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Checking references for intended status: Informational ---------------------------------------------------------------------------- ** Obsolete normative reference: RFC 5751 (Obsoleted by RFC 8551) -- Obsolete informational reference (is this intentional?): RFC 3501 (Obsoleted by RFC 9051) Summary: 1 error (**), 0 flaws (~~), 1 warning (==), 2 comments (--). 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 August 24, 2016 5 Expires: February 25, 2017 7 Considerations for protecting Email header with S/MIME 8 draft-melnikov-smime-header-signing-03 10 Abstract 12 This document describes best practices for handling of Email header 13 protected by S/MIME. It also adds a new Content-Type parameter to 14 help distinguish an S/MIME protected forwarded message from an S/MIME 15 construct protecting message header. 17 Status of This Memo 19 This Internet-Draft is submitted in full conformance with the 20 provisions of BCP 78 and BCP 79. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF). Note that other groups may also distribute 24 working documents as Internet-Drafts. The list of current Internet- 25 Drafts is at http://datatracker.ietf.org/drafts/current/. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 This Internet-Draft will expire on February 25, 2017. 34 Copyright Notice 36 Copyright (c) 2016 IETF Trust and the persons identified as the 37 document authors. All rights reserved. 39 This document is subject to BCP 78 and the IETF Trust's Legal 40 Provisions Relating to IETF Documents 41 (http://trustee.ietf.org/license-info) in effect on the date of 42 publication of this document. Please review these documents 43 carefully, as they describe your rights and restrictions with respect 44 to this document. Code Components extracted from this document must 45 include Simplified BSD License text as described in Section 4.e of 46 the Trust Legal Provisions and are provided without warranty as 47 described in the Simplified BSD License. 49 Table of Contents 51 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 52 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 53 3. Recommendations for handling of S/MIME protected header . . . 3 54 4. New Content-Type header field parameter: forwarded . . . . . 4 55 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 56 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 57 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 7.1. Normative References . . . . . . . . . . . . . . . . . . 5 59 7.2. Informative References . . . . . . . . . . . . . . . . . 6 60 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 7 61 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 63 1. Introduction 65 S/MIME [RFC5751] is typically used to protect (sign and/or encrypt) 66 Email message body parts, but not header of corresponding Email 67 messages. Header fields may contain confidential information or 68 information whose validity need protecting from disclosure or 69 modification. [RFC5751] describes how to protect the Email message 70 header [RFC5322], by wrapping the message inside a message/rfc822 71 container [RFC2045]: 73 In order to protect outer, non-content-related message header 74 fields (for instance, the "Subject", "To", "From", and "Cc" 75 fields), the sending client MAY wrap a full MIME message in a 76 message/rfc822 wrapper in order to apply S/MIME security services 77 to these header fields. It is up to the receiving client to 78 decide how to present this "inner" header along with the 79 unprotected "outer" header. 81 When an S/MIME message is received, if the top-level protected 82 MIME entity has a Content-Type of message/rfc822, it can be 83 assumed that the intent was to provide header protection. This 84 entity SHOULD be presented as the top-level message, taking into 85 account header merging issues as previously discussed. 87 While the above advice helps in protecting message header fields, it 88 doesn't provide enough guidance on what information should and should 89 not be included in outer (unprotected) header and how information 90 from outer and inner headers should be presented to users. This 91 document describes best UI and other practices for handling of 92 messages wrapped inside message/rfc822 body parts. The goal of this 93 document is to improve interoperability and minimize damage caused by 94 possible differences between inner and outer headers. 96 2. Conventions Used in This Document 98 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 99 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 100 document are to be interpreted as described in [RFC2119]. 102 3. Recommendations for handling of S/MIME protected header 104 When generating S/MIME messages which protect header fields by 105 wrapping a message inside message/rfc822 wrapper: 107 1. If a header field is being encrypted because it is sensitive, its 108 value MUST NOT be included in the outer header. If the header 109 field is mandatory according to RFC 5322, a stub value (or a 110 value indicating that the outer value is not to be used) is to be 111 included. 