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Versions: 01 02 RFC 1847

Network Working Group                                          Jim Galvin
INTERNET DRAFT                                               Sandy Murphy
draft-ietf-pem-sigenc-02.txt                                Steve Crocker
                                                                Ned Freed
                                                            November 1994


                     Security Multiparts for MIME:
                Multipart/Signed and Multipart/Encrypted



Status of this Memo

This document is an Internet Draft.  Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas, and
its Working Groups.  Note that other groups may also distribute working
documents as Internet Drafts.

Internet Drafts are valid for a maximum of six months and may be
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them other than as ``work in progress''.

To learn the current status of any Internet Draft, please check the
1id-abstracts.txt listing contained in one of the Internet Drafts Shadow
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Coast), munnari.oz.au (Pacific Rim), or nic.nordu.net (Europe).

Abstract

This document defines two new content types for specifying the
application of security services to MIME message bodies.  MIME, an
acronym for "Multipurpose Internet Mail Extensions", defines the format
of the contents of Internet mail messages and provides for multi-part
textual and non-textual message bodies.  The new content types are
subtypes of multipart: signed and encrypted.  Each will contain two body
parts: one for the protected data and one for the control information
necessary to remove the protection.  The type and contents of the
control information body parts are determined by the value of the
protocol parameter of the enclosing multipart/signed or
multipart/encrypted content type, which is required to be present.








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1.  Introduction

An Internet electronic mail message consists of two parts: the headers
and the body.  The headers form a collection of field/value pairs
structured according to RFC822 [1], whilst the body, if structured, is
defined according to MIME [2].  The basic MIME specification does not
provide specific security protection.

This document defines a framework whereby security protection provided
by other protocols may be used with MIME in a complementary fashion.
The resulting combined service provides security for single-part and
multi-part textual and non-textual messages.

The framework is provided by defining two new security subtypes of the
MIME multipart content type: signed and encrypted.  In each of the
security subtypes, there are exactly two related body parts: one for the
protected data and one for the control information.  This should allow
the framework to be used by all protocols providing signature and
encryption services.

A three-step process is specified for the origination and reception of
the multipart contents.  The details of the processing performed during
each step is specified by the protocol being used.

By registering new values for the required protocol parameter, the
framework is easily extended to accommodate a variety of protocols.

A MIME agent that includes support for this framework will be able to
parse protected body parts, even when the value of the protocol
parameter is unrecognized and the body part can not be displayed to the
user.

2.  Definition of Security Subtypes of Multipart

The multipart/signed content type specifies how to support
authentication and integrity services via digital signature.  The
control information is carried in the second of the two required body
parts.

The multipart/encrypted content type specifies how to support
confidentiality via encryption.  The control information is carried in
the first of the two required body parts.








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2.1.  Definition of Multipart/Signed

(1)  MIME type name: multipart

(2)  MIME subtype name: signed

(3)  Required parameters: boundary, protocol, and micalg

(4)  Optional parameters: none

(5)  Encoding considerations: 7bit, 8bit, or binary depending on
     transport requirements and encoding of the digitally signed data

(6)  Security considerations: Must be treated as opaque while in transit

The multipart/signed content type contains exactly two body parts.  The
first body part is the body part over which the digital signature was
created, including its content type label.  The second body part
contains the control information necessary to verify the digital
signature.  The first body part may contain any valid MIME content type,
labelled accordingly.  The second body part is labelled according to the
value of the protocol parameter.

The attribute token for the protocol parameter is "protocol", i.e.,

    parameter := "protocol" "=" value


The value token is comprised of the type and sub-type tokens of the
Content-Type: header of the second body part, i.e.,

    value := type "/" subtype

where the type and subtype tokens are defined by the MIME [2]
specification.  The semantics of the protocol parameter are defined
according to its value.

The attribute token for the micalg parameter is "micalg", i.e.,

    parameter := "micalg" "=" value


The Message Integrity Check (MIC) is the name given to the quantity
computed over the body part with a message digest or hash function, in
support of the digital signature service.  Valid value tokens are





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defined by the specification for the value of the protocol parameter.
The value may be a comma (",") separated list of tokens, indicating the
presence of multiple MIC algorithms.  As a result, the comma (",")
character is explicitly excluded from the list of characters that may be
included in a token used as a value of the micalg parameter.  If
multiple MIC algorithms are specified, the purpose and use of the
multiple algorithms is defined by the protocol.  If the MIC algorithm is
also specified in the control information and the value there does not
agree with the value in this parameter, it must be treated as an error.

