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Versions: (RFC 2633) 00 01 02 03 04 05 06 07 08 09 RFC 3851

Internet Draft                                Editor:  Blake Ramsdell,
draft-ietf-smime-rfc2633bis-07.txt            Sendmail, Inc.
February 15, 2004
Expires August 15, 2004


               S/MIME Version 3.1 Message Specification

Status of this memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

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."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.


1. Introduction

S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a
consistent 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 data confidentiality (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 specification defines how to create a MIME body part that has
been cryptographically enhanced according to CMS [CMS], which is
derived from PKCS #7 [PKCS-7]. This specification also defines the
application/pkcs7-mime MIME type that can be used to transport those
body parts.

This specification also discusses how to use the multipart/signed MIME
type defined in [MIME-SECURE] to transport S/MIME signed messages.
multipart/signed is used in conjunction with the
application/pkcs7-signature MIME type, which is used to transport
a detached S/MIME signature.

In order to create S/MIME messages, an S/MIME agent MUST follow the
specifications in this document, as well as the specifications
listed in the Cryptographic Message Syntax [CMS].

Throughout this specification, 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

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [MUSTSHOULD].


1.3 Definitions

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

ASN.1: Abstract Syntax Notation One, as defined in CCITT X.208
[X.208-88].

BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209
[X.209-88].

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 [X.509-88].

7-bit data: Text data with lines less than 998 characters long, where
none of the characters have the 8th bit set, and there are no NULL
characters. <CR> and <LF> occur only as part of a <CR><LF> end of line
delimiter.

8-bit data: Text data with lines less than 998 characters, and where
none of the characters are NULL characters. <CR> and <LF> occur only
as part of a <CR><LF> end of line delimiter.

Binary data: Arbitrary data.

Transfer Encoding: A reversible transformation made on data so 8-bit
or binary data may be sent via a channel that only transmits 7-bit
data.

Receiving agent: software that interprets and processes S/MIME CMS
objects, MIME body parts that contain CMS content types, or both.

Sending agent: software that creates S/MIME CMS content types, MIME
body parts that contain CMS content types, or both.

S/MIME agent: user software that is a receiving agent, a sending
agent, or both.


1.4 Compatibility with Prior Practice of S/MIME

S/MIME version 3.1 agents should attempt to have the greatest
interoperability possible with agents for prior versions of S/MIME.
S/MIME version 2 is described in RFC 2311 through RFC 2315, inclusive
and S/MIME version 3 is described in RFC 2630 through RFC 2634
inclusive. RFC 2311 also has historical information about the
development of S/MIME.


1.5 Changes Since S/MIME v3.0

The RSA public key algorithm was changed to a MUST implement key
wrapping algorithm, and the Diffie-Hellman algorithm changed to a
SHOULD implement.

The AES symmetric encryption algorithm has been included as a SHOULD
implement.

The RSA public key algorithm was changed to a MUST implement signature
algorithm.

Ambiguous language about the use of "empty" SignedData messages to
transmit certificates was clarified to reflect that transmission of
certificate revocation lists is also allowed.

The use of binary encoding for some MIME entities is now explicitly
discussed.

Header protection through the use of the message/rfc822 MIME type has
been added.

Use of the CompressedData CMS type is allowed, along with required
MIME type and file extension additions.


1.6 Discussion of This Specification

This specification 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. CMS Options

CMS allows for a wide variety of options in content and algorithm
support. This section puts forth a number of support requirements and
recommendations in order to achieve a base level of interoperability
among all S/MIME implementations. [CMS] provides additional details
regarding the use of the cryptographic algorithms.


2.1 DigestAlgorithmIdentifier

Sending and receiving agents MUST support SHA-1 [CMSALG]. Receiving
agents SHOULD support MD5 [CMSALG] for the purpose of providing
backward compatibility with MD5-digested S/MIME v2 SignedData objects.


2.2 SignatureAlgorithmIdentifier

Receiving agents MUST support id-dsa-with-sha1 defined in [CMSALG].
The algorithm parameters MUST be absent (not encoded as NULL).
Receiving agents MUST support rsaEncryption, defined in [CMSALG].

