Internet Draft                                       Paul  Hoffman, IMC
draft-ietf-smime-x400transport-01.txt               Chris Bonatti, IECA
November 22, 2000
Expires May 22, 2001

               Transporting S/MIME Objects in X.400

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This document describes protocol options for conveying CMS-protected
objects associated with S/MIME version 3 over an X.400 message transfer

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.

1.1 Terminology

The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", and
"MAY" in this document are to be interpreted as described in RFC 2119

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.

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.

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 the
body part type. The body part type is an ASN.1 OID or Integer that
denotes the syntax and semantics of the body part in question.

2.2 Carrying S/MIME as X.400 Content

When transporting a CMS object in X.400, the preferred approach (except
as discussed in section 2.3 below) is to convey the object as X.400
message content. This section describes how S/MIME CMS objects are
conveyed as the content part of X.400 messages. This mechanism is
suitable for transport of CMS-protected messages regardless of the mail
content that has been encapsulated.

Implementations MUST include the CMS object in the content field of the
X.400 message.

If the CMS object is covered by an outer MIME wrapper, the content-type
field of the P1 envelope MUST be set to the following CMS-defined value:

id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
    rsadsi(113549) pkcs(1) pkcs7(7) 1 }

If the CMS object is not covered by an outer MIME wrapper, the
content-type field of the P1 envelope MUST be set to the following
CMS-defined value:

id-ct-contentInfo  OBJECT IDENTIFIER ::= { iso(1) member-body(2)
    us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
    content-types(1) 6}

2.3 Carrying S/MIME as IPMS Body Parts

Under some circumstances S/MIME CMS objects MAY be conveyed within
select body parts of the content. Implementations generally SHOULD NOT
embed CMS objects within X.400 body parts because of the dependency on
the support provided by the content type. There is no guarantee that all
X.400 content types will necessarily include structured content, much
less body parts. Furthermore, the structure of different X.400 body
parts may vary to the extent that it is difficult to universally specify
the conveyance of CMS objects. Nevertheless, one notable exception is

In instances when CMS objects are forwarded as part of a message
forwarding function, use of a body part is necessary. When forwarding a
CMS object in an IPMS or IPMS-compatible body part, implementations MUST
use the content-body-part as formally defined by [X.400], as shown below
for reference.

content-body-part {ExtendedContentType:content-type}
        PARAMETERS {ForwardedContentParameters IDENTIFIED BY
            {id-ep-content -- concatenated with content-type -- }},
        DATA {Content IDENTIFIED BY
            {id-et-content -- concatenated with content-type -- }} }

ForwardedContentParameters ::= SET {
    delivery-time     [0] MessageDeliveryTime OPTIONAL,
    delivery-envelope [1] OtherMessageDeliveryFields OPTIONAL,
    mts-identifier    [2] MessageDeliveryIdentifier OPTIONAL}

id-ep-content ::= {joint-iso-itu-t(2) mhs(6) ipms(1) ep(11) 17}

The implementation MUST copy the CMS object to be forwarded into the
Content field of the content-body-part. The direct-reference field of
the body part MUST include the OID formed by the concatenation of the
id-ep-content value and the following CMS-defined value.

id-ct-contentInfo  OBJECT IDENTIFIER ::=
    { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
    pkcs-9(9) smime(16) content-types(1) 6}

The ForwardedContentParameters are optional and MAY be supported at the
discretion of the implementor.

2.4 Transfer Encoding

According to various S/MIME specifications for message wrapping, CMS
objects MAY optionally be wrapped in MIME to dynamically support 7-bit
transport. This outer wrapping is not required for X.400 transport, and
generally SHOULD NOT be applied in a homogeneous X.400 environment.
Heterogeneous mail systems or other factors MAY require the presence of
this outer MIME wrapper

3. Security Considerations

This entire document discusses the topic of conveying security protocol
structures. Additional security issues are identified in section 5 of
[MSG], section 6 of [ESS] and the Security Considerations section of

A. References

[CMS] Housley, R., "Cryptographic Message Syntax", RFC 2630, June 1999.

[MSG] Ramsdell, B., Editor "S/MIME Version 3 Message Specification", RFC
2633, June 1999.

[MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.

[PKCS-7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version
1.5", RFC 2315, March 1998.

[X.400] ITU-T X.400 Series of Recommendations, Information technology -
Message Handling Systems (MHS). X.400: System and Service Overview;
X.402: Overall Architecture; X.411: Message Transfer System: Abstract
Service Definition and Procedures; X.420: Interpersonal Messaging
System; 1996.

B. Differences between version -00 and -01

Many small corrections from Bill Ottaway.

C. Editors' Addresses

Paul Hoffman
Internet Mail Consortium
127 Segre Place
Santa Cruz, CA  95060  USA

Chris Bonatti
IECA, Inc.