    NOTE: The presence of the micalg parameter on the
    multipart/signed content type header is explicitly intended to
    support one-pass processing.  MIME implementations may locate
    the second body part by inputting the first body part and
    computing the specified MIC values until the boundary
    identifying the second body part is found.


The entire multipart/signed body part must be treated as opaque while it
is in transit from an originator to a recipient.  Intermediate message
transfer agents must not alter the content of a multipart/signed in any
way, including, but not limited to, changing the content transfer
encoding of the body part or any of its encapsulated body parts.

When creating a multipart/signed body part, the following sequence of
steps describes the processing necessary.  It must be emphasized that
these steps are descriptive, not prescriptive, and in no way impose
restrictions or requirements on implementations of this specification.

(1)  The content of the body part to be protected is prepared according
     to a local convention.  The content is then transformed into a MIME
     body part, including an appropriate set of headers.

(2)  The body part (content and headers) to be digitally signed is
     prepared for signature according to the value of the protocol
     parameter.  The MIME headers of the signed body part are included
     in the signature to protect the integrity of the MIME labelling of
     the data that is signed.

(3)  The prepared body part is made available to the signature creation
     process according to a local convention.  The signature creation
     process must make available to a MIME implementation two data
     streams: the control information necessary to verify the signature,
     which the MIME implementation will place in the second body part,
     and the digitally signed body part, which the MIME implementation





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     will use as the first body part.

When receiving a multipart/signed body part, the following sequence of
steps describes the processing necessary.  It must be emphasized that
these steps are descriptive, not prescriptive, and in no way impose
restrictions or requirements on implementations of this specification.

(1)  The first body part and the control information in the second body
     part must be prepared for the signature verification process
     according to the value of the protocol parameter.

(2)  The prepared body parts must be made available to the signature
     verification process according to a local convention.  The
     signature verification process must make available to the MIME
     implementation the result of the signature verification and the
     body part that was digitally signed.

         NOTE: The result of the signature verification is likely to
         include a testament of the success or failure of the
         verification.  Also, in the usual case, the body part
         returned after signature verification will be the same as
         the body part that was received.  We do not insist that
         this be the case to allow for protocols that may modify the
         body part during the signature processing.


(3)  The result of the signature verification process is made available
     to the user and the MIME implementation continues processing with
     the verified body part, i.e., the body part returned by the
     signature verification process.

2.2.  Definition of Multipart/Encrypted

(1)  MIME type name: multipart

(2)  MIME subtype name: encrypted

(3)  Required parameters: boundary, protocol

(4)  Optional parameters: none

(5)  Encoding considerations: 7bit, 8bit, or binary depending on
     transport requirements and encoding of the encrypted data







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(6)  Security considerations: none

The multipart/encrypted content type contains exactly two body parts.
The first body part contains the control information necessary to
decrypt the data in the second body part and is labelled according to
the value of the protocol parameter.  The second body part contains the
data which was encrypted and is always labelled application/octet-
stream.

The attribute token for the protocol parameter is "protocol", i.e.,

    parameter := "protocol" "=" value


The value token is comprised of the type and sub-type tokens of the
Content-Type: header of the first body part, i.e.,

    value := type "/" subtype

where the type and subtype tokens are defined by the MIME [2]
specification.  The semantics of the protocol parameter are defined
according to its value.

When creating a multipart/encrypted body part, the following sequence of
steps describes the processing necessary.  It must be emphasized that
these steps are descriptive, not prescriptive, and in no way impose
restrictions or requirements on implementations of this specification.

(1)  The contents of the body part to be protected is prepared according
     to a local convention.  The contents are then transformed into a
     MIME body part, including an appropriate set of headers.

(2)  The body part (content and headers) to be encrypted is prepared for
     encryption according to the value of the protocol parameter.  The
     MIME headers of the encrypted body part are included in the
     encryption to protect from disclosure the MIME labelling of the
     data that is encrypted.

