Securing X.400 Content with Secure/Multipurpose Internet Mail Extensions (S/MIME)
RFC 3854

Network Working Group                                         P. Hoffman
Request for Comments: 3854                                           IMC
Category: Standards Track                                     C. Bonatti
                                                                    IECA
                                                                A. Eggen
                                                                     FFI
                                                               July 2004

     Securing X.400 Content with Secure/Multipurpose Internet Mail
                          Extensions (S/MIME)

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 a protocol for adding cryptographic signature
   and encryption services to X.400 content with Secure/Multipurpose
   Internet Mail Extensions (S/MIME).

1.  Introduction

   The techniques described in the Cryptographic Message Syntax [CMS]
   specification are general enough to support many different content
   types.  The [CMS] specification thus provides many options for
   providing different security mechanisms.  In order to ensure
   interoperability of systems within the X.400 community, it is
   necessary to specify the use of CMS features to protect X.400 content
   (called "CMS-X.400" in this document).

1.1.  Specification Overview

   This document is intended to be similar to the S/MIME Version 3.1
   Message Specification [MSG] except that it is tailored to the
   requirements of X.400 content rather than Multipurpose Internet Mail
   Extensions (MIME).

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   This document defines how to create an X.400 content type that has
   been cryptographically enhanced according to [CMS].  In order to
   create S/MIME messages carrying X.400 content, an S/MIME agent has to
   follow specifications in this document, as well as the specifications
   listed in [CMS].  This memo also defines new parameter values for the
   application/pkcs7-mime MIME type that can be used to transport those
   body parts.

   Throughout this document, there are requirements and recommendations
   made for how receiving agents handle incoming messages.  There are
   separate requirements and recommendations for how sending agents
   create outgoing messages.  In general, the best strategy is to "be
   liberal in what you receive and conservative in what you send".  Most
   of the requirements are placed on the handling of incoming messages
   while the recommendations are mostly on the creation of outgoing
   messages.

   This document does not address transport of CMS-X.400 content.  It is
   assumed that CMS-X.400 content would be transported by Internet mail
   systems, X.400, or other suitable transport.

   This document describes applying security services to the content of
   entire X.400 messages, which may or may not be IPMS messages.  These
   objects can be carried by several means, including SMTP-based mail
   and X.400 mail.  Note that cooperating S/MIME agents must support
   common forms of message content in order to achieve interoperability.

   If the CMS objects are sent as parts of an RFC 822 message, a
   standard MIXER gateway [MIXER] will most likely choose to encapsulate
   the message.  This is not likely to be a format that is usable by an
   X.400 recipient.  MIXER is specifically focused on translation
   between X.420 Interpersonal Messages and non-secure RFC822/MIME
   messages.  The discussion of security-related body parts in sections
   7.3 and 7.4 of [BODYMAP] is relevant to CMS messages.

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

1.2.  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].

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1.3.  Definitions

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

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

   BER:               Basic Encoding Rules for ASN.1, as defined in
                      ISO/IEC 8825-1.

   Certificate:       A type that binds an entity's distinguished name
                      to a public key with a digital signature.

   DER:               Distinguished Encoding Rules for ASN.1, as defined
                      in ISO/IEC 8825-1.

   7-bit data:        Text data with lines less than 998 characters
                      long, where none of the characters have the 8th
                      bit set, and there are no NULL characters.  <CR>
                      and <LF> occur only as part of a <CR><LF> end of
                      line delimiter.

   8-bit data:        Text data with lines less than 998 characters, and
                      where none of the characters are NULL characters.
                      <CR> and <LF> occur only as part of a <CR><LF> end
                      of line delimiter.

   Binary data:       Arbitrary data.

   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.

   Receiving agent:   Software that interprets and processes S/MIME CMS
                      objects.

   Sending agent:     Software that creates S/MIME CMS objects.

   S/MIME agent:      User software that is a receiving agent, a sending
                      agent, or both.

1.4.  Compatibility with Prior Practice of S/MIME

   There are believed to be no existing X.400 implementations that
   support S/MIME version 2.  Further, signed interoperability between
   X.400 and MIME systems that support S/MIME version 2 is not believed

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   to be easily achievable.  Therefore backward compatibility with
   S/MIME version 2 is not considered to be a requirement for this
   document.

