimum, the message originator MUST
460 populate each entity's GeneralNames with the address to which the signed
461 receipt should be sent.�Optionally, the message originator MAY also
462 populate each entity's GeneralNames with other GeneralName instances (such
463 as directoryName).
465 5. The completed receiptRequest attribute is placed in the signedAttributes
466 field of the SignerInfo object.
468 2.2.1 Multiple Receipt Requests
470 There can be multiple SignerInfos within a SignedData object, and each
471 SignerInfo may include signedAttributes. Therefore, a single SignedData
472 object may include multiple SignerInfos, each SignerInfo having a
473 receiptRequest attribute. For example, an originator can send a signed
474 message with two SignerInfos, one containing a DSS signature, the other
475 containing an RSA signature.
477 Each recipient SHOULD return only one signed receipt.
479 Not all of the SignerInfos need to include receipt requests, but in all of
480 the SignerInfos that do contain receipt requests, the receipt requests MUST
481 be identical.
483 2.2.2 Information Needed to Validate Signed Receipts
485 The sending agent MUST retain one or both of the following items to support
486 the validation of signed receipts returned by the recipients.
488 - the original signedData object requesting the signed receipt
490 - the message signature digest value used to generate the original
491 signedData signerInfo signature value and the digest value of the
492 Receipt content containing values included in the original signedData
493 object. If signed receipts are requested from multiple recipients, then
494 retaining these digest values is a performance enhancement because the
495 sending agent can reuse the saved values when verifying each returned
496 signed receipt.
498 2.3 Receipt Request Processing
500 A receiptRequest is associated only with the SignerInfo object in which the
501 receipt request attribute is directly attached. Processing software SHOULD
502 examine the signedAttributes field of each of the SignerInfos for which it
503 verifies a signature in the innermost signedData object to determine if a
504 receipt is requested. This may result in the receiving agent processing
505 multiple receiptRequest attributes included in a single SignedData object.
507 Before processing a receiptRequest signedAttribute, the receiving agent
508 MUST verify the signature of the SignerInfo which covers the receiptRequest
509 attribute. A recipient MUST NOT process a receiptRequest attribute that has
510 not been verified. Because all receiptRequest attributes in a SignedData
511 object must be identical, the receiving application fully processes (as
512 described in the following paragraphs) the first receiptRequest attribute
513 that it encounters in a SignerInfo that it verifies, and it then ensures
514 that all other receiptRequest attributes in signerInfos that it verifies
515 are identical to the first one encountered. If there are verified
516 ReceiptRequest attributes which conflict, then the processing software MUST
517 NOT return any signed receipt. A signed receipt SHOULD be returned if any
518 signerInfo containing a receiptRequest attribute can be validated, even if
519 other signerInfos containing the same receiptRequest attribute cannot be
520 validated because they are signed using an algorithm not supported by the
521 receiving agent.
523 If a receiptRequest attribute is absent from the signed attributes, then a
524 signed receipt has not been requested from any of the message recipients
525 and MUST NOT be created. If a receiptRequest attribute is present in the
526 signed attributes, then a signed receipt has been requested from some or
527 all of the message recipients. Note that in some cases, a receiving agent
528 might receive two almost-identical messages, one with a receipt request and
529 the other without one. In this case, the receiving agent SHOULD send a
530 signed receipt for the message that requests a signed receipt.
532 If a receiptRequest attribute is present in the signed attributes, the
533 following process SHOULD be used to determine if a message recipient has
534 been requested to return a signed receipt.
536 1. If an mlExpansionHistory attribute is present in the outermost
537 signedData block, do one of the following two steps, based on the absence
538 or presence of mlReceiptPolicy:
540 1.1. If an mlReceiptPolicy value is absent from the last MLData
541 element, a Mail List receipt policy has not been specified and the
542 processing software SHOULD examine the receiptRequest attribute value
543 to determine if a receipt should be created and returned.
545 1.2. If an mlReceiptPolicy value is present in the last MLData element,
546 do one of the following two steps, based on the value of
547 mlReceiptPolicy:
549 1.2.1. If the mlReceiptPolicy value is none, then the receipt
550 policy of the Mail List supersedes the originator's request for a
551 signed receipt and a signed receipt MUST NOT be created.
553 1.2.2. If the mlReceiptPolicy value is insteadOf or inAdditionTo,
554 the processing software SHOULD examine the receiptsFrom value from
555 the receiptRequest attribute to determine if a receipt should be
556 created and returned. If a receipt is created, the insteadOf and
557 inAdditionTo fields identify entities that SHOULD be sent the
558 receipt instead of or in addition to the originator.
560 2. If the receiptsFrom value of the receiptRequest attribute is
561 allOrFirstTier, do one of the following two steps based on the value of
562 allOrFirstTier.
564 2.1. If the value of allOrFirstTier is allReceipts, then a signed
565 receipt SHOULD be created.
567 2.2. If the value of allOrFirstTier is firstTierRecipients, do one of
568 the following two steps based on the presence of an mlExpansionHistory
569 attribute in an outer signedData block:
571 2.2.1. If an mlExpansionHistory attribute is present, then this
572 recipient is not a first tier recipient and a signed receipt MUST
573 NOT be created.
575 2.2.2. If an mlExpansionHistory attribute is not present, then a
576 signed receipt SHOULD be created.
578 3. If the receiptsFrom value of the receiptRequest attribute is a
579 receiptList:
581 3.1. If receiptList contains one of the GeneralNames of the recipient,
582 then a signed receipt should be created.
584 3.2. If receiptList does not contain one of the GeneralNames of the
585 recipient, then a signed receipt MUST NOT be created.
587 A flow chart for the above steps to be executed for each signerInfo for
588 which the receiving agent verifies the signature would be:
590 0. Receipt Request attribute present?
591 YES -> 1.
592 NO -> STOP
593 1. Has mlExpansionHistory in outer signedData?
594 YES -> 1.1.
595 NO -> 2.
596 1.1. mlReceiptPolicy absent?
597 YES -> 2.
598 NO -> 1.2.
599 1.2. Pick based on value of mlReceiptPolicy.
600 none -> 1.2.1.
601 insteadOf or inAdditionTo -> 1.2.2.
602 1.2.1. STOP.
603 1.2.2. Examine receiptsFrom to determine if a receipt should be created,
604 create it if required, send it to recipients designated by
605 mlReceiptPolicy, then -> STOP.
606 2. Is value of receiptsFrom allOrFirstTier?
607 YES -> Pick based on value of allOrFirstTier.
608 allReceipts -> 2.1.
609 firstTierRecipients -> 2.2.
610 NO -> 3.
611 2.1. Create a receipt, then -> STOP.
612 2.2. Has mlExpansionHistory in the outer signedData block?
613 YES -> 2.2.1.
614 NO -> 2.2.2.
615 2.2.1. STOP.
616 2.2.2. Create a receipt, then -> STOP.
617 3. Is receiptsFrom value of receiptRequest a receiptList?
618 YES -> 3.1.
619 NO -> STOP.
620 3.1. Does receiptList contain the recipient?
621 YES -> Create a receipt, then -> STOP.
622 NO -> 3.2.
623 3.2. STOP.
625 2.4 Signed Receipt Creation
627 A signed receipt is a signedData object encapsulating a Receipt content
628 (also called a "signedData/Receipt"). Signed receipts are created as
629 follows:
631 1. The signature of the original signedData signerInfo that includes the
632 receiptRequest signed attribute MUST be successfully verified before
633 creating the signedData/Receipt.
635 1.1. The content of the original signedData object is digested as
636 described in [CMS]. The resulting digest value is then compared with
637 the value of the messageDigest attribute included in the
638 signedAttributes of the original signedData signerInfo. If these digest
639 values are different, then the signature verification process fails and
640 the signedData/Receipt MUST NOT be created.
642 1.2. The ASN.1 DER encoded signedAttributes (including messageDigest,
643 receiptRequest and, possibly, other signed attributes) in the original
644 signedData signerInfo are digested as described in [CMS]. The resulting
645 digest value, called msgSigDigest, is then used to verify the signature
646 of the original signedData signerInfo. If the signature verification
647 fails, then the signedData/Receipt MUST NOT be created.
649 2. A Receipt structure is created.
651 2.1. The value of the Receipt version field is set to 1.
653 2.2. The object identifier from the contentType attribute included in
654 the original signedData signerInfo that includes the receiptRequest
655 attribute is copied into the Receipt contentType.
657 2.3. The original signedData signerInfo receiptRequest
658 signedContentIdentifier is copied into the Receipt
659 signedContentIdentifier.
661 2.4. The signature value from the original signedData signerInfo that
662 includes the receiptRequest attribute is copied into the Receipt
663 originatorSignatureValue.
665 3. The Receipt structure is ASN.1 DER encoded to produce a data stream, D1.
