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S/MIME Version 3 Message Specification

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 2633.
Author Blake C. Ramsdell
Last updated 2020-01-21 (Latest revision 1999-04-26)
RFC stream Internet Engineering Task Force (IETF)
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IESG IESG state RFC 2633 (Proposed Standard)
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Internet Draft                                 Editor:  Blake Ramsdell,
draft-ietf-smime-msg-08.txt                    Worldtalk
April 23, 1999                                 
Expires October 23, 1999                       

                S/MIME Version 3 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

The list of Internet-Draft Shadow Directories can be accessed at

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 privacy and data security (using encryption).

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

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

1.1 Specification Overview

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

This draft defines how to create a MIME body part that has been
cryptographically enhanced according to CMS [CMS], which is derived
from PKCS #7 [PKCS-7]. This draft also defines the application/pkcs7-
mime MIME type that can be used to transport those body parts.

This draft also 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.

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

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

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described 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

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

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

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

Sending agent: software that creates S/MIME CMS objects, MIME body
parts that contain CMS objects, 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 agents should attempt to have the greatest
interoperability possible with S/MIME version 2 agents. S/MIME version
2 is described in RFC 2311 through RFC 2315, inclusive. RFC 2311 also
has historical information about the development of 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:

with the single word


in the body of the message. There is a Web site for the mailing list
at <>.

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 [SHA1].  Receiving
agents SHOULD support MD5 [MD5] for the purpose of providing backward
compatibility with MD5-digested S/MIME v2 SignedData objects.

2.2 SignatureAlgorithmIdentifier

Sending and receiving agents MUST support id-dsa defined in [DSS].
The algorithm parameters MUST be absent (not encoded as NULL).

Receiving agents SHOULD support rsaEncryption, defined in [PKCS-1].

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

Note that S/MIME v2 clients are only capable of verifying digital
signatures using the rsaEncryption algorithm.

2.3 KeyEncryptionAlgorithmIdentifier

Sending and receiving agents MUST support Diffie-Hellman defined in

Receiving agents SHOULD 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 SHOULD support rsaEncryption.

Note that S/MIME v2 clients are only capable of decrypting content
encryption keys using the rsaEncryption algorithm.

2.4 General Syntax

CMS defines multiple content types.  Of these, only the Data,
SignedData, and EnvelopedData content types are currently used for

2.4.1 Data Content Type

Sending agents MUST use the id-data content type identifier to
indicate the message content which has had security services applied
to it. 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 privacy protection to a message. A
sender needs to have access to a public key for each
intended message recipient to use this service. This content type does
not provide authentication.

2.5 Attribute SignerInfo Type

The SignerInfo type allows the inclusion of 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
- signingTime (section 2.5.1 in this document)
- sMIMECapabilities (section 2.5.2 in this document)
- sMIMEEncryptionKeyPreference (section 2.5.3 in this document)

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

Sending agents SHOULD generate one instance of the signingCertificate
signed attribute in each S/MIME message.

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.

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. 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.

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"). 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.

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

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

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

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 (OIDs Used with S/MIME) that is centrally
maintained and is separate from this draft. The list of OIDs is
maintained by the Internet Mail Consortium at <
smime/oids.html>. Note that all OIDs associated with the MUST and
SHOULD implement algorithms are included in section A of this

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

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

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

2.5.3 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

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. 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 requires the use of SignerInfo version 1, that is the
issuerAndSerialNumber CHOICE MUST be used for SignerIdentifier.

2.7 ContentEncryptionAlgorithmIdentifier

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

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

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

- If the receiving agent has not yet created a list of capabilities
   for the sender's public key, then, after verifying the signature
   on the incoming message and checking the timestamp, the receiving
   agent SHOULD create a new list containing at least the signing
   time and the symmetric capabilities.
 - If such a list already exists, the receiving agent SHOULD verify
   that the signing time in the incoming message is greater than
   the signing time stored in the list and that the signature is
   valid. If so, the receiving agent SHOULD update both the signing
   time and capabilities in the list. Values of the signing time that
   lie far in the future (that is, a greater discrepancy than any
   reasonable clock skew), or a 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

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

Sections through 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. 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. Rule 2: Unknown Capabilities, Known Use of Encryption

 - 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
 - and the last encrypted message received from the recipient had a
   trusted signature on it,
then the outgoing message SHOULD use the same encryption algorithm as
was used on the last signed and encrypted message received from the
recipient. Rule 3: Unknown Capabilities, Unknown Version of S/MIME

 - 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

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
objects. Several MIME types as well as several CMS objects are used.
The data to be secured is always a canonical MIME entity. The MIME
entity and other data, such as certificates and algorithm identifiers,
are given to 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

3.1 Preparing the MIME Entity for Signing or Enveloping

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

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

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

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.

3.1.1 Canonicalization

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

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

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
- 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

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

    =A1Hola Michael!

    How do you like the new S/MIME specification?

    I agree. It's generally a good idea to encode lines that begin
    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

    Content-Type: image/jpeg
    Content-Transfer-Encoding: base64



3.2 The application/pkcs7-mime Type

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

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

Note that this discussion refers to the transfer encoding of the 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,      .p7m
Application/pkcs7-mime (degenerate       .p7c
signedData "certs-only" message)
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

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

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 infomation
about the contained content. This draft defines the following smime-

Name                   Security                Inner Content
enveloped-data         EnvelopedData           id-data
signed-data            SignedData              id-data
certs-only             SignedData              none

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

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." would
be DES40).

