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Internet Draft                                       Paul  Hoffman, IMC
draft-ietf-smime-x400wrap-03.txt                    Chris Bonatti, IECA
July 19, 2001                                         Anders Eggen, FFI
Expires in six months

                    Securing X.400 Content with S/MIME


Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
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      The list of current Internet-Drafts can be accessed at
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Abstract

This document describes a protocol for adding cryptographic signature
and encryption services to X.400 content.


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 Message
Specification [MSG] except that it is tailored to the requirements of
X.400 content rather than Multipurpose Internet Mail Extensions (MIME).

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.

1.2 Terminology

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

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


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 [SHA1].

2.2 SignatureAlgorithmIdentifier

Sending and receiving agents MUST support id-dsa defined in [DSS]. The
algorithm parameters MUST be absent (not encoded as NULL).

Receiving agents MAY support rsaEncryption, defined in [PKCS-1].

Sending agents MAY support rsaEncryption. Outgoing messages are signed
with a user's private key.

2.3 KeyEncryptionAlgorithmIdentifier

Sending and receiving agents MUST support rsaEncryption. Incoming
encrypted messages contain symmetric keys which are to be decrypted with
a user's private key.

Sending and receiving agents MAY support Diffie-Hellman defined in [DH].

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.

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 it
does 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.


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.

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 draft 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 protected X.400 content MUST be placed in the SignedData
encapContentInfo eContent field. The eContent field MUST either (a)
contain the contentType OID and MUST NOT be MIME-wrapped or (b) contain
the id-data OID and a MIME-wrapped content. The object identifier for
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. The resulting CMS object MAY optionally be
wrapped in a MIME encoding.

Note that if SMTP [SMTP] 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.

3.2.1 MIME Wrapping to Dynamically Support 7-bit Transport

The signedData object MAY optionally be wrapped in MIME to dynamically
support 7-bit transport. In this case the application/pkcs7-mime type as
defined in S/MIME Version 3 Message Specification [MSG] SHOULD be used
with the following parameters:

Content-Type: application/pkcs7-mime; smime-type=signed-data
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.

The protected X.400 content 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. The resulting CMS object MAY
optionally be wrapped in a MIME encoding.

Note that if SMTP is used to transport the resulting enveloped-only
message then the optional MIME encoding SHOULD be used. If other binary
transport (e.g., X.400) 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 to
dynamically support 7-bit transport. In this case, the
application/pkcs7-mime type as defined in S/MIME Version 3 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].

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 protected ASN.1 encoded X.400 content 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".

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 is only for signed).
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.


4. Use of Certificates

4.1 Certificate Enrollment

S/MIME v3 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 [CERT3].

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.


5. Security Considerations

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


A. References

[CERT3] Ramsdell, B., Editor, "S/MIME Version 3 Certificate
Handling", RFC 2632, June 1999.

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

[DH] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC 2631,
June 1999.

[DSS] NIST FIPS PUB 186, "Digital Signature Standard", 18 May 1994.

[ESS] Hoffman, P., Editor "Enhanced Security Services for S/MIME",
RFC 2634, 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", BCP14, RFC 2119, March 1997.

[PKCS-1] Kaliski, B., "PKCS #1: RSA Encryption Version 2.0", RFC
2437, October 1998.

[SHA1] NIST FIPS PUB 180-1, "Secure Hash Standard," National
Institute of Standards and Technology, U.S. Department of Commerce, 31
May 1994.

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

[TRANSPORT] Hoffman, P. and Bonatti, C., "Transporting S/MIME Objects in
X.400", work in progress (will progress with this document).

[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. Editor's Address

Paul Hoffman
Internet Mail Consortium
127 Segre Place
Santa Cruz, CA  95060  USA
phoffman@imc.org

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

Anders Eggen
Forsvarets Forskningsinstitutt
Postboks 25
2027 Kjeller, Norway
anders.eggen@ffi.no

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