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 © 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).

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].

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.

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.

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.

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.

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.

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.

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].

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".

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).

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 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.
   
   [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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