   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.  CMS Options

   CMS allows for a wide variety of options in content and algorithm
   support.  This section puts forth a number of support requirements
   and recommendations in order to achieve a base level of
   interoperability among all CMS-X.400 implementations.  [CMS] provides
   additional details regarding the use of the cryptographic algorithms.

2.1.  DigestAlgorithmIdentifier

   Sending and receiving agents MUST support SHA-1 [CMSALG].

2.2.  SignatureAlgorithmIdentifier

   Receiving agents MUST support id-dsa-with-sha1 defined in [CMSALG].
   The algorithm parameters MUST be absent (not encoded as NULL).
   Receiving agents MUST support rsaEncryption, defined in [CMSALG].

   Sending agents MUST support either id-dsa-with-sha1 or rsaEncryption.

2.3.  KeyEncryptionAlgorithmIdentifier

   Sending and receiving agents MUST support rsaEncryption, defined in
   [CMSALG].

   Sending and receiving agents SHOULD support Diffie-Hellman defined in
   [CMSALG].

2.4.  General Syntax

   The general syntax of CMS objects consist of an instance of the
   ContentInfo structure containing one of several defined CMS content
   types.  CMS defines multiple content types.  Of these, only the
   SignedData and EnvelopedData content types are used for CMS-X.400.

2.4.1.  SignedData Content Type

   Sending agents MUST use the signedData content type to apply a
   digital signature to a message or, in a degenerate case where there
   is no signature information, to convey certificates.

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2.4.2.  EnvelopedData Content Type

   Senders MUST use the envelopedData content type to apply privacy
   protection to a message.  A sender needs to have access to a public
   key for each intended message recipient to use this service.  This
   content type does not provide authentication.

2.5.  Attribute SignerInfo Type

   The SignerInfo type allows the inclusion of unsigned and signed
   attributes to be included along with a signature.

   Receiving agents MUST be able to handle zero or one instance of each
   of the signed attributes listed here.  Sending agents SHOULD generate
   one instance of each of the following signed attributes in each CMS-
   X400 message:

   - signingTime
   - sMIMECapabilities
   - sMIMEEncryptionKeyPreference

   Requirements for processing of these attributes MUST be in accordance
   with the S/MIME Message Specification [MSG].  Handling of the
   signingTime attribute MUST comply with clause 2.5.1 of [MSG].
   Handling of the sMIMECapabilities attribute MUST comply with clause
   2.5.2 of [MSG].  Handling of the sMIMEEncryptionKeyPreference
   attribute MUST comply with clause 2.5.3 of [MSG].

   Further, receiving agents SHOULD be able to handle zero or one
   instance in the signed attributes of the signingCertificate attribute
   [ESS].

   Sending agents SHOULD generate one instance of the signingCertificate
   signed attribute in each CMS-X400 message.

   Additional attributes and values for these attributes may be defined
   in the future.  Receiving agents SHOULD handle attributes or values
   that they do not recognize in a graceful manner.

   Sending agents that include signed attributes that are not listed
   here SHOULD display those attributes to the user, so that the user is
   aware of all of the data being signed.

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2.6.  ContentEncryptionAlgorithmIdentifier

   Sending and receiving agents MUST support encryption and decryption
   with DES EDE3 CBC, hereinafter called "tripleDES" [CMSALG].  Sending
   and receiving agents SHOULD support encryption and decryption with
   AES [CMSAES] at a key size of 128, 192 and 256 bits.

3.  Creating S/MIME Messages

   This section describes the S/MIME message formats and how they can be
   used to secure X.400 contents.  The S/MIME messages are a combination
   of X.400 contents and CMS objects (i.e., a ContentInfo structure
   containing one of the CMS-defined content types).  The X.400 content
   and other data, such as certificates and algorithm identifiers, are
   given to CMS processing facilities which produces a CMS object.  This
   document also describes how nested, secured S/MIME messages should be
   formatted when encapsulating an X.400 content, and provides an
   example of how a triple-wrapped S/MIME message over X.400 content
   should be created if backwards compatibility with S/MIME version 2 is
   of no concern.

   S/MIME provides one format for enveloped-only data, several formats
   for signed-only data, and several formats for signed and enveloped
   data.  The different formats are required to accommodate several
   environments, in particular for signed messages.  Only one of these
   signed formats is applicable to X.400.

