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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Missing Reference: 'FWS' is mentioned on line 255, but not defined == Unused Reference: 'RFC5321' is defined on line 360, but no explicit reference was found in the text == Unused Reference: 'RFC5652' is defined on line 372, but no explicit reference was found in the text == Unused Reference: 'I-D.melnikov-smime-msa-to-mda' is defined on line 403, but no explicit reference was found in the text ** Obsolete normative reference: RFC 5750 (Obsoleted by RFC 8550) ** Obsolete normative reference: RFC 5751 (Obsoleted by RFC 8551) Summary: 2 errors (**), 0 flaws (~~), 5 warnings (==), 1 comment (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group A. Melnikov 3 Internet-Draft Isode Ltd 4 Intended status: Informational May 19, 2014 5 Expires: November 20, 2014 7 Draft and Release using Internet Email 8 draft-melnikov-mmhs-authorizing-users-05 10 Abstract 12 This document describes a procedure for when an Military Message 13 Handling System (MMHS) message is composed by one user and is only 14 released to the mail transfer system when one or more authorizing 15 users authorize release of the message by adding the MMHS- 16 Authorizing-Users header field. The resulting message can be 17 optionally signed by the sender and/or reviewer, allowing recipients 18 to verify both the original signature (if any) and review signatures. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on November 20, 2014. 37 Copyright Notice 39 Copyright (c) 2014 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 55 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 56 3. Draft and Release procedure . . . . . . . . . . . . . . . . . 3 57 3.1. Handling of Initial Message Submission by MSA . . . . . . 3 58 3.2. Review by Authorizing User(s) . . . . . . . . . . . . . . 3 59 3.2.1. Processing of Encrypted Messages . . . . . . . . . . 4 60 3.2.2. Authorizing S/MIME signatures . . . . . . . . . . . . 5 61 3.3. Role of other Messaging Agents . . . . . . . . . . . . . 5 62 3.3.1. Border MTA at the sender's domain . . . . . . . . . . 5 63 3.3.2. MDA at the sender's domain . . . . . . . . . . . . . 5 64 4. MMHS-Authorizing-Users header field . . . . . . . . . . . . . 5 65 5. Updated MIXER mapping . . . . . . . . . . . . . . . . . . . . 6 66 5.1. Mapping from RFC 5322/MIME to X.400 . . . . . . . . . . . 6 67 5.2. Mapping from X.400 to RFC 5322/MIME . . . . . . . . . . . 6 68 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 69 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 70 7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . 7 71 7.2. Intentionally Malformed Header Fields . . . . . . . . . . 8 72 8. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 8 73 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 74 9.1. Normative References . . . . . . . . . . . . . . . . . . 8 75 9.2. Informative References . . . . . . . . . . . . . . . . . 9 76 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10 78 1. Introduction 80 In some secure environments email messages can't be released to the 81 MTS (Mail Transfer System) and, thus delivered to recipients, unless 82 they are authorized by one or more authorizing users (e.g. Releasing 83 Officers or Release Authorities). This document describes how this 84 mechanism can be realized by an additional Internet Email [RFC5322] 85 header field and optionally protected using S/MIME [RFC5750] 86 [RFC5751] or DKIM [RFC6376]. 88 This document describes a procedure for how an email message composed 89 by one user can be released to the MTS when one or more authorizing 90 users authorize and optionally countersign the message. The header 91 communicates which users authorized the message. If signed, the 92 resulting message allows recipients to verify both the original (if 93 any) and counter S/MIME signatures. The list of authorizing users is 94 specified in the MMHS-Authorizing-Users header field Section 4. The 95 original S/MIME signature generated by the sender (if any) should be 96 unaffected by additional S/MIME review signatures. 98 2. Conventions Used in This Document 100 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 101 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 102 document are to be interpreted as described in [RFC2119]. 