Sending agents MUST support either id-dsa-with-sha1 or rsaEncryption.

If using rsaEncryption, sending and receiving agents MUST support the
algorithms in section 2.1 as specified.

Note that S/MIME v3 clients might only implement signing or signature
verification using id-dsa-with-sha1, and might also use id-dsa as an
AlgorithmIdentifier in this field. Receiving clients SHOULD recognize
id-dsa as equivalent to id-dsa-with-sha1, and sending clients MUST use
id-dsa-with-sha1 if using that algorithm. Also note that S/MIME v2
clients are only required to verify digital signatures using the
rsaEncryption algorithm with SHA-1 or MD5, and may not implement
id-dsa-with-sha1 at all.


2.3 KeyEncryptionAlgorithmIdentifier

Sending and receiving agents MUST support rsaEncryption, defined in
[CMSALG].

Sending and receiving agents SHOULD support Diffie-Hellman defined in
[CMSALG], using the ephemeral-static mode.

Note that S/MIME v3 clients might only implement key encryption and
decryption using the Diffie-Hellman algorithm. Also note that S/MIME
v2 clients are only capable of decrypting content-encryption keys
using the rsaEncryption algorithm.


2.4 General Syntax

There are several CMS content types. Of these, only the Data,
SignedData, EnvelopedData and CompressedData content types are
currently used for S/MIME.


2.4.1 Data Content Type

Sending agents MUST use the id-data content type identifier to
identify the "inner" MIME message content. For example, when applying
a digital signature to MIME data, the CMS signedData encapContentInfo
eContentType MUST include the id-data object identifier and the MIME
content MUST be stored in the SignedData encapContentInfo eContent
OCTET STRING (unless the sending agent is using multipart/signed, in
which case the eContent is absent, per section 3.4.3 of this
document). As another example, when applying encryption to MIME data,
the CMS EnvelopedData encryptedContentInfo ContentType MUST include
the id-data object identifier and the encrypted MIME content MUST be
stored in the envelopedData encryptedContentInfo encryptedContent
OCTET STRING.


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 data confidentiality 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 CompressedData Content Type

This content type is used to apply data compression to a message. This
content type does not provide authentication or privacy, and is only
used to reduce message size.

See section 3.6 for further guidance on the use of this type in
conjunction with other CMS types.


2.5 Attribute SignerInfo Type

The SignerInfo type allows the inclusion of unsigned and signed
attributes to be included along with a signature.

Receiving agents MUST be able to handle zero or one instance of each
of the signed attributes listed here. Sending agents SHOULD generate
one instance of each of the following signed attributes in each S/MIME
message:

- signingTime (section 2.5.1 in this document)
- sMIMECapabilities (section 2.5.2 in this document)
- sMIMEEncryptionKeyPreference (section 2.5.3 in this document)
- id-messageDigest (section 11.2 in [CMS])
- id-contentType (section 11.1 in [CMS])

Further, receiving agents SHOULD be able to handle zero or one
instance in the signingCertificate signed attribute, as defined in
section 5 of [ESS].

Sending agents SHOULD generate one instance of the signingCertificate
signed attribute in each SignerInfo structure.

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.

Interactive sending agents that include signed attributes that are not
listed here SHOULD display those attributes to the user, so that the
user is aware of all of the data being signed.


2.5.1 Signing-Time Attribute

The signing-time attribute is used to convey the time that a message
was signed. The time of signing will most likely be created by a
message originator and therefore is only as trustworthy as the
originator.

Sending agents MUST encode signing time through the year 2049 as
UTCTime; signing times in 2050 or later MUST be encoded as
GeneralizedTime. When the UTCTime CHOICE is used, S/MIME agents MUST
interpret the year field (YY) as follows:

if YY is greater than or equal to 50, the year is interpreted as 19YY;
if YY is less than 50, the year is interpreted as 20YY.


2.5.2 SMIMECapabilities Attribute

The SMIMECapabilities attribute includes signature algorithms (such as
"sha1WithRSAEncryption"), symmetric algorithms (such as "DES-EDE3-
CBC"), and key encipherment algorithms (such as "rsaEncryption").
There are also several identifiers which indicate support for other
optional features such as binary encoding and compression. 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.

