Network Working Group S. Dusse

Request for Comments: 2311 RSA Data Security

Category: Informational P. Hoffman

Internet Mail Consortium

B. Ramsdell

Worldtalk

L. Lundblade

Qualcomm

L. Repka

Netscape

March 1998

S/MIME Version 2 Message Specification

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. Distribution of this

memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

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.

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Please note: The information in this document is historical material

being published for the public record. It is not an IETF standard.

The use of the word "standard" in this document indicates a standard

for adopters of S/MIME version 2, not an IETF standard.

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 memo defines how to create a MIME body part that has been

cryptographically enhanced according to PKCS #7 [PKCS-7]. This memo

also defines the application/pkcs7-mime MIME type that can be used to

transport those body parts. This memo also defines how to create

certification requests that conform to PKCS #10 [PKCS-10], and the

application/pkcs10 MIME type for transporting those requests.

This memo also discusses how to use the multipart/signed MIME type

defined in [MIME-SECURE] to transport S/MIME signed messages. This

memo also defines the application/pkcs7-signature MIME type, which is

also used to transport S/MIME signed messages. This specification is

compatible with PKCS #7 in that it uses the data types defined by

PKCS #7.

In order to create S/MIME messages, an agent has to follow

specifications in this memo, as well as some of the specifications

listed in the following documents:

- "PKCS #1: RSA Encryption", [PKCS-1]

- "PKCS #7: Cryptographic Message Syntax", [PKCS-7]

- "PKCS #10: Certification Request Syntax", [PKCS-10]

Throughout this memo, 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

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data. An automated process that sends an encrypted message might not

be able to receive an encrypted message at all, for example. Thus,

the requirements and recommendations for the two types of agents are

listed separately when appropriate.

1.2 Terminology

Throughout this memo, the terms MUST, MUST NOT, SHOULD, and SHOULD

NOT are used in capital letters. This conforms to the definitions in

[MUSTSHOULD]. [MUSTSHOULD] defines the use of these key words to

help make the intent of standards track documents as clear as

possible. The same key words are used in this document to help

implementors achieve interoperability.

1.3 Definitions

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

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

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

Certificate: A type that binds an entity's distinguished name to a

public key with a digital signature.

DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT

X.509.

7-bit data: Text data with lines less than 998 characters long, where

none of the characters have the 8th bit set, and there are no NULL

characters. <CR> and <LF> occur only as part of a <CR><LF> end of

line delimiter.

8-bit data: Text data with lines less than 998 characters, and where

none of the characters are NULL characters. <CR> and <LF> occur only

as part of a <CR><LF> end of line delimiter.

Binary data: Arbitrary data.

Transfer Encoding: A reversible transformation made on data so 8-bit

or binary data may be sent via a channel that only transmits 7-bit

data.

1.4 Compatibility with Prior Practice of S/MIME

Appendix C contains important information about how S/MIME agents

following this specification should act in order to have the greatest

interoperability with earlier implementations of S/MIME.

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2. PKCS #7 Options

The PKCS #7 message format allows for a wide variety of options in

content and algorithm support. This section puts forth a number of

support requirements and recommendations in order to achieve a base

level of interoperability among all S/MIME implementations.

2.1 DigestAlgorithmIdentifier

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

Sending agents SHOULD use SHA-1.

2.2 DigestEncryptionAlgorithmIdentifier

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

Receiving agents MUST support verification of signatures using RSA

public key sizes from 512 bits to 1024 bits.

Sending agents MUST support rsaEncryption. Outgoing messages are

signed with a user's private key. The size of the private key is

determined during key generation.

2.3 KeyEncryptionAlgorithmIdentifier

Receiving agents MUST support rsaEncryption. Incoming encrypted

messages contain symmetric keys which are to be decrypted with a

user's private key. The size of the private key is determined during

key generation.

Sending agents MUST support rsaEncryption. Sending agents MUST

support encryption of symmetric keys with RSA public keys at key

sizes from 512 bits to 1024 bits.

2.4 General Syntax

The PKCS #7 defines six distinct content types: "data", "signedData",

"envelopedData", "signedAndEnvelopedData", "digestedData", and

"encryptedData". Receiving agents MUST support the "data",

"signedData" and "envelopedData" content types. Sending agents may or

may not send out any of the content types, depending on the services

that the agent supports.

2.4.1 Data Content Type

Sending agents MUST use the "data" content type as the content within

other content types to indicate the message content which has had

security services applied to it.

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2.4.2 SignedData Content Type

Sending agents MUST use the signedData content type to apply a

digital signature to a message or, in a degenerate case where there

is no signature information, to convey certificates.