   Note that canonicalization is not required for X.400 content because
   it is a binary rather than text encoding, and only the "embedded"
   content version is used.  These dramatically simplify the description
   of S/MIME productions.

   The reader of this section is expected to understand X.400 as
   described in [X.400] and S/MIME as described in [CMS] and [ESS].

3.1.  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.

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   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.

   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.

   Another piece of information borne by the envelope identifies the
   types of encoded information represented in the content.  An encoded
   information type (EIT) is an identifier (an ASN.1 Object Identifier
   or Integer) that denotes the medium and format (e.g., IA5 text or
   Group 3 facsimile) of individual portions of the content.  It further
   enables the MTS to determine the message's deliverability to
   particular users, and to identify opportunities for it to make the
   message deliverable by converting a portion of the content from one
   EIT to another.

   This document describes how S/MIME CMS is used to secure the content
   part of X.400 messages.

3.2.  Creating a Signed-only Message with X.400 Content

   The SignedData format as described in the Cryptographic Message
   Syntax [CMS] MUST be used for signing of X.400 contents.

   The X.400 content to be protected MUST be placed in the SignedData
   encapContentInfo eContent field.  Note that this X.400 content SHOULD
   maintain the encoding defined by the content type, but SHOULD NOT be
   MIME wrapped.  The object identifier for the content type of the
   protected X.400 content MUST be placed in the SignedData
   encapContentInfo eContentType field.

   The signedData object is encapsulated by a ContentInfo SEQUENCE with
   a contentType of id-signedData.

   Note that if SMTP [SMTP] is used to transport the resulting signed-
   only message then the optional MIME encoding SHOULD be used.  If
   binary transports such as X.400 are used then the optional MIME
   encoding SHOULD NOT be used.

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   There are many reasons for this requirement.  An outer MIME wrapper
   should not be used in X.400.  Further, there are places where X.400
   systems will interact with SMTP/MIME systems where the outer MIME
   wrapper might be necessary.  Because this wrapping is outside the
   security wrappers, any gateway system that might bridge the gap
   between the two systems will be smart enough to apply or remove the
   outer MIME wrapper as appropriate.

3.2.1.  MIME Wrapping to Dynamically Support 7-bit Transport

   The signedData object MAY optionally be wrapped in MIME.  This allows
   the system to support 7-bit transport when required.  This outer MIME
   wrapper MAY be dynamically added or removed throughout the delivery
   path since it is outside the signature and encryption wrappers.  In
   this case the application/pkcs7-mime type as defined in S/MIME
   Version 3.1 Message Specification [MSG] SHOULD be used with the
   following parameters:

   Content-Type: application/pkcs7-mime; smime-type=signed-x400
   Content-Transfer-Encoding: base64

   If the application/pkcs7-mime MIME type is used to support 7-bit
   transport, the steps to create this format are:

   Step 1.  The X.400 content to be signed is ASN.1 encoded.

   Step 2.  The ASN.1 encoded X.400 content and other required data is
   processed into a CMS object of type SignedData.  The SignedData
   structure is encapsulated by a ContentInfo SEQUENCE with a
   contentType of id-signedData.

   Step 3.  The CMS object is inserted into an application/pkcs7-mime
   MIME entity.

   The smime-type parameter for messages using application/pkcs7-mime
   with SignedData is "signed-x400" as defined in [TRANSPORT].

3.3.  Creating an Enveloped-only Message with X.400 Content

   This section describes the format for enveloping an X.400 content
   without signing it.  It is important to note that sending enveloped
   but not signed messages does not provide for data integrity.  It is
   possible to replace ciphertext in such a way that the processed
   message will still be valid, but the meaning is altered.

   The EnvelopedData format as described in [CMS] is used for
   confidentiality of the X.400 contents.

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   The X.400 content to be protected MUST be placed in the EnvelopedData
   encryptedContentInfo encryptedContent field.  Note that this X.400
   content SHOULD maintain the encoding defined by the content type, but
   SHOULD NOT be MIME wrapped.  The object identifier for content type
   of the protected X.400 content MUST be placed in the EnvelopedData
   encryptedContentInfo contentType field.

   The envelopedData object is encapsulated by a ContentInfo SEQUENCE
   with a contentType of id-envelopedData.

   Note that if SMTP is used to transport the resulting enveloped-only
   message then the optional MIME encoding SHOULD be used.  If other
   transport (e.g., X.400) that is optimized for binary content is used
   then the optional MIME encoding SHOULD NOT be used.