If present, the SMIMECapabilities attribute MUST be a SignedAttribute;
it MUST NOT be an UnsignedAttribute. CMS defines SignedAttributes as a
SET OF Attribute. The SignedAttributes in a signerInfo MUST NOT
include multiple instances of the SMIMECapabilities attribute. CMS
defines the ASN.1 syntax for Attribute to include attrValues SET OF
AttributeValue. A SMIMECapabilities attribute MUST only include a
single instance of AttributeValue. There MUST NOT be zero or multiple
instances of AttributeValue present in the attrValues SET OF
AttributeValue.

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
object identifiers (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. Because of the requirement for identical encoding,
individuals documenting algorithms to be used in the SMIMECapabilities
attribute should explicitly document the correct byte sequence for the
common cases.

For any capability, 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.

The OIDs used with S/MIME are assigned in several different RFCs. Note
that all OIDs associated with the MUST and SHOULD implement algorithms
are included in section A of this document.

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 specification,
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.

Section 2.7.1 explains a strategy for caching capabilities.


2.5.2.1 SMIMECapability For the RC2 Algorithm

For the RC2 algorithm preference SMIMECapability, the capabilityID
MUST be set to the value rC2-CBC as defined in [CMSALG]. The
parameters field MUST contain SMIMECapabilitiesParametersForRC2CBC
(see appendix A).

Please note that the SMIMECapabilitiesParametersForRC2CBC is a single
INTEGER which contains the effective key length (NOT the corresponding
RC2 parameter version value). So, for example, for RC2 with a 128-bit
effective key length, the parameter would be encoded as the INTEGER
value 128, NOT the corresponding parameter version of 58.


2.5.3 Encryption Key Preference Attribute

The encryption key preference attribute allows the signer to
unambiguously describe which of the signer's certificates has the
signer's preferred encryption key. This attribute is designed to
enhance behavior for interoperating with those clients which use
separate keys for encryption and signing. This attribute is used to
convey to anyone viewing the attribute which of the listed
certificates should be used for encrypting a session key for future
encrypted messages.

If present, the SMIMEEncryptionKeyPreference attribute MUST be a
SignedAttribute; it MUST NOT be an UnsignedAttribute. CMS defines
SignedAttributes as a SET OF Attribute. The SignedAttributes in a
signerInfo MUST NOT include multiple instances of the
SMIMEEncryptionKeyPreference attribute. CMS defines the ASN.1 syntax
for Attribute to include attrValues SET OF AttributeValue. A
SMIMEEncryptionKeyPreference attribute MUST only include a single
instance of AttributeValue. There MUST NOT be zero or multiple
instances of AttributeValue present in the attrValues SET OF
AttributeValue.

The sending agent SHOULD include the referenced certificate in the set
of certificates included in the signed message if this attribute is
used. The certificate may be omitted if it has been previously made
available to the receiving agent. Sending agents SHOULD use this
attribute if the commonly used or preferred encryption certificate is
not the same as the certificate used to sign the message.

Receiving agents SHOULD store the preference data if the signature on
the message is valid and the signing time is greater than the
currently stored value. (As with the SMIMECapabilities, the clock skew
should be checked and the data not used if the skew is too great.)
Receiving agents SHOULD respect the sender's encryption key preference
attribute if possible. This however represents only a preference and
the receiving agent may use any certificate in replying to the sender
that is valid.

Section 2.7.1 explains a strategy for caching preference data.


2.5.3.1 Selection of Recipient Key Management Certificate

In order to determine the key management certificate to be used when
sending a future CMS envelopedData message for a particular recipient,
the following steps SHOULD be followed:

- If an SMIMEEncryptionKeyPreference attribute is found in a
  signedData object received from the desired recipient, this
  identifies the X.509 certificate that should be used as the X.509
  key management certificate for the recipient.

- If an SMIMEEncryptionKeyPreference attribute is not found in a
  signedData object received from the desired recipient, the set of
  X.509 certificates should be searched for a X.509 certificate with
  the same subject name as the signing X.509 certificate which can be
  used for key management.

