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Internet Draft                              Steve Dusse,
draft-dusse-smime-msg-03.txt                RSA Data Security
September 5, 1997                           Paul Hoffman,
Expires in six months                       Internet Mail Consortium
                                            Blake Ramsdell,
                                            Deming Internet Security
                                            Laurence Lundblade,
                                            Qualcomm
                                            Lisa Repka,
                                            Netscape

                 S/MIME Message Specification

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 draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite 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 the Internet-Drafts Shadow
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munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).


1. Introduction

S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a
standard way to send and receive secure MIME data. Based on the
popular Internet MIME standard, S/MIME provides the following
cryptographic security services for electronic messaging applications:
authentication, message integrity and non-repudiation of origin (using
digital signatures) and privacy and data security (using encryption).

S/MIME can be used by traditional mail user agents (MUAs) to add
cryptographic security services to mail that is sent, and to interpret
cryptographic security services in mail that is received. However,
S/MIME is not restricted to mail; it can be used with any transport
mechanism that transports MIME data, such as HTTP. As such, S/MIME
takes advantage of the object-based features of MIME and allows secure
messages to be exchanged in mixed-transport systems.

Further, S/MIME can be used in automated message transfer agents that
use cryptographic security services that do not require any human
intervention, such as the signing of software-generated documents and
the encryption of FAX messages sent over the Internet.

1.1 Specification Overview

This document describes a protocol for adding cryptographic signature
and encryption services to MIME data. The MIME standard
[MIME-SPEC] provides a general structure for the content type of
Internet messages and allows extensions for new content type
applications.

This draft defines how to create a MIME body part that has been
cryptographically enhanced according to PKCS #7 [PKCS-7]. This draft
also defines the application/pkcs7-mime MIME type that can be used to
transport those body parts. This draft also defines how to create
certification requests that conform to PKCS #10 [PKCS-10], and the
application/pkcs10 MIME type for transporting those requests.

This draft also discusses how to use the multipart/signed MIME type
defined in [MIME-SECURE] to transport S/MIME signed messages. This
draft also defines the application/pkcs7-signature MIME type, which is
also used to transport S/MIME signed messages. This specification is
compatible with PKCS #7 in that it uses the data types defined by PKCS
#7.

In order to create S/MIME messages, an agent has to follow
specifications in this draft, as well as some of the specifications
listed in the following pre-standards works:
 - "PKCS #1: RSA Encryption Standard", [PKCS-1].
 - "PKCS #7: Cryptographic Message Syntax Standard", [PKCS-7]
 - "PKCS #10: Certification Request Syntax Standard", [PKCS-10].

Throughout this draft, 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.

The separation for requirements on receiving agents and sending agents
also derives from the likelihood that there will be S/MIME systems
that involve software other than traditional Internet mail clients.
S/MIME can be used with any system that transports MIME data. An
automated process that sends an encrypted message might not be able to
receive an encrypted message at all, for example. Thus, the
requirements and recommendations for the two types of agents are
listed separately when appropriate.

1.2 Terminology

Throughout this draft, the terms MUST, MUST NOT, SHOULD, and SHOULD
NOT are used in capital letters. This conforms to the definitions in
[MUSTSHOULD].

1.3 Definitions

For the purposes of this draft, the following definitions apply.

ASN.1: Abstract Syntax Notation One, as defined in CCITT X.208.

BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209.

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 CCITT
X.509, Section 8.7.

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.

1.4 Compatibility with Pre-standards S/MIME

Appendix C contains important information about how standards-based
S/MIME agents should act in order to have the greatest
interoperability with pre-standards S/MIME.

1.5 Discussion of This Draft

This draft is being discussed on the "ietf-smime" mailing list.
To subscribe, send a message to:
     ietf-smime-request@imc.org
with the single word
     subscribe
in the body of the message. There is a Web site for the mailing list
at <http://www.imc.org/ietf-smime/>.


2. PKCS #7 Options

The PKCS #7 message format 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 S/MIME implementations.

2.1 DigestAlgorithmIdentifier

Receiving agents MUST support SHA-1 [SHA1] and MD5 [MD5].

Sending agents SHOULD use SHA-1.

2.2 DigestEncryptionAlgorithmIdentifier

Receiving agents MUST support rsaEncryption, defined in [PKCS-1].
Receiving agents MUST support verification of signatures using RSA
public key sizes from 512 bits to 1024 bits.

Sending agents MUST support rsaEncryption. Outgoing messages are
signed with a user's private key. The size of the private key is
determined during key generation.

2.3 KeyEncryptionAlgorithmIdentifier

Receiving agents MUST support rsaEncryption. Incoming encrypted
messages contain symmetric keys which are to be decrypted with a
user's private key. The size of the private key is determined during
key generation.

Sending agents MUST support rsaEncryption. Sending agents MUST support
encryption of symmetric keys with RSA public keys at key sizes from
512 bits to 1024 bits.

2.4 General Syntax

The PKCS #7 defines six distinct content types: "data", "signedData",
"envelopedData", "signedAndEnvelopedData", "digestedData", and
"encryptedData". Receiving agents MUST support the "data",
"signedData" and "envelopedData" content types. Sending agents may or
may not send out any of the content types, depending on the services
that the agent supports.

2.4.1 Data Content Type

Sending agents MUST use the "data" content type as the content within
other content types to indicate the message content which has had
security services applied to it.

2.4.2 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.

2.4.3 EnvelopedData Content Type

This content type is used 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 unauthenticated and
authenticated attributes to be included along with a signature.

Receiving agents MUST be able to handle and display zero or one
instance of each of the signed attributes described in this section.

Sending agents SHOULD be able to generate one instance of each of the
signed attributes described in this section, and SHOULD include these
attributes in each signed message sent.

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.

2.5.1 Signing-Time Attribute

The signing-time attribute is used to convey the time that a message
was signed. Until there are trusted timestamping services, the time of
signing will most likely be created by a message originator and
therefore is only as trustworthy as the originator. The syntax of the
signing-time attribute is:

SigningTime ::= UTCTime

2.5.2 SMIMECapabilities Attribute

The SMIMECapabilities attribute includes signature algorithms (such as
"md5WithRSAEncryption"), symmetric algorithms (such as "DES-CBC"), and
key encipherment algorithms (such as "rsaEncryption"). It also
includes a non-algorithm capability which is the preference
for signedData. The SMIMECapabilities were designed to be flexible and
extensible so that, in the future, a means of identifying other
capabilities and preferences such as certificates can be added in a
way that will not cause current clients to break.