(3)  The prepared body part is made available to the encryption process
     according to a local convention.  The encryption process must make
     available to a MIME implementation two data streams: the control
     information necessary to decrypt the body part, which the MIME
     implementation will place in the first body part, and the encrypted
     body part, which the MIME implementation will place in the second
     body part and label application/octet-stream.  Thus, when used in a





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     multipart/encrypted, the application/octet-stream data is comprised
     of a nested MIME body part.

When receiving a multipart/encrypted body part, the following sequence
of steps describes the processing necessary.  It must be emphasized that
these steps are descriptive, not prescriptive, and in no way impose
restrictions or requirements on implementations of this specification.

(1)  The second body part and the control information in the first body
     part must be prepared for the decryption process according to the
     value of the protocol parameter.

(2)  The prepared body parts must be made available to the decryption
     process according to a local convention.  The decryption process
     must make available to the MIME implementation the result of the
     decryption and the decrypted form of the encrypted body part.

         NOTE: The result of the decryption process is likely to
         include a testament of the success or failure of the
         decryption.  Failure may be due to an inability to locate
         the proper decryption key or the proper recipient field,
         etc.


(3)  The result of the decryption process is made available to the user
     and the MIME implementation continues processing with the decrypted
     body part, i.e., the body part returned by the decryption process.

         NOTE: A MIME implementation will not be able to display the
         received form of the second body part because the
         application of encryption will transform the body part.
         This transformation will not be described in the MIME
         headers (Content-Type: and Content-Transfer-Encoding:) but,
         rather, will be described in the content of the first body
         part.  Therefore, an implementation should wait until the
         encryption has been removed before attempting to display
         the content.


3.  Definition of Control Information Content Types

This document requires the definition of two additional content types,
one each for when a digital signature is applied to a body part and for
when encryption is applied to a body part.  These content types are to
be used in multipart/signed and multipart/encrypted body parts,





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respectively, as indicated above.  Their definition must be included in
the specification defining the use of the security protocol indicated in
the protocol parameter.

4.  Definition of Key Management Content Types

It is recognized that many cryptographic techniques require a mechanism
for distributing the cryptographic keys used in support of their
services.  As a result, many security protocols will define two
additional body parts: one for the purpose of requesting cryptographic
keys and one for the purpose of distributing cryptographic keys.

5.  Security Considerations

This specification describes an enhancement to MIME to support signed
and encrypted body parts.  In that context this entire document is about
security.

6.  Acknowledgements

David H. Crocker suggested the use of a multipart structure for MIME-PEM
interaction.

7.  References

[1]  David H. Crocker.  Standard for the Format of ARPA Internet Text
     Messages.  RFC 822, University of Delaware, August 1982.

[2]  Nathaniel Borenstein and Ned Freed. MIME (Multipurpose Internet
     Mail Extension) Part One: Mechanisms for Specifying and Describing
     the Format of Internet Message Bodies.  RFC 1521, Bellcore and
     Innosoft, September 1993.  Obsoletes RFC 1341.

8.  Authors' Addresses

Jim Galvin
email:  galvin@tis.com

Sandy Murphy
email:  sandy@tis.com

Steve Crocker
email:  crocker@tis.com

Trusted Information Systems





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3060 Washington Road
Glenwood, MD  21738
Tel:    +1 301 854 6889
FAX:    +1 301 854 5363

Ned Freed
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790
Tel:    +1 818 919 3600
FAX:    +1 818 919 3614
email:  ned@innosoft.com






































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Table of Contents


  Status of this Memo .............................................    1
  Abstract ........................................................    1
1  Introduction ...................................................    2
2  Definition of Security Subtypes of Multipart ...................    2
2.1  Definition of Multipart/Signed ...............................    3
2.2  Definition of Multipart/Encrypted ............................    5
3  Definition of Control Information Content Types ................    7
4  Definition of Key Management Content Types .....................    8
5  Security Considerations ........................................    8
6  Acknowledgements ...............................................    8
7  References .....................................................    8
8  Authors' Addresses .............................................    8



































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