Internet Draft                              Steve Dusse,
draft-dusse-smime-msg-01.txt                RSA Data Security
May 5, 1997                                 Paul Hoffman,
Expires November 5, 1997                    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
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ftp.isi.edu (US West Coast).

. 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 has been
cryptographically enhanced according to PKCS #7 [PKCS-7]. This draft
also defines the application/pkcs7-mime MIME type that can 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 request.

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 #9: Selected Attribute Types", [PKCS-9].
 - "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 importnat 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-subscribe-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/>.

. 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 and 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
 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", "signedAndEnvelopedData", 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 Signed-data Content Type

Sending agents MUST use the Signed-data content type to apply a
digital signature to a message or, in a degenerate case where
there is no signature information, to convey information pertaining to
certificates.

2.4.3 Enveloped-data 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.

2.4.4 Signed-and-enveloped-data Content Type

This content type is used to apply a digital signature as well as
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 should only be used for compatibility with [PEM].
The separate application of signing then enveloping SHOULD be used in
all other cases.

2.5 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 and/or encrypted message sent.

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 defined in [PKCS-9].

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 SHOULD 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. Also, additional attributes and values for those
attributes may be defined in the future. Receiving agents MUST handle
a SMIMECapabilities object that has attributes or values that it does
not recognize in a graceful manner.

2.6 ContentEncryptionAlgorithmIdentifier

Receiving agents MUST support decryption and encryption using the
"FOO" algorithm at a key size of 40 bits, hereinafter called "FOO/40".
Receiving agents SHOULD support decryption using DES EDE3 CBC,
hereinafter called "tripleDES".

Sending agents SHOULD support encryption with F00/40 and tripleDES.

[[[Note for draft -01: Obviously, there is no "FOO" algorithm. There
is a debate on the mailing list whether the specification of any
40-bit algorithm in the draft would cause weak security. The use of
FOO/40 in this draft should help focus on that topic, hopefully to
either convince readers that no weak security is mandated, or to fix
any case where weak security is accidentally mandated. Because of
statements from RSA, it is unclear which 40-bit algorithm will be
used. The current candidates are RC2 at 40 bits (if RSA removes its
trade secret claims on RC2) and DES at 40 bits. For draft -01, it does
not matter which of these algorithms are chosen. A future version of
this draft will specify the algorithm fully or will remove the 40-bit
algorithm altogether.]]]

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 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 capabilities lists in messages whose
   signature could not be verified, SHOULD 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 FOO/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 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 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 FOO/40.

2.6.3 Choosing Weak Encryption

Like all algorithms that use 40 bit keys, FOO/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 FOO/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.

. Creating S/MIME Messages

This section describes how to create S/MIME messages using various
body parts, some of which are cryptographically processed as described
in Section 2. The reader of this section is expected to understand
MIME as described in [MIME-SPEC] and [MIME-SECURE].

3.1 Content-Type application/pkcs7-mime

This subsection defines the format of data used in
application/pkcs7-mime.

3.1.1 Format of the application/pkcs7-mime Body

PKCS #7 defines a general ASN.1 type, ContentInfo, for use in
exchanging cryptographic information. PKCS #7 also defines several
content types which can be used within a ContentInfo. For our purposes
here, the most important are SignedData (for exchanging digitally
signed data), EnvelopedData (for exchanging digitally enveloped data)
and SignedAndEnvelopedData (for exchanging data both digitally signed
and enveloped).

Therefore, when the MIME content type application/pkcs7-mime is used,
the body MUST be a ContentInfo as defined by [PKCS-7], encoded using
the Basic Encoding Rules (BER). The PKCS #7 content type SHOULD be
SignedData, EnvelopedData or SignedAndEnvelopedData, but use of other
content types defined by [PKCS-7] are also allowed.

Since BER is specified, instead of the more restrictive DER, an
application may use techniques such as indefinite-length encoding.
This is especially useful for transferring large data or streamed data
where the total length is not known in advance.