The semantics of the SMIMECapabilites attribute specify a partial list
as to what the client announcing the SMIMECapabilites can support. A
client does not have to list every capability it supports, and
probably should not list all its capabilities so that the capabilities
list doesn't get too long. In an SMIMECapabilities attribute, the OIDs
are listed in order of their preference, but SHOULD be logically
separated along the lines of their categories (signature algorithms,
symmetric algorithms, key encipherment algorithms, etc.)

The structure of the SMIMECapabilities attribute is to facilitate
simple table lookups and binary comparisons in order to determine
matches. For instance, the DER-encoding for the SMIMECapability for
DES EDE3 CBC MUST be identically encoded regardless of the
implementation.

In the case of symmetric algorithms, the associated parameters for the
OID MUST specify all of the parameters necessary to differentiate
between two instances of the same algorithm. For instance, the number
of rounds and block size for RC5 must be specified in addition to the
key length.

There is a list of OIDs (the registered SMIMECapabilities list) that
is centrally maintained and is separate from this draft. Upon moving
this draft to standards track, the IANA will maintain the list of
OIDs. Until this draft becomes a draft standard, the list of OIDs is
maintained by the Internet Mail Consortium at
<http://www.imc.org/ietf-smime/oids.html>.

The OIDs that correspond to algorithms SHOULD use the same OID as the
actual algorithm, except in the case where the algorithm usage is
ambiguous from the OID. For instance, in an earlier draft,
rsaEncryption was ambiguous because it could refer to either a
signature algorithm or a key encipherment algorithm. In the event that
an OID is ambiguous, it needs to be arbitrated by the maintainer of
the registered SMIMECapabilities list as to which type of algorithm
will use the OID, and a new OID MUST be allocated under the
SMIMECapabilities OID to satisfy the other use of the OID.

The registered SMIMECapabilities list specifies the parameters for
OIDs that need them, most notably key lengths in the case of
variable-length symmetric ciphers. In the event that there are no
differentiating parameters for a particular OID, the parameters MUST
be omitted, and MUST NOT be encoded as NULL.

Additional values for the SMIMECapabilities attribute may be defined
in the future. Receiving agents MUST handle a SMIMECapabilities object
that has values that it does not recognize in a graceful manner.

2.6 ContentEncryptionAlgorithmIdentifier

Receiving agents MUST support decryption using the RC2 algorithm [RC2]
at a key size of 40 bits, hereinafter called "RC2/40". Receiving
agents SHOULD support decryption using DES EDE3 CBC, hereinafter
called "tripleDES" [3DES] [DES].

Sending agents SHOULD support encryption with RC2/40 and tripleDES.

2.6.1 Deciding Which Encryption Method To Use

When a sending agent creates an encrypted message, it has to decide
which type of encryption to use. The decision process involves using
information garnered from the capabilities lists included in messages
received from the recipient, as well as out-of-band information such
as private agreements, user preferences, legal restrictions, and so
on.

Section 2.5 defines a method by which a sending agent can optionally
announce, among other things, its decrypting capabilities in its order
of preference. The following method for processing and remembering the
encryption capabilities attribute in incoming signed messages SHOULD
be used.

 - If the receiving agent has not yet created a list of capabilities
   for the sender's public key, then, after verifying the signature
   on the incoming message and checking the timestamp, the receiving
   agent SHOULD create a new list containing at least the signing
   time and the symmetric capabilities.

 - If such a list already exists, the receiving agent SHOULD verify
   that the signing time in the incoming message is greater than
   the signing time stored in the list and that the signature is
   valid. If so, the receiving agent SHOULD update both the signing
   time and capabilities in the list. Values of the signing time that
   lie far in the future (that is, a greater discrepancy than any
   reasonable clock skew), or a capabilitie lists in messages whose
   signature could not be verified, MUST NOT be accepted.

The list of capabilities SHOULD be stored for future use in creating
messages.

Before sending a message, the sending agent MUST decide whether it is
willing to use weak encryption for the particular data in the message.
If the sending agent decides that weak encryption is unacceptable for
this data, then the sending agent MUST NOT use a weak algorithm such
as RC2/40. The decision to use or not use weak encryption overrides
any other decision in this section about which encryption algorithm to
use.

Sections 2.6.2.1 through 2.6.2.4 describe the decisions a sending
agent SHOULD use in deciding which type of encryption should be
applied to a message. These rules are ordered, so the sending agent
SHOULD make its decision in the order given.

2.6.2.1 Rule 1: Known Capabilities

If the sending agent has received a set of capabilities from the
recipient for the message the agent is about to encrypt, then the
sending agent SHOULD use that information by selecting the first
capability in the list (that is, the capability most preferred by the
intended recipient) for which the sending agent knows how to encrypt.
The sending agent SHOULD use one of the capabilities in the list if
the agent reasonably expects the recipient to be able to decrypt the
message.

2.6.2.2 Rule 2: Unknown Capabilities, Known Use of Encryption

If:
 - the sending agent has no knowledge of the encryption capabilities
   of the recipient,
 - and the sending agent has received at least one message from the
   recipient,
 - and the last encrypted message received from the recipient had a
   trusted signature on it,
then the outgoing message SHOULD use the same encryption algorithm as
was used on the last signed and encrypted message received from the
recipient.

2.6.2.3 Rule 3: Unknown Capabilities, Risk of Failed Decryption

If:
 - the sending agent has no knowledge of the encryption capabilities
   of the recipient,
 - and the sending agent is willing to risk that the recipient may
   not be able to decrypt the message,
then the sending agent SHOULD use tripleDES.

2.6.2.4 Rule 4: Unknown Capabilities, No Risk of Failed Decryption

If:
 - the sending agent has no knowledge of the encryption capabilities
   of the recipient,
 - and the sending agent is not willing to risk that the recipient
   may not be able to decrypt the message,
then the sending agent MUST use RC2/40.

2.6.3 Choosing Weak Encryption

Like all algorithms that use 40 bit keys, RC2/40 is considered by many
to be weak encryption. A sending agent that is controlled by a human
SHOULD allow a human sender to determine the risks of sending data
using RC2/40 or a similarly weak encryption algorithm before sending
the data, and possibly allow the human to use a stronger encryption
method such as tripleDES.


3. Creating S/MIME Messages

This section describes the S/MIME message formats and how they are
created. S/MIME messages are a combination of MIME bodies and PKCS
objects. Several MIME types as well as several PKCS objects are used.
The data to be secured is always a canonical MIME entity. The MIME
entity and other data, such as certificates and algorithm identifiers,
are given to PKCS processing facilities which produces a PKCS object.
The PKCS object is then finally wrapped in MIME.

S/MIME provides one format for enveloped-only data, several formats
for signed-only data, and several formats for signed and enveloped
data. Several formats are required to accommodate several environments,
in particular for signed messages. The criteria for choosing among
these formats are also described.

