path: root/doc/rfc/rfc3855.txt

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.




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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.




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   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-protected message 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}

2.2.1.  Carrying Plaintext MIME objects as X.400 Content

   When transporting a plaintext MIME object in X.400, the preferred
   approach is to convey the object as X.400 message content.  The






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   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 }

2.3.  Carrying S/MIME as IPMS Body Parts

   Under some circumstances S/MIME CMS-protected messages can be
   conveyed within select body parts of the content.  Implementations
   generally SHOULD NOT embed CMS objects within X.400 body parts, but
   should instead convey them as content as described in section 2.2.
   Nevertheless, one notable exception is necessary for the case of
   forwarding.

   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}
       EXTENDED-BODY-PART-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}

   id-et-content ::= {joint-iso-itu-t(2) mhs(6) ipms(1) et(4) 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-et-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}

   For example, to forward any CMS object the DATA component of the body
   part would be identified by { 2 6 1 4 17 1 2 840 113549 1 9 16 1 6 }.



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   The ForwardedContentParameters are optional and MAY be supported at
   the discretion of the implementor.  The OID value id-et-content MAY
   also be included in the original-encoded-information-types field of
   the X.400 message envelope at the discretion of the sending S/MIME
   agent.

   In this instance, the content-type field of the P1 envelope MUST be
   set to the value associate with the forwarding content (e.g., integer
   22 for IPMS).

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

2.5.  Encoded Information Type Indication

   In [MSG], the application/pkcs7-mime content type and optional
   "smime-type" parameter are used to convey details about the security
   applied (signed or enveloped) along with information about the
   contained content.  This may aid receiving S/MIME implementations in
   correctly processing the secured content.  Additional values of
   smime-type are defined in [ESS].  In an X.400 transport environment,
   MIME typing is not available.  Therefore the equivalent semantic is
   conveyed using the Encoded Information Types (EITs).  The EITs are
   conveyed in the original-encoded-information-types field of the X.400
   message envelope.  This memo defines the following smime-types.




















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   +-----------------------------------------------------+
   |                                                     |
   |     smime-type           EIT Value (OID)            |
   | CMS protection type       Inner Content             |
   |                                                     |
   +-----------------------------------------------------+
   |                                                     |
   |  enveloped-data        id-eit-envelopedData         |
   |  EnvelopedData         Data                         |
   |                                                     |
   |  signed-data           id-eit-signedData            |
   |  SignedData            Data                         |
   |                                                     |
   |  certs-only            id-eit-certsOnly             |
   |  SignedData            empty (zero-length content)  |
   |                                                     |
   |  signed-receipt        id-eit-signedReceipt         |
   |  SignedData            Receipt                      |
   |                                                     |
   |  enveloped-x400        id-eit-envelopedx400         |
   |  EnvelopedData         X.400 content                |
   |                                                     |
   |  signed-x400           id-eit-signedx400            |
   |  SignedData            X.400 content                |
   |                                                     |
   |  compressed-data       id-eit-compressedData        |
   |  CompressedData        RFC 3274 compression wrapper |
   |                                                     |
   +-----------------------------------------------------+

   Sending agents SHOULD include the appropriate S/MIME EIT OID value.
   Receiving agents SHOULD recognize S/MIME OID values in the EITs
   field, and process the message appropriately according to local
   procedures.

   In order that consistency can be obtained in future S/MIME EIT
   assignments, the following guidelines should be followed when
   assigning new EIT values.  Values assigned for S/MIME EITs should
   correspond to assigned smime-type values on a one-to-one basis.  The
   restrictions of section 3.2.2 of [MSG] therefore apply.  S/MIME EIT
   values may coexist with other EIT values intended to further qualify
   the makeup of the protected content.