667 4. D1 is digested. The resulting digest value is included as the
668 messageDigest attribute in the signedAttributes of the signerInfo which
669 will eventually contain the signedData/Receipt signature value.
671 5. The digest value (msgSigDigest) calculated in Step 1 to verify the
672 signature of the original signedData signerInfo is included as the
673 msgSigDigest attribute in the signedAttributes of the signerInfo which will
674 eventually contain the signedData/Receipt signature value.
676 6. A contentType attribute including the id-ct-receipt object identifier
677 MUST be created and added to the signed attributes of the signerInfo which
678 will eventually contain the signedData/Receipt signature value.
680 7. A signingTime attribute indicating the time that the signedData/Receipt
681 is signed SHOULD be created and added to the signed attributes of the
682 signerInfo which will eventually contain the signedData/Receipt signature
683 value. Other attributes (except receiptRequest) may be added to the
684 signedAttributes of the signerInfo.
686 8. The signedAttributes (messageDigest, msgSigDigest, contentType and,
687 possibly, others) of the signerInfo are ASN.1 DER encoded and digested as
688 described in CMS, Section 5.3. The resulting digest value is used to
689 calculate the signature value which is then included in the
690 signedData/Receipt signerInfo.
692 9. The ASN.1 DER encoded Receipt content MUST be directly encoded within
693 the signedData encapContentInfo eContent OCTET STRING defined in [CMS]. The
694 id-ct-receipt object identifier MUST be included in the signedData
695 encapContentInfo eContentType. This results in a single ASN.1 encoded
696 object composed of a signedData including the Receipt content. The Data
697 content type MUST NOT be used. The Receipt content MUST NOT be encapsulated
698 in a MIME header or any other header prior to being encoded as part of the
699 signedData object.
701 10. The signedData/Receipt is then put in an application/pkcs7-mime MIME
702 wrapper with the smime-type parameter set to "signed-receipt". This will
703 allow for identification of signed receipts without having to crack the
704 ASN.1 body. The smime-type parameter would still be set as normal in any
705 layer wrapped around this message.
707 11. If the signedData/Receipt is to be encrypted within an envelopedData
708 object, then an outer signedData object MUST be created that encapsulates
709 the envelopedData object, and a contentHints attribute with contentType set
710 to the id-ct-receipt object identifier MUST be included in the outer
711 signedData SignerInfo signedAttributes. When a receiving agent processes
712 the outer signedData object, the presence of the id-ct-receipt OID in the
713 contentHints contentType indicates that a signedData/Receipt is encrypted
714 within the envelopedData object encapsulated by the outer signedData.
716 All agents that support the generation of ESS signed receipts MUST provide
717 the ability to send encrypted signed receipts (that is, a
718 signedData/Receipt encapsulated within an envelopedData). The agent MAY
719 send an encrypted signed receipt in response to an
720 envelopedData-encapsulated signedData requesting a signed receipt. It is a
721 matter of local policy regarding whether or not the signed receipt should
722 be encrypted. The ESS signed receipt includes the message digest value
723 calculated for the original signedData object that requested the signed
724 receipt. If the original signedData object was sent encrypted within an
725 envelopedData object and the ESS signed receipt is sent unencrypted, then
726 the message digest value calculated for the original encrypted signedData
727 object is sent unencrypted. The responder should consider this when
728 deciding whether or not to encrypt the ESS signed receipt.
730 2.4.1 MLExpansionHistory Attributes and Receipts
732 An MLExpansionHistory attribute MUST NOT be included in the attributes of a
733 SignerInfo in a SignedData object that encapsulates a Receipt content. This
734 is true because when a SignedData/Receipt is sent to an MLA for
735 distribution, then the MLA must always encapsulate the received
736 SignedData/Receipt in an outer SignedData in which the MLA will include the
737 MLExpansionHistory attribute. The MLA cannot change the signedAttributes of
738 the received SignedData/Receipt object, so it can't add the
739 MLExpansionHistory to the SignedData/Receipt.
741 2.5 Determining the Recipients of the Signed Receipt
743 If a signed receipt was created by the process described in the sections
744 above, then the software MUST use the following process to determine to
745 whom the signed receipt should be sent.
747 1. The receiptsTo field must be present in the receiptRequest attribute.
748 The software initiates the sequence of recipients with the value(s) of
749 receiptsTo.
751 2. If the MlExpansionHistory attribute is present in the outer SignedData
752 block, and the last MLData contains an MLReceiptPolicy value of insteadOf,
753 then the software replaces the sequence of recipients with the value(s) of
754 insteadOf.
756 3. If the MlExpansionHistory attribute is present in the outer SignedData
757 block and the last MLData contains an MLReceiptPolicy value of
758 inAdditionTo, then the software adds the value(s) of inAdditionTo to the
759 sequence of recipients.
761 2.6. Signed Receipt Validation
763 A signed receipt is communicated as a single ASN.1 encoded object composed
764 of a signedData object directly including a Receipt content. It is
765 identified by the presence of the id-ct-receipt object identifier in the
766 encapContentInfo eContentType value of the signedData object including the
767 Receipt content.
769 A signedData/Receipt is validated as follows:
771 1. ASN.1 decode the signedData object including the Receipt content.
773 2. Extract the contentType, signedContentIdentifier, and
774 originatorSignatureValue from the decoded Receipt structure to identify the
775 original signedData signerInfo that requested the signedData/Receipt.
777 3. Acquire the message signature digest value calculated by the sender to
778 generate the signature value included in the original signedData signerInfo
779 that requested the signedData/Receipt.
781 3.1. If the sender-calculated message signature digest value has been
782 saved locally by the sender, it must be located and retrieved.
784 3.2. If it has not been saved, then it must be re-calculated based on
785 the original signedData content and signedAttributes as described in
786 [CMS].
788 4. The message signature digest value calculated by the sender is then
789 compared with the value of the msgSigDigest signedAttribute included in the
790 signedData/Receipt signerInfo. If these digest values are identical, then
791 that proves that the message signature digest value calculated by the
792 recipient based on the received original signedData object is the same as
793 that calculated by the sender. This proves that the recipient received
794 exactly the same original signedData content and signedAttributes as sent
795 by the sender because that is the only way that the recipient could have
796 calculated the same message signature digest value as calculated by the
797 sender. If the digest values are different, then the signedData/Receipt
798 signature verification process fails.
800 5. Acquire the digest value calculated by the sender for the Receipt
801 content constructed by the sender (including the contentType,
802 signedContentIdentifier, and signature value that were included in the
803 original signedData signerInfo that requested the signedData/Receipt).
805 5.1. If the sender-calculated Receipt content digest value has been
806 saved locally by the sender, it must be located and retrieved.
808 5.2. If it has not been saved, then it must be re-calculated. As
809 described in section 2.4 above, step 2, create a Receipt structure
810 including the contentType, signedContentIdentifier and signature value
811 that were included in the original signedData signerInfo that requested
812 the signed receipt. The Receipt structure is then ASN.1 DER encoded to
813 produce a data stream which is then digested to produce the Receipt
814 content digest value.
816 6. The Receipt content digest value calculated by the sender is then
817 compared with the value of the messageDigest signedAttribute included in
818 the signedData/Receipt signerInfo. If these digest values are identical,
819 then that proves that the values included in the Receipt content by the
820 recipient are identical to those that were included in the original
821 signedData signerInfo that requested the signedData/Receipt. This proves
822 that the recipient received the original signedData signed by the sender,
823 because that is the only way that the recipient could have obtained the
824 original signedData signerInfo signature value for inclusion in the Receipt
825 content. If the digest values are different, then the signedData/Receipt
826 signature verification process fails.
828 7. The ASN.1 DER encoded signedAttributes of the signedData/Receipt
829 signerInfo are digested as described in [CMS].
831 8. The resulting digest value is then used to verify the signature value
832 included in the signedData/Receipt signerInfo. If the signature
833 verification is successful, then that proves the integrity of the
834 signedData/receipt signerInfo signedAttributes and authenticates the
835 identity of the signer of the signedData/Receipt signerInfo. Note that the
836 signedAttributes include the recipient-calculated Receipt content digest
837 value (messageDigest attribute) and recipient-calculated message signature
838 digest value (msgSigDigest attribute). Therefore, the aforementioned
839 comparison of the sender-generated and recipient-generated digest values
840 combined with the successful signedData/Receipt signature verification
841 proves that the recipient received the exact original signedData content
842 and signedAttributes (proven by msgSigDigest attribute) that were signed by
843 the sender of the original signedData object (proven by messageDigest
844 attribute). If the signature verification fails, then the
845 signedData/Receipt signature verification process fails.
847 The signature verification process for each signature algorithm that is
848 used in conjunction with the CMS protocol is specific to the algorithm.
849 These processes are described in documents specific to the algorithms.
851 2.7 Receipt Request Syntax
853 A receiptRequest attribute value has ASN.1 type ReceiptRequest. Use the
854 receiptRequest attribute only within the signed attributes associated with
855 a signed message.