2.4.3 EnvelopedData Content Type

This content type is used to apply privacy protection to a message. A

sender needs to have access to a public key for each intended message

recipient to use this service. This content type does not provide

authentication.

2.5 Attribute SignerInfo Type

The SignerInfo type allows the inclusion of unauthenticated and

authenticated attributes to be included along with a signature.

Receiving agents MUST be able to handle zero or one instance of each

of the signed attributes described in this section.

Sending agents SHOULD be able to generate one instance of each of the

signed attributes described in this section, and SHOULD include these

attributes in each signed message sent.

Additional attributes and values for these attributes may be defined

in the future. Receiving agents SHOULD handle attributes or values

that it does not recognize in a graceful manner.

2.5.1 Signing-Time Attribute

The signing-time attribute is used to convey the time that a message

was signed. Until there are trusted timestamping services, the time

of signing will most likely be created by a message originator and

therefore is only as trustworthy as the originator.

Sending agents MUST encode signing time through the year 2049 as

UTCTime; signing times in 2050 or later MUST be encoded as

GeneralizedTime. 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 S/MIME Capabilities Attribute

The S/MIME capabilities attribute includes signature algorithms (such

as "md5WithRSAEncryption"), symmetric algorithms (such as "DES-CBC"),

and key encipherment algorithms (such as "rsaEncryption"). It also

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includes a non-algorithm capability which is the preference for

signedData. SMIMECapabilities was designed to be flexible and

extensible so that, in the future, a means of identifying other

capabilities and preferences such as certificates can be added in a

way that will not cause current clients to break.

The semantics of the S/MIME capabilites 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

encoding, 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 SMIMECapabilities was designed to facilitate simple

table lookups and binary comparisons in order to determine matches.

For instance, the DER-encoding for the SMIMECapability for DES EDE3

CBC MUST be identically encoded regardless of the implementation.

In the case of symmetric algorithms, the associated parameters for

the OID MUST specify all of the parameters necessary to differentiate

between two instances of the same algorithm. For instance, the number

of rounds and block size for RC5 must be specified in addition to the

key length.

There is a list of OIDs (the registered SMIMECapability list) that is

centrally maintained and is separate from this memo. The list of OIDs

is maintained by the Internet Mail Consortium at

<http://www.imc.org/ietf-smime/oids.html>.

The OIDs that correspond to algorithms SHOULD use the same OID as the

actual algorithm, except in the case where the algorithm usage is

ambiguous from the OID. For instance, in an earlier memo,

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 S/MIME capabilities 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 S/MIME capabilities 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.

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Additional values for SMIMECapability may be defined in the future.

Receiving agents MUST handle a SMIMECapabilities object that has

values that it does not recognize in a graceful manner.

2.6 ContentEncryptionAlgorithmIdentifier

Receiving agents MUST support decryption using the RC2 [RC2] or a

compatible algorithm at a key size of 40 bits, hereinafter called

"RC2/40". Receiving agents SHOULD support decryption using DES EDE3

CBC, hereinafter called "tripleDES" [3DES] [DES].

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

2.6.1 Deciding Which Encryption Method To Use

When a sending agent creates an encrypted message, it has to decide

which type of encryption to use. The decision process involves using

information garnered from the capabilities lists included in messages

received from the recipient, as well as out-of-band information such

as private agreements, user preferences, legal restrictions, and so

on.

Section 2.5 defines a method by which a sending agent can optionally

announce, among other things, its decrypting capabilities in its

order of preference. The following method for processing and

remembering the encryption capabilities attribute in incoming signed

messages SHOULD be used.

- If the receiving agent has not yet created a list of capabilities

for the sender's public key, then, after verifying the signature

on the incoming message and checking the timestamp, the receiving

agent SHOULD create a new list containing at least the signing

time and the symmetric capabilities.

- If such a list already exists, the receiving agent SHOULD verify

that the signing time in the incoming message is greater than the

signing time stored in the list and that the signature is valid.

If so, the receiving agent SHOULD update both the signing time and

capabilities in the list. Values of the signing time that lie far

in the future (that is, a greater discrepancy than any reasonable

clock skew), or a capabilitie lists in messages whose signature

could not be verified, MUST NOT be accepted.

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

messages.

Before sending a message, the sending agent MUST decide whether it is

willing to use weak encryption for the particular data in the

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message. If the sending agent decides that weak encryption is

unacceptable for this data, then the sending agent MUST NOT use a

weak algorithm such as RC2/40. The decision to use or not use weak

encryption overrides any other decision in this section about which

encryption algorithm to use.