3.3.1.  MIME Wrapping to Dynamically Support 7-bits Transport

   The envelopedData object MAY optionally be wrapped in MIME.  This
   allows the system to support 7-bit transport when required.  This
   outer MIME wrapper MAY be dynamically added or removed throughout the
   delivery path since it is outside the signature and encryption
   wrappers.  In this case, the application/pkcs7-mime type as defined
   in S/MIME Version 3.1 Message Specification [MSG] SHOULD be used with
   the following parameters:

   Content-Type: application/pkcs7-mime; smime-type=enveloped-x400
   Content-Transfer-Encoding: base64

   If the application/pkcs7-mime MIME type is used to support 7-bit
   transport, the steps to create this format are:

   Step 1.  The X.400 content to be enveloped is ASN.1 encoded.

   Step 2.  The ASN.1 encoded X.400 content and other required data is
   processed into a CMS object of type EnvelopedData.  In addition to
   encrypting a copy of the content-encryption key for each recipient, a
   copy of the content encryption key SHOULD be encrypted for the
   originator and included in the EnvelopedData (see [CMS] Section 6).
   The EnvelopedData structure is encapsulated by a ContentInfo SEQUENCE
   with a contentType of id-envelopedData.

   Step 3.  The CMS object is inserted into an application/pkcs7-mime
   MIME entity to allow for 7-bit transport.

   If the application/pkcs7-mime MIME entity is used, the smime-type
   parameter for enveloped-only messages is "enveloped-x400" as defined
   in [TRANSPORT].

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3.4.  Nested CMS Structures

   To achieve signing and enveloping, any of the signed-only and
   encrypted-only CMS objects may be nested.

   When nesting is used, backwards compatibility with S/MIME version 2
   requires that each layer of the nested message are identified with
   the OID id-data, and when id-data is used a MIME wrapper is required.
   This can potentially lead to an enormous amount of overhead and
   should be avoided.  Because S/MIME version 2 compatibility is of no
   concern, implementations SHOULD directly encode the encapsulated
   object as the eContent of the current structure.

   MIME wrapping to support 7-bit transport is optional and need only be
   used around the outermost CMS structure.  In this case, the
   application/pkcs7 content type MUST be used.

   An S/MIME implementation MUST be able to receive and process
   arbitrarily nested CMS structures within reasonable resource limits
   of the recipient computer.

3.4.1.  Creating a Triple Wrapped Message With an X.400 Content

   The Enhanced Security Services for S/MIME [ESS] document provides
   examples of how nested, secured S/MIME messages are formatted.  ESS
   provides an example of how a triple-wrapped S/MIME message is
   formatted using application/pkcs7-mime for the signatures.

   This section explains how an X.400 content may be conveyed within a
   Triple Wrapped Message because S/MIME version 2 compatibility is of
   no concern:

   Step 1.  Start with the X.400 content (called the "original
   content").  The X.400 content MUST be ASN.1 encoded, but SHOULD NOT
   be MIME wrapped.

   Step 2.  Place the ASN.1 encoded X.400 content to be protected in the
   SignedData encapContentInfo eContent field.  Add any attributes that
   are to be signed.

   Step 3.  Sign the result of step 2 (the original content).  The
   SignedData encapContentInfo eContentType MUST contain the object
   identifier of the X.400 content.

   Step 4.  Encrypt the result of step 3 as a single block.  The
   EnvelopedData encryptedContentInfo contentType MUST be set to id-
   signedData.  This is called the "encrypted body".

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   Step 5.  Using the same logic as in step 2 and 3 above, sign the
   result of step 4 (the encrypted body) as a single block.  The
   SignedData encapContentInfo eContentType MUST be set to id-
   envelopedData.  The outer SignedData structure is encapsulated by a
   ContentInfo SEQUENCE with a contentType of id-signedData.

   Step 6.  The resulting message is called the "outer signature", and
   is also the triple wrapped message.

   MIME wrapping to support 7-bit transport is optional and MUST only be
   used around the outermost CMS structure.  In this case, the
   application/pkcs7-mime content type MUST be used.  The smime-type in
   the case of adding a MIME wrapper MUST be consistent with that
   appropriate to the innermost protection layer.