857 ReceiptRequest ::= SEQUENCE {
858 signedContentIdentifier ContentIdentifier,
859 receiptsFrom ReceiptsFrom,
860 receiptsTo SEQUENCE SIZE (1..ub-receiptsTo)) OF GeneralNames }
862 ub-receiptsTo INTEGER ::= 16
864 id-aa-receiptRequest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
865 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 1}
867 ContentIdentifier ::= OCTET STRING
869 id-aa-contentIdentifier OBJECT IDENTIFIER ::= { iso(1) member-body(2)
870 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 7}
872 A signedContentIdentifier MUST be created by the message originator when
873 creating a receipt request. To ensure global uniqueness, the minimal
874 signedContentIdentifier SHOULD contain a concatenation of user-specific
875 identification information (such as a user name or public keying material
876 identification information), a GeneralizedTime string, and a random number.
878 The receiptsFrom field is used by the originator to specify the recipients
879 requested to return a signed receipt. A CHOICE is provided to allow
880 specification of:
881 - receipts from all recipients are requested
882 - receipts from first tier (recipients that did not receive the
883 message as members of a mailing list) recipients are requested
884 - receipts from a specific list of recipients are requested
886 ReceiptsFrom ::= CHOICE {
887 allOrFirstTier [0] AllOrFirstTier,
888 -- formerly "allOrNone [0]AllOrNone"
889 receiptList [1] SEQUENCE OF GeneralNames }
891 AllOrFirstTier ::= INTEGER { -- Formerly AllOrNone
892 allReceipts (0),
893 firstTierRecipients (1) }
895 The receiptsTo field is used by the originator to identify the user(s) to
896 whom the identified recipient should send signed receipts. The message
897 originator MUST populate the receiptsTo field with a GeneralNames for each
898 entity to whom the recipient should send the signed receipt.�If the message
899 originator wants the recipient to send the signed receipt to the
900 originator, then the originator MUST include a GeneralNames for itself in
901 the receiptsTo field.
903 2.8 Receipt Syntax
905 Receipts are represented using a new content type, Receipt. The Receipt
906 content type shall have ASN.1 type Receipt. Receipts must be encapsulated
907 within a SignedData message.
909 Receipt ::= SEQUENCE {
910 version Version, -- Version is imported from [CMS]
911 contentType ContentType,
912 signedContentIdentifier ContentIdentifier,
913 originatorSignatureValue OCTET STRING }
915 id-ct-receipt OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
916 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-ct(1) 1}
918 The version field defines the syntax version number, which is 1 for this
919 version of the standard.
921 2.9 Content Hints
923 Many applications find it useful to have information that describes the
924 innermost signed content of a multi-layer message available on the
925 outermost signature layer. The contentHints attribute provides such
926 information.
928 Content-hints attribute values have ASN.1 type contentHints.
930 ContentHints ::= SEQUENCE {
931 � contentDescription UTF8String SIZE (1..MAX) OPTIONAL,
932 � contentType ContentType }
934 id-aa-contentHint OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
935 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 4}
937 The contentDescription field may be used to provide information that the
938 recipient may use to select protected messages for processing, such as a
939 message subject. If this field is set, then the attribute is expected to
940 appear on the signedData object enclosing an envelopedData object and not
941 on the inner signedData object. The SIZE (1..MAX) construct constrains the
942 sequence to have at least one entry. MAX indicates the upper bound is
943 unspecified. Implementations are free to choose an upper bound that suits
944 their environment.
946 Messages which contain a signedData object wrapped around an envelopedData
947 object, thus masking the inner content type of the message, SHOULD include
948 a contentHints attribute, except for the case of the data content type.
949 Specific message content types may either force or preclude the inclusion
950 of the contentHints attribute. For example, when a signedData/Receipt is
951 encrypted within an envelopedData object, an outer signedData object MUST
952 be created that encapsulates the envelopedData object and a contentHints
953 attribute with contentType set to the id-ct-receipt object identifier MUST
954 be included in the outer signedData SignerInfo signedAttributes.
956 2.10� Message Signature Digest Attribute
958 The msgSigDigest attribute can only be used in the signed attributes of a
959 signed receipt.�It contains the digest of the ASN.1 DER encoded
960 signedAttributes included in the original signedData that requested the
961 signed receipt.�Only one msgSigDigest attribute can appear in an signed
962 attributes set. It is defined as follows:
964 msgSigDigest ::= OCTET STRING
966 id-aa-msgSigDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
967 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 5}
969 2.11 Signed Content Reference Attribute
971 The contentReference attribute is a link from one SignedData to another. It
972 may be used to link a reply to the original message to which it refers, or
973 to incorporate by reference one SignedData into another. The first
974 SignedData MUST include a contentIdentifier signed attribute, which SHOULD
975 be constructed as specified in section 2.7. The second SignedData links to
976 the first by including a ContentReference signed attribute containing the
977 content type, content identifier, and signature value from the first
978 SignedData.
980 ContentReference ::= SEQUENCE {
981 contentType ContentType,
982 signedContentIdentifier ContentIdentifier,
983 originatorSignatureValue OCTET STRING }
985 id-aa-contentReference OBJECT IDENTIFIER ::= { iso(1) member-body(2)
986 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 10 }
988 3. Security Labels
990 This section describes the syntax to be used for security labels that can
991 optionally be associated with S/MIME encapsulated data. A security label is
992 a set of security information regarding the sensitivity of the content that
993 is protected by S/MIME encapsulation.
995 "Authorization" is the act of granting rights and/or privileges to users
996 permitting them access to an object. "Access control" is a means of
997 enforcing these authorizations. The sensitivity information in a security
998 label can be compared with a user's authorizations to determine if the user
999 is allowed to access the content that is protected by S/MIME encapsulation.
1001 Security labels may be used for other purposes such as a source of routing
1002 information. The labels are often priority based ("secret", "confidential",
1003 "restricted", and so on) or role-based, describing which kind of people can
1004 see the information ("patient's health-care team", "medical billing
1005 agents", "unrestricted", and so on).
1007 3.1 Security Label Processing Rules
1009 A sending agent may include a security label attribute in the signed
1010 attributes of a signedData object. A receiving agent examines the security
1011 label on a received message and determines whether or not the recipient is
1012 allowed to see the contents of the message.
1014 3.1.1 Adding Security Labels
1016 A sending agent that is using security labels MUST put the security label
1017 attribute in the signedAttributes field of a SignerInfo block. The security
1018 label attribute MUST NOT be included in the unsigned attributes. Integrity
1019 and authentication security services MUST be applied to the security label,
1020 therefore it MUST be included as an signed attribute, if used. This causes
1021 the security label attribute to be part of the data that is hashed to form
1022 the SignerInfo signature value. A SignerInfo block MUST NOT have more than
1023 one security label signed attribute.
1025 When there are multiple SignedData blocks applied to a message, a security
1026 label attribute may be included in either the inner signature, outer
1027 signature, or both. A security label signed attribute may be included in a
1028 signedAttributes field within the inner SignedData block. The inner
1029 security label will include the sensitivities of the original content and
1030 will be used for access control decisions related to the plaintext
1031 encapsulated content. The inner signature provides authentication of the
1032 inner security label and cryptographically protects the original signer's
1033 inner security label of the original content.
1035 When the originator signs the plaintext content and signed attributes, the
1036 inner security label is bound to the plaintext content. An intermediate
1037 entity cannot change the inner security label without invalidating the
1038 inner signature. The confidentiality security service can be applied to the
1039 inner security label by encrypting the entire inner signedData object
1040 within an EnvelopedData block.
1042 A security label signed attribute may also be included in a
1043 signedAttributes field within the outer SignedData block. The outer
1044 security label will include the sensitivities of the encrypted message and
1045 will be used for access control decisions related to the encrypted message
1046 and for routing decisions. The outer signature provides authentication of
1047 the outer security label (as well as for the encapsulated content which may
1048 include nested S/MIME messages).
1050 There can be multiple SignerInfos within a SignedData object, and each
1051 SignerInfo may include signedAttributes. Therefore, a single SignedData
1052 object may include multiple eSSSecurityLabels, each SignerInfo having an
1053 eSSSecurityLabel attribute. For example, an originator can send a signed
1054 message with two SignerInfos, one containing a DSS signature, the other
1055 containing an RSA signature. If any of the SignerInfos included in a
1056 SignedData object include an eSSSecurityLabel attribute, then all of the
1057 SignerInfos in that SignedData object MUST include an eSSSecurityLabel
1058 attribute and the value of each MUST be identical.
1060 3.1.2 Processing Security Labels
1062 Before processing an eSSSecurityLabel signedAttribute, the receiving agent
1063 MUST verify the signature of the SignerInfo which covers the
1064 eSSSecurityLabel attribute. A recipient MUST NOT process an
1065 eSSSecurityLabel attribute that has not been verified.