104 The formal syntax uses the Augmented Backus-Naur Form (ABNF) 105 [RFC5234] notation including the core rules defined in Appendix B of 106 RFC 5234 [RFC5234]. Terms not defined in this document are taken 107 from [RFC5322]. 109 3. Draft and Release procedure 111 3.1. Handling of Initial Message Submission by MSA 113 The original email message to be sent doesn't include the MMHS- 114 Authorizing-Users header field. It may or may not include sender's S 115 /MIME signature. 117 The message to be sent is first submitted over SMTP [RFC6409]. The 118 specific mechanism for how it arrives to authorizing user(s) is not 119 specified in this document. One possibility is for the Message 120 Submission Agent (MSA) to redirect all email messages not addressed 121 to authorizing users and not submitted by authorizing users to a 122 preconfigured mailbox that can be accessed by authorizing user(s). 123 Another possibility is for the MSA to redirect all email messages 124 without the MMHS-Authorizing-Users header field and/or corresponding 125 S/MIME review signatures to a preconfigured mailbox that can be 126 accessed by authorizing user(s). 128 In order to prevent a malicious sender from bypassing or altering 129 Draft and Release procedure, MSA MUST check that MMHS-Authorizing- 130 Users header field (if present) is syntactically valid, contains 131 email addresses of entities authorized to act as authorizing users 132 and, when review signatures are used, that every entity listed has 133 one or more matching review signature (or signature) which is valid. 135 3.2. Review by Authorizing User(s) 137 Each user agent that is used by an authorized user MUST perform the 138 following steps (if there are multiple authorizing users, these steps 139 are repeated for each): 141 1. Verify the origination of the message. The exact mechanism to do 142 that is out of scope for this document, but one example is by 143 verifying the S/MIME signature and making sure that it matches 144 the sender of the message, as described in [RFC5750] [RFC5751]. 146 Another example is by verifying a DKIM signature [RFC6376] that 147 covers From/Sender header fields. 149 2. Check if the message already contains the MMHS-Authorizing-Users 150 header field with the email address of the authorizing user. 151 (This can happen if email system is misconfigured and thus 152 contains a loop, or if a malicious sender or attacker is trying 153 to affect authorization procedure.) If the message doesn't 154 contain the email address of the authorizing user in the MMHS- 155 Authorizing-Users header field, then go to the next step. If 156 MMHS-Authorizing-Users header field contains the email address of 157 the authorizing user, verify validity of the header field (for 158 example by checking for S/MIME signature/review signature or for 159 DKIM signature). If the validity of the MMHS-Authorizing-Users 160 header field can be verified, go to step 5 below. Otherwise 161 strip the MMHS-Authorizing-Users header field or return the 162 message to sender (bounce). 164 3. Allow the authorizing user to review content of the message. 165 Some of the checks can be automated (for example search for 166 keywords). (See Section 3.2.1 for additional considerations.) 167 If based on the check the authorizing user is happy to release 168 the message to MTS (or to the next authorizing user, if multiple 169 authorizations are required), the UA should enable the 170 authorizing user to protect additions to the MMHS-Authorizing- 171 Users header field, for example by allowing to add S/MIME review 172 signature (if S/MIME is used for protecting MMHS-Authorizing- 173 Users header field. See Section 3.2.2 for more details). If the 174 authorizing user wants to block the message, it can be discarded 175 or returned to sender. The authorizing user can also choose to 176 forward the message to another authorizing user for approval. 178 4. If there is an existing MMHS-Authorizing-Users header field 179 containing the email address of the authorizing user, skip this 180 step. Otherwise, insert a new MMHS-Authorizing-Users header 181 field (if absent) containing the email address of the authorizing 182 user or append the email address of the authorizing user to the 183 end of the existing MMHS-Authorizing-Users header field. 185 5. The (possibly) updated email message is either released to the 186 MTS, or to the next authorizing user, as per email system 187 configuration. 