The reader of this section is expected to understand MIME as described
in [MIME-SPEC], [MIME-SECURE] and [APP-MIME].

3.1 Preparing the MIME Entity for Signing or Enveloping

S/MIME is used to secure MIME entities. A MIME entity may be a
sub-part, sub-parts of a message, or the whole message with all its
sub-parts. A MIME entity that is the whole message includes only the
MIME headers and MIME body, and does not include the RFC-822 headers.
Note that S/MIME can also be used to secure MIME entities used in
applications other than Internet mail.

The MIME entity that is secured and described in this section can be
thought of as the "inside" MIME entity. That is, it is the "innermost"
object in what is possibly a larger MIME message. Processing "outside"
MIME entities into PKCS #7 objects is described in Section 3.2, 3.4
and elsewhere.

The procedure for preparing a MIME entity is given in [MIME-SPEC]. The
same procedure is used here with some additional restrictions when
signing. Description of the procedures from [MIME-SPEC] are repeated
here, but the reader should refer to that document for the exact
procedure. This section also describes additional requirements.

A single procedure is used for creating MIME entities that are to be
signed, enveloped, or both signed and enveloped. Some additional steps
are recommended to defend against known corruptions that can occur
during mail transport that are of particular importance for
clear-signing using the multipart/signed format. It is recommended that
these additional steps be performed on enveloped messages, or signed
and enveloped messages in order that the message can be forwarded to
any environment without modification.

These steps are descriptive rather than prescriptive. The implementor
is free to use any procedure as long as the result is the same.

  Step 1. The MIME entity is prepared according to the local
          conventions

  Step 2. The leaf parts of the MIME entity are converted to canonical
          form

  Step 3. Appropriate transfer encoding is applied to the leaves of
          the MIME entity

When an S/MIME message is received, the security services on the
message are removed, and the result is the MIME entity. That MIME
entity is typically passed to a MIME-capable user agent where, it is
further decoded and presented to the user or receiving application.

3.1.1 Canonicalization

Each MIME entity MUST be converted to a canonical form that is uniquely
and unambiguously representable in the environment where the signature
is created and the environment where the signature will be verified.
MIME entities MUST be canonicalized for enveloping as well as signing.

The exact details of canonicalization depend on the actual MIME type
and subtype of an entity, and are not described here. Instead, the
standard for the particular MIME type should be consulted. For
example, canonicalization of type text/plain is different from
canonicalization of audio/basic. Other than text types, most types
have only one representation regardless of computing platform or
environment which can be considered their canonical representation. In
general, canonicalization will be performed by the sending agent
rather than the S/MIME implementation.

The most common and important canonicalization is for text, which is
often represented differently in different environments. MIME entities
of major type "text" must have both their line endings and character
set canonicalized. The line ending must be the pair of characters
<CR><LF>, and the character set should be a registered character set.
The details of the canonicalization are specified in [MIME-SPEC].

Note that some character sets such as ISO-2022 have multiple
representations for the same characters. When preparing such text for
signing, the canonical representation specified for the character set
MUST be used.

3.1.2 Transfer Encoding

When generating any of the secured MIME entities below, except the
signing using the multipart/signed format, no transfer encoding at all
is required.  S/MIME implementations MUST be able to deal with binary
MIME objects. If no Content-Transfer-Encoding header is present, the
transfer encoding should be considered binary.

S/MIME implementations SHOULD however use transfer encoding described
in section 3.1.3 for all MIME entities they secure. The reason for
securing only 7-bit MIME entities, even for enveloped data that are
not exposed to the transport, is that it allows the MIME entity to be
handled in any environment without changing it. For example, a trusted
gateway might remove the envelope, but not the signature, of a
message, and then forward the signed message on to the end recipient
so that they can verify the signatures directly. If the transport
internal to the site is not 8-bit clean, such as on a wide-area
network with a single mail gateway, verifying the signature will not
be possible unless the original MIME entity was only 7-bit data.

3.1.3 Transfer Encoding for Signing Using multipart/signed

If a multipart/signed entity is EVER to be transmitted over the standard
Internet SMTP infrastructure or other transport that is constrained to
7-bit text, it MUST have transfer encoding applied so that it is
represented as 7-bit text. MIME entities that are 7-bit data already
need no transfer encoding. Entities such as 8-bit text and binary data
can be encoded with quoted-printable or base-64 transfer encoding.

The primary reason for the 7-bit requirement is that the Internet mail
transport infrastructure cannot guarantee transport of 8-bit or binary
data. Even though many segments of the transport infrastructure now
handle 8-bit and even binary data, it is sometimes not possible to
know whether the transport path is 8-bit clear. If a mail message with
8-bit data were to encounter a message transfer agent that can not
transmit 8-bit or binary data, the agent has three options, none of
which are acceptable for a clear-signed message:
 - The agent could change the transfer encoding; this would
   invalidate the signature.
 - The agent could transmit the data anyway, which would most likely
   result in the 8th bit being corrupted; this too would invalidate
   the signature.
 - The agent could return the message to the sender.

[MIME-SECURE] prohibits an agent from changing the transfer encoding
of the first part of a multipart/signed message. If a compliant agent
that can not transmit 8-bit or binary data encounters a
multipart/signed message with 8-bit or binary data in the first part,
it would have to return the message to the sender as undeliverable.

3.1.4 Sample Canonical MIME Entity

This example shows a multipart/mixed message with full transfer
encoding. This message contains a text part and an attachment. The
sample message text includes characters that are not US-ASCII and thus
must be transfer encoded. Though not shown here, the end of each line
is <CR><LF>. The line ending of the MIME headers, the text, and
transfer encoded parts, all must be <CR><LF>.

Note that this example is not of a proper S/MIME message.

    Content-Type: multipart/mixed; boundary=bar

    --bar
    Content-Type: text/plain; charset=iso-8859-1
    Content-Transfer-Encoding: quoted-printable

    A1Hola Michael!

    How do you like the new S/MIME standard?

    I agree. It's generally a good idea to encode lines that begin with
    From=20because some mail transport agents will insert a greater-
    than (>) sign, thus invalidating the signature.

    Also, in some cases it might be desirable to encode any   =20
    trailing whitespace that occurs on lines in order to ensure  =20
    that the message signature is not invalidated when passing =20
    a gateway that modifies such whitespace (like BITNET). =20

    --bar
    Content-Type: application/willy-waggle

    iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
    jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
    uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
    HOxEa44b+EI=
    =ndaj
    --bar--

3.2 The application/pkcs7-mime Type

The application/pkcs7-mime type is used to carry PKCS #7 objects of
several types including envelopedData and signedData. The details of
constructing these entities is described in subsequent sections. This
section describes the general characteristics of the
application/pkcs7-mime type.