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2.5.1.  Enveloped Data

   The enveloped data EIT indicates that the X.400 content field
   contains a MIME type that has been protected by the CMS enveloped-
   data content type in accordance with [MSG].  The resulting enveloped
   data CMS content is conveyed in accordance with section 2.2.  This
   EIT should be indicated by the following OID value:

      id-eit-envelopedData  OBJECT IDENTIFIER ::=
          { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
          pkcs-9(9) smime(16) id-eit(10) id-eit-envelopedData(1) }

2.5.2.  Signed Data

   The signed data EIT indicates that the X.400 content field contains a
   MIME type that has been protected by the CMS signed-data content type
   in accordance with [MSG].  The resulting signed data CMS content is
   conveyed in accordance with section 2.2.  This EIT should be
   indicated by the following OID value:

      id-eit-signedData  OBJECT IDENTIFIER ::=
           { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
           pkcs-9(9) smime(16) id-eit(10) id-eit-signedData(2) }

2.5.3.  Certs Only

   The certs-only message is used to transport certificates and/or CRLs,
   such as in response to a registration request.  This is described in
   [CERT31].  The certs-only message consists of a single instance of
   CMS content of type signed-data.  The encapContentInfo eContent field
   MUST be absent and signerInfos field MUST be empty.  The resulting
   certs-only CMS content is conveyed in accordance with section 2.2.
   This EIT should be indicated by the following OID value:

      id-eit-certsOnly  OBJECT IDENTIFIER ::=
          { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
          pkcs-9(9) smime(16) id-eit(10) id-eit-certsOnly(3) }

2.5.4.  Signed Receipt

   The signed receipt EIT indicates that the X.400 content field
   contains a Receipt content that has been protected by the CMS
   signed-data content type in accordance with [ESS].  The resulting CMS
   signed-data content is conveyed in accordance with section 2.2.  This
   EIT should be indicated by the following OID value:






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      id-eit-signedReceipt  OBJECT IDENTIFIER ::=
          { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
          pkcs-9(9) smime(16) id-eit(10) id-eit-signedReceipt(4) }

2.5.5.  Enveloped X.400

   The enveloped X.400 EIT indicates that the X.400 content field
   contains X.400 content that has been protected by the CMS enveloped-
   data content type in accordance with [X400WRAP].  The resulting
   enveloped X.400 CMS content is conveyed in accordance with section
   2.2.  This EIT should be indicated by the following OID value:

      id-eit-envelopedX400  OBJECT IDENTIFIER ::=
          { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
          pkcs-9(9) smime(16) id-eit(10) id-eit-envelopedX400(5) }

2.5.6.  Signed X.400

   The signed X.400 EIT indicates that the X.400 content field contains
   X.400 content that has been protected by the CMS signed-data content
   type in accordance with [X400WRAP].  The resulting signed X.400 CMS
   content is conveyed in accordance with section 2.2.  This EIT should
   be indicated by the following OID value:

      id-eit-signedX400  OBJECT IDENTIFIER ::=
          { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
          pkcs-9(9) smime(16) id-eit(10) id-eit-signedX400(6) }

2.5.7.  Compressed Data

   The compressed data EIT indicates that the X.400 content field
   contains a another type that has been compressed by the compressed-
   data content type in accordance with [COMPRESS].  The resulting CMS
   content is conveyed in accordance with section 2.2.  This EIT should
   be indicated by the following OID value:

      id-eit-compressedData  OBJECT IDENTIFIER ::=
          { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
          pkcs-9(9) smime(16) id-eit(10) id-eit-compressedData(7) }

2.6.  Interaction with X.400 Elements of Service

   Care should be taken in the selection of X.400 services to be used in
   conjunction with CMS objects.  Services affecting conversion of the
   content, expansion of Distribution Lists (DLs), and message
   redirection can interact badly with services provided by the
   "EnvelopedData" and "SignedData" CMS content types.




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2.6.1.  MTS Conversion Services

   MTS conversion is not applicable to the scenario of this document
   because such conversion is incompatible with CMS protection
   mechanisms.  X.400 systems that implement conversion services should
   generally be unable to attempt conversion of CMS content types
   because those types do not conform to X.420 structure rules.
   Nevertheless, when transporting CMS objects within an X.400
   environment, the Conversion Prohibition service SHOULD be selected.