Data produced by BER is 8-bit, but many transports are limited to
7-bit data. Therefore, in most situations, a suitable 7-bit
Content-Transfer-Encoding is needed, such as base64.

PKCS #7 has a provision for "detached data", where, for example, the
SignedData in the ContentInfo contains only the signature information,
but not the actual data which is signed (which is transferred
separately). S/MIME agents using application/pkcs7-mime MUST NOT use
detached data, and the data MUST be present within the ContentInfo.

PKCS #7 provides for a degenerate case of SignedData which can be used
for disseminating certificates and CRLs. This is explicitly allowed in
application/pkcs7-mime. In this case, the content is omitted from the
ContentInfo and there are no entries in the SignerInfos, leaving only
entries for certificates and CRLs. Typically, this is used to convey
certificates in response to a certification request, or CRLs in
response to a CRL retrieval request. The MIME agent SHOULD process and
store the certificates and CRLs and may choose to display a
confirmation to the user.

The requirement that the signed or enveloped data has to itself be a
MIME body part is the reason the protocol defined here is called
"application/pkcs7-mime" and not "application/pkcs7". A protocol for
transferring a PKCS #7 object with arbitrary signed or enveloped
content could be defined, but is outside the scope of this draft.

3.1.2 Format of the Signed or Enveloped application/pkcs7-mime Data

PKCS #7 places no requirements on the format of the data which is
signed or enveloped. However, for use in application/pkcs7-mime, the
signed or enveloped data is itself a MIME entity. Therefore, when a
MIME agent receives an application/pkcs7-mime object, the result of
removing the signature or envelope can be passed directly to the
normal MIME-processing software.

Because the MIME entity being signed or enveloped is likely to be
transferred to and processed on a different platform than it was
created, the data MUST be in MIME canonical format.

The most common difference between platforms is the end of line
delimiter. There are other considerations for the selection of the
transfer encoding for the MIME entity that affect the ability to
verify the signature of a received MIME entity, especially if it is to
be subsequently forwarded. The data that is signed or enveloped has to
be recovered by the recipient in the exact form it was produced by the
sender, so agents MUST be sure that this data be in a canonical format
which can be processed by any platform. Converting a message into
canonical format is covered in [MIME-SPEC].

When creating a full MIME entity, the encoded sub-entities are
inserted in the full MIME entity. This step consists essentially of
concatenating the sub-entities interspersed with the appropriate MIME
Content-type, Content-Transfer-Encoding, and related headers and part
boundaries. Note that MIME headers and boundaries are all text with
canonical line endings.

3.1.3 Outside Content Transfer Encoding

The body of application/pkcs7-mime is a BER-encoded PKCS #7
ContentInfo. However, as mentioned above, the result of BER-encoding
is binary data, not 7-bit text. If S/MIME is being used in a system
that requires 7-bit text, such as SMTP, a content transfer encoding
must be used. In general, base64 Content-Transfer-Encoding can be
used, although all that is required is that the transfer encoding be
such that application/pkcs7-mime entity can be transferred intact. (It
is even possible that the transfer encoding will be changed by the
message transfer agent, because this is explicitly permitted.)

3.1.4 Receiving an application/pkcs7-mime Body Part

These are the steps for receiving an application/pkcs7-mime message.

Step 1. Remove any content transfer encoding.

Step 2. Examine the PKCS #7 ContentInfo to determine the PKCS #7
content type.

Step 3. Process the inner content type according to PKCS #7.

Step 4. Pass the result to a standard MIME processor which removes
transfer encoding, splits multipart entities, and possibly converts
the parts to local format.

3.2 Content-Type application/pkcs10

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.

3.2.1 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.

3.2.2 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 a 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
     Content-Transfer-Encoding: base64

     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.

3.3 Use of the Multipart/Signed Content-Type

[MIME-SECURE] defines the multipart/signed content type explicitly for
clear-signed messages. Clear signing means the content is in the clear
so that recipients with systems that do not process PKCS information
can still read the message even if they cannot verify the signature.
The multipart/signed MIME type is a multipart type with exactly two
parts. The first part carries the MIME entity being signed, and the
second part carries the signature.