113 2. The outer header SHOULD be minimal in order to avoid disclosure 114 of confidential information. It is recommended that the outer 115 header only contains "Date" (set to the same value as in the 116 inner header, or, if the Date value is also sensitive, to Monday 117 9am of the same week), possibly "Subject" and "To"/"Bcc" header 118 fields. In particular, Keywords, In-Reply-To and References 119 header fields SHOULD NOT be included in the outer header; "To" 120 and "Cc" header fields should be omitted and replaced with "Bcc: 121 undisclosed-recipients;". [[CREF1: What about Message-ID? Is 122 there a way to protect it or create a separate value in the 123 encrypted part?]] 125 But note that having key header fields duplicated in the outer 126 header is convenient for many message stores (e.g. IMAP) and 127 clients that can't decode S/MIME encrypted messages. In 128 particular, Subject/To/Cc/Bcc information is returned in IMAP 129 ENVELOPE FETCH data item [RFC3501], which is frequently used by 130 IMAP clients in order to avoid parsing message header. 132 3. The "Subject" header field value of the outer header SHOULD 133 either be identical to the inner "Subject" header field value, or 134 contain a clear indication that the outer value is not to be used 135 for display (the inner header field value would contain the true 136 value). 138 Note that recommendations listed above only apply to non MIME header 139 fields (header fields with names not starting with "Content-" 140 prefix). 142 Note that the above recommendations can also affect antispam 143 processing. 145 When displaying S/MIME messages which protect header fields by 146 wrapping a message inside message/rfc822 wrapper: 148 1. The outer headers might be tampered with, so a receiving client 149 SHOULD ignore them. If a header field is present in the inner 150 header, only the inner header field value MUST be displayed (and 151 the corresponding outer value must be ignored). If a particular 152 header field is only present in the outer header, it MAY be 153 ignored (not displayed) or it MAY be displayed with a clear 154 indicator that it is not trustworthy(*). 156 (*) - the latter case only applies if the header field is not 157 protected is some other way, for example by DKIM. 159 4. New Content-Type header field parameter: forwarded 161 This document defines a new Content-Type header field parameter 162 [RFC2045] with name "forwarded". The parameter value is case- 163 insensitive and can be either "yes" or "no". (The default value 164 being "yes"). The parameter is only meaningful with media type 165 "message/rfc822" and "message/global" [RFC6532] when used within S/ 166 MIME encrypted body parts. The value "yes" means that the message 167 nested inside message/rfc822 is a forwarded message and not a 168 construct created solely to protect the inner header. 170 Instructions in [RFC5751] describing how to protect the Email message 171 header [RFC5322], by wrapping the message inside a message/rfc822 172 container [RFC2045] are thus updated to read: 174 In order to protect outer, non-content-related message header 175 fields (for instance, the "Subject", "To", "From", and "Cc" 176 fields), the sending client MAY wrap a full MIME message in a 177 message/rfc822 wrapper in order to apply S/MIME security services 178 to these header fields. It is up to the receiving client to 179 decide how to present this "inner" header along with the 180 unprotected "outer" header. 182 When an S/MIME message is received, if the top-level protected 183 MIME entity has a Content-Type of message/rfc822 or message/global 184 without the "forwarded" parameter or with the "forwarded" 185 parameter set to "no", it can be assumed that the intent was to 186 provide header protection. This entity SHOULD be presented as the 187 top-level message, taking into account header merging issues as 188 previously discussed. 190 5. IANA Considerations 192 This document requests no action from IANA. RFC Editor should delete 193 this section before publication. 195 6. Security Considerations 197 This document talks about UI considerations, including security 198 considerations, when processing wrapped message/rfc822 messages 199 protecting header fields. One of the goals of this document is to 200 specify UI for displaying such messages which is less confusing/ 201 misleading and thus more secure. 203 The document is not defining new protocol, so it doesn't create any 204 new security concerns not already covered by S/MIME [RFC5751], MIME 205 [RFC2045] and Email [RFC5322] in general. 207 7. References 209 7.1. Normative References 211 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 212 Requirement Levels", BCP 14, RFC 2119, 213 DOI 10.17487/RFC2119, March 1997, 214 . 216 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 217 Extensions (MIME) Part One: Format of Internet Message 218 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 219 . 221 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 222 DOI 10.17487/RFC5322, October 2008, 223 . 225 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 226 Mail Extensions (S/MIME) Version 3.2 Message 227 Specification", RFC 5751, DOI 10.17487/RFC5751, January 228 2010, . 230 [RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized 231 Email Headers", RFC 6532, DOI 10.17487/RFC6532, February 232 2012, . 234 7.2. Informative References 236 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 237 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 238 . 240 Appendix A. Acknowledgements 242 Thank you to Wei Chuang, Steve Kille, David Wilson and Robert 243 Williams for suggestions, comments and corrections on this document. 244 Some ideas were also taken from Daniel Kahn Gillmor's email on the 245 OpenPGP mailing list. 247 David Wilson came up with the idea of defining a new Content-Type 248 header field parameter to distinguish forwarded messages from inner 249 header field protection constructs. 251 Author's Address 253 Alexey Melnikov 254 Isode Ltd 255 14 Castle Mews 256 Hampton, Middlesex TW12 2NP 257 UK 259 EMail: Alexey.Melnikov@isode.com