- Or use some other method of determining the user's key management
  key. If a X.509 key management certificate is not found, then
  encryption cannot be done with the signer of the message. If
  multiple X.509 key management certificates are found, the S/MIME
  agent can make an arbitrary choice between them.


2.6 SignerIdentifier SignerInfo Type

S/MIME v3.1 and S/MIME v3 requires the use of SignerInfo version 1,
that is the issuerAndSerialNumber CHOICE MUST be used for
SignerIdentifier.

S/MIME v3.1 implementations MUST allow for the use of the choice of
subjectKeyIdentifier in messages. Messages that use this choice cannot
be read by S/MIME v2 clients, and MUST be handled gracefully in S/MIME
v3.1 clients.

It is important to understand that some certificates use a value for
subjectKeyIdentifier that is not suitable for uniquely identifying a
certificate. Implementations MUST be prepared for multiple
certificates for potentially different entities to have the same value
for subjectKeyIdentifier, and MUST be prepared to try each matching
certificate during signature verification before indicating an error
condition.


2.7 ContentEncryptionAlgorithmIdentifier

Sending and receiving agents MUST support encryption and decryption
with DES EDE3 CBC, hereinafter called "tripleDES" [CMSALG]. Receiving
agents SHOULD support encryption and decryption using the RC2 [CMSALG]
or a compatible algorithm at a key size of 40 bits, hereinafter called
"RC2/40". Sending and receiving agents SHOULD support encryption and
decryption with AES [CMSAES] at a key size of 128, 192 and 256 bits.


2.7.1 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 2.5.2 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 capabilities list 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 2.7.2.1 through 2.7.2.4 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.


2.7.1.1 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.


2.7.1.2 Rule 2: Unknown Capabilities, Unknown Version of S/MIME

If the following two conditions are met:
 - the sending agent has no knowledge of the encryption capabilities
   of the recipient,
 - and the sending agent has no knowledge of the version of S/MIME
   of the recipient,
then the sending agent SHOULD use tripleDES because it is a stronger
algorithm and is required by S/MIME v3. If the sending agent chooses
not to use tripleDES in this step, it SHOULD use RC2/40.


2.7.2 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.


2.7.3 Multiple Recipients

If a sending agent is composing an encrypted message to a group of
recipients where the encryption capabilities of some of the recipients
do not overlap, the sending agent is forced to send more than one
message. It should be noted that if the sending agent chooses to send
a message encrypted with a strong algorithm, and then send the same
message encrypted with a weak algorithm, someone watching the
communications channel may be able to learn the contents of the
strongly-encrypted message simply by decrypting the weakly-encrypted
message.


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 CMS
content types. Several MIME types as well as several CMS content types
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 CMS processing facilities which produces a
CMS object. The CMS object is then finally wrapped in MIME. The
Enhanced Security Services for S/MIME [ESS] document provides examples
of how nested, secured S/MIME messages are formatted. ESS provides an
example of how a triple-wrapped S/MIME message is formatted using
multipart/signed and application/pkcs7-mime for the signatures.

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] and [MIME-SECURE].


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. If protection of the RFC-822
headers is required, the use of the message/rfc822 MIME type is
explained later in this section.

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 CMS content types 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 processed, 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.

In order to protect outer, non-content related message headers (for
instance, the "Subject", "To", "From" and "CC" fields), the sending
client MAY wrap a full MIME message in a message/rfc822 wrapper in
order to apply S/MIME security services to these headers. It is up to
the receiving client to decide how to present these "inner" headers
along with the unprotected "outer" headers.

When an S/MIME message is received, if the top-level protected MIME
entity has a Content-Type of message/rfc822, it can be assumed that
the intent was to provide header protection. This entity should be
presented as the top-level message, taking into account header merging
issues as previously discussed.


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 and
compressing 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 non-security part
of 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 charset should be a registered charset [CHARSETS].
The details of the canonicalization are specified in [MIME-SPEC]. The
chosen charset SHOULD be named in the charset parameter so that the
receiving agent can unambiguously determine the charset used.