1067 A receiving agent MUST process the eSSSecurityLabel attribute, if present,
1068 in each SignerInfo in the SignedData object for which it verifies the
1069 signature. This may result in the receiving agent processing multiple
1070 eSSSecurityLabels included in a single SignedData object. Because all
1071 eSSSecurityLabels in a SignedData object must be identical, the receiving
1072 agent processes (such as performing access control) on the first
1073 eSSSecurityLabel that it encounters in a SignerInfo that it verifies, and
1074 then ensures that all other eSSSecurityLabels in signerInfos that it
1075 verifies are identical to the first one encountered. If the
1076 eSSSecurityLabels in the signerInfos that it verifies are not all
1077 identical, then the receiving agent MUST warn the user of this condition.
1079 Receiving agents SHOULD have a local policy regarding whether or not to
1080 show the inner content of a signedData object that includes an
1081 eSSSecurityLabel security-policy-identifier that the processing software
1082 does not recognize. If the receiving agent does not recognize the
1083 eSSSecurityLabel security-policy-identifier value, then it SHOULD stop
1084 processing the message and indicate an error.
1086 3.2 Syntax of eSSSecurityLabel
1088 The eSSSecurityLabel syntax is derived directly from [MTSABS] ASN.1 module.
1089 (The MTSAbstractService module begins with "DEFINITIONS IMPLICIT TAGS
1090 ::=".) Further, the eSSSecurityLabel syntax is compatible with that used in
1091 [MSP4].
1093 ESSSecurityLabel ::= SET {
1094 security-policy-identifier SecurityPolicyIdentifier,
1095 security-classification SecurityClassification OPTIONAL,
1096 privacy-mark ESSPrivacyMark OPTIONAL,
1097 security-categories SecurityCategories OPTIONAL }
1099 id-aa-securityLabel OBJECT IDENTIFIER ::= { iso(1) member-body(2)
1100 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 2}
1102 SecurityPolicyIdentifier ::= OBJECT IDENTIFIER
1104 SecurityClassification ::= INTEGER {
1105 unmarked (0),
1106 unclassified (1),
1107 restricted (2),
1108 confidential (3),
1109 secret (4),
1110 top-secret (5) } (0..ub-integer-options)
1112 ub-integer-options INTEGER ::= 256
1114 ESSPrivacyMark ::= CHOICE {
1115 ��� pString����� PrintableString SIZE (1..ub-privacy-mark-length),
1116 ��� utf8String�� UTF8String SIZE (1..MAX)
1117 }
1119 ub-privacy-mark-length INTEGER ::= 128
1121 SecurityCategories ::= SET SIZE (1..ub-security-categories) OF
1122 SecurityCategory
1124 ub-security-categories INTEGER ::= 64
1126 SecurityCategory ::= SEQUENCE {
1127 type [0] OBJECT IDENTIFIER,
1128 value [1] ANY DEFINED BY type -- defined by type
1129 }
1131 --Note: The aforementioned SecurityCategory syntax produces identical
1132 --hex encodings as the following SecurityCategory syntax that is
1133 --documented in the X.411 specification:
1134 --
1135 --SecurityCategory ::= SEQUENCE {
1136 -- type [0] SECURITY-CATEGORY,
1137 -- value [1] ANY DEFINED BY type }
1138 --
1139 --SECURITY-CATEGORY MACRO ::=
1140 --BEGIN
1141 --TYPE NOTATION ::= type | empty
1142 --VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER)
1143 --END
1145 3.3 Security Label Components
1147 This section gives more detail on the the various components of the
1148 eSSSecurityLabel syntax.
1150 3.3.1 Security Policy Identifier
1152 A security policy is a set of criteria for the provision of security
1153 services. The eSSSecurityLabel security-policy-identifier is used to
1154 identify the security policy in force to which the security label relates.
1155 It indicates the semantics of the other security label components.
1157 3.3.2 Security Classification
1159 This specification defines the use of the Security Classification field
1160 exactly as is specified in the X.411 Recommendation, which states in part:
1162 If present, a security-classification may have one of a hierarchical
1163 list of values. The basic security-classification hierarchy is defined
1164 in this Recommendation, but the use of these values is defined by the
1165 security-policy in force. Additional values of security-classification,
1166 and their position in the hierarchy, may also be defined by a
1167 security-policy as a local matter or by bilateral agreement. The basic
1168 security-classification hierarchy is, in ascending order: unmarked,
1169 unclassified, restricted, confidential, secret, top-secret.
1171 This means that the security policy in force (identified by the
1172 eSSSecurityLabel security-policy-identifier) defines the
1173 SecurityClassification integer values and their meanings.
1175 An organization can develop its own security policy that defines the
1176 SecurityClassification INTEGER values and their meanings. However, the
1177 general interpretation of the X.411 specification is that the values of 0
1178 through 5 are reserved for the "basic hierarchy" values of unmarked,
1179 unclassified, restricted, confidential, secret, and top-secret. Note that
1180 X.411 does not provide the rules for how these values are used to label
1181 data and how access control is performed using these values.
1183 There is no universal definition of the rules for using these "basic
1184 hierarchy" values. Each organization (or group of organizations) will
1185 define a security policy which documents how the "basic hierarchy" values
1186 are used (if at all) and how access control is enforced (if at all) within
1187 their domain.
1189 Therefore, the security-classification value MUST be accompanied by a
1190 security-policy-identifier value to define the rules for its use. For
1191 example, a company's "secret" classification may convey a different meaning
1192 than the US Government "secret" classification. In summary, a security
1193 policy SHOULD NOT use integers 0 through 5 for other than their X.411
1194 meanings, and SHOULD instead use other values in a hierarchical fashion.
1196 Note that the set of valid security-classification values MUST be
1197 hierarchical, but these values do not necessarily need to be in ascending
1198 numerical order. Further, the values do not need to be contiguous.
1200 For example, in the Defense Message System 1.0 security policy, the
1201 security-classification value of 11 indicates Sensitive-But-Unclassified
1202 and 5 indicates top-secret. The hierarchy of sensitivity ranks top-secret
1203 as more sensitive than Sensitive-But-Unclassified even though the numerical
1204 value of top-secret is less than Sensitive-But-Unclassified.
1206 (Of course, if security-classification values are both hierarchical and in
1207 ascending order, a casual reader of the security policy is more likely to
1208 understand it.)
1210 An example of a security policy that does not use any of the X.411 values
1211 might be:
1212 10 -- anyone
1213 15 -- Morgan Corporation and its contractors
1214 20 -- Morgan Corporation employees
1215 25 -- Morgan Corporation board of directors
1217 An example of a security policy that uses part of the X.411 hierarchy might
1218 be:
1219 0 -- unmarked
1220 1 -- unclassified, can be read by everyone
1221 2 -- restricted to Timberwolf Productions staff
1222 6 -- can only be read to Timberwolf Productions executives
1224 3.3.3 Privacy Mark
1226 If present, the eSSSecurityLabel privacy-mark is not used for access
1227 control. The content of the eSSSecurityLabel privacy-mark may be defined by
1228 the security policy in force (identified by the eSSSecurityLabel
1229 security-policy-identifier) which may define a list of values to be used.
1230 Alternately, the value may be determined by the originator of the
1231 security-label.
1233 3.3.4 Security Categories
1235 If present, the eSSSecurityLabel security-categories provide further
1236 granularity for the sensitivity of the message. The security policy in
1237 force (identified by the eSSSecurityLabel security-policy-identifier) is
1238 used to indicate the syntaxes that are allowed to be present in the
1239 eSSSecurityLabel security-categories. Alternately, the security-categories
1240 and their values may be defined by bilateral agreement.
1242 3.4 Equivalent Security Labels
1244 Because organizations are allowed to define their own security policies,
1245 many different security policies will exist. Some organizations may wish to
1246 create equivalencies between their security policies with the security
1247 policies of other organizations. For example, the Acme Company and the
1248 Widget Corporation may reach a bilateral agreement that the "Acme private"
1249 security-classification value is equivalent to the "Widget sensitive"
1250 security-classification value.
1252 Receiving agents MUST NOT process an equivalentLabels attribute in a
1253 message if the agent does not trust the signer of that attribute to
1254 translate the original eSSSecurityLabel values to the security policy
1255 included in the equivalentLabels attribute. Receiving agents have the
1256 option to process equivalentLabels attributes but do not have to. It is
1257 acceptable for a receiving agent to only process eSSSecurityLabels. All
1258 receiving agents SHOULD recognize equivalentLabels attributes even if they
1259 do not process them.