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 CMS object is inserted into an application/pkcs7-mime MIME

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;
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename=smime.p7m


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

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

3.4.2 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 CMS object is inserted into an application/pkcs7-mime MIME

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;
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename=smime.p7m


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. The application/pkcs7-signature MIME Type

This MIME type always contains 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". 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

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
MD5         md5
SHA-1       sha1
Any other   unknown

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

    Content-Type: multipart/signed;
       micalg=sha1; boundary=boundary42

    Content-Type: text/plain

    This is a clear-signed message.

    Content-Type: application/pkcs7-signature; name=smime.p7s
    Content-Transfer-Encoding: base64
    Content-Disposition: attachment; filename=smime.p7s



3.5 Signing and Encrypting

To achieve signing and enveloping, any of the signed-only and
encrypted-only formats may be nested. This is allowed because the
above formats are all MIME entities, and because they all secure MIME

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

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

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.

3.6 Creating a Certificates-only Message

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

Step 1. The certificates are made available to the 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 CMS signedData object is enclosed in an application/pkcs7-
mime MIME entity

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

3.7 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.
The IETF's PKIX Working Group is preparing another method for
requesting certificates; however, that work was not finished at the
time of this draft. S/MIME v3 does not specify how to request a
certificate, but instead mandates that every sending agent already has
a certificate. Standardization of certificate management is being
pursued separately in the IETF.

3.8 Identifying an S/MIME Message

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

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

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

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

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

4. Certificate Processing

A receiving agent MUST provide some certificate retrieval mechanism in
order to gain access to certificates for recipients of digital
envelopes. This draft does not cover how S/MIME agents handle
certificates, only what they do after a certificate has been validated
or rejected. S/MIME certification issues are covered in [CERT3].

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

If an S/MIME agent needs to generate a key pair, then the S/MIME agent
or some related administrative utility or function MUST be capable of
generating separate DH and DSS public/private key pairs on behalf of
the user. Each key pair 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 a key pair, then the S/MIME agent
or some related administrative utility or function SHOULD generate RSA
key pairs.

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

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

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

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

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.

A. ASN.1 Module

  { iso(1) member-body(2) us(840) rsadsi(113549)
         pkcs(1) pkcs-9(9) smime(16) modules(0) smime(4) }


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

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

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

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

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

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

SMIMECapabilities ::= SEQUENCE OF SMIMECapability

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

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

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

-- The Content Encryption Algorithms defined for SMIME are:

-- Triple-DES is the manditory algorithm with CBCParameter being the

   {iso(1) member-body(2) us(840) rsadsi(113549)
encryptionAlgorithm(3) 7}

CBCParameter ::= IV


--  RC2 (or compatable) is an optional algorithm w/ RC2-CBC-paramter
as the

   {iso(1) member-body(2) us(840) rsadsi(113549)
encryptionAlgorithm(3) 2}

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

RC2-CBC-parameter ::=  SEQUENCE {
   rc2ParameterVersion  INTEGER,
   iv                   IV}

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

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

-- Digest Algorithms:

--       {iso(1) member-body(2) us(840) rsadsi(113549)
digestAlgorithm(2) 5}

---      {iso(1) identified-organization(3) oiw(14) secsig(3)
--        26}

-- Asymmetric Encryption Algorithms
-- rsaEncryption OBJECT IDENTIFIER ::=
--   {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
--   {joint-iso-ccitt(2) ds(5) algorithm(8) encryptionAlgorithm(1) 1}
--    {iso(1) member-body(2) us(840) x9-57(10040) x9cm(4) 1 }

-- Signature Algorithms
-- md2WithRSAEncryption OBJECT IDENTIFIER ::=
--    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
-- md5WithRSAEncryption OBJECT IDENTIFIER ::=
--    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
-- sha-1WithRSAEncryption OBJECT IDENTIFIER ::=
--    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
-- id-dsa-with-sha1 OBJECT IDENTIFIER ::=
--    {iso(1) member-body(2) us(840) x9-57(10040) x9cm(4) 3}

-- Other Signed Attributes
-- signingTime OBJECT IDENTIFIER ::=
--    {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
--    See [CMS] for a description of how to encode the attribute


B. References

[3DES] ANSI X9.52-1998, "Triple Data Encryption Algorithm Modes of
Operation", American National Standards Institute, 1998.

[CERT3] "S/MIME Version 3 Certificate Handling", Internet Draft draft-

[CHARSETS] Character sets assigned by IANA. See <

[CMS] "Cryptographic Message Syntax", Internet Draft draft-ietf-smime-

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

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

[DH] "Diffie-Hellman Key Agreement Method", Internet Draft draft-ietf-

[DSS] NIST FIPS PUB 186, "Digital Signature Standard", 18 May 1994.

[ESS] "Enhanced Security Services for S/MIME", Internet draft, draft-

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

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

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

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

[PKCS-1] "PKCS #1: RSA Encryption Version 1.5", RFC 2313

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

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

[RC2] "A Description of the RC2 (r) Encryption Algorithm", RFC 2268

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

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

C. 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. Without v2, there wouldn't be a v3.

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.

Dave Crocker
Bill Flanigan
Paul Hoffman
Russ Housley
John Pawling
Jim Schaad

D. Changes from last draft

Changed section 1.3 to contain correct reference language for
MUSTSHOULD (Thomas Narten)
Renumbered section F to section E (Blake Ramsdell)
Changed section E to update author's address (Blake Ramsdell)
Changed <TBD> in section C to actual list (Blake Ramsdell)

E. Editor’s address

Blake Ramsdell
17720 NE 65th St Ste 201
Redmond, WA 98052
+1 425 376 0225