Note that some charsets such as ISO-2022 have multiple representations
for the same characters. When preparing such text for signing, the
canonical representation specified for the charset 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 7BIT.

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.

S/MIME implementations which "know" that all intended recipient(s) are
capable of handling inner (all but the outermost) binary MIME objects
SHOULD use binary encoding as opposed to a 7-bit-safe transfer
encoding for the inner entities. The use of a 7-bit-safe encoding
(such as base64) would unnecessarily expand the message size.
Implementations MAY "know" that recipient implementations are capable
of handling inner binary MIME entities either by interpreting the
id-cap-preferBinaryInside sMIMECapabilities attribute, by prior
agreement, or by other means.

If one or more intended recipients are unable to handle inner binary
MIME objects, or if this capability in unknown for any of the intended
recipients, S/MIME implementations SHOULD use transfer encoding
described in section 3.1.3 for all MIME entities they secure.


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 an 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 specification?

    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: image/jpeg
    Content-Transfer-Encoding: base64

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

    --bar--


3.2 The application/pkcs7-mime Type

The application/pkcs7-mime type is used to carry CMS content types
including envelopedData, signedData and compressedData. The details of
constructing these entities is described in subsequent sections. This
section describes the general characteristics of the
application/pkcs7-mime type.

The carried CMS object always contains a MIME entity that is prepared
as described in section 3.1 if the eContentType is id-data. Other
contents may be carried when the eContentType contains different
values. See [ESS] for an example of this with signed receipts.

Since CMS content types 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 CMS
object or "outside" MIME entity. It is completely distinct from, and
unrelated to, the transfer encoding of the MIME entity secured by the
CMS object, the "inside" object, which is described in section 3.1.

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 [CONTDISP] 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 (signedData, envelopedData)      .p7m

Application/pkcs7-mime (degenerate signedData           .p7c
certificate management message)

Application/pkcs7-mime (compressedData)                 .p7z

Application/pkcs7-signature                             .p7s

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.


3.2.2 The smime-type parameter

The application/pkcs7-mime content type defines the optional "smime-
type" parameter. The intent of this parameter is to convey details
about the security applied (signed or enveloped) along with
information about the contained content. This specification defines
the following smime-types.

Name                   CMS type                Inner Content

enveloped-data         EnvelopedData           id-data

signed-data            SignedData              id-data

certs-only             SignedData              none

compressed-data        CompressedData          id-data

In order that consistency can be obtained with future, the following
guidelines should be followed when assigning a new smime-type
parameter.

1. If both signing and encryption can be applied to the content, then
two values for smime-type SHOULD be assigned "signed-*" and
"encrypted-*". If one operation can be assigned then this may be
omitted. Thus since "certs-only" can only be signed, "signed-" is
omitted.

2. A common string for a content oid should be assigned. We use "data"
for the id-data content OID when MIME is the inner content.

3. If no common string is assigned. Then the common string of
"OID.<oid>" is recommended (for example, "OID.1.3.6.1.5.5.7.6.1" would
be DES40).

It is explicitly intended that this field be a suitable hint for mail
client applications to indicate whether a message is "signed" or
"encrypted" without having to tunnel into the CMS payload.


3.3 Creating an Enveloped-only Message

This section describes the format for enveloping a MIME entity without
signing it. It is important to note that sending enveloped but not
signed messages does not provide for data integrity. It is possible to
replace ciphertext in such a way that the processed message will still
be valid, but the meaning may be altered.

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
CMS object of type envelopedData. In addition to encrypting a copy of
the content-encryption key for each recipient, a copy of the content-
encryption key SHOULD be encrypted for the originator and included in
the envelopedData (see CMS Section 6).

Step 3. The envelopedData object is wrapped in a CMS ContentInfo
object.

Step 4. The ContentInfo 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:
application/pkcs7-mime with SignedData, and multipart/signed. In
general, the multipart/signed form is preferred for sending, and
receiving agents SHOULD be able to handle both.


3.4.1 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, the signedData
format protects the message content from being changed by benign
intermediate agents. Such agents might do line wrapping or
content-transfer encoding changes which would break the signature.