Sections 2.6.2.1 through 2.6.2.4 describe the decisions a sending

agent SHOULD use in deciding which type of encryption should be

applied to a message. These rules are ordered, so the sending agent

SHOULD make its decision in the order given.

2.6.2.1 Rule 1: Known Capabilities

If the sending agent has received a set of capabilities from the

recipient for the message the agent is about to encrypt, then the

sending agent SHOULD use that information by selecting the first

capability in the list (that is, the capability most preferred by the

intended recipient) for which the sending agent knows how to encrypt.

The sending agent SHOULD use one of the capabilities in the list if

the agent reasonably expects the recipient to be able to decrypt the

message.

2.6.2.2 Rule 2: Unknown Capabilities, Known Use of Encryption

If:

- the sending agent has no knowledge of the encryption capabilities

of the recipient,

- and the sending agent has received at least one message from the

recipient,

- and the last encrypted message received from the recipient had a

trusted signature on it,

then the outgoing message SHOULD use the same encryption algorithm as

was used on the last signed and encrypted message received from the

recipient.

2.6.2.3 Rule 3: Unknown Capabilities, Risk of Failed Decryption

If:

- the sending agent has no knowledge of the encryption capabilities

of the recipient,

- and the sending agent is willing to risk that the recipient may

not be able to decrypt the message,

then the sending agent SHOULD use tripleDES.

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

If:

- the sending agent has no knowledge of the encryption capabilities

of the recipient,

- and the sending agent is not willing to risk that the recipient

may not be able to decrypt the message,

then the sending agent MUST use RC2/40.

2.6.3 Choosing Weak Encryption

Like all algorithms that use 40 bit keys, RC2/40 is considered by

many to be weak encryption. A sending agent that is controlled by a

human SHOULD allow a human sender to determine the risks of sending

data using RC2/40 or a similarly weak encryption algorithm before

sending the data, and possibly allow the human to use a stronger

encryption method such as tripleDES.

2.6.4 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 can decipher 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 PKCS

objects. Several MIME types as well as several PKCS objects are used.

The data to be secured is always a canonical MIME entity. The MIME

entity and other data, such as certificates and algorithm

identifiers, are given to PKCS processing facilities which produces a

PKCS object. The PKCS object is then finally wrapped in MIME.

S/MIME provides one format for enveloped-only data, several formats

for signed-only data, and several formats for signed and enveloped

data. Several formats are required to accommodate several

environments, in particular for signed messages. The criteria for

choosing among these formats are also described.

The reader of this section is expected to understand MIME as

described in [MIME-SPEC] and [MIME-SECURE].

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3.1 Preparing the MIME Entity for Signing or Enveloping

S/MIME is used to secure MIME entities. A MIME entity may be a sub-

part, sub-parts of a message, or the whole message with all its sub-

parts. A MIME entity that is the whole message includes only the MIME

headers and MIME body, and does not include the RFC-822 headers. Note

that S/MIME can also be used to secure MIME entities used in

applications other than Internet mail.

The MIME entity that is secured and described in this section can be

thought of as the "inside" MIME entity. That is, it is the

"innermost" object in what is possibly a larger MIME message.

Processing "outside" MIME entities into PKCS #7 objects is described

in Section 3.2, 3.4 and elsewhere.

The procedure for preparing a MIME entity is given in [MIME-SPEC].

The same procedure is used here with some additional restrictions

when signing. Description of the procedures from [MIME-SPEC] are

repeated here, but the reader should refer to that document for the

exact procedure. This section also describes additional requirements.

A single procedure is used for creating MIME entities that are to be

signed, enveloped, or both signed and enveloped. Some additional

steps are recommended to defend against known corruptions that can

occur during mail transport that are of particular importance for

clear-signing using the multipart/signed format. It is recommended

that these additional steps be performed on enveloped messages, or

signed and enveloped messages in order that the message can be

forwarded to any environment without modification.

These steps are descriptive rather than prescriptive. The implementor

is free to use any procedure as long as the result is the same.

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

conventions

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

canonical form

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

the MIME entity

When an S/MIME message is received, the security services on the

message are removed, and the result is the MIME entity. That MIME

entity is typically passed to a MIME-capable user agent where, it is

further decoded and presented to the user or receiving application.

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3.1.1 Canonicalization

Each MIME entity MUST be converted to a canonical form that is

uniquely and unambiguously representable in the environment where the

signature is created and the environment where the signature will be

verified. MIME entities MUST be canonicalized for enveloping as well

as signing.

