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Checking references for intended status: Informational ---------------------------------------------------------------------------- == Missing Reference: 'FWS' is mentioned on line 249, but not defined == Unused Reference: 'RFC5321' is defined on line 353, 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 (~~), 3 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 October 18, 2013 5 Expires: April 21, 2014 7 Draft and Release using Internet Email 8 draft-melnikov-mmhs-authorizing-users-03 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 countersigned, allowing recipients to verify both the 18 original signature (if any) and countersignatures. 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 April 21, 2014. 37 Copyright Notice 39 Copyright (c) 2013 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. S/MIME countersignatures . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . 9 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). This document describes how this mechanism can be 84 realized by an additional Internet Email [RFC5322] header field and 85 optionally using S/MIME [RFC5750] [RFC5751]. 87 This document describes a procedure for how an email message composed 88 by one user can be released to the MTS when one or more authorizing 89 users authorize and optionally countersign the message. The header 90 communicates which users authorized the message. If signed, the 91 resulting message allows recipients to verify both the original (if 92 any) and counter S/MIME signatures. The list of authorizing users is 93 specified in the MMHS-Authorizing-Users header field Section 4. The 94 original S/MIME signature generated by the sender (if any) should be 95 unaffected by additional S/MIME countersignatures [RFC5652]. 97 2. Conventions Used in This Document 99 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 100 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 101 document are to be interpreted as described in [RFC2119]. 103 The formal syntax uses the Augmented Backus-Naur Form (ABNF) 104 [RFC5234] notation including the core rules defined in Appendix B of 105 RFC 5234 [RFC5234]. Terms not defined in this document are taken 106 from [RFC5322]. 108 3. Draft and Release procedure 110 3.1. Handling of Initial Message Submission by MSA 112 The original email message to be sent doesn't include the MMHS- 113 Authorizing-Users header field. It may or may not include sender's S 114 /MIME signature. 116 The message to be sent is first submitted over SMTP [RFC6409]. The 117 specific mechanism for how it arrives to authorizing user(s) is not 118 specified in this document. One possibility is for the Message 119 Submission Agent (MSA) to redirect all email messages not addressed 120 to authorizing users and not submitted by authorizing users to a 121 preconfigured mailbox that can be accessed by authorizing user(s). 122 Another possibility is for the MSA to redirect all email messages 123 without the MMHS-Authorizing-Users header field and/or corresponding 124 S/MIME countersignatures to a preconfigured mailbox that can be 125 accessed by authorizing user(s). 127 In order to prevent a malicious sender from bypassing or altering 128 Draft and Release procedure, MSA MUST check that MMHS-Authorizing- 129 Users header field (if present) is syntactically valid, contains 130 email addresses of entities authorized to act as authorization users 131 and that every entity listed has one or more matching 132 countersignature (if countersignatures are used to protect the MMHS- 133 Authorizing-Users header field). 135 3.2. Review by Authorizing User(s) 137 Each user agent that is used by an authorized user has to 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 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 countersignature or for DKIM 159 signature). If the validity of the MMHS-Authorizing-Users header 160 field can be verified, go to step 5 below. Otherwise strip the 161 MMHS-Authorizing-Users header field or return the message to 162 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 172 countersignature (if S/MIME is used for protecting MMHS- 173 Authorizing-Users header field. See Section 3.2.2 for more 174 details). If the authorizing user wants to block the message, it 175 can be discarded or returned to sender, and no further steps from 176 this list should take place. The authorizing user can also 177 choose to forward the message to another authorizing user for 178 approval. 180 4. If there is an existing MMHS-Authorizing-Users header field 181 containing the email address of the authorizing user, skip this 182 step. Othrwise insert a new MMHS-Authorizing-Users header field 183 (if absent) containing the email address of the authorizing user 184 or append the email address of the authorizing user to the end of 185 the existing MMHS-Authorizing-Users header field. 187 5. The (possibly) updated email message is either released to the 188 MTS, or to the next authorizing user, as per email system 189 configuration. 191 3.2.1. Processing of Encrypted Messages 192 Any encrypted message sent in an environment where Draft and Release 193 procedure is in force needs to be also encrypted to all authorizing 194 users, so that they can perform review of the message. A message 195 that can't be decrypted by an authorizing user MUST be returned to 196 sender. 198 3.2.2. S/MIME countersignatures 200 If a message is signed multiple times (for example using different 201 cryptographic algorithms), all of the signatures that can be verified 202 by an authorizing user SHOULD be countersigned. A recipient of the 203 message should consider any chain of countersignatures that matches 204 MMHS-Authorizing-Users header field values as valid, if all 205 signatures in the chain verify. 207 When both encryption and signing is used, countersignature SHOULD be 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 that countersigned this email message (for example 236 using S/MIME) before it was authorized for release to MTS. Each user 237 is described by her/his email address. 