3.4.2 Signing Using application/pkcs7-mime with 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
CMS object of type signedData

Step 3. The signedData object is wrapped in a CMS ContentInfo
object.

Step 4. The ContentInfo object is inserted into an
application/pkcs7-mime MIME entity.

The smime-type parameter for messages using application/pkcs7-mime
with 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


3.4.3 Signing Using the multipart/signed Format

This format is a clear-signing format. Recipients without any S/MIME
or CMS 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 signed; the second part contains the "detached
signature" CMS SignedData object in which the encapContentInfo
eContent field is absent.


3.4.3.1 The application/pkcs7-signature MIME Type

This MIME type always contains a CMS ContentInfo containing a single
CMS object of type signedData. The signedData encapContentInfo
eContent field MUST be absent. The signerInfos field contains the
signatures for the MIME entity.

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


3.4.3.2 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 CMS processing in order to
obtain an object of type signedData in which the encapContentInfo
eContent field is absent.

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" CMS
SignedData object 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(s) used in the calculation of the Message
Integrity Check. If multiple message digest algorithms are used they
MUST be separated by commas per [MIME-SECURE]. The values to be placed
in the micalg parameter SHOULD be from the following:

Algorithm   Value
used

MD5         md5
SHA-1       sha1
SHA-256     sha256
SHA-384     sha384
SHA-512     sha512
Any other   (defined separately in algorithm profile or "unknown"
             if not defined)

(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.

The SHA-256, SHA-384 and SHA-512 algorithms [FIPS180-2] are not
currently supported in S/MIME, and are included here for completeness.


3.4.3.3 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--

The content that is digested (the first part of the multipart/signed)
are the bytes:

43 6f 6e 74 65 6e 74 2d 54 79 70 65 3a 20 74 65 78 74 2f 70 6c 61 69
6e 0d 0a 0d 0a 54 68 69 73 20 69 73 20 61 20 63 6c 65 61 72 2d 73 69
67 6e 65 64 20 6d 65 73 73 61 67 65 2e 0d 0a


3.5 Creating an Compressed-only Message

This section describes the format for compressing a MIME entity.
Please note that versions of S/MIME prior to 3.1 did not specify any
use of compressedData, and will not recognize it.  The use of a
capability to indicate the ability to receive compressedData is
described in [CMSCOMPR] and is the preferred method for compatibility.

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
CMS object of type compressedData.

Step 3. The compressedData object is wrapped in a CMS ContentInfo
object.

Step 4. The ContentInfo object is inserted into an
application/pkcs7-mime MIME entity.

The smime-type parameter for compressed-only messages is "compressed-
data". The file extension for this type of message is ".p7z".

A sample message would be:

    Content-Type: application/pkcs7-mime; smime-type=compressed-data;
         name=smime.p7z
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename=smime.p7z

    rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
    7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
    f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
    0GhIGfHfQbnj756YT64V


3.6 Multiple Operations

The signed-only, encrypted-only, and compressed-only MIME formats can
be nested. This works because these formats are all MIME entities that
encapsulate other MIME entities.

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 apply any of the signing, encrypting and compressing
operations in any order. 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.

There are security ramifications to choosing whether to sign first or
encrypt first. A recipient of a message that is encrypted and then
signed can validate that the encrypted block was unaltered, but cannot
determine any relationship between the signer and the unencrypted
contents of the message. A recipient of a message that is signed-then-
encrypted can assume that the signed message itself has not been
altered, but that a careful attacker may have changed the
unauthenticated portions of the encrypted message.

When using compression, keep the following guidelines in mind:

- Compression of binary encoded encrypted data is discouraged, since
  it will not yield significant compression. Base64 encrypted data
  could very well benefit, however.
- If a lossy compression algorithm is used with signing, you will need
  to compress first, then sign.


3.7 Creating a Certificate Management Message

The certificate management message or MIME entity is used to transport
certificates and/or certificate revocation lists, such as in response
to a registration request.