1261 3.4.1 Creating Equivalent Labels
1263 The EquivalentLabels signed attribute is defined as:
1265 EquivalentLabels ::= SEQUENCE OF ESSSecurityLabel
1267 id-aa-equivalentLabels OBJECT IDENTIFIER ::= { iso(1) member-body(2)
1268 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 9}
1270 As stated earlier, the ESSSecurityLabel contains the sensitivity values
1271 selected by the original signer of the signedData. If an ESSSecurityLabel
1272 is present in a signerInfo, all signerInfos in the signedData MUST contain
1273 an ESSSecurityLabel and they MUST all be identical. In addition to an
1274 ESSSecurityLabel, a signerInfo MAY also include an equivalentLabels signed
1275 attribute. If present, the equivalentLabels attribute MUST include one or
1276 more security labels that are believed by the signer to be semantically
1277 equivalent to the ESSSecurityLabel attribute included in the same
1278 signerInfo.
1280 All security-policy object identifiers MUST be unique in the set of
1281 ESSSecurityLabel and EquivalentLabels security labels. Before using an
1282 EquivalentLabels attribute, an agent MUST ensure that all security-policy
1283 OIDs are unique in the security label or labels included in the
1284 EquivalentLabels. Once the agent selects the security label (within the
1285 EquivalentLabels) to be used for processing, then the security-policy OID
1286 of the selected EquivalentLabels security label MUST be compared with the
1287 ESSSecurityLabel security-policy OID to ensure that they are unique.
1289 In the case that an ESSSecurityLabel attribute is not included in a
1290 signerInfo, then an EquivalentLabels attribute may still be included. For
1291 example, in the Acme security policy, the absence of an ESSSecurityLabel
1292 could be defined to equate to a security label composed of the Acme
1293 security-policy OID and the "unmarked" security-classification.
1295 Note that equivalentLabels MUST NOT be used to convey security labels that
1296 are semantically different from the ESSSecurityLabel included in the
1297 signerInfos in the signedData. If an entity needs to apply a security label
1298 that is semantically different from the ESSSecurityLabel, then it MUST
1299 include the sematically different security label in an outer signedData
1300 object that encapsulates the signedData object that includes the
1301 ESSSecurityLabel.
1303 If present, the equivalentLabels attribute MUST be an signed attribute; it
1304 MUST NOT be an unsigned attribute. CMS defines signedAttributes as a SET OF
1305 Attribute. A signerInfo MUST NOT include multiple instances of the
1306 equivalentLabels attribute. CMS defines the ASN.1 syntax for the signed
1307 attributes to include attrValues SET OF AttributeValue. A equivalentLabels
1308 attribute MUST only include a single instance of AttributeValue. There MUST
1309 NOT be zero or multiple instances of AttributeValue present in the
1310 attrValues SET OF AttributeValue.
1312 3.4.2 Processing Equivalent Labels
1314 A receiving agent SHOULD process the ESSSecurityLabel before processing any
1315 EquivalentLabels. If the policy in the ESSSecurityLabel is understood by
1316 the receiving agent, it MUST process that label and MUST ignore all
1317 EquivalentLabels.
1319 When processing an EquivalentLabels attribute, the receiving agent MUST
1320 validate the signature on the EquivalentLabels attribute. A receiving agent
1321 MUST NOT act on an equivalentLabels attribute for which the signature could
1322 not be validated, and MUST NOT act on an equivalentLabels attribute unless
1323 that attribute is signed by an entity trusted to to translate the original
1324 eSSSecurityLabel values to to the security policy included in the
1325 equivalentLabels attribute. Determining who is allowed to specify
1326 equivalence mappings is a local policy. If a message has more than one
1327 EquivalentLabels attribute, the receiving agent SHOULD process the first
1328 one that it reads and validates that contains the security policy of
1329 interest to the receiving agent.
1331 4. Mail List Management
1333 Sending agents must create recipient-specific data structures for each
1334 recipient of an encrypted message. This process can impair performance for
1335 messages sent to a large number of recipients. Thus, Mail List Agents
1336 (MLAs) that can take a single message and perform the recipient-specific
1337 encryption for every recipient are often desired.
1339 An MLA appears to the message originator as a normal message recipient, but
1340 the MLA acts as a message expansion point for a Mail List (ML). The sender
1341 of a message directs the message to the MLA, which then redistributes the
1342 message to the members of the ML. This process offloads the per-recipient
1343 processing from individual user agents and allows for more efficient
1344 management of large MLs. MLs are true message recipients served by MLAs
1345 that provide cryptographic and expansion services for the mailing list.
1347 In addition to cryptographic handling of messages, secure mailing lists
1348 also have to prevent mail loops. A mail loop is where one mailing list is a
1349 member of a second mailing list, and the second mailing list is a member of
1350 the first. A message will go from one list to the other in a
1351 rapidly-cascading succession of mail that will be distributed to all other
1352 members of both lists.
1354 To prevent mail loops, MLAs use the mlExpansionHistory attribute of the
1355 outer signature of a triple wrapped message. The mlExpansionHistory
1356 attribute is essentially a list of every MLA that has processed the
1357 message. If an MLA sees its own unique entity identifier in the list, it
1358 knows that a loop has been formed, and does not send the message to the
1359 list again.
1361 4.1 Mail List Expansion
1363 Mail list expansion processing is noted in the value of the
1364 mlExpansionHistory attribute, located in the signed attributes of the MLA's
1365 SignerInfo block. The MLA creates or updates the signed mlExpansionHistory
1366 attribute value each time the MLA expands and signs a message for members
1367 of a mail list.
1369 The MLA MUST add an MLData record containing the MLA's identification
1370 information, date and time of expansion, and optional receipt policy to the
1371 end of the mail list expansion history sequence. If the mlExpansionHistory
1372 attribute is absent, then the MLA MUST add the attribute and the current
1373 expansion becomes the first element of the sequence. If the
1374 mlExpansionHistory attribute is present, then the MLA MUST add the current
1375 expansion information to the end of the existing MLExpansionHistory
1376 sequence. Only one mlExpansionHistory attribute can be included in the
1377 signedAttributes of a SignerInfo.
1379 Note that if the mlExpansionHistory attribute is absent, then the recipient
1380 is a first tier message recipient.
1382 There can be multiple SignerInfos within a SignedData object, and each
1383 SignerInfo may include signedAttributes. Therefore, a single SignedData
1384 object may include multiple SignerInfos, each SignerInfo having a
1385 mlExpansionHistory attribute. For example, an MLA can send a signed message
1386 with two SignerInfos, one containing a DSS signature, the other containing
1387 an RSA signature.
1389 If an MLA creates a SignerInfo that includes an mlExpansionHistory
1390 attribute, then all of the SignerInfos created by the MLA for that
1391 SignedData object MUST include an mlExpansionHistory attribute, and the
1392 value of each MUST be identical. Note that other agents might later add
1393 SignerInfo attributes to the SignedData block, and those additional
1394 SignerInfos might not include mlExpansionHistory attributes.
1396 A recipient MUST verify the signature of the SignerInfo which covers the
1397 mlExpansionHistory attribute before processing the mlExpansionHistory, and
1398 MUST NOT process the mlExpansionHistory attribute unless the signature over
1399 it has been verified. If a SignedData object has more than one SignerInfo
1400 that has an mlExpansionHistory attribute, the recipient MUST compare the
1401 mlExpansionHistory attributes in all the SignerInfos that it has verified,
1402 and MUST NOT process the mlExpansionHistory attribute unless every verified
1403 mlExpansionHistory attribute in the SignedData block is identical. If the
1404 mlExpansionHistory attributes in the verified signerInfos are not all
1405 identical, then the receiving agent MUST stop processing the message and
1406 SHOULD notify the user or MLA administrator of this error condition. In the
1407 mlExpansionHistory processing, SignerInfos that do not have an
1408 mlExpansionHistory attribute are ignored.
1410 4.1.1 Detecting Mail List Expansion Loops
1412 Prior to expanding a message, the MLA examines the value of any existing
1413 mail list expansion history attribute to detect an expansion loop. An
1414 expansion loop exists when a message expanded by a specific MLA for a
1415 specific mail list is redelivered to the same MLA for the same mail list.
1417 Expansion loops are detected by examining the mailListIdentifier field of
1418 each MLData entry found in the mail list expansion history. If an MLA finds
1419 its own identification information, then the MLA must discontinue expansion
1420 processing and should provide warning of an expansion loop to a human mail
1421 list administrator. The mail list administrator is responsible for
1422 correcting the loop condition.
1424 4.2 Mail List Agent Processing
1426 The first few paragraphs of this section provide a high-level description
1427 of MLA processing. The rest of the section provides a detailed description
1428 of MLA processing.
