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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 October 21, 2014 5 Expires: April 24, 2015 7 Considerations for protecting Email header with S/MIME 8 draft-melnikov-smime-header-signing-00 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 April 24, 2015. 34 Copyright Notice 36 Copyright (c) 2014 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 . . . . . . . . . . . . . . . . . . . . . 4 56 6. Security Considerations . . . . . . . . . . . . . . . . . . . 4 57 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 7.1. Normative References . . . . . . . . . . . . . . . . . . 5 59 7.2. Informative References . . . . . . . . . . . . . . . . . 5 60 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 6 62 1. Introduction 64 S/MIME [RFC5751] is typically used to protect (sign and/on encrypt) 65 Email message body parts, but not header of corresponding Email 66 messages. Header fields may contain confidential information or 67 information whose validity need protecting from disclosure or 68 modification. [RFC5751] describes how to protect the Email message 69 header [RFC5322], by wrapping the message inside a message/rfc822 70 container [RFC2045]: 72 In order to protect outer, non-content-related message header 73 fields (for instance, the "Subject", "To", "From", and "Cc" 74 fields), the sending client MAY wrap a full MIME message in a 75 message/rfc822 wrapper in order to apply S/MIME security services 76 to these header fields. It is up to the receiving client to 77 decide how to present this "inner" header along with the 78 unprotected "outer" header. 80 When an S/MIME message is received, if the top-level protected 81 MIME entity has a Content-Type of message/rfc822, it can be 82 assumed that the intent was to provide header protection. This 83 entity SHOULD be presented as the top-level message, taking into 84 account header merging issues as previously discussed. 86 While the above advice helps in protecting message header fields, it 87 doesn't provide enough guidance on what information should and should 88 not be included in outer (unprotected) header and how information 89 from outer and inner headers should be presented to users. This 90 document describes best UI and other practices for handling of 91 messages wrapped inside message/rfc822 body parts. The goal of this 92 document is to improve interoperability and minimize damage caused by 93 possible differences between inner and outer headers. 95 2. Conventions Used in This Document 97 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 98 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 99 document are to be interpreted as described in [RFC2119]. 101 3. Recommendations for handling of S/MIME protected header 103 When generating S/MIME messages which protect header fields by 104 wrapping a message inside message/rfc822 wrapper: 106 1. If a header field is being encrypted because it is sensitive, its 107 value MUST NOT be included in the outer header. If the header 108 field is mandatory according to RFC 5322, a stub value (or a 109 value indicating that the outer value is not to be used) is to be 110 included. 112 2. The outer header SHOULD be minimal in order to avoid disclosure 113 of confidential information. It is recommended that the outer 114 header only contains "Date" (set to the same value as in the 115 inner header), possibly "Subject" and "To"/"Bcc" header fields. 116 But note that having key header fields duplicated in the outer 117 header is convenient for many message stores (e.g. IMAP) and 118 clients that can't decode S/MIME encrypted messages. In 119 particular, Subject/To/Cc/Bcc information is returned in IMAP 120 ENVELOPE FETCH data item [RFC3501], which is frequently used by 121 IMAP clients in order to avoid parsing message header. 123 3. The "Subject" header field value of the outer header SHOULD 124 either be identical to the inner "Subject" header field value, or 125 contain a clear indication that the outer value is not to be used 126 for display (the inner header field value would contain the true 127 value). 129 Note that recommendations listed above only apply to non MIME header 130 fields (header fields with names not starting with "Content-" 131 prefix). 133 When displaying S/MIME messages which protect header fields by 134 wrapping a message inside message/rfc822 wrapper: 136 1. The outer headers might be tampered with, so a receiving client 137 SHOULD ignore them. If a header field is present in the inner 138 header, only the inner header field value MUST be displayed (and 139 the corresponding outer value must be ignored). If a particular 140 header field is only present in the outer header, it MAY be 141 ignored (not displayed) or it MAY be displayed with a clear 142 indicator that it is not trustworthy(*). 144 (*) - the latter case only applies if the header field is not 145 protected is some other way, for example by DKIM. 147 4. New Content-Type header field parameter: forwarded 149 This document defines a new Content-Type header field parameter 150 [RFC2045] with name "forwarded". The parameter value is case- 151 insensitive and can be either "yes" or "no". (The default value 152 being "no"). The parameter is only meaningful with media type 153 "message/rfc822" when used within S/MIME encrypted body parts. The 154 value "yes" means that the message nested inside message/rfc822 is a 155 forwarded message and not a construct created solely to protect the 156 inner header. 158 Instructions in [RFC5751] describing how to protect the Email message 159 header [RFC5322], by wrapping the message inside a message/rfc822 160 container [RFC2045] are thus updated to read: 162 In order to protect outer, non-content-related message header 163 fields (for instance, the "Subject", "To", "From", and "Cc" 164 fields), the sending client MAY wrap a full MIME message in a 165 message/rfc822 wrapper in order to apply S/MIME security services 166 to these header fields. It is up to the receiving client to 167 decide how to present this "inner" header along with the 168 unprotected "outer" header. 170 When an S/MIME message is received, if the top-level protected 171 MIME entity has a Content-Type of message/rfc822 without the 172 "forwarded" parameter or with the "forwarded" parameter set to 173 "no", it can be assumed that the intent was to provide header 174 protection. This entity SHOULD be presented as the top-level 175 message, taking into account header merging issues as previously 176 discussed. 178 5. IANA Considerations 180 This document requests no action from IANA. RFC Editor should delete 181 this section before publication. 183 6. Security Considerations 185 This document talks about UI considerations, including security 186 considerations, when processing wrapped message/rfc822 messages 187 protecting header fields. One of the goals of this document is to 188 specify UI for displaying such messages which is less confusing/ 189 misleading and thus more secure. 191 The document is not defining new protocol, so it doesn't create any 192 new security concerns not already covered by S/MIME [RFC5751], MIME 193 [RFC2045] and Email [RFC5322] in general. 195 7. References 197 7.1. Normative References 199 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 200 Requirement Levels", BCP 14, RFC 2119, March 1997. 202 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 203 Extensions (MIME) Part One: Format of Internet Message 204 Bodies", RFC 2045, November 1996. 206 [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, 207 October 2008. 209 [RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet 210 Mail Extensions (S/MIME) Version 3.2 Message 211 Specification", RFC 5751, January 2010. 213 7.2. Informative References 215 [RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 216 4rev1", RFC 3501, March 2003. 218 Appendix A. Acknowledgements 220 Thank you to Steve Kille and David Wilson for suggestions, comments 221 and corrections on this document. 223 David Wilson came up with the idea of defining a new Content-Type 224 header field parameter to distinguish forwarded messages from inner 225 header field protection constructs. 227 Author's Address 229 Alexey Melnikov 230 Isode Ltd 231 14 Castle Mews 232 Hampton, Middlesex TW12 2NP 233 UK 235 EMail: Alexey.Melnikov@isode.com