   In some instances, an smime-type will be created that only reflects
   one security service (such as certs-only, which applies only to
   signed-only messages).  However, as new layers are wrapped, this
   smime-type SHOULD be propagated upwards.  Thus if a certs-only
   message were to be encrypted, or wrapped in a new SignedData
   structure, the smime-type of certs-only should be propagated up to
   the next MIME wrapper.  In other words, the innermost type is
   reflected outwards.

3.5.  Carrying Plaintext X.400 Content in SMTP

   While the objectives of this document focus on protecting X.400
   content with CMS wrappers, it is a reality that users do not
   generally send all message using security.  It therefore stands to
   reason that a means to carry non-secured X.400 content over the
   chosen transport system must be seamlessly provided.  While
   transporting X.400 content in an X.400 system is trivial, carrying
   X.400 content in SMTP requires additional definition.

   Content-Type: application/x400-content; content-type = 1*DIGIT *( "."
   1*DIGIT)

   where the content-type parameter value is either a single integer
   (for a built-in content-type) or an OID in dotted notation (for an
   extended content-type).

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4.  Use of Certificates

4.1.  Certificate Enrollment

   S/MIME v3.1 does not specify how to get a certificate from a
   certificate authority, but instead mandates that every sending agent
   already has a certificate.  The PKIX Working Group has, at the time
   of this writing, produced two separate standards for certificate
   enrollment: CMP (RFC 2510) and CMC (RFC 2792).

4.2.  Certificate Processing

   A receiving agent MUST provide some certificate retrieval mechanism
   in order to gain access to certificates for recipients of digital
   envelopes.  This document does not cover how S/MIME agents handle
   certificates, only what they do after a certificate has been
   validated or rejected.  S/MIME certification issues are covered in
   [CERT31].

   At a minimum, for initial S/MIME deployment, a user agent could
   automatically generate a message to an intended recipient requesting
   that recipient's certificate in a signed return message.  Receiving
   and sending agents SHOULD also provide a mechanism to allow a user to
   "store and protect" certificates for correspondents in such a way so
   as to guarantee their later retrieval.

4.3.  Certificate Name Use for X.400 Content

   End-entity certificates used in the context of this document MAY
   contain an X.400 address as described in [X.400].  The address must
   be in the form of an "ORAddress".  The X.400 address SHOULD be in the
   subjectAltName extension, and SHOULD NOT be in the subject
   distinguished name.

   Sending agents SHOULD make the originator address in the X.400
   content (e.g., the "originator" field in P22) match an X.400 address
   in the signer's certificate.

   Receiving agents MUST recognize X.400 addresses in the subjectAltName
   field.

   Receiving agents SHOULD check that the originator address in the
   X.400 content matches an X.400 address in the signer's certificate,
   if X.400 addresses are present in the certificate and an originator
   address is available in the content.  A receiving agent SHOULD
   provide some explicit alternate processing of the message if this

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   comparison fails, which may be to display a message that shows the
   recipient the addresses in the certificate or other certificate
   details.

   The subject alternative name extension is used in S/MIME as the
   preferred means to convey the X.400 address(es) that correspond to
   the entity for this certificate.  Any X.400 addresses present MUST be
   encoded using the x400Address CHOICE of the GeneralName type.  Since
   the SubjectAltName type is a SEQUENCE OF GeneralName, multiple X.400
   addresses MAY be present.

5.  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].

6.  References

6.1.  Normative References

   [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.

   [CMSAES]     Schaad, J., "Use of the AES Encryption Algorithm in
                CMS", RFC 3565, July 2003.

   [CMSALG]     Housley, R., "Cryptographic Message Syntax (CMS)
                Algorithms", RFC 3370, August 2002.

   [ESS]        Hoffman, P., Editor "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.

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

   [TRANSPORT]  Hoffman, P. and C. Bonatti, "Transporting
                Secure/Multipurpose Internet Mail Extensions (S/MIME)
                Objects in X.400", RFC 3855, July 2004.

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   [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.

6.2.  Informative References

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

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

   [SMTP]       Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
                April, 2001.

7.  Editors' 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

   Anders Eggen
   Forsvarets Forskningsinstitutt
   Postboks 25
   2027 Kjeller, Norway

   EMail: anders.eggen@ffi.no

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8.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
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Hoffman, et al.             Standards Track                    [Page 15]