1430 MLA message processing depends on the structure of the S/MIME layers in the
1431 message sent to the MLA for expansion. In addition to sending triple
1432 wrapped messages to an MLA, an entity can send other types of messages to
1433 an MLA, such as:
1434 - a single wrapped signedData or envelopedData message
1435 - a double wrapped message (such as signed and enveloped, enveloped and
1436 signed, or signed and signed, and so on)
1437 - a quadruple-wrapped message (such as if a well-formed triple wrapped
1438 message was sent through a gateway that added an outer SignedData layer)
1440 In all cases, the MLA MUST parse all layers of the received message to
1441 determine if there are any signedData layers that include an
1442 eSSSecurityLabel signedAttribute. This may include decrypting an
1443 EnvelopedData layer to determine if an encapsulated SignedData layer
1444 includes an eSSSecurityLabel attribute. The MLA MUST fully process each
1445 eSSSecurityLabel attribute found in the various signedData layers,
1446 including performing access control checks, before distributing the message
1447 to the ML members. The details of the access control checks are beyond the
1448 scope of this document. The MLA MUST verify the signature of the signerInfo
1449 including the eSSSecurityLabel attribute before using it.
1451 In all cases, the MLA MUST sign the message to be sent to the ML members in
1452 a new "outer" signedData layer. The MLA MUST add or update an
1453 mlExpansionHistory attribute in the "outer" signedData that it creates to
1454 document MLA processing. If there was an "outer" signedData layer included
1455 in the original message received by the MLA, then the MLA-created "outer"
1456 signedData layer MUST include each signed attribute present in the
1457 original "outer" signedData layer, unless the MLA explicitly replaces an
1458 attribute (such as signingTime or mlExpansionHistory) with a new value.
1460 When an S/MIME message is received by the MLA, the MLA MUST first determine
1461 which received signedData layer, if any, is the "outer" signedData layer.
1462 To identify the received "outer" signedData layer, the MLA MUST verify the
1463 signature and fully process the signedAttributes in each of the
1464 outer signedData layers (working from the outside in) to determine if any
1465 of them either include an mlExpansionHistory attribute or encapsulate an
1466 envelopedData object.
1468 The MLA's search for the "outer" signedData layer is completed when it
1469 finds one of the following:
1470 - the "outer" signedData layer that includes an mlExpansionHistory
1471 attribute or encapsulates an envelopedData object
1472 - an envelopedData layer
1473 - the original content (that is, a layer that is neither envelopedData nor
1474 signedData).
1476 If the MLA finds an "outer" signedData layer, then the MLA MUST perform
1477 the following steps:
1479 1. Strip off all of the signedData layers that encapsulated the "outer"
1480 signedData layer
1482 2. Strip off the "outer" signedData layer itself (after remembering the
1483 included signedAttributes)
1485 3. Expand the envelopedData (if present)
1487 4. Sign the message to be sent to the ML members in a new "outer"
1488 signedData layer that includes the signedAttributes (unless explicitly
1489 replaced) from the original, received "outer" signedData layer.
1491 If the MLA finds an "outer" signedData layer that includes an
1492 mlExpansionHistory attribute AND the MLA subsequently finds an
1493 envelopedData layer buried deeper with the layers of the received message,
1494 then the MLA MUST strip off all of the signedData layers down to the
1495 envelopedData layer (including stripping off the original "outer"
1496 signedData layer) and MUST sign the expanded envelopedData in a new "outer"
1497 signedData layer that includes the signedAttributes (unless explicitly
1498 replaced) from the original, received "outer" signedData layer.
1500 If the MLA does not find an "outer" signedData layer AND does not find an
1501 envelopedData layer, then the MLA MUST sign the original, received message
1502 in a new "outer" signedData layer. If the MLA does not find an "outer"
1503 signedData AND does find an envelopedData layer then it MUST expand the
1504 envelopedData layer, if present, and sign it in a new "outer" signedData
1505 layer.
1507 4.2.1 Examples of Rule Processing
1509 The following examples help explain the rules above:
1511 1) A message (S1(Original Content)) (where S = SignedData) is sent to the
1512 MLA in which the signedData layer does not include an MLExpansionHistory
1513 attribute. The MLA verifies and fully processes the signedAttributes in S1.
1514 The MLA decides that there is not an original, received "outer" signedData
1515 layer since it finds the original content, but never finds an envelopedData
1516 and never finds an mlExpansionHistory attribute. The MLA calculates a new
1517 signedData layer, S2, resulting in the following message sent to the ML
1518 recipients: (S2(S1(Original Content))). The MLA includes an
1519 mlExpansionHistory attribute in S2.
1521 2) A message (S3(S2(S1(Original Content)))) is sent to the MLA in which
1522 none of the signedData layers includes an MLExpansionHistory attribute. The
1523 MLA verifies and fully processes the signedAttributes in S3, S2 and S1. The
1524 MLA decides that there is not an original, received "outer" signedData
1525 layer since it finds the original content, but never finds an envelopedData
1526 and never finds an mlExpansionHistory attribute. The MLA calculates a new
1527 signedData layer, S4, resulting in the following message sent to the ML
1528 recipients: (S4(S3(S2(S1(Original Content))))). The MLA includes an
1529 mlExpansionHistory attribute in S4.
1531 3) A message (E1(S1(Original Content))) (where E = envelopedData) is sent
1532 to the MLA in which S1 does not include an MLExpansionHistory attribute.
1533 The MLA decides that there is not an original, received "outer" signedData
1534 layer since it finds the E1 as the outer layer. The MLA expands the
1535 recipientInformation in E1. The MLA calculates a new signedData layer, S2,
1536 resulting in the following message sent to the ML recipients:
1537 (S2(E1(S1(Original Content)))). The MLA includes an mlExpansionHistory
1538 attribute in S2.
1540 4) A message (S2(E1(S1(Original Content)))) is sent to the MLA in which S2
1541 includes an MLExpansionHistory attribute. The MLA verifies the signature
1542 and fully processes the signedAttributes in S2. The MLA finds the
1543 mlExpansionHistory attribute in S2, so it decides that S2 is the "outer"
1544 signedData. The MLA remembers the signedAttributes included in S2 for later
1545 inclusion in the new outer signedData that it applies to the message. The
1546 MLA strips off S2. The MLA then expands the recipientInformation in E1
1547 (this invalidates the signature in S2 which is why it was stripped). The
1548 MLA calculates a new signedData layer, S3, resulting in the following
1549 message sent to the ML recipients: (S3(E1(S1(Original Content)))). The MLA
1550 includes in S3 the attributes from S2 (unless it specifically replaces an
1551 attribute value) including an updated mlExpansionHistory attribute.
1553 5) A message (S3(S2(E1(S1(Original Content))))) is sent to the MLA in which
1554 none of the signedData layers include an MLExpansionHistory attribute. The
1555 MLA verifies the signature and fully processes the signedAttributes in S3
1556 and S2. When the MLA encounters E1, then it decides that S2 is the "outer"
1557 signedData since S2 encapsulates E1. The MLA remembers the signedAttributes
1558 included in S2 for later inclusion in the new outer signedData that it
1559 applies to the message. The MLA strips off S3 and S2. The MLA then expands
1560 the recipientInformation in E1 (this invalidates the signatures in S3 and
1561 S2 which is why they were stripped). The MLA calculates a new signedData
1562 layer, S4, resulting in the following message sent to the ML recipients:
1563 (S4(E1(S1(Original Content)))). The MLA includes in S4 the attributes from
1564 S2 (unless it specifically replaces an attribute value) and includes a new
1565 mlExpansionHistory attribute.
1567 6) A message (S3(S2(E1(S1(Original Content))))) is sent to the MLA in which
1568 S3 includes an MLExpansionHistory attribute. In this case, the MLA verifies
1569 the signature and fully processes the signedAttributes in S3. The MLA finds
1570 the mlExpansionHistory in S3, so it decides that S3 is the "outer"
1571 signedData. The MLA remembers the signedAttributes included in S3 for later
1572 inclusion in the new outer signedData that it applies to the message. The
1573 MLA keeps on parsing encapsulated layers because it must determine if there
1574 are any eSSSecurityLabel attributes contained within. The MLA verifies the
1575 signature and fully processes the signedAttributes in S2. When the MLA
1576 encounters E1, then it strips off S3 and S2. The MLA then expands the
1577 recipientInformation in E1 (this invalidates the signatures in S3 and S2
1578 which is why they were stripped). The MLA calculates a new signedData
1579 layer, S4, resulting in the following message sent to the ML recipients:
1580 (S4(E1(S1(Original Content)))). The MLA includes in S4 the attributes from
1581 S3 (unless it specifically replaces an attribute value) including an
1582 updated mlExpansionHistory attribute.
1584 4.2.3 Processing Choices
1586 The processing used depends on the type of the outermost layer of the
1587 message. There are three cases for the type of the outermost data:
1588 - EnvelopedData
1589 - SignedData
1590 - data
1592 4.2.3.1 Processing for EnvelopedData
1594 1. The MLA locates its own RecipientInfo and uses the information it
1595 contains to obtain the message key.
1597 2. The MLA removes the existing recipientInfos field and replaces it with a
1598 new recipientInfos value built from RecipientInfo structures created for
1599 each member of the mailing list. The MLA also removes the existing
1600 originatorInfo field and replaces it with a new originatorInfo value built
1601 from information describing the MLA.