2.6.2.  Message Redirection Services

   X.400 message redirection services can have an indirect impact on the
   application of the CMS "EnvelopedData" content type.  Several
   different forms of redirection are possible in X.400, including:

   -  Originator Requested Alternate Recipient (ORAR)
   -  Alternate Recipient Assignment
   -  Redirection of Incoming Messages

   In addition, any auto-forwarding services that are not security-aware
   may share the same problem.  An auto-forwarding implementation that
   removes the EnvelopedData and reapplies it for the forwarded
   recipient is not affected by this problem.  The normal case is that
   the private key is not available when the human user is not present,
   thus decryption is not possible.  However, if the private key is
   present, forwarding can be used instead.

   When the "EnvelopedData" content type is used to protect message
   contents, an instance of RecipientInfo is needed for each recipient
   and alternate recipient in order to ensure the desired access to the
   message.  A RecipientInfo for the originator is a good practice just
   in case the MTS returns the whole message.

   In the event that ORAR is used, the originator is aware of the
   identity of the alternate recipient and SHOULD include a
   corresponding RecipientInfo element.  For other forms of redirection
   (including non-security-aware auto-forwarding) the alternate
   recipient must either have access to the intended recipient's keys
   (not recommended) or must relay the message to the intended recipient
   by other means.

2.6.3.  DL Expansion

   X.400 DLs can have an indirect impact on the application of the CMS
   "EnvelopedData" content type.  When the "EnvelopedData" content type
   is used to protect message contents, an instance of RecipientInfo is
   needed for each recipient in order to ensure the desired access to



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   the message.  Messages to a DL would typically include only a single
   RecipientInfo associated with the DL.  Unlike Mail Lists (MLs)
   described in [ESS], however, X.400 DLs are not generally security-
   aware and do not regenerate RecipientInfo elements for the DL
   members.  It is recommended that a security-aware ML conforming to
   [ESS] be used in preference to X.400 DLs.  When transporting CMS
   objects within an X.400 environment, the DL Expansion Prohibited
   service SHOULD be selected.

3.  Security Considerations

   This specification introduces no new security concerns to the CMS or
   S/MIME models.  Security issues are identified in section 5 of [MSG],
   section 6 of [ESS] and the Security Considerations section of [CMS].

4.  References

4.1.  Normative References

   [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.

   [CERT31]     Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
                Extensions (S/MIME) Version 3.1 Certificate Handling",
                RFC 3850, July 2004.

   [CMS]        Housley, R., "Cryptographic Message Syntax (CMS)", RFC
                3852, July 2004.

   [COMPRESS]   Gutmann, P., "Compressed Data Content Type for
                Cryptographic Message Syntax (CMS)", RFC 3274, June
                2002.

   [ESS]        Hoffman, P., Ed., "Enhanced Security Services for
                S/MIME", RFC 2634, June 1999.

   [MSG]        Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
                Extensions (S/MIME) Version 3.1 Message Specification",
                RFC 3851, July 2004.

   [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.





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4.2.  Informative References

   [BODYMAP]    Alvestrand, H., "Mapping between X.400 and RFC-822/MIME
                Message Bodies", RFC 2157, January 1998.

   [MIXER]      Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay):
                Mapping between X.400 and RFC 822/MIME", RFC 2156,
                January 1998.

   [X400WRAP]   Hoffman, P., Bonatti, C., and A. Eggen, "Securing X.400
                Content with Secure/Multipurpose Internet Mail
                Extensions (S/MIME), RFC 3854, July 2004.

5.  Authors' Addresses

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

   EMail: phoffman@imc.org


   Chris Bonatti
   IECA, Inc.
   15309 Turkey Foot Road
   Darnestown, MD  20878-3640  USA

   EMail: bonattic@ieca.com






















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6.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
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   on the procedures with respect to rights in RFC documents can be
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
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   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.









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