The exact details of canonicalization depend on the actual MIME type

and subtype of an entity, and are not described here. Instead, the

standard for the particular MIME type should be consulted. For

example, canonicalization of type text/plain is different from

canonicalization of audio/basic. Other than text types, most types

have only one representation regardless of computing platform or

environment which can be considered their canonical representation.

In general, canonicalization will be performed by the sending agent

rather than the S/MIME implementation.

The most common and important canonicalization is for text, which is

often represented differently in different environments. MIME

entities of major type "text" must have both their line endings and

character set canonicalized. The line ending must be the pair of

characters <CR><LF>, and the 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

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recipient so that they can verify the signatures directly. If the

transport internal to the site is not 8-bit clean, such as on a

wide-area network with a single mail gateway, verifying the signature

will not be possible unless the original MIME entity was only 7-bit

data.

3.1.3 Transfer Encoding for Signing Using multipart/signed

If a multipart/signed entity is EVER to be transmitted over the

standard Internet SMTP infrastructure or other transport that is

constrained to 7-bit text, it MUST have transfer encoding applied so

that it is represented as 7-bit text. MIME entities that are 7-bit

data already need no transfer encoding. Entities such as 8-bit text

and binary data can be encoded with quoted-printable or base-64

transfer encoding.

The primary reason for the 7-bit requirement is that the Internet

mail transport infrastructure cannot guarantee transport of 8-bit or

binary data. Even though many segments of the transport

infrastructure now handle 8-bit and even binary data, it is sometimes

not possible to know whether the transport path is 8-bit clear. If a

mail message with 8-bit data were to encounter a message transfer

agent that can not transmit 8-bit or binary data, the agent has three

options, none of which are acceptable for a clear-signed message:

- The agent could change the transfer encoding; this would

invalidate the signature.

- The agent could transmit the data anyway, which would most likely

result in the 8th bit being corrupted; this too would invalidate

the signature.

- The agent could return the message to the sender.

[MIME-SECURE] prohibits an agent from changing the transfer encoding

of the first part of a multipart/signed message. If a compliant agent

that can not transmit 8-bit or binary data encounters a

multipart/signed message with 8-bit or binary data in the first part,

it would have to return the message to the sender as undeliverable.

3.1.4 Sample Canonical MIME Entity

This example shows a multipart/mixed message with full transfer

encoding. This message contains a text part and an attachment. The

sample message text includes characters that are not US-ASCII and

thus must be transfer encoded. Though not shown here, the end of each

line is <CR><LF>. The line ending of the MIME headers, the text, and

transfer encoded parts, all must be <CR><LF>.

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

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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?

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

From=20because some mail transport agents will insert a greater-

than (>) sign, thus invalidating the signature.

Also, in some cases it might be desirable to encode any =20

trailing whitespace that occurs on lines in order to ensure =20

that the message signature is not invalidated when passing =20

a gateway that modifies such whitespace (like BITNET). =20

--bar

Content-Type: 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 PKCS #7 objects of

several types including envelopedData and signedData. The details of

constructing these entities is described in subsequent sections. This

section describes the general characteristics of the

application/pkcs7-mime type.

This MIME type always carries a single PKCS #7 object. The PKCS #7

object must always be BER encoding of the ASN.1 syntax describing the

object. The contentInfo field of the carried PKCS #7 object always

contains a MIME entity that is prepared as described in section 3.1.

The contentInfo field must never be empty.

Since PKCS #7 objects are binary data, in most cases base-64 transfer

encoding is appropriate, in particular when used with SMTP transport.

The transfer encoding used depends on the transport through which the

object is to be sent, and is not a characteristic of the MIME type.

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Note that this discussion refers to the transfer encoding of the PKCS

#7 object or "outside" MIME entity. It is completely distinct from,

and unrelated to, the transfer encoding of the MIME entity secured by

the PKCS #7 object, the "inside" object, which is described in

section 3.1.

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 .p7m

(signedData, envelopedData)

application/pkcs7-mime .p7c

(degenerate signedData

"certs-only" message)

application/pkcs7-signature .p7s

application/pkcs10 .p10

In addition, the file name SHOULD be limited to eight characters

followed by a three letter extension. The eight character filename

base can be any distinct name; the use of the filename base "smime"

SHOULD be used to indicate that the MIME entity is associated with

S/MIME.