This MIME type always carries a single PKCS #7 object. The PKCS #7
object must always be BER encoding of the ASN.1 syntax describing the
object. The contentInfo field of the carried PKCS #7 object always
contains a MIME entity that is prepared as described in section 3.1.
The contentInfo field must never be empty.

Since PKCS #7 objects are binary data, in most cases base-64 transfer
encoding is appropriate, in particular when used with SMTP transport.
The transfer encoding used depends on the transport through which the
object is to be sent, and is not a characteristic of the MIME type.

Note that this discussion refers to the transfer encoding of the
PKCS #7 object or "outside" MIME entity. It is completely distinct
from, and unrelated to, the transfer encoding of the MIME entity
secured by the PKCS #7 object, the "inside" object, which is described
in section 3.1.

[Note: the following has been added to the spec, and is not in earlier
versions. It should not cause any incompatibilities with pre-standards
S/MIME implementations, and should help receiving agents.]

Because there are several types of application/pkcs7-mime objects, a
sending agent SHOULD do as much as possible to help a receiving agent
know about the contents of the object without forcing the receiving
agent to decode the ASN.1 for the object. The MIME headers of all
application/pkcs7-mime objects SHOULD include the optional
"smime-type" parameter, as described in the following sections.

3.2.1 The name and filename Parameters

For the application/pkcs7-mime, sending agents SHOULD emit the
optional "name" parameter to the Content-Type field for compatibility
with older systems. Sending agents SHOULD also emit the optional
Content-Disposition field with the "filename" parameter. If a sending
agent emits the above parameters, the value of the parameters SHOULD
be a file name with the appropriate extension:

MIME Type                      File Extension

application/pkcs7-mime              .p7m
(signedData, envelopedData)

application/pkcs7-mime              .p7c
(degenerate signedData
"certs-only" message)

application/pkcs7-signature         .p7s

application/pkcs10                  .p10

In addition, the file name SHOULD be limited to eight characters
followed by a three letter extension. The eight character filename
base can be any distinct name; the use of the filename base "smime"
SHOULD be used to indicate that the MIME entity is associated with
S/MIME.

Including a file name serves two purposes. It facilitates easier use
of S/MIME objects as files on disk. It also can convey type
information across gateways. When a MIME entity of type
application/pkcs7-mime (for example) arrives at a gateway that has no
special knowledge of S/MIME, it will default the entity's MIME type to
application/octet-stream and treat it as a generic attachment, thus
losing the type information. However, the suggested filename for an
attachment is often carried across a gateway. This often allows the
receiving systems to determine the appropriate application to hand the
attachment off to, in this case a stand-alone S/MIME processing
application. Note that this mechanism is provided as a convenience for
implementations in certain environments. A proper S/MIME
implementation MUST use the MIME types and MUST not rely on the file
extensions.

3.3 Creating an Enveloped-only Message

This section describes the format for enveloping a MIME entity without
signing it.

  Step 1. The MIME entity to be enveloped is prepared according to
          section 3.1.

  Step 2. The MIME entity and other required data is processed into a
          PKCS #7 object of type envelopedData.

  Step 3. The PKCS #7 object is inserted into an application/pkcs7-mime
          MIME entity.

The smime-type parameter for enveloped-only messages is
"enveloped-data". The file extension for this type of message is
".p7m".

A sample message would be:

    Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
         name=smime.p7m
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename=smime.p7m

    rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
    7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
    f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
    0GhIGfHfQbnj756YT64V

3.4 Creating a Signed-only Message

There are two formats for signed messages defined for S/MIME. The
criteria for choosing among them are given in section 3.8.

3.4.1 Signing Using application/pkcs7-mime and SignedData

This signing format uses the application/pkcs7-mime MIME type. The
steps to create this format are:

  Step 1. The MIME entity is prepared according to section 3.1

  Step 2. The MIME entity and other required data is processed into a
          PKCS #7 object of type signedData

  Step 3. The PKCS #7 object is inserted into an
          application/pkcs7-mime MIME entity

The smime-type parameter for messages using application/pkcs7-mime and
SignedData is "signed-data". The file extension for this type of
message is ".p7m".

A sample message would be:

    Content-Type: application/pkcs7-mime; smime-type=signed-data;
         name=smime.p7m
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename=smime.p7m

    567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7
    77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH
    HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh
    6YT64V0GhIGfHfQbnj75

3.4.2 Signing Using the multipart/signed Format

This format is a clear-signing format. Recipients without any S/MIME
or PKCS processing facilities are able to view the message. It makes
use of the multipart/signed MIME type described in [MIME-SECURE]. The
multipart/signed MIME type has two parts. The first part contains the
MIME entity that is to be signed; the second part contains the
signature, which is a PKCS #7 detached signature.

3.4.2.1 The application/pkcs7-signature MIME type

This MIME type always contains a single PKCS #7 object of type
signedData. The contentInfo field of the PKCS #7 object must be
empty. The signerInfos field contains the signatures for the MIME
entity. The details of the registered type are given in Appendix E.

The file extension for signed-only messages using
application/pkcs7-signature  is ".p7s".

3.4.2.2 Creating a multipart/signed Message

  Step 1. The MIME entity to be signed is prepared according to
          section 3.1, taking special care for clear-signing.

  Step 2. The MIME entity is presented to PKCS #7 processing in order
          to obtain an object of type signedData with an empty
          contentInfo field.

  Step 3. The MIME entity is inserted into the first part of a
          multipart/signed message with no processing other than
          that described in section 3.1.

  Step 4. Transfer encoding is applied to the detached signature and
          it is inserted into a MIME entity of type
          application/pkcs7-signature

  Step 5. The MIME entity of the application/pkcs7-signature is
          inserted into the second part of the multipart/signed entity

The multipart/signed Content type has two required parameters: the
protocol parameter and the micalg parameter.

The protocol parameter MUST be "application/pkcs7-signature". Note
that quotation marks are required around the protocol parameter
because MIME requires that the "/" character in the parameter value
MUST be quoted.

The micalg parameter allows for one-pass processing when the signature
is being verified. The value of the micalg parameter is dependent on
the message digest algorithm used in the calculation of the Message
Integrity Check. The value of the micalg parameter SHOULD be one
of the following:

Algorithm used     Value
--------------     ---------
MD5                md5
SHA-1              sha1
any other          unknown

(Historical note: some early implementations of S/MIME emitted and
expected "rsa-md5" and "rsa-sha1" for the micalg parameter.) Receiving
agents SHOULD be able to recover gracefully from a micalg parameter
value that they do not recognize.