The multipart/signed Content type has two parameters: - The protocol
parameter (required) - The micalg parameter (optional)

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 optional 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.

An example of a multipart/signed message is:

       Content-Type: multipart/signed;
          protocol="application/pkcs7-signature";
          micalg=rsa-md5; boundary=boundary42

       --boundary42
       Content-Type: text/plain

       This is a clear-signed message
       --boundary42
       Content-Type: application/pkcs7-signature
       Content-Transfer-Encoding: base64

       ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
       4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
       n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
       7GhIGfHfYT64VQbnj756
       --boundary42--

3.3.1 Preparation of the Data for Signing

The data is prepared as described in section 3.2 with the following
extra considerations for transfer encoding. All parts of the MIME
entity MUST be encoded with 7-bit, quoted-printable, or base64
transfer encoding before computing the signature. No binary or 8-bit
data is permitted. This means that receiving agents, even those on
8-bit-clean transport systems, MUST be able to remove 7-bit,
quoted-printable, or base64 transfer encoding.

The current Internet mail transport infrastructure cannot guarantee
transport of unencoded binary or 8-bit data and because the digest for
the signature is computed on the fully encoded message. If a message
with 8-bit data were to encounter a  message transfer agent (MTA) that
can not transmit 8-bit or binary data it has three options, none of
which are acceptable for a clear-signed message.
 - The MTA could change the transfer encoding, but that would
   invalidate the signature.
 - The MTA could transmit the data anyway, which would most likely
   result in the 8th bit being stripped, also resulting in the
   signature being invalidated.
 - The MTA could return the message to the sender.

[MIME-SECURE] prohibits an MTA from changing the transfer encoding of
the first part of a multipart/signed message. If a compliant MTA
encountered 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.

Sending agents may choose to use quoted printable transfer encoding in
some situations where it may not otherwise seem necessary to better
preserve the data so the signature can be verified. For example,
trailing spaces should always be quoted-printable encoded because some
mail systems strip them if they are not transferred with encoding.
Also the word "From" followed by a space at the beginning of a line
should be encoded as some mail systems convert this to ">From".
Because of the restriction on 8-bit transport it is also necessary to
use quoted printable encoding for text using an 8-bit character set
such as ISO-8859-1.

3.3.2 The application/pkcs7-signature Type

The second part of the multipart/signed message MUST be type
application/pkcs7-signature. The data in the second part is the PKCS
#7 detached signature. A detached signature is a PKCS #7 data object
of type SignedData with the signerInfos field containing signatures on
some external data and the ContentInfo field empty. In this case, the
external data is the fully encoded MIME entity described above and
placed in the first part of the multipart/signed message.

3.3.3 Procedure for Sending a Clear Signed Message

Step 1. The data is prepared as described above so the transfer
encoding of all the parts is either 7-bit, quoted-printable or base64.

Step 2. The PKCS #7 detached signature for the data is computed.

Step 3. The content data is inserted into the first part of the
multipart/signed MIME entity.

Step 4. The detached signature is inserted into the second part of the
multipart/signed message using appropriate transfer encoding.

3.3.4 Procedure for Receiving a Clear Signed Message

Step 1. The agent separates the two parts of the multipart/signed
entity.

Step 2. The message digest of the first part of the multipart/signed
message is computed.

Step 3. The application/pkcs7-signature information in the second part
is processed along with the digest obtained in the first step to
verify the signature.

Step 4. The MIME processing and decoding of the first part proceeds
and the message is presented to the user along with the result of the
signature verification.

3.4 Nesting of S/MIME Security Services

MIME allows for an arbitrarily complex structure that is frequently
limited in practice to a basic functional subset. While the MIME
specification allows an arbitrary depth of MIME entities nested within
other MIME entities, the addition of security services to MIME data
makes arbitrary nesting difficult to implement and challenging
to convey to a user in a meaningful way. As with other areas in the
S/MIME specification, requirements and recommendations are made which
attempt to balance the concerns of utility versus simplicity.