Step 1. The certificates and/or certificate revocation lists are made
available to the CMS generating process which creates a CMS object of
type signedData. The signedData encapContentInfo eContent field MUST
be absent and signerInfos field MUST be empty.

Step 2. The signedData object is wrapped in a CMS ContentInfo
object.

Step 3. The ContentInfo object is enclosed in an application/pkcs7-
mime MIME entity

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


3.8 Registration Requests

A sending agent that signs messages MUST have a certificate for the
signature so that a receiving agent can verify the signature. There
are many ways of getting certificates, such as through an exchange
with a certificate authority, through a hardware token or diskette,
and so on.

S/MIME v2 [SMIMEV2] specified a method for "registering" public keys
with certificate authorities using an application/pkcs10 body part.
Since that time, the IETF PKIX Working Group has developed other
methods for requesting certificates. However, S/MIME v3.1 does not
require a particular certificate request mechanism.


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 of the criteria listed 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:   multipart/signed
parameters:  protocol="application/pkcs7-signature"
file suffix: any

MIME type:   application/octet-stream
parameters:  any
file suffix: p7m, p7s, p7c, p7z


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 specification does not cover how S/MIME agents handle
certificates, only what they do after a certificate has been validated
or rejected. S/MIME certification issues are covered in [CERT31].

At a minimum, for initial S/MIME deployment, a user agent could
automatically generate a message to an intended recipient requesting
that recipient's certificate in a signed return message. Receiving and
sending agents SHOULD also provide a mechanism to allow a user to
"store and protect" certificates for correspondents in such a way so
as to guarantee their later retrieval.


4.1 Key Pair Generation

All generated key pairs MUST be generated from a good source of non-
deterministic random input [RANDOM] and the private key MUST be
protected in a secure fashion.

If an S/MIME agent needs to generate an RSA key pair, then the S/MIME
agent or some related administrative utility or function SHOULD
generate RSA key pairs using the following guidelines. A user agent
SHOULD generate RSA key pairs at a minimum key size of 768 bits. A
user agent MUST NOT generate RSA key pairs less than 512 bits long.
Creating keys longer than 1024 bits may cause some older S/MIME
receiving agents to not be able to verify signatures, but gives better
security and is therefore valuable. A receiving agent SHOULD be able
to verify signatures with keys of any size over 512 bits. 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

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
specification gives a framework for using those estimates in choosing
algorithms.

If a sending agent is sending the same message using different
strengths of cryptography, an attacker watching the communications
channel may be able to determine the contents of the strongly-
encrypted message by decrypting the weakly-encrypted version. In other
words, a sender should not send a copy of a message using weaker
cryptography than they would use for the original of the message.

Modification of the ciphertext can go undetected if authentication is
not also used, which is the case when sending EnvelopedData without
wrapping it in SignedData or enclosing SignedData within it.

See RFC 3218 [MMA] for more information about thwarting the adaptive
chosen ciphertext vulnerability in PKCS #1 Version 1.5
implementations.

In some circumstances the use of the Diffie-Hellman key agreement
scheme in a prime order subgroup of a large prime p is vulnerable to
certain attacks known as "small-subgroup" attacks. Methods exist,
however, to prevent these attacks. These methods are described in RFC
2785 [DHSUB].


A. ASN.1 Module

SecureMimeMessageV3dot1
  { iso(1) member-body(2) us(840) rsadsi(113549)
         pkcs(1) pkcs-9(9) smime(16) modules(0) msg-v3dot1(21) }

DEFINITIONS IMPLICIT TAGS ::=
BEGIN

IMPORTS
-- Cryptographic Message Syntax
    SubjectKeyIdentifier, IssuerAndSerialNumber,
    RecipientKeyIdentifier
        FROM    CryptographicMessageSyntax
               { iso(1) member-body(2) us(840) rsadsi(113549)
                 pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2001(14) };


--  id-aa is the arc with all new authenticated and unauthenticated
--  attributes produced the by S/MIME Working Group

id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840)
        rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)}

-- S/MIME Capabilities provides a method of broadcasting the symetric
-- capabilities understood.  Algorithms should be ordered by
-- preference and grouped by type