3.4.2.3 Sample multipart/signed message

    Content-Type: multipart/signed;
       protocol="application/pkcs7-signature";
       micalg=sha1; boundary=boundary42

    --boundary42
    Content-Type: text/plain

    This is a clear-signed message.
    --boundary42
    Content-Type: application/pkcs7-signature; name=smime.p7s
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename=smime.p7s

    ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
    4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
    n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
    7GhIGfHfYT64VQbnj756
    --boundary42--

3.4.2.4 Encapsulation Using application/mime

In some cases, multipart/signed entities are automatically decomposed
in such a way as to make computing the hash of the first part, the
signed part, impossible; in such a situation, the signature becomes
unverifiable. In order to prevent such decomposition until the MIME
entity can be processed in a proper S/MIME environment, a
multipart/signed entity may be encapsulated in an application/mime
entity.

All S/MIME implementations SHOULD be able to generate and receive
application/mime encapsulations of multipart/signed entities which
have their signature of type application/pkcs7-mime. In particular, on
receipt of a MIME entity of type application/mime with the type
parameter "multipart/signed" and the protocol parameter
"application/pkcs7-mime", a receiving agent SHOULD be able to process
the entity correctly. This is required even if the local environment
has facilities for processing application/mime because the
application/mime standard requires that the encapsulated entity only
be processed on request of the user, or if processing software can
process the entity completely and correctly. In this case, an S/MIME
facility can always process the entity completely and SHOULD do so.

The steps to create an application/mime encapsulation of a
multipart/signed entity are:

  Step 1. Prepare a multipart/signed message as described in
          section 3.4.2.2

  Step 2. Insert the multipart/signed entity into an application/mime
          according to [APP-MIME]. This requires that the parameters
          of the multipart/signed entity be included as parameters
          on the application/mime entity.

Note that messages using application/mime are subject to the same
encoding rules as message/* and multipart/* types. The encoding of the
application/mime part MUST NOT be binary.

In addition, the application/mime entity SHOULD have a name parameter
giving a file name ending with ".aps". It SHOULD also have a
content-disposition parameter with the same filename. The ".aps"
extension SHOULD be used exclusively for application/mime encapsulated
multipart/signed entities containing a signature of type
application/pkcs7-signature. This is necessary so that the receiving
agent can correctly dispatch to software that verifies S/MIME
signatures in environments where the MIME type and parameters have been
lost or can't be used for such dispatch. Basically, the file extension
becomes the sole carrier of type information.

A sample application/mime encapsulation of a signed message might be:

    Content-type: application/mime; content-type="multipart/signed";
         protocol="application/pkcs7-signature";
         micalg=sha1; name=smime.aps
    Content-disposition: attachment; filename=smime.aps

    Content-Type: multipart/signed;
       protocol="application/pkcs7-signature";
       micalg=sha1; boundary=boundary42

    --boundary42
    Content-Type: text/plain

    This is a very short clear-signed message. However, at least you
    can read it!

    --boundary42
    Content-Type: application/pkcs7-signature
    Content-Transfer-Encoding: base64

    ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
    4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
    n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
    7GhIGfHfYT64VQbnj756
    --boundary42--


3.4.2.5 Encapsulation in an Non-MIME Environment

While this standard primarily addresses the Internet, it is useful to
compose and receive S/MIME secured messages in non-MIME
environments. This is particularly the case when it is desired that
security be implemented end-to-end. Other discussion here addresses
the receipt of S/MIME messages in non-MIME environments. Here the
composition of multipart/signed entities is addressed.

When a message is to be sent in such an environment, the
multipart/signed entity is created as described above. That entity is
then treated as an opaque stream of bits and added to the message as
an attachment. It must have a file name that ends with ".aps", as this
is the sole mechanism for recognizing it as an S/MIME message by the
receiving agent.

When this message arrives in a MIME environment, it is likely to have
a MIME type of application/octet-stream, with MIME parameters giving
the filename for the attachment. If the intervening gateway has
carried the file type, it will end in ".aps" and be recognized as an
S/MIME message.

3.5 Signing and Encrypting

To achieve signing and enveloping, any of the signed-only and
encrypted-only formats may be nested. This is allowed because the
above formats are all MIME entities, and because they all secure MIME
entities. In addition, PKCS #7 provides a data type for enveloped and
signed data, and its use is described here.

An S/MIME implementation MUST be able to receive and process
arbitrarily nested S/MIME within reasonable resource limits of the
recipient computer.

It is possible to either sign a message first, or to envelope the
message first. It is up to the implementor and the user to choose.
When signing first, the signatories are then securely obscured by the
enveloping. When enveloping first the signatories are exposed, but it
is possible to verify signatures without removing the enveloping. This
may be useful in an environment were automatic signature verification
is desired, as no private key material is required to verify a
signature.

3.6 Creating a Certificates-only Message

The certificates only message or MIME entity is used to transport
certificates, such as in response to a registration request. This
format can also be used to convey CRLs.

  Step 1. The certificates are made available to the PKCS #7 generating
          process which creates a PKCS #7 object of type signedData.
          The contentInfo and signerInfos fields must be empty.

  Step 2. The PKCS #7 signedData object is enclosed in an
          application/pkcs7-mime MIME entity

The smime-type parameter for a certs-only message is "certs-only".
The file extension for this type of message is ".p7c".

3.7 Creating a Registration Request

A typical application which allows a user to generate cryptographic
information has to submit that information to a certification
authority, who transforms it into a certificate. PKCS #10 describes a
syntax for certification requests. The application/pkcs10 body type
MUST be used to transfer a PKCS #10 certification request.

The details of certification requests and the process of obtaining a
certificate are beyond the scope of this draft. Instead, only the
format of data used in application/pkcs10 is defined.

3.7.1 Format of the application/pkcs10 Body

PKCS #10 defines the ASN.1 type CertificationRequest for use in
submitting a certification request. Therefore, when the MIME content
type application/pkcs10 is used, the body MUST be a
CertificationRequest, encoded using the Basic Encoding Rules (BER).

Although BER is specified, instead of the more restrictive DER, a
typical application will use DER since the CertificationRequest's
CertificationRequestInfo has to be DER-encoded in order to be signed.
A robust application SHOULD output DER, but allow BER or DER on input.

Data produced by BER or DER is 8-bit, but many transports are limited
to 7-bit data. Therefore, a suitable 7-bit Content-Transfer-Encoding
SHOULD be applied. The base64 Content-Transfer-Encoding SHOULD be used
with application/pkcs10, although any 7-bit transfer encoding may
work.

3.7.2 Sending and Receiving an application/pkcs10 Body Part

For sending a certificate-signing request, the application/pkcs10
message format MUST be used to convey a PKCS #10 certificate-signing
request. Note that for sending certificates and CRLs messages
without any signed content, the application/pkcs7-mime message format
MUST be used to convey a degenerate PKCS #7 signedData "certs-only"
message.

