RFC1847 - Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted

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　　Network Working Group J. Galvin
Request For Comments: 1847 S. Murphy
Category: Standards Track Trusted Information Systems
S. Crocker
CyberCash, Inc.
N. Freed
Innosoft International, Inc.
October 1995

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

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.

Abstract

This document defines a framework within which security services may
be applied to MIME body parts. 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.

Table of Contents

1. Introduction .............................................. 2
2. Definition of Security Subtypes of Multipart .............. 2
2.1 Definition of Multipart/Signed .......................... 3
2.2 Definition of Multipart/Encrypted ....................... 6
3. Definition of Control Information Content Types ........... 9
4. Definition of Key Management Content Types ................ 9
5. Security Considerations ................................... 10
6. Acknowledgements .......................................... 10
7. References ................................................ 10
8. Authors' Addresses ........................................ 11

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 STD 11, 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. By itself, it does not specify security protection. A MIME
agent must include support for both the framework defined here and a
mechanism to interact with a security protocol defined in a separate
document. 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. 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.
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
recognize a security multipart body part and to identify its
protected data and control information body parts. If the value of
the protocol parameter is unrecognized the MIME agent will not be
able to process the security multipart. However, a MIME agent may
continue to process any other body parts that may be present.

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.

A three-step process is described for the origination and reception
of the multipart/signed and multipart/encrypted contents. The
details of the processing performed during each step is left to be
specified by the security protocol being used.

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) 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 MIME headers. 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, labeled accordingly. The second body part is labeled 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
defined by the specification for the value of the protocol parameter.
The value may be a comma (",") separated list of tokens, indicating
the use 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 contents of the multipart/signed container 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.

The signature in a multipart/signed only applies to the material that
is actually within the multipart/signed object. In particular, it
does not apply to any enclosing message material, nor does it apply
to entities that are referenced (e.g. via a MIME message/external-
body) by rather than included in the signed content.

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 in canonical MIME format,
including an appropriate set of MIME headers.

In addition, if the multipart/signed object is EVER to be
transferred over the standard Internet SMTP infrastructure, the
resulting MIME body is constrained to 7 bits -- that is, the use
of material requiring either 8bit or binary
content-transfer-encoding is NOT allowed. Such 8bit or binary
material MUST be encoded using either the quoted-printable or
base64 encodings.

This requirement exists because it is not generally possible,
given the current characteristics of Internet SMTP, for a
message originator to guarantee that a message will travel only
along paths capable of carrying 8bit or binary material.

SMTP clients normally have the option of either converting the
message to eliminate the use of 8bit or binary encoding or
returning a nondelivery notification to the originator.
However, conversion is not viable in the case of signed objects
since conversion would necessarily invalidate the signature.
This leaves a nondelivery notification as the only available
option, which is not acceptable.

(2) The body part (headers and content) 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
labeling 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 label according to the value of the protocol
parameter, and the digitally signed body part, which the MIME
implementation will use as the first body part.

When receiving a multipart/signed body part, the following sequence
of steps describes the processing necessary to verify the signature
or signatures. 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.

The following example is an illustration of a multipart/signed body
part. It is necessarily incomplete since the control information is
defined by the security protocol, which must be specified in a
separate document.

Content-Type: multipart/signed; protocol="TYPE/STYPE";
micalg="MICALG"; boundary="Signed Boundary"

--Signed Boundary
Content-Type: text/plain; charset="us-ascii"

This is some text to be signed although it could be
any type of data, labeled accordingly, of course.

--Signed Boundary
Content-Type: TYPE/STYPE

CONTROL INFORMATION for protocol "TYPE/STYPE" would be here

--Signed Boundary--

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) 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 labeled according to
the value of the protocol parameter. The second body part contains
the data which was encrypted and is always labeled
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 in canonical MIME format, including an appropriate
set of MIME headers.

(2) The body part (headers and content) 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 labeling 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 label
according to the value of the protocol parameter, 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 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 to decrypt the
enclosed data. 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. Implementors should note that the data, if any, of a
failed decryption process is pretty much guaranteed to be
garbage.

(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.

The following example is an illustration of a multipart/encrypted
body part. It is necessarily incomplete since the control
information is defined by the security protocol, which must be
specified in a separate document.

Content-Type: multipart/encrypted; protocol="TYPE/STYPE";
boundary="Encrypted Boundary"

--Encrypted Boundary
Content-Type: TYPE/STYPE

CONTROL INFORMATION for protocol "TYPE/STYPE" would be here

--Encrypted Boundary
Content-Type: application/octet-stream

Content-Type: text/plain; charset="us-ascii"

All of this indented text, including the indented headers,
would be unreadable since it would have been encrypted by
the protocol "TYPE/STYPE". Also, this encrypted data could
be any type of data, labeled accordingly, of course.

--Encrypted Boundary--

3. Definition of Control Information Content Types

This document defines a framework within which security services may
be applied to MIME body parts. A minimal MIME implementation will be
able to recognize multipart/signed and multipart/encrypted body parts
and be able to identify the protected data and control information
body parts within them.

Complete support for security services requires the MIME agent to
recognize the value of the protocol parameter and to continue
processing based on its value. The value of the protocol parameter
is the same value used to label the content type of the control
information.

The value of the protocol parameter and the resulting processing
required must be specified in the document defining the security
protocol used. That document must also precisely specify the
contents of the control information body part.

4. Definition of Key Management Content Types

This specification recognizes that the complete specification of a
MIME-based security protocol must include a mechanism for
distributing the cryptographic material used in support of the
security services. For example, a digital signature service
implemented with asymmetric cryptography requires that a signer's
public key be available to the signee.

One possible mechanism for distributing cryptographic material is to
define two additional body parts: one for the purpose of requesting
cryptographic information and one for the purpose of returning the
cryptographic information requested. The specification of a security
protocol may include a definition of two such body parts or it may
specify an alternate mechanism for the distribution of cryptographic
material.

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 the
MIME and PEM interaction.

7. References

[1] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC822, University of Delaware, August 1982.

[2] Borenstein, N., and N. Freed, "MIME (Multipurpose Internet Mail
Extension) Part One: Mechanisms for Specifying and Describing the
Format of Internet Message Bodies", RFC1521, Bellcore and
Innosoft, September 1993.

8. Authors' Addresses

Jim Galvin
Trusted Information Systems
3060 Washington Road
Glenwood, MD 21738

Phone: +1 301 854 6889
Fax: +1 301 854 5363
EMail: galvin@tis.com

Sandy Murphy
Trusted Information Systems
3060 Washington Road
Glenwood, MD 21738

Phone: +1 301 854 6889
Fax: +1 301 854 5363
EMail: sandy@tis.com

Steve Crocker
CyberCash, Inc.
2086 Hunters Crest Way
Vienna, VA 22181

Phone:: +1 703 620 1222
Fax: +1 703 391 2651
EMail: crocker@cybercash.com

Ned Freed
Innosoft International, Inc.
1050 East Garvey Avenue South
West Covina, CA 91790

Phone: +1 818 919 3600
Fax: +1 818 919 3614

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