Considerations for protecting Email header with S/MIME
draft-melnikov-smime-header-signing-05

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Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Intended status: Informational                           October 1, 2017
Expires: April 4, 2018

         Considerations for protecting Email header with S/MIME
                 draft-melnikov-smime-header-signing-05

Abstract

   This document describes best practices for handling of Email header
   protected by S/MIME.  It also adds a new Content-Type parameter to
   help distinguish an S/MIME protected forwarded message from an S/MIME
   construct protecting message header.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 4, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   3
   3.  Recommendations for handling of S/MIME protected header . . .   3
   4.  New Content-Type header field parameter: forwarded  . . . . .   4
   5.  Example message with S/MIME header protection . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   S/MIME [RFC5751] is typically used to protect (sign and/or encrypt)
   Email message body parts, but not header of corresponding Email
   messages.  Header fields may contain confidential information or
   information whose validity need protecting from disclosure or
   modification.  [RFC5751] describes how to protect the Email message
   header [RFC5322], by wrapping the message inside a message/rfc822
   container [RFC2045]:

      In order to protect outer, non-content-related message header
      fields (for instance, the "Subject", "To", "From", and "Cc"
      fields), the sending client MAY wrap a full MIME message in a
      message/rfc822 wrapper in order to apply S/MIME security services
      to these header fields.  It is up to the receiving client to
      decide how to present this "inner" header along with the
      unprotected "outer" header.

      When an S/MIME message is received, if the top-level protected
      MIME entity has a Content-Type of message/rfc822, it can be
      assumed that the intent was to provide header protection.  This
      entity SHOULD be presented as the top-level message, taking into
      account header merging issues as previously discussed.

   While the above advice helps in protecting message header fields, it
   doesn't provide enough guidance on what information should and should
   not be included in outer (unprotected) header and how information
   from outer and inner headers should be presented to users.  This
   document describes best UI and other practices for handling of
   messages wrapped inside message/rfc822 body parts.  The goal of this
   document is to improve interoperability and minimize damage caused by
   possible differences between inner and outer headers.

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