If a user chooses to both sign and envelope a message to another user,
this can be accomplished in two ways:
 - by creating a PKCS #7 signedAndEnvelopedData content type
 - by separately creating a signedData content type then using the
   result as input to create an envelopedData content type

Concerns have been voiced about the fact that the
signedAndEnvelopedData content type exposes the identities of the
S/MIME signatories (i.e. the issuerAndSerialNumber for each signatory
is not protected in the envelope).

In order to accommodate the protection of signatory information within
enveloped messages all sending and receiving agents MUST support the
nesting of signed messages within enveloped messages. If an incoming
enveloped message is decrypted and the resulting MIME entity is a
signed application/pkcs7-mime, then the user agent SHOULD
automatically process the resulting MIME entity and present the
signature status and corresponding information to the user.

Likewise, if a user chooses to sign and encrypt an outgoing message,
then the user agent SHOULD automatically create an
application/pkcs7-mime signed message then use the resulting message
as input to the creation of an application/pkcs7-mime enveloped
message. In the case of this signed/enveloped nesting, the inner
signed message SHOULD be left in its binary state and that the content
transfer encoding on the inner signed message be indicated as binary.

There are a number of useful security functions that can be achieved
by allowing additional nesting of security services. An receiving
agent SHOULD allow for automatic processing of one or two additional
layers of S/MIME entities nested in other S/MIME entities. Where
additional nesting is not automatically handled, some provision SHOULD
be made to handle additional nesting manually, such as through some
explicit user action such as the resubmission of the resulting message
to the user agent.

3.5 Choosing the Message Type for Outgoing Signed Messages

There are two distinct mechanisms for conveying a signed message:

 - a signed, self-contained application/pkcs7-mime construct
 - a multipart/signed construct (where the signature is conveyed
   separate from the signed data in an application/pkcs7-signature
   body part)

Sending agents MUST support creation of both application/pkcs7-mime
and multipart/signed messages. Therefore, receiving agents MUST be
able to process signatures from both types of messages.

In many Internet mail transactions, there is a reasonable expectation
that MIME data will be delivered intact. However, because of the
limitations inherent in a significant portion of existing mail
infrastructure, not all S/MIME-enabled agents will be able to
correctly receive and/or process multipart/signed messages. For
example, some older gateways and message transfer agents will treat
unknown multipart messages such as multipart/signed as multipart/mixed
and discard the MIME packaging for the signed message, leading to
unverifiable signatures.

One of the greatest values of the multipart/signed construct is in the
ability of agents which are not S/MIME-enabled to be able to handle
the (otherwise unencoded) body of the message that was signed. Thus,
multipart/signed SHOULD be used when a signed message is being sent to
a set of recipients where the S/MIME capabilities are not all known.

If the sending agent believes that a particular recipient might not be
able to receive a multipart/signed message successfully due to
improper gateways, the agent can either:
 - wrap the multipart/signed message in another MIME envelope using
   application/mime, as described in [MIME-APP]
 - use application/pkcs7-mime instead of multipart/signed

Both methods have advantages and drawbacks. Because of the newness of
the specification, few gateways would try to break open an
application/mime message, and this should thus shield the contents
fairly well. Further, because application/mime messages are plain
text, non-MIME recipients of such a message would still be able to
read it (albeit after visually parsing the message). However, because
of the newness of the specification, few of the currently-implemented
S/MIME receiving agents would know what to do with an application/mime
message. All receiving S/MIME agents do know how to interpret
application/pkcs7-mime messages, so using this scheme will lead to
good interpretation by S/MIME-aware agents. However the contents of an
application/pkcs7-mime message are indecipherable to non-S/MIME
receiving agents.

When all of the intended recipients are known to be S/MIME-capable,
the sending agent SHOULD use application/pkcs7-mime because it has a
greater possibility of successful receipt through unknown gateways.
The availability of certificates for all intended message recipients
and/or S/MIME-specific entries in an addressbook or database may be a
good indication that all recipients are S/MIME-capable.