1603 3. The MLA encapsulates the expanded encrypted message in a SignedData
1604 block, adding an mlExpansionHistory attribute as described in the "Mail
1605 List Expansion" section to document the expansion.
1607 4. The MLA signs the new message and delivers the updated message to mail
1608 list members to complete MLA processing.
1610 4.2.3.2 Processing for SignedData
1612 MLA processing of multi-layer messages depends on the type of data in each
1613 of the layers. Step 3 below specifies that different processing will take
1614 place depending on the type of CMS message that has been signed. That is,
1615 it needs to know the type of data at the next inner layer, which may or may
1616 not be the innermost layer.
1618 1. The MLA verifies the signature value found in the outermost SignedData
1619 layer associated with the signed data. MLA processing of the message
1620 terminates if the message signature is invalid.
1622 2. If the outermost SignedData layer includes an signed mlExpansionHistory
1623 attribute the MLA checks for an expansion loop as described in the
1624 "Detecting Mail List Expansion Loops" section.
1626 3. Determine the type of the data that has been signed. That is, look at
1627 the type of data on the layer just below the SignedData, which may or may
1628 not be the "innermost" layer. Based on the type of data, perform either
1629 step 3.1 (EnvelopedData), step 3.2 (SignedData), or step 3.3 (all other
1630 types).
1632 3.1. If the signed data is EnvelopedData, the MLA performs expansion
1633 processing of the encrypted message as described previously. Note that
1634 this process invalidates the signature value in the outermost
1635 SignedData layer associated with the original encrypted message.
1636 Proceed to section 3.2 with the result of the expansion.
1638 3.2. If the signed data is SignedData, or is the result of expanding an
1639 EnvelopedData block in step 3.1:
1641 3.2.1. The MLA strips the existing outermost SignedData layer after
1642 remembering the value of the mlExpansionHistory and all other
1643 signed attributes in that layer, if present.
1645 3.2.2. If the signed data is EnvelopedData (from step 3.1), the MLA
1646 encapsulates the expanded encrypted message in a new outermost
1647 SignedData layer. On the other hand, if the signed data is
1648 SignedData (from step 3.2), the MLA encapsulates the signed data in
1649 a new outermost SignedData layer.
1651 3.2.3. The outermost signedData layer created by the MLA replaces
1652 the original outermost signedData layer.�The MLA MUST create an
1653 signed attribute list for the new outermost signedData layer which
1654 MUST include each signed attribute present in the original
1655 outermost signedData layer, unless the MLA explicitly replaces one
1656 or more particular attributes with new value.�A special case is the
1657 mlExpansionHistory attribute.�The MLA MUST add an
1658 mlExpansionHistory signed attribute to the outer signedData layer
1659 as follows:
1661 3.2.3.1. If the original outermost SignedData layer included an
1662 mlExpansionHistory attribute, the attribute's value is copied
1663 and updated with the current ML expansion information as
1664 described in the "Mail List Expansion" section.
1666 3.2.3.2. If the original outermost SignedData layer did not
1667 include an mlExpansionHistory attribute, a new attribute value
1668 is created with the current ML expansion information as
1669 described in the "Mail List Expansion" section.
1671 3.3. If the signed data is not EnvelopedData or SignedData:
1673 3.3.1. The MLA encapsulates the received signedData object in an
1674 outer SignedData object, and adds an mlExpansionHistory attribute
1675 to the outer SignedData object containing the current ML expansion
1676 information as described in the "Mail List Expansion" section.
1678 4. The MLA signs the new message and delivers the updated message to mail
1679 list members to complete MLA processing.
1681 A flow chart for the above steps would be:
1683 1. Has a valid signature?
1684 YES -> 2.
1685 NO -> STOP.
1686 2. Does outermost SignedData layer
1687 contain mlExpansionHistory?
1688 YES -> Check it, then -> 3.
1689 NO -> 3.
1690 3. Check type of data just below outermost
1691 SignedData.
1692 EnvelopedData -> 3.1.
1693 SignedData -> 3.2.
1694 all others -> 3.3.
1695 3.1. Expand the encrypted message, then -> 3.2.
1696 3.2. -> 3.2.1.
1697 3.2.1. Strip outermost SignedData layer, note value of mlExpansionHistory
1698 and other signed attributes, then -> 3.2.2.
1699 3.2.2. Encapsulate in new signature, then -> 3.2.3.
1700 3.2.3. Create new signedData layer. Was there an old mlExpansionHistory?
1701 YES -> copy the old mlExpansionHistory values, then -> 4.
1702 NO -> create new mlExpansionHistory value, then -> 4.
1703 3.3. Encapsulate in a SignedData layer and add an mlExpansionHistory
1704 attribute, then -> 4.
1705 4. Sign message, deliver it, STOP.
1707 4.2.3.3 Processing for data
1709 1. The MLA encapsulates the message in a SignedData layer, and adds an
1710 mlExpansionHistory attribute containing the current ML expansion
1711 information as described in the "Mail List Expansion" section.
1713 2. The MLA signs the new message and delivers the updated message to mail
1714 list members to complete MLA processing.
1716 4.3 Mail List Agent Signed Receipt Policy Processing
1718 If a mailing list (B) is a member of another mailing list (A), list B often
1719 needs to propagate forward the mailing list receipt policy of A. As a
1720 general rule, a mailing list should be conservative in propagating forward
1721 the mailing list receipt policy because the ultimate recipient need only
1722 process the last item in the ML expansion history. The MLA builds the
1723 expansion history to meet this requirement.
1725 The following table describes the outcome of the union of mailing list A's
1726 policy (the rows in the table) and mailing list B's policy (the columns in
1727 the table).
1729 | B's policy
1730 A's policy | none insteadOf inAdditionTo missing
1731 -------------------------------------------------------------------------
1732 none | none none none none
1733 insteadOf | none insteadOf(B) *1 insteadOf(A)
1734 inAdditionTo | none insteadOf(B) *2 inAdditionTo(A)
1735 missing | none insteadOf(B) inAdditionTo(B) missing
1737 *1 = insteadOf(insteadOf(A) + inAdditionTo(B))
1738 *2 = inAdditionTo(inAdditionTo(A) + inAdditionTo(B))
1740 4.4 Mail List Expansion History Syntax
1742 An mlExpansionHistory attribute value has ASN.1 type MLExpansionHistory. If
1743 there are more than ub-ml-expansion-history mailing lists in the sequence,
1744 the processing agent should provide notification of the error to a human
1745 mail list administrator.�The mail list administrator is responsible for
1746 correcting the overflow condition.
1748 MLExpansionHistory ::= SEQUENCE
1749 SIZE (1..ub-ml-expansion-history) OF MLData
1751 id-aa-mlExpandHistory OBJECT IDENTIFIER ::= { iso(1) member-body(2)
1752 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 3}
1754 ub-ml-expansion-history INTEGER ::= 64
1756 MLData contains the expansion history describing each MLA that has
1757 processed a message. As an MLA distributes a message to members of an ML,
1758 the MLA records its unique identifier, date and time of expansion, and
1759 receipt policy in an MLData structure.
1761 MLData ::= SEQUENCE {
1762 mailListIdentifier EntityIdentifier,
1763 -- EntityIdentifier is imported from [CMS]
1764 expansionTime GeneralizedTime,
1765 mlReceiptPolicy MLReceiptPolicy OPTIONAL }
1767 The receipt policy of the ML can withdraw the originator's request for the
1768 return of a signed receipt. However, if the originator of the message has
1769 not requested a signed receipt, the MLA cannot request a signed receipt. In
1770 the event that a ML's signed receipt policy supersedes the originator's
1771 request for signed receipts, such that the originator will not receive any
1772 signed receipts, then the MLA MAY inform the originator of that fact.
1774 When present, the mlReceiptPolicy specifies a receipt policy that
1775 supersedes the originator's request for signed receipts. The policy can be
1776 one of three possibilities: receipts MUST NOT be returned (none); receipts
1777 should be returned to an alternate list of recipients, instead of to the
1778 originator (insteadOf); or receipts should be returned to a list of
1779 recipients in addition to the originator (inAdditionTo).
1781 MLReceiptPolicy ::= CHOICE {
1782 none [0] NULL,
1783 insteadOf [1] SEQUENCE SIZE (1..MAX) OF GeneralNames,
1784 inAdditionTo [2] SEQUENCE SIZE (1..MAX) OF GeneralNames }
1786 5. Security Considerations
1788 This entire document discusses security.
1790 A. ASN.1 Module
1792 ExtendedSecurityServices
1793 { iso(1) member-body(2) us(840) rsadsi(113549)
1794 pkcs(1) pkcs-9(9) smime(16) modules(0) ess(2) }
1796 DEFINITIONS IMPLICIT TAGS ::=
1797 BEGIN
1799 IMPORTS
1801 -- Cryptographic Message Syntax (CMS)
1802 ContentType, EntityIdentifier, SubjectKeyIdentifier, Version
1803 FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840)
1804 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) }
1806 -- X.509
1807 GeneralNames FROM CertificateExtensions
1808 {joint-iso-ccitt ds(5) module(1) certificateExtensions(26) 0};
1810 -- Extended Security Services
1812 -- The construct "SEQUENCE SIZE (1..MAX) OF" appears in several ASN.1
1813 -- constructs in this module. A valid ASN.1 SEQUENCE can have zero or
1814 -- more entries. The SIZE (1..MAX) construct constrains the SEQUENCE to
1815 -- have at least one entry. MAX indicates the upper bound is unspecified.