189 3.2.1. Processing of Encrypted Messages 191 Any encrypted message sent in an environment where Draft and Release 192 procedure is in force needs to be also encrypted to all authorizing 193 users, so that they can perform review of the message. A message 194 that can't be decrypted by an authorizing user MUST be returned to 195 sender. 197 3.2.2. Authorizing S/MIME signatures 199 If a message is signed multiple times (for example using different 200 cryptographic algorithms), all of the signatures that can be verified 201 by an authorizing user SHOULD be signed with a review signature 202 (authorizing signatures). A recipient of the message should consider 203 any chain of review signatures that matches MMHS-Authorizing-Users 204 header field values as valid, only if all signatures in the chain 205 verify. 207 When triple wrapping [RFC2634] is used, authorizing signatures are 208 applied to the outer level, so that it can be verified by MTAs 209 without the need to decrypt content. 211 3.3. Role of other Messaging Agents 213 3.3.1. Border MTA at the sender's domain 215 Sender's domain border MTAs are responsible for ensuring that all 216 messages that leave sender's domain were properly authorized by 217 authorizing user(s), as determined by the sender's domain email 218 system configuration. They verify presence and validity of MMHS- 219 Authorizing-Users header field in outgoing messages, as well as 220 validity of associated signatures on the message. 222 3.3.2. MDA at the sender's domain 224 If a message being sent is to be delivered within the sender's 225 domain, Message Delivery Agents (MDAs) are responsible for ensuring 226 that the message was properly authorized by authorizing user(s), as 227 determined by the sender's domain email system configuration. They 228 verify presence and validity of MMHS-Authorizing-Users header field 229 in the message, as well as validity of associated signatures on the 230 message. 232 4. MMHS-Authorizing-Users header field 234 The MMHS-Authorizing-Users header field specifies the list of 235 authorizing users (or entities(*)) that countersigned this email 236 message (for example using S/MIME) before it was authorized for 237 release to MTS. Each user/entity is described by her/his/its email 238 address. 240 (*) Note that in some environments identities of authorizing users 241 are required to be hidden from recipients of email messages, so upon 242 receipt MMHS-Authorizing-Users might contain email address associated 243 with a group of possible users. 245 The MMHS-Authorizing-Users header field specified in this document 246 MUST NOT appear more than once in message headers. (An email message 247 that contains multiple MMHS-Authorizing-Users is malformed. An agent 248 processing such malformed message SHOULD either return it to sender 249 (if possible) or fix the message so that it only contains one copy of 250 the header field.) [[An alternative is to allow for multiple copies 251 of the header field and treat them as additive. This might work 252 better with DKIM!]] 254 MMHS-Authorizing-Users = "MMHS-Authorizing-Users:" 255 [FWS] mailbox-list [FWS] CRLF 257 mailbox-list = 259 5. Updated MIXER mapping 261 This section updates MIXER mapping specified in [RFC2156]. 263 5.1. Mapping from RFC 5322/MIME to X.400 265 In the absence of the MMHS-Authorizing-Users header field, From and 266 Sender header fields are mapped to their X.400 equivalents as 267 specified in [RFC2156]. 269 If MMHS-Authorizing-Users header field is present: 271 1. The first From header field address is mapped to 272 IPMS.Heading.originator if there is no Sender header field and 273 the remaining From header field addresses + the MMHS-Authorizing- 274 Users header field address(es) are mapped to IPMS.Heading 275 .authorizing-users. If a Sender header field is present, the 276 From header field address(es) and the MMHS-Authorizing-Users 277 header field address(es) are mapped to IPMS.Heading.authorizing- 278 users. 280 2. The Sender header field (if present) is mapped to 281 IPMS.Heading.originator. 283 5.2. Mapping from X.400 to RFC 5322/MIME 285 Mapping from X.400 to Internet is controlled by whether or not a 286 particular message is considered to be a military message. A message 287 is considered to be a military message (as defined by ACP 123 288 [ACP123] and also specified in STANAG 4406 [STANAG-4406]) if there 289 are any MMHS heading extensions present. Alternatively, this MAY be 290 done by configuration (i.e. all messages can be considered to be 291 military messages). 