Including a file name serves two purposes. It facilitates easier use

of S/MIME objects as files on disk. It also can convey type

information across gateways. When a MIME entity of type

application/pkcs7-mime (for example) arrives at a gateway that has no

special knowledge of S/MIME, it will default the entity's MIME type

to application/octet-stream and treat it as a generic attachment,

thus losing the type information. However, the suggested filename for

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an attachment is often carried across a gateway. This often allows

the receiving systems to determine the appropriate application to

hand the attachment off to, in this case a stand-alone S/MIME

processing application. Note that this mechanism is provided as a

convenience for implementations in certain environments. A proper

S/MIME implementation MUST use the MIME types and MUST NOT rely on

the file extensions.

3.3 Creating an Enveloped-only Message

This section describes the format for enveloping a MIME entity

without signing it.

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

section 3.1.

Step 2. The MIME entity and other required data is processed into a

PKCS #7 object of type envelopedData.

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

mime MIME entity.

The smime-type parameter for enveloped-only messages is "enveloped-

data". The file extension for this type of message is ".p7m".

A sample message would be:

Content-Type: application/pkcs7-mime; smime-type=enveloped-data;

name=smime.p7m

Content-Transfer-Encoding: base64

Content-Disposition: attachment; filename=smime.p7m

rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6

7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H

f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4

0GhIGfHfQbnj756YT64V

3.4 Creating a Signed-only Message

There are two formats for signed messages defined for S/MIME:

application/pkcs7-mime and 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

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receivers and the relative importance of receivers with S/MIME

facilities being able to verify the signature versus the importance

of receivers without S/MIME software being able to view the message.

Messages signed using the multipart/signed format can always be

viewed by the receiver whether they have S/MIME software or not. They

can also be viewed whether they are using a MIME-native user agent or

they have messages translated by a gateway. In this context, "be

viewed" means the ability to process the message essentially as if it

were not a signed message, including any other MIME structure the

message might have.

Messages signed using the signedData format cannot be viewed by a

recipient unless they have S/MIME facilities. However, if they have

S/MIME facilities, these messages can always be verified if they were

not changed in transit.

3.4.2 Signing Using application/pkcs7-mime and SignedData

This signing format uses the application/pkcs7-mime MIME type. The

steps to create this format are:

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

Step 2. The MIME entity and other required data is processed into a

PKCS #7 object of type signedData

Step 3. The PKCS #7 object is inserted into an

application/pkcs7-mime MIME entity

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

and SignedData is "signed-data". The file extension for this type of

message is ".p7m".

A sample message would be:

Content-Type: application/pkcs7-mime; smime-type=signed-data;

name=smime.p7m

Content-Transfer-Encoding: base64

Content-Disposition: attachment; filename=smime.p7m

567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7

77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH

HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh

6YT64V0GhIGfHfQbnj75

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3.4.3 Signing Using the multipart/signed Format

This format is a clear-signing format. Recipients without any S/MIME

or PKCS processing facilities are able to view the message. It makes

use of the multipart/signed MIME type described in [MIME-SECURE]. The

multipart/signed MIME type has two parts. The first part contains the

MIME entity that is to be signed; the second part contains the

signature, which is a PKCS #7 detached signature.

3.4.3.1 The application/pkcs7-signature MIME Type

This MIME type always contains a single PKCS #7 object of type

signedData. The contentInfo field of the PKCS #7 object must be

empty. The signerInfos field contains the signatures for the MIME

entity. The details of the registered type are given in Appendix D.

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 PKCS #7 processing in order

to obtain an object of type signedData with an empty

contentInfo field.

Step 3. The MIME entity is inserted into the first part of a

multipart/signed message with no processing other than that

described in section 3.1.

Step 4. Transfer encoding is applied to the detached signature and

it is inserted into a MIME entity of type

application/pkcs7-signature

Step 5. The MIME entity of the application/pkcs7-signature is

inserted into the second part of the multipart/signed

entity

The multipart/signed Content type has two required parameters: the

protocol parameter and the micalg parameter.

The protocol parameter MUST be "application/pkcs7-signature". Note

that quotation marks are required around the protocol parameter

because MIME requires that the "/" character in the parameter value

MUST be quoted.

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The micalg parameter allows for one-pass processing when the

signature is being verified. The value of the micalg parameter is

dependent on the message digest algorithm used in the calculation of

the Message Integrity Check. The value of the micalg parameter SHOULD

be one of the following:

Algorithm used Value

-------------- ---------

MD5 md5

SHA-1 sha1

any other unknown

(Historical note: some early implementations of S/MIME emitted and

expected "rsa-md5" and "rsa-sha1" for the micalg parameter.)

Receiving agents SHOULD be able to recover gracefully from a micalg

parameter value that they do not recognize.

3.4.3.3 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--

3.5 Signing and Encrypting