1816 -- Implementations are free to choose an upper bound that suits their
1817 -- environment.
1819 UTF8String ::= [UNIVERSAL 12] IMPLICIT OCTET STRING
1820 -- The contents are formatted as described in [UTF8]
1822 -- Section 2.7
1824 ReceiptRequest ::= SEQUENCE {
1825 signedContentIdentifier ContentIdentifier,
1826 receiptsFrom ReceiptsFrom,
1827 receiptsTo SEQUENCE SIZE (1..ub-receiptsTo) OF GeneralNames }
1829 ub-receiptsTo INTEGER ::= 16
1831 id-aa-receiptRequest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
1832 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 1}
1834 ContentIdentifier ::= OCTET STRING
1836 id-aa-contentIdentifier OBJECT IDENTIFIER ::= { iso(1) member-body(2)
1837 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 7}
1839 ReceiptsFrom ::= CHOICE {
1840 allOrFirstTier [0] AllOrFirstTier,
1841 -- formerly "allOrNone [0]AllOrNone"
1842 receiptList [1] SEQUENCE OF GeneralNames }
1844 AllOrFirstTier ::= INTEGER { -- Formerly AllOrNone
1845 allReceipts (0),
1846 firstTierRecipients (1) }
1848 -- Section 2.8
1850 Receipt ::= SEQUENCE {
1851 version Version, -- Version is imported from [CMS]
1852 contentType ContentType,
1853 signedContentIdentifier ContentIdentifier,
1854 originatorSignatureValue OCTET STRING }
1856 id-ct-receipt OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
1857 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-ct(1) 1}
1859 -- Section 2.9
1861 ContentHints ::= SEQUENCE {
1862 � contentDescription UTF8String SIZE (1..MAX) OPTIONAL,
1863 � contentType ContentType }
1865 id-aa-contentHint OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
1866 rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 4}
1868 -- Section 2.10
1870 MsgSigDigest ::= OCTET STRING
1872 id-aa-msgSigDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
1873 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 5}
1875 -- Section 2.11
1877 ContentReference ::= SEQUENCE {
1878 contentType ContentType,
1879 signedContentIdentifier ContentIdentifier,
1880 originatorSignatureValue OCTET STRING }
1882 id-aa-contentReference OBJECT IDENTIFIER ::= { iso(1) member-body(2)
1883 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 10 }
1885 -- Section 3.2
1887 ESSSecurityLabel ::= SET {
1888 security-policy-identifier SecurityPolicyIdentifier,
1889 security-classification SecurityClassification OPTIONAL,
1890 privacy-mark ESSPrivacyMark OPTIONAL,
1891 security-categories SecurityCategories OPTIONAL }
1893 id-aa-securityLabel OBJECT IDENTIFIER ::= { iso(1) member-body(2)
1894 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 2}
1896 SecurityPolicyIdentifier ::= OBJECT IDENTIFIER
1898 SecurityClassification ::= INTEGER {
1899 unmarked (0),
1900 unclassified (1),
1901 restricted (2),
1902 confidential (3),
1903 secret (4),
1904 top-secret (5) } (0..ub-integer-options)
1906 ub-integer-options INTEGER ::= 256
1908 ESSPrivacyMark ::= CHOICE {
1909 ��� pString����� PrintableString SIZE (1..ub-privacy-mark-length),
1910 ��� utf8String�� UTF8String SIZE (1..MAX)
1911 }
1913 ub-privacy-mark-length INTEGER ::= 128
1915 SecurityCategories ::= SET SIZE (1..ub-security-categories) OF
1916 SecurityCategory
1918 ub-security-categories INTEGER ::= 64
1920 SecurityCategory ::= SEQUENCE {
1921 type [0] OBJECT IDENTIFIER,
1922 value [1] ANY DEFINED BY type -- defined by type
1923 }
1925 --Note: The aforementioned SecurityCategory syntax produces identical
1926 --hex encodings as the following SecurityCategory syntax that is
1927 --documented in the X.411 specification:
1928 --
1929 --SecurityCategory ::= SEQUENCE {
1930 -- type [0] SECURITY-CATEGORY,
1931 -- value [1] ANY DEFINED BY type }
1932 --
1933 --SECURITY-CATEGORY MACRO ::=
1934 --BEGIN
1935 --TYPE NOTATION ::= type | empty
1936 --VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER)
1937 --END
1939 -- Section 3.4
1941 EquivalentLabels ::= SEQUENCE OF ESSSecurityLabel
1943 id-aa-equivalentLabels OBJECT IDENTIFIER ::= { iso(1) member-body(2)
1944 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 9}
1946 -- Section 4.4
1948 MLExpansionHistory ::= SEQUENCE
1949 SIZE (1..ub-ml-expansion-history) OF MLData
1951 id-aa-mlExpandHistory OBJECT IDENTIFIER ::= { iso(1) member-body(2)
1952 us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) 3}
1954 ub-ml-expansion-history INTEGER ::= 64
1956 MLData ::= SEQUENCE {
1957 mailListIdentifier EntityIdentifier,
1958 -- EntityIdentifier is imported from [CMS]
1959 expansionTime GeneralizedTime,
1960 mlReceiptPolicy MLReceiptPolicy OPTIONAL }
1962 MLReceiptPolicy ::= CHOICE {
1963 none [0] NULL,
1964 insteadOf [1] SEQUENCE SIZE (1..MAX) OF GeneralNames,
1965 inAdditionTo [2] SEQUENCE SIZE (1..MAX) OF GeneralNames }
1967 END -- of ExtendedSecurityServices
1969 B. References
1971 [ASN1-1988] "Recommendation X.208: Specification of Abstract Syntax
1972 Notation One (ASN.1)"
1974 [ASN1-1994] "Recommendation X.680: Specification of Abstract Syntax
1975 Notation One (ASN.1)"
1977 [CMS] "Cryptographic Message Syntax", Internet Draft
1978 draft-ietf-smime-cms-xx.
1980 [MSP4] "Secure Data Network System (SDNS) Message Security Protocol (MSP)
1981 4.0", Specification SDN.701, Revision A, 1997-02-06.
1983 [MTSABS] "1988 International Telecommunication Union (ITU) Data
1984 Communication Networks Message Handling Systems: Message Transfer System:
1985 Abstract Service Definition and Procedures, Volume VIII, Fascicle VIII.7,
1986 Recommendation X.411"; MTSAbstractService {joint-iso-ccitt mhs-motis(6)
1987 mts(3) modules(0) mts-abstract-service(1)}
1989 [PKCS7-1.5] "PKCS #7: Cryptographic Message Syntax", RFC 2315.
1991 [SMIME2] "S/MIME Version 2 Message Specification", RFC 2311, and
1992 "S/MIME Version 2 Certificate Handling", RFC 2312.
1994 [SMIME3] "S/MIME Version 3 Message Specification", Internet Draft
1995 draft-ietf-smime-msg-xx, and "S/MIME Version 3 Certificate Handling",
1996 Internet Draft draft-ietf-smime-cert-xx.
1998 [UTF8] "UTF-8, a transformation format of ISO 10646", RFC 2279.
2000 C. Acknowledgments
2002 The first draft of this work was prepared by David Solo. John Pawling did a
2003 huge amount of very detailed revision work during the many phases of the
2004 document.
2006 Many other people have contributed hard work to this draft, including:
2007 Bancroft Scott
2008 Bengt Ackzell
2009 Blake Ramsdell
2010 Carlisle Adams
2011 David Kemp
2012 Jim Schaad
2013 Russ Housley
2014 Scott Hollenbeck
2015 Steve Dusse
2017 D. Open Issues
2019 None.
2021 E. Changes from draft-ietf-smime-ess-06 to draft-ietf-smime-ess-07
2023 1.3.4: Reworded second and fifth paragraphs a bit. Also added
2024 OID for sMIMEEncryptionKeyPreference.
2026 3.4: Clarified intention in second paragraph. Also slightly updated wording
2027 in fifth paragraph.
2029 3.4.2: Clarified intention in second paragraph.
2031 F: Changed my area code
2033 F. Editor's Address
2035 Paul Hoffman
2036 Internet Mail Consortium
2037 127 Segre Place
2038 Santa Cruz, CA 95060
2039 (831) 426-9827
2040 phoffman@imc.org