To send an application/pkcs10 body, the application generates the
cryptographic information for the user. The details of the
cryptographic information are beyond the scope of this draft.

  Step 1. The cryptographic information is placed within a PKCS #10
          CertificationRequest.

  Step 2. The CertificationRequest is encoded according to BER or DER
          (typically, DER).

  Step 3. As a typical step, the DER-encoded CertificationRequest is
          also base64 encoded so that it is 7-bit data suitable for
          transfer in SMTP. This then becomes the body of an
          application/pkcs10 body part.

The result might look like this:

    Content-Type: application/pkcs10; name=smime.p10
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename=smime.p10

    rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
    7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
    f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
    0GhIGfHfQbnj756YT64V

A typical application only needs to send a certification request. It
is a certification authority that has to receive and process the
request. The steps for recovering the CertificationRequest from the
message are straightforward but are not presented here. The procedures
for processing the certification request are beyond the scope of this
document.

3.8 Choosing a Format for Signed-only Messages

There are no hard-and-fast rules when a particular signed-only format
should be chosen because it depends on the capabilities of all the
receivers and the relative importance of receivers with S/MIME
facilities being able to verify the signature versus the importance of
receivers without S/MIME software being able to view the message.

Messages signed using the multipart/signed format can always be viewed
by the receiver whether they have S/MIME software or not. They can
also be viewed whether they are using a MIME-native user agent or they
have messages translated by a gateway. In this context, "be viewed"
means the ability to process the message essentially as if it were not
a signed message, including any other MIME structure the message might
have.

Messages signed using the signedData format cannot be viewed by a
recipient unless they have S/MIME facilities. However, if they have
S/MIME facilities, these messages can always be verified if they were
not changed in transit.

3.8.1 Rationale for Multiple Signing Formats

The rationale behind the multiple formats for signing has to do with
the MIME subtype defaulting rules of the application and multipart
top-level types, and the behavior of currently deployed gateways and
mail user agents.

Ideally, the multipart/signed format would be the only format used
because it provides a truly backwards compatible way to sign MIME
entities. In a pure MIME environment with very capable user agents,
this would be possible. The world, however, is more complex than this.

One problem with the multipart/signed format occurs with gateways to
non-MIME environments. In these environments, the gateway will
generally not be S/MIME aware, will not recognize the multipart/signed
type, and will default its treatment to multipart/mixed as is
prescribed by the MIME standard. The real problem occurs when the
gateway also applies conversions to the MIME structure of the original
message that is being signed and is contained in the first part of the
multipart/signed structure, such as the gateway converting text and
attachments to the local format. Because the signature is over the
MIME structure of the original message, but the original message is
now decomposed and transformed, the signature cannot be verified.
Because MIME encoding of a particular set of body parts can be done in
many different ways, there is no way to reconstruct the original MIME
entity over which the signature was computed.

A similar problem occurs when an attempt is made to combine an
existing user agent with a stand-alone S/MIME facility. Typical user
agents do not have the ability to make a multipart sub-entity
available to a stand-alone application in the same way they make leaf
MIME entities available to "viewer" applications. This user agent
behavior is not required by the MIME standard and thus not widely
implemented. The result is that it is impossible for most user agents
to hand off the entire multipart/signed entity to a stand-alone
application.

3.8.2 Solutions to the Problem

To work around these two problems, the application/pkcs7-mime type can
be used. When going through a gateway, it will be defaulted to the
MIME type of application/octet-stream and treated as a single opaque
entity. That is, the message will be treated as an attachment of
unknown type, converted into the local representation for an
attachment and thus can be made available to an S/MIME facility
completely intact. A similar result is achieved when a user agent
similarly treats the application/pkcs7-mime MIME entity as a simple
leaf node of the MIME structure and makes it available to viewer
applications.

Another way to work around these problems is to encapsulate the
multipart/signed MIME entity in a MIME entity of type
application/mime. The result is similar to that obtained using
application/pkcs7-mime. When the application/mime entity arrives at a
gateway that does not recognize it, its type will be defaulted to
application/octet-stream and it will be treated as a single opaque
entity. A similar situation will happen with a receiving client that
does not recognize the entity. It will usually be treated as a file
attachment. It can then be made available to the S/MIME facility.

The major difference between the two alternatives
(application/pkcs7-mime or multipart/signed wrapped with
application/mime ) is when the S/MIME facility opens the attachment.
In the latter case, the S/MIME agent will find a multipart/signed
entity rather than a BER encoded PKCS7-object. Considering the two
representations abstractly, the only difference is syntax.

The application/mime standard is a general mechanism for encapsulating
MIME, and in particular delaying its interpretation until it can be
done in the appropriate environment or at the request of the user. The
application/mime specification does not permit a user agent to
automatically interpret the encapsulated MIME unless it can be
processed entirely and properly. The parameters to the
application/mime entity give the type of the encapsulated entity so it
can be determined whether or not the entity can be processed before it
is expanded.

Application/mime is a general encapsulation mechanism that can be
built into a gateway or user agent, allowing expansion of the
encapsulated entity under user control. Because it is a general
mechanism, it is in many cases more likely to be available than an
S/MIME facility. Thus, it enables users to expand or to verify signed
messages based on their local facilities and choices. It provides
exactly the same advantages that the application/pkcs7-mime with
signedData does. It also has the added benefit of allowing expansion
in non-S/MIME environments and expansion under the recipient's
control.

3.8.3 Deciding Which Format To Use

The following table gives criteria for selecting the signature message
format in order of preference if the criteria is met:

    mulipart/signed
        If it is unknown whether or not all the recipients have
           S/MIME processing facilities and
        It is unknown whether or not they have the ability to process
           the application/mime type and
        It is more important that the message be read by all
           recipients than that it be verifiable

    application/mime
        It is known that all recipients have the ability to process
          messages of the type application/mime
        It does not matter whether or not they have S/MIME facilities

    signedData
        It is known that all recipients have S/MIME facilities

The sender may determine whether or not a recipient has S/MIME
facilities by keeping track of messages they have received from that
person in an address book or other facility. If they have received
S/MIME messages from a particular address, it is safe to conclude that
S/MIME messages may be sent to that address.

3.9 Identifying an S/MIME Message

Because S/MIME takes into account interoperation in non-MIME
environments, several different mechanisms are employed to carry the
type information, and it becomes a bit difficult to identify S/MIME
messages. The following table lists criteria for determining whether
or not a message is an S/MIME message. A message is considered an
S/MIME message if it matches any below.

The file suffix in the table below comes from the "name" parameter in
the content-type header, or the "filename" parameter on the
content-disposition header. These parameters that give the file suffix
are not listed below as part of the parameter section.