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

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

SMIMECapabilities ::= SEQUENCE OF SMIMECapability

-- Encryption Key Preference provides a method of broadcasting the
-- prefered encryption certificate.

id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11}

SMIMEEncryptionKeyPreference ::= CHOICE {
   issuerAndSerialNumber   [0] IssuerAndSerialNumber,
   receipentKeyId          [1] RecipientKeyIdentifier,
   subjectAltKeyIdentifier [2] SubjectKeyIdentifier
}

id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
   us(840) rsadsi(113549) pkcs(1) pkcs9(9) 16 }

id-cap  OBJECT IDENTIFIER ::= { id-smime 11 }

-- The preferBinaryInside indicates an ability to receive messages
-- with binary encoding inside the CMS wrapper

id-cap-preferBinaryInside  OBJECT IDENTIFIER ::= { id-cap 1 }

--  The following list the OIDs to be used with S/MIME V3

-- Signature Algorithms Not Found in [CMSALG]
--
-- md2WithRSAEncryption OBJECT IDENTIFIER ::=
--    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
--     2}
--
-- Other Signed Attributes
--
-- signingTime OBJECT IDENTIFIER ::=
--    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
--     5}
--    See [CMS] for a description of how to encode the attribute
--    value.

SMIMECapabilitiesParametersForRC2CBC ::= INTEGER
--        (RC2 Key Length (number of bits))

END


B. Normative References

[CERT31] "S/MIME Version 3.1 Certificate Handling", Internet Draft
draft-ietf-smime-rfc2632bis

[CHARSETS] Character sets assigned by IANA. See <ftp://ftp.isi.edu/in-
notes/iana/assignments/character-sets>.

[CMS] "Cryptographic Message Syntax", RFC 3369

[CMSAES] "Use of the AES Encryption Algorithm in CMS",
draft-ietf-smime-aes-alg-07

[CMSALG] "Cryptographic Message Syntax (CMS) Algorithms", RFC 3370

[CMSCOMPR] "Compressed Data Content Type for Cryptographic Message
Syntax (CMS)", RFC 3274

[CONTDISP] "Communicating Presentation Information in Internet
Messages: The Content-Disposition Header Field", RFC 2183

[ESS] "Enhanced Security Services for S/MIME", RFC 2634

[FIPS180-2] "Secure Hash Signature Standard (SHS)", National Institute
of Standards and Technology (NIST). FIPS Publication 180-2

[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

[X.208-88] CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.

[X.209-88] CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One (ASN.1). 1988.

[X.509-88] CCITT. Recommendation X.509: The Directory - Authentication
Framework. 1988.


C. Informative References

[DHSUB] "Methods for Avoiding the "Small-Subgroup" Attacks on the
Diffie-Hellman Key Agreement Method for S/MIME", RFC 2785

[MMA] "Preventing the Million Message Attack on CMS", RFC 3218

[PKCS-7] "PKCS #7: Cryptographic Message Syntax Version 1.5", RFC 2315

[RANDOM] "Randomness Recommendations for Security", RFC 1750

[SMIMEV2] "S/MIME Version 2 Message Specification", RFC 2311


D. Acknowledgements

Many thanks go out to the other authors of the S/MIME Version 2
Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence
Lundblade and Lisa Repka.

A number of the members of the S/MIME Working Group have also worked
very hard and contributed to this document. Any list of people is
doomed to omission, and for that I apologize. In alphabetical order,
the following people stand out in my mind due to the fact that they
made direct contributions to this document.

Tony Capel
Piers Chivers
Dave Crocker
Bill Flanigan
Peter Gutmann
Paul Hoffman
Russ Housley
William Ottaway
John Pawling
Jim Schaad


E. Editor's address

Blake Ramsdell
Sendmail, Inc.
704 228th Ave NE #775
Sammamish, WA  98074

blake@sendmail.com


F. Changes from last draft

Updated editor contact info (Blake Ramsdell)

Separated normative and informative references (Jim Schaad)

Added language clarifying non-uniqueness of subjectKeyIdentifier (List
discussion)

smime-type clarifications (List discussion)

RC2 S/MIME capability documented (Jim Schaad)


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