3.5.1 Sending to Mailing Lists

Sending agents often send messages to recipients on public mailing
lists and systems outside traditional Internet mail like Usenet. As
stated earlier, one of the important benefits of multipart/signed
messages is that their text portions can be displayed (but not
verified) by non-S/MIME and by non-MIME recipients. If
application/pkcs7-mime is used instead of multipart/signed, the
message will only be readable by recipients with S/MIME tools.
Therefore, implementors of S/MIME agents SHOULD attempt to ensure that
application/pkcs7-mime messages are never sent to public forums which
have participation from non-S/MIME-enabled users.

If a sending agent cannot generate multipart/signed, and the user of
that agent wants to sign a message that is being sent to a recipient
who has not explicitly advertised support for application/pkcs7-mime,
then the sender SHOULD be warned that some users may not be able to
read the message at all if sent in signed form, and that they should
only proceed if they are confident that all recipients have S/MIME
capability.

Because you generally cannot know all the capabilities of all the
recipients of a mailing list or similar forum, the use of
application/pkcs7-mime SHOULD be strongly discouraged for messages
sent to these types of lists and, if allowed at all, appropriate
warnings SHOULD be issued.

3.5.2  Choice Matrix Summary

This chart describes when a sending agent SHOULD use each MIME type:

MIME Type                        When to Use

application/pkcs7-mime           When sending envelopedData

                                 When sending signedData to
                                 only S/MIME-capable recipients
                                 where multipart/signed is expected
                                 to not arrive intact

                                 When sending a certs-only
                                 message

multipart/signed                 Default for  sending a signed
                                 message; required when sending a
                                 signed message to a mix of
                                 recipients where S/MIME-
                                 capabilities are not known for
                                 all recipients

application/mime                 When sending multipart/signed to
                                 recipient who may or may not be
                                 S/MIME-capable and the message
                                 is expected to not arrive intact

application/pkcs10               When sending a certification
                                 request to a certification authority

3.6 Relationship to File-Based MIME Security

The use of S/MIME is not limited to the on-line mail environment. By
associating standard file extensions to the various S/MIME content
types, useful automatic conversion can occur between agents and
file-based S/MIME processing capabilities in a receiving agent or at
the operating system level. The MIME Content-Type header field has an
optional parameter, "name", that is useful for saving body parts on
disk at the recipient's site. The MIME Content-Disposition header
field, with the value "attachment" also has an optional parameter,
"filename", for saving body parts on disk at the recipient's site.

Sending agents SHOULD emit the optional name parameter to the
Content-Type field for the for compatibility with older systems.
Sending agents SHOULD emit the optional Content-Disposition field with
the filename parameter. If a sending agent emits a Content-Disposition
field with the filename parameter, it MUST use the file extensions
listed below that is consistent with the content type. Both of these
parameters SHOULD be set to the same filename with extension.

For support of legacy systems (i.e. DOS) the filename should be
limited to eight characters followed by a period followed by a three
letter extension. The eight character filename base can be any
distinct name; the use of the filename "smime" SHOULD be used to
indicate that the MIME entity is associated with S/MIME.

The filename extensions MUST correspond to the S/MIME message types
from this table:

S/MIME Type                     File Extension

application/pkcs7-signature         .p7s


application/pkcs7-mime              .p7m
(for signedData and
envelopedData)

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

application/pkcs10                  .p10

For instance:

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

<base64 data goes here>

If this message were of type application/pkcs10, then the filename
would be something like "smime.p10".

If S/MIME constructs are saved to or imported from disk, the same file
extensions SHOULD be used. This allows for static registry of the
filename extension and a particular associated "helper" application in
certain application environments.

Certain older MIME agents and gateways automatically convert unknown
subtypes of the MIME "application" type to application/octet-stream.
Sending agents including the optional name parameter and the optional
Content-Disposition: attachment;filename= parameter give at least some
indication that the included MIME entity is an S/MIME entity. S/MIME
agents and "helper" applications SHOULD be capable of processing
application/octet-stream messages with recognized S/MIME filename
extensions either automatically or by explicit user action.