293 For non military messages, mapping from X.400 as specified in 294 [RFC2156] is used. 296 For military messages, the following mapping is used: 298 1. IPMS.Heading.originator is mapped to From header field. 300 2. The IPMS.Heading.authorizing-users is mapped to MMHS-Authorizing- 301 Users header field. 303 6. IANA Considerations 305 IANA is requested to add the MMHS-Authorizing-Users header field 306 specified in Section 4 to the "Permanent Message Header Field Names", 307 defined by Registration Procedures for Message Header Fields 308 [RFC3864]. The registration template is as follows: 310 Header field name: MMHS-Authorizing-Users 312 Applicable protocol: mail ([RFC5322]) 314 Status: Standard 316 Author/Change controller: IETF 318 Specification document(s): [[RFC XXXX]] 320 Related information: 322 7. Security Considerations 324 7.1. Forged Header Fields 326 A malicious sender may add/change an MMHS-Authorizing-Users header 327 field to bypass or alter message authorization procedure invoked for 328 messages with no MMHS-Authorizing-Users header field. For that 329 reason it is important for agents and clients that rely on validity 330 of MMHS-Authorizing-Users header field to also verify review 331 signature (or a similar protection mechanism), that confirms that a 332 particular person or entity authorized release of a message. 334 7.2. Intentionally Malformed Header Fields 336 It is possible for an attacker to add an MMHS-Authorizing-Users 337 header field that is extraordinarily large or otherwise malformed in 338 an attempt to discover or exploit weaknesses in header field parsing 339 code. Implementers must thoroughly verify all such header fields 340 received from MTAs and be robust against intentionally as well as 341 unintentionally malformed header fields. 343 8. Open Issues 345 Netnews Approved header field has the same syntax and semantics as 346 the one described here. Should it be used (and be formally 347 registered for email) instead? 349 9. References 351 9.1. Normative References 353 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 354 Requirement Levels", BCP 14, RFC 2119, March 1997. 356 [RFC2156] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): 357 Mapping between X.400 and RFC 822/MIME", RFC 2156, January 358 1998. 360 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 361 October 2008. 363 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 364 October 2008. 366 [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", 367 STD 72, RFC 6409, November 2011. 369 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 370 Specifications: ABNF", STD 68, RFC 5234, January 2008. 372 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, 373 RFC 5652, September 2009. 375 [RFC2634] Hoffman, P., "Enhanced Security Services for S/MIME", RFC 376 2634, June 1999. 378 [RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 379 Mail Extensions (S/MIME) Version 3.2 Certificate 380 Handling", RFC 5750, January 2010. 382 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 383 Mail Extensions (S/MIME) Version 3.2 Message 384 Specification", RFC 5751, January 2010. 386 [RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys 387 Identified Mail (DKIM) Signatures", STD 76, RFC 6376, 388 September 2011. 390 [ACP123] CCEB, , "Common Messaging strategy and procedures", ACP 391 123, May 2009. 393 9.2. Informative References 395 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 396 Procedures for Message Header Fields", BCP 90, RFC 3864, 397 September 2004. 399 [STANAG-4406] 400 NATO, , "STANAG 4406 Edition 2: Military Message Handling 401 System", STANAG 4406, March 2005. 403 [I-D.melnikov-smime-msa-to-mda] 404 Ottaway, W. and A. Melnikov, "Domain-based signing and 405 encryption using S/MIME", draft-melnikov-smime-msa-to- 406 mda-04 (work in progress), March 2014. 408 Appendix A. Acknowledgements 410 Many thanks for reviews and text provided by Steve Kille, Jim Schaad, 411 Russ Housley and David Wilson. 413 Some text in this document was copied from RFC 7001. 415 Author's Address 417 Alexey Melnikov 418 Isode Ltd 419 5 Castle Business Village 420 36 Station Road 421 Hampton, Middlesex TW12 2BX 422 UK 424 EMail: Alexey.Melnikov@isode.com