239 The MMHS-Authorizing-Users header field specified in this document 240 MUST NOT appear more than once in message headers. (An email message 241 that contains multiple MMHS-Authorizing-Users is malformed. An agent 242 processing such malformed message SHOULD either return it to sender 243 (if possible) or fix the message so that it only contains one copy of 244 the header field.) [[An alternative is to allow for multiple copies 245 of the header field and treat them as additive. This might work 246 better with DKIM!]] 248 MMHS-Authorizing-Users = "MMHS-Authorizing-Users:" 249 [FWS] mailbox-list [FWS] CRLF 251 mailbox-list = 253 5. Updated MIXER mapping 255 This section updates MIXER mapping specified in [RFC2156]. 257 5.1. Mapping from RFC 5322/MIME to X.400 259 In the absence of the MMHS-Authorizing-Users header field, From and 260 Sender header fields are mapped to their X.400 equivalents as 261 specified in [RFC2156]. 263 If MMHS-Authorizing-Users header field is present: 265 1. The first From header field address is mapped to 266 IPMS.Heading.originator if there is no Sender header field and 267 the remaining From header field addresses + the MMHS-Authorizing- 268 Users header field address(es) are mapped to IPMS.Heading 269 .authorizing-users. If a Sender header field is present, the 270 From header field address(es) and the MMHS-Authorizing-Users 271 header field address(es) are mapped to IPMS.Heading.authorizing- 272 users. 274 2. The Sender header field (if present) is mapped to 275 IPMS.Heading.originator. 277 5.2. Mapping from X.400 to RFC 5322/MIME 278 Mapping from X.400 to Internet is controlled by whether or not a 279 particular message is considered to be a military message. A message 280 is considered to be a military message (as defined by ACP 123 281 [ACP123] and also specified in STANAG 4406 [STANAG-4406]) if there 282 are any MMHS heading extensions present. Alternatively, this MAY be 283 done by configuration (i.e. all messages can be considered to be 284 military messages). 286 For non military messages, mapping from X.400 as specified in 287 [RFC2156] is used. 289 For military messages, the following mapping is used: 291 1. IPMS.Heading.originator is mapped to From header field. 293 2. The IPMS.Heading.authorizing-users is mapped to MMHS-Authorizing- 294 Users header field. 296 6. IANA Considerations 298 IANA is requested to add the MMHS-Authorizing-Users header field 299 specified in Section 4 to the "Permanent Message Header Field Names", 300 defined by Registration Procedures for Message Header Fields 301 [RFC3864]. The registration template is as follows: 303 Header field name: MMHS-Authorizing-Users 305 Applicable protocol: mail ([RFC5322]) 307 Status: Standard 309 Author/Change controller: IETF 311 Specification document(s): [[RFC XXXX]] 313 Related information: 315 7. Security Considerations 317 7.1. Forged Header Fields 319 A malicious sender may add/change an MMHS-Authorizing-Users header 320 field to bypass or alter message authorization procedure invoked for 321 messages with no MMHS-Authorizing-Users header field. For that 322 reason it is important for agents and clients that rely on validity 323 of MMHS-Authorizing-Users header field to also verify 324 countersignature (or a similar protection mechanism), that confirms 325 that a particular person or entity authorized release of a message. 327 7.2. Intentionally Malformed Header Fields 329 It is possible for an attacker to add an MMHS-Authorizing-Users 330 header field that is extraordinarily large or otherwise malformed in 331 an attempt to discover or exploit weaknesses in header field parsing 332 code. Implementers must thoroughly verify all such header fields 333 received from MTAs and be robust against intentionally as well as 334 unintentionally malformed header fields. 336 8. Open Issues 338 Netnews Approved header field has the same syntax and semantics as 339 the one described here. Should it be used (and be formally 340 registered for email) instead? 342 9. References 344 9.1. Normative References 346 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 347 Requirement Levels", BCP 14, RFC 2119, March 1997. 349 [RFC2156] Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): 350 Mapping between X.400 and RFC 822/MIME", RFC 2156, January 351 1998. 353 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 354 October 2008. 356 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 357 October 2008. 359 [RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail", 360 STD 72, RFC 6409, November 2011. 362 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 363 Specifications: ABNF", STD 68, RFC 5234, January 2008. 365 [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, 366 RFC 5652, September 2009. 368 [RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 369 Mail Extensions (S/MIME) Version 3.2 Certificate 370 Handling", RFC 5750, January 2010. 372 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 373 Mail Extensions (S/MIME) Version 3.2 Message 374 Specification", RFC 5751, January 2010. 376 [RFC6376] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys 377 Identified Mail (DKIM) Signatures", STD 76, RFC 6376, 378 September 2011. 380 [ACP123] CCEB, ., "Common Messaging strategy and procedures", ACP 381 123, May 2009. 383 9.2. Informative References 385 [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration 386 Procedures for Message Header Fields", BCP 90, RFC 3864, 387 September 2004. 389 [STANAG-4406] 390 NATO, ., "STANAG 4406 Edition 2: Military Message Handling 391 System", STANAG 4406, March 2005. 393 Appendix A. Acknowledgements 395 Many thanks for reviews and text provided by Steve Kille, Jim Schaad 396 and David Wilson. 398 Some text in this document was copied from RFC 7001. 400 Author's Address 402 Alexey Melnikov 403 Isode Ltd 404 5 Castle Business Village 405 36 Station Road 406 Hampton, Middlesex TW12 2BX 407 UK 409 EMail: Alexey.Melnikov@isode.com