. 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
 bits and a maximum key size of 1024 bits. A user agent SHOULD 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.

. 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 feasable, 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.


Appendix A - Object Identifiers & 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

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}

B. References

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

[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

[PEM] "Privacy-Enhanced Mail (PEM) basics", RFC 1421

[PKCS-1], "PKCS #1: RSA Encryption", draft has been submitted for RFC
status

[PKCS-7], "PKCS #7: Cryptographic Message Syntax", draft has been
submitted for RFC status

[PKCS-9], "PKCS #9: Selected Attribute Types", submission of draft
pending

[PKCS-10], "PKCS #10: Certification Request Syntax", draft has been
submitted for RFC status

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-manditory 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 -00 and -01 revisions of
this draft:

Some instances of "MUST" were de-capitalized or changed to words like
"need to" because they didn't specify absolute requirements.

Changed many instances of "MIME message" to "MIME data" to clarify
the fact that MIME parts are data, not messages.

Added Section 1.2 about MUST and SHOULD.

Added Section 1.5 about the mailing list and Web site for the draft.

Added "and SHOULD include these attributes in each signed and/or
encrypted message sent" to the last sentence of section 2.5.

Changed Section 2.6 into a single protocol. This caused major shifts
and lots of rewording in the section. Removed completely the concept
of "profiles."

Temporarily changed "RC2 CBC, key size 40 bits" to "FOO/40" and
explained why.

Removed MUST support in Section 2.6 for everything other than FOO/40,
and changed tripleDES to SHOULD.

Removed SHOULD lists for receiving and sending agents in Section 2.6.

Capitalized the SHOULDs in the capabilities list discussion in
previous Section 2.6.3. Also, reworded parts to reflect that all
capabilities should be recorded.

Added wording in previous Section 2.6.3.3 that messages used to
determine the encryption capabilities must have trusted signatures.

Changed the wording in Section 4.1 to reflect what is
cryptographically sound; also shortened discussion of US export.

Added Section 5 on security, and described the rationale for using
40-bit keys as the basis for S/MIME and why it is impossible to
accurately choose a key size for a particular message.

Removed OIDs from Appendix A.1 that were removed from Section 2.6.

Clarified the wording in Appendix G about the status of the S/MIME
name.

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: none

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: none

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: none

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

Section 3 is being reordered to make it easier to implement from. At
the same time, we need to look at:
 - new wording for section 3.5's "improper gateways" discussion.
   Need to make it clear that multipart/signed is always strongly
   preferred.
 - add back the description of how to do content transfer encoding.
 - look at whether nesting should be restricted like it is now.
 - whether or not to allow 8-bit data in signedData and envelopedData
   body parts, or must everything be forced to 7-bit first.

Make the micalg parameter required, not optional.

What to do if sending to multiple people with different known
capabilities? What if sending to a group, some of whom have
known capabilities, others with unknown?

Do we need better heuristics for determining the encryption
capabilities of a recipient? What about guessing based on
key length?

References to the encryption and hash algorithms.

Use of the S/MIME trademark.

Include an appendix with the proper requests for new MIME subtypes.

Need to list text values for the micalg parameter of multipart/signed.
RFC 1848 lists only "RSA-MD2" and "RSA-MD5", not "SHA-1".

What does PEM compatibility mean/entail, and do we care?

Look at the use of PKCS-7 "data" format.

Remove and/or deprecate signedAndEnveloped in the draft.

Remove the reference to PKCS-9.

G.  Trademarks

RSA Data Security, Inc., owns the US trademark for the name "S/MIME"
and for a logo associated with that name. RSA Data Security, Inc., is
considering allowing the use of the name for work done in the IETF.
The name "S/MIME" may or may not be used in future versions of this
draft.

H. Acknowledgements

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

I. Authors' addresses

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

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

Blake Ramsdell
Deming Internet Security
 NE 20th St., Suite C
Bellevue, WA 98005
(206) 882-8861
blaker@deming.com

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

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