Network Working Group P. Hoffman

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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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.

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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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RFC 3855 Transporting S/MIME Objects in X.400 July 2004

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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RFC 3855 Transporting S/MIME Objects in X.400 July 2004

+-----------------------------------------------------+
| |
| 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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RFC 3855 Transporting S/MIME Objects in X.400 July 2004

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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RFC 3855 Transporting S/MIME Objects in X.400 July 2004

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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RFC 3855 Transporting S/MIME Objects in X.400 July 2004

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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RFC 3855 Transporting S/MIME Objects in X.400 July 2004

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: [email protected]

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

EMail: [email protected]

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RFC 3855 Transporting S/MIME Objects in X.400 July 2004

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
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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.

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Acknowledgement

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

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