Transporting Secure/Multipurpose Internet Mail Extensions (S/MIME) Objects in X.400
RFC 3855

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Network Working Group                                         P. Hoffman
Request for Comments: 3855                                           IMC
Category: Standards Track                                     C. Bonatti
                                                                    IECA
                                                               July 2004

             Transporting Secure/Multipurpose Internet Mail
                  Extensions (S/MIME) Objects in X.400

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document describes protocol options for conveying objects that
   have been protected using the Cryptographic Message Syntax (CMS) and
   Secure/Multipurpose Internet Mail Extensions (S/MIME) version 3.1
   over an X.400 message transfer system.

1. Introduction

   The techniques described in the Cryptographic Message Syntax [CMS]
   specification and message specifications can reasonably be
   transported via a variety of electronic mail systems.  This
   specification defines the options and values necessary to enable
   interoperable transport of S/MIME messages over an X.400 system.

   This document describes a mechanism for using CMS objects as the
   message content of X.400 messages in a native X.400 environment.
   This means that gateways or other functions that expect to deal with
   IPMS, such as those specified in [MIXER] and [BODYMAP], cannot do
   anything with these messages.  Note that cooperating S/MIME agents
   must support common forms of message content in order to achieve
   interoperability.

   Definition of gateway services to support relay of CMS object between
   X.400 and SMTP environments is beyond the scope of this document.

Hoffman & Bonatti           Standards Track                     [Page 1]
RFC 3855          Transporting S/MIME Objects in X.400         July 2004

1.1.  Terminology

   The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
   and "MAY" in this document are to be interpreted as described in BCP
   14, RFC 2119 [MUSTSHOULD].

1.2.  Definitions

   For the purposes of this document, the following definitions apply.

   ASN.1: Abstract Syntax Notation One, as defined in ISO/IEC 8824.

   Object Identifier (OID): A globally unique identifier value
   consisting of a sequence of integer values assigned through
   distributed registration as specified by ISO/IEC 8824.

   Transfer Encoding: A reversible transformation made on data so 8-bit
   or binary data may be sent via a channel that only transmits 7-bit
   data.

1.3.  Compatibility with Existing S/MIME Implementations

   It is a goal of this document to, if possible, maintain backward
   compatibility with existing X.400 implementations that employ S/MIME
   v3.1 wrappers.

2.  S/MIME Packaging

2.1.  The X.400 Message Structure

   This section reviews the X.400 message format.  An X.400 message has
   two parts, the envelope and the content, as described in X.402
   [X.400]:

   Envelope --  An information object whose composition varies from one
      transmittal step to another and that variously identifies the
      message's originator and potential recipients, documents its
      previous conveyance and directs its subsequent conveyance by the
      Message Transfer System (MTS), and characterizes its content.

   Content -- The content is the piece of information that the
      originating User Agent wants to be delivered to one or more
      recipients.  The MTS neither examines nor modifies the content,
      except for conversion, during its conveyance of the message.  MTS
      conversion is not applicable to the scenario of this document
      because such conversion is incompatible with CMS protection
      mechanisms.

Hoffman & Bonatti           Standards Track                     [Page 2]
RFC 3855          Transporting S/MIME Objects in X.400         July 2004

   One piece of information borne by the envelope identifies the type of
   the content.  The content type is an identifier (an ASN.1 OID or
   Integer) that denotes the syntax and semantics of the content
   overall.  This identifier enables the MTS to determine the message's
   deliverability to particular users, and enables User Agents and
   Message Stores to interpret and process the content.

   Some X.400 content types further refine the structure of content as a
   set of heading elements and body parts.  An example of this is the
   Interpersonal Messaging System (IPMS).  The IPMS content structure is
   able to convey zero or more arbitrary body parts each identified by