MIME type:   application/pkcs7-mime
parameters:  any
file suffix: any

MIME type:   application/pkcs10
parameters:  any
file suffix: any

MIME type:   multipart/signed
parameters:  protocol="application/pkcs7-signature"
file suffix: any

MIME type:   application/mime
parameters:  content-type="multipart/signed";
                protocol="application/pkcs7-signature"
file suffix: any

MIME type:   application/octet-stream
parameters:  any
file suffix: p7m, p7s, aps, p7c, p10


4. Certificate Processing

A receiving agent MUST provide some certificate retrieval mechanism in
order to gain access to certificates for recipients of digital
envelopes. This draft 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 a different
document.

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.

4.1 Key Pair Generation

An S/MIME agent or some related administrative utility or function MUST
be capable of generating RSA key pairs on behalf of the user. Each key
pair MUST be generated from a good source of non-deterministic
random input and protected in a secure fashion.

A user agent SHOULD generate RSA key pairs at a minimum key size of
768 bits and a maximum key size of 1024 bits. A user agent MUST NOT
generate RSA key pairs less than 512 bits long. Some agents created in
the United States have chosen to create 512 bit keys in order to get
more advantageous export licenses. However, 512 bit keys are
considered by many to be cryptographically insecure.

Implementors should be aware that multiple (active) key pairs may be
associated with a single individual. For example, one key pair may be
used to support confidentiality, while a different key pair may be
used for authentication.


5. Security

This entire draft discusses security. Security issues not covered in
other parts of the draft include:

40-bit encryption is considered weak by most cryptographers. Using
weak cryptography in S/MIME offers little actual security over sending
plaintext. However, other features of S/MIME, such as the
specification of tripleDES and the ability to announce stronger
cryptographic capabilities to parties with whom you communicate, allow
senders to create messages that use strong encryption. Using weak
cryptography is never recommended unless the only alternative is no
cryptography. When feasible, sending and receiving agents should
inform senders and recipients the relative cryptographic strength of
messages.

It is impossible for most software or people to estimate the value of
a message. Further, it is impossible for most software or people to
estimate the actual cost of decrypting a message that is encrypted
with a key of a particular size. Further, it is quite difficult to
determine the cost of a failed decryption if a recipient cannot decode
a message. Thus, choosing between different key sizes (or choosing
whether to just use plaintext) is also impossible. However, decisions
based on these criteria are made all the time, and therefore this
draft gives a framework for using those estimates in choosing
algorithms.


A. Object Identifiers and Syntax

The syntax for SMIMECapability is:

SMIMECapability ::= SEQUENCE {
    capabilityID OBJECT IDENTIFIER,
    parameters OPTIONAL ANY DEFINED BY capabilityID }

SMIMECapabilities ::= SEQUENCE OF SMIMECapability

A.1 Content Encryption Algorithms

RC2-CBC OBJECT IDENTIFIER ::=
     {iso(1) member-body(2) us(840) rsadsi(113549) encryptionAlgorithm(3) 2}

For the effective-key-bits (key size) other than 32 and less than
256, the RC2-CBC algorithm parameters are encoded as:

RC2-CBC parameter ::=  SEQUENCE {
     rc2ParameterVersion  INTEGER,
     iv                   OCTET STRING (8)}

For the effective-key-bits of 40, 64, and 128, the
rc2ParameterVersion values are 160, 120, 58 respectively.

DES-EDE3-CBC OBJECT IDENTIFIER ::=
     {iso(1) member-body(2) us(840) rsadsi(113549) encryptionAlgorithm(3) 7}

For DES-CBC and DES-EDE3-CBC, the parameter should be encoded as:

CBCParameter :: IV

where IV ::= OCTET STRING -- 8 octets.

A.2 Digest Algorithms

md5 OBJECT IDENTIFIER ::=
     {iso(1) member-body(2) us(840) rsadsi(113549) digestAlgorithm(2) 5}

sha-1 OBJECT IDENTIFIER ::=
     {iso(1) identified-organization(3) oiw(14) secsig(3) algorithm(2) 26}

A.3 Asymmetric Encryption Algorithms

rsaEncryption OBJECT IDENTIFIER ::=
     {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1}

rsa OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) algorithm(8) encryptionAlgorithm(1) 1}

A.3 Signature Algorithms

md2WithRSAEncryption OBJECT IDENTIFIER ::=
     {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 2}

md5WithRSAEncryption OBJECT IDENTIFIER ::=
     {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 4}

sha-1WithRSAEncryption OBJECT IDENTIFIER ::=
     {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 5}

A.4 Signed Attributes

signingTime OBJECT IDENTIFIER ::=
     {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 5}

SMIMECapabilities OBJECT IDENTIFIER ::=
    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}


B. References

[3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES,"
IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41.

[APP-MIME] "Wrapping MIME Objects:  Application/MIME", Internet
Draft draft-crocker-wrap-01.txt.

[DES] ANSI X3.106, "American National Standard for Information
Systems-Data Link Encryption," American National Standards Institute,
1983.

[MD5] "The MD5 Message Digest Algorithm", RFC 1321

[MIME-SPEC] The primary definition of MIME. "MIME Part 1: Format of
Internet Message Bodies", RFC 2045; "MIME Part 2: Media Types", RFC
2046; "MIME Part 3: Message Header Extensions for Non-ASCII Text", RFC
2047; "MIME Part 4: Registration Procedures", RFC 2048; "MIME Part 5:
Conformance Criteria and Examples", RFC 2049

[MIME-SECURE] "Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847

[MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119

[PKCS-1] "PKCS #1: RSA Encryption", Internet Draft
draft-hoffman-pkcs-rsa-encrypt

[PKCS-7] "PKCS #7: Cryptographic Message Syntax", Internet Draft
draft-hoffman-pkcs-crypt-msg

[PKCS-10] "PKCS #10: Certification Request Syntax", Internet Draft
draft-hoffman-pkcs-certif-req

[RC2] "Description of the RC2 Encryption Algorithm", Internet Draft
draft-rivest-rc2desc

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


C. Compatibility with Pre-standards S/MIME

S/MIME was originally developed by RSA Data Security, Inc. Many
developers implemented S/MIME agents before the standard was turned
over to the IETF. All S/MIME receiving agents SHOULD make every attempt to
interoperate with pre-standards S/MIME sending agents.

C.1 Pre-standards MIME Types

Pre-standard S/MIME agents used the following MIME types:

application/x-pkcs7-mime
application/x-pkcs7-signature
application/x-pkcs10

In each case, the "x-" subtypes correspond to the subtypes described
in this document without the "x-".

C.2 Pre-standards Profiles

Pre-standard S/MIME had two profiles for encryption: "restricted" and
"unrestricted". The difference between these profiles historically
came about due to US Government export regulations, as described at
the end of this section. It is expected that in the future, there will
be few agents that only use the restricted profile.

Briefly, the restricted profile required the ability to encrypt and
decrypt using RSA's trade-secret RC2 algorithm in CBC mode with 40-bit
keys. The unrestricted profile required the ability to encrypt and
decrypt using RSA's trade-secret RC2 algorithm in CBC mode with 40-bit
keys, and to encrypt and decrypt using tripleDES. The restricted
profile also had non-mandatory suggestions for other algorithms, but
these were not widely implemented.

It is important to note that many current implementations of S/MIME
use the restricted profile.

C.2.1 Historical Reasons for the Existence of Two Encryption Profiles

Due to US Government export regulations, an S/MIME agent which
supports a strong content encryption algorithm such as DES would not
be freely exportable outside of North America. US software
manufacturers have been compelled to incorporate an exportable or
"restricted" content encryption algorithm in order to create a widely
exportable version of their product. S/MIME agents created in the US
and intended for US domestic use (or use under special State
Department export licenses) can utilize stronger, "unrestricted"
content encryption. However, in order to achieve interoperability,
such agents need to support whatever exportable algorithm is
incorporated in restricted S/MIME agents.

The RC2 symmetric encryption algorithm has been approved by the US
Government for "expedited" export licensing at certain key sizes.
Consequently, support for the RC2 algorithm in CBC mode is required
for baseline interoperability in all S/MIME implementations. Support
for other strong symmetric encryption algorithms such as RC5 CBC, DES
CBC and DES EDE3-CBC for content encryption is strongly encouraged
where possible.


D. Revision History

The following changes were made between the -02 and -03 revisions of
this draft:

Fixed many typos throughout the document found by Lisa Repka.

Fixed typos in the mailing list address and Section 3.

Added references to the cryptographic algorithms.

Clarified that the degnerate case is in Section 2.4.2.

Removed Section 2.4.4 and the reference to PEM since SignedAndEncrypted
was only mentioned as a SHOULD NOT.

Removed "and/or encrypted" from Section 2.5. Added paragraph about
handling new attributes and values. Added similar wording to the end
of Section 2.5.2.

Removed "and encryption" from the first sentence of Section 2.6 since
the sentence is about receiving agents.

Added paragraph at the end of Section 3.1.1 about canonicalization
in character sets.

Changed the last sentence of 3.2.1 to say implementations must not
rely on the extension.

Made micalg required in Section 3.4.2.2 and defined its values.

Added paragraph in Section 3.4.2.4 prohibiting binary in app/mime.

Added "application/pkcs10" to Section 3.9.

Added text at the end of Section 4.1 discussing multiple key pairs.
Also change the "SHOULD NOT" to "MUST NOT" generate keys under 512
bits.

In Appendix A, lowercased the "us" to conform to ASN.1.

In Appendix E, added "name" as an optional parameter in each
registration.

Fixed Appendix G with RSA-approved wording.


E. Request for New MIME Subtypes

E.1 application/pkcs7-mime

To: ietf-types@iana.org
Subject: Registration of MIME media type application/pkcs7-mime

MIME media type name: application

MIME subtype name: pkcs7-mime

Required parameters: none

Optional parameters: name

Encoding considerations: Will be binary data, therefore should use
base64 encoding

Security considerations: Described in [PKCS-7]

Interoperability considerations: Designed to carry data formatted
with PKCS-7, as described in [PKCS-7]

Published specification: draft-dusse-smime-msg-xx

Applications which use this media type: Secure Internet mail and
other secure data transports.

Additional information:
File extension(s): .p7m and .p7c
Macintosh File Type Code(s):

Person & email address to contact for further information:
Steve Dusse, spock@rsa.com

Intended usage: COMMON

E.2 application/pkcs7-signature

To: ietf-types@iana.org
Subject: Registration of MIME media type application/pkcs7-signature

MIME media type name: application

MIME subtype name: pkcs7-signature

Required parameters: none

Optional parameters: name

Encoding considerations: Will be binary data, therefore should use
base64 encoding

Security considerations: Described in [PKCS-7]

Interoperability considerations: Designed to carry digital
signatures with PKCS-7, as described in [PKCS-7]

Published specification: draft-dusse-smime-msg-xx

Applications which use this media type: Secure Internet mail and
other secure data transports.

Additional information:
File extension(s): .p7s
Macintosh File Type Code(s):

Person & email address to contact for further information:
Steve Dusse, spock@rsa.com

Intended usage: COMMON

E.3 application/pkcs10

To: ietf-types@iana.org
Subject: Registration of MIME media type application/pkcs10

MIME media type name: application

MIME subtype name: pkcs10

Required parameters: none

Optional parameters: name

Encoding considerations: Will be binary data, therefore should use
base64 encoding

Security considerations: Described in [PKCS-10]

Interoperability considerations: Designed to carry digital
certificates formatted with PKCS-10, as described in [PKCS-10]

Published specification: draft-dusse-smime-msg-xx

Applications which use this media type: Secure Internet mail and
other transports where certificates are required.

Additional information:
File extension(s): .p10
Macintosh File Type Code(s):

Person & email address to contact for further information:
Steve Dusse, spock@rsa.com

Intended usage: COMMON


F. Open Issues

Should we leave in or take out the application/mime discussion in
Section 3? There are no implementations that use it today, and it is
still unclear how to describe to a user when to chose it over plain
multipart/signed.

Should we discuss how gateways should interact with S/MIME here or in
a different document?


G.  Trademarks

"S/MIME" is a trademark of RSA Data Security, Inc. Use of this mark is
available for companies' products which comply with this version of
this specification.


H. Acknowledgements

Significant contributions to the content of this draft were made by
many people, including Jeff Thompson and Jeff Weinstein.


I. Authors' addresses

Steve Dusse
RSA Data Security, Inc.
100 Marine Parkway, #500
Redwood City, CA  94065  USA
(415) 595-8782
spock@rsa.com

Paul Hoffman
Internet Mail Consortium
127 Segre Place
Santa Cruz, CA  95060
(408) 426-9827
phoffman@imc.org

Blake Ramsdell
Deming Internet Security
13122 NE 20th St., Suite C
Bellevue, WA 98005
(425) 882-8861
blaker@deming.com

Laurence Lundblade
QUALCOMM Incorporated
Eudora Division
6455 Lusk Boulevard
San Diego, California 92121-2779
(800) 238-3672
lgl@qualcomm.com

Lisa Repka
Netscape Communications Corporation
501 East Middlefield Road
Mountain View, CA  94043
(415) 254-1900
repka@netscape.com


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