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RFC2633 - S/MIME Version 3 Message Specification

王朝other·作者佚名  2008-05-31
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Network Working Group B. Ramsdell, Editor

Request for Comments: 2633 Worldtalk

Category: Standards Track June 1999

S/MIME Version 3 Message Specification

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). 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.

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 CMS [CMS], which is derived

from 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 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.

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

specifications in this memo, as well as the specifications listed in

the Cryptographic Message Syntax [CMS].

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

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

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

the requirements and recommendations for the two types of agents are

listed separately when appropriate.

1.2 Terminology

The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

document are to be interpreted as described in [MUSTSHOULD].

1.3 Definitions

For the purposes of this 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.

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 RFC2311 through RFC2315, inclusive. RFC

2311 also has historical information about the development of S/MIME.

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

[DH].

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

S/MIME.

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 message:

- signingTime (section 2.5.1 in this document)

- sMIMECapabilities (section 2.5.2 in this document)

- sMIMEEncryptionKeyPreference (section 2.5.3 in this document)

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 AttributeValue.

The semantics of the SMIMECapabilites attribute specify a partial

list as to what the client announcing the SMIMECapabilites can

support. A client does not have to list every capability it supports,

and probably should not list all its capabilities so that the

capabilities list doesn't get too long. In an SMIMECapabilities

attribute, the OIDs are listed in order of their preference, but

SHOULD be logically separated along the lines of their categories

(signature algorithms, symmetric algorithms, key encipherment

algorithms, etc.)

The structure of the SMIMECapabilities attribute is to facilitate

simple table lookups and binary comparisons in order to determine

matches. For instance, the DER-encoding for the SMIMECapability for

DES EDE3 CBC MUST be identically encoded regardless of the

implementation.

In the case of symmetric algorithms, the associated parameters for

the OID MUST specify all of the parameters necessary to differentiate

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

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

key length.

There is a list of OIDs (OIDs Used with S/MIME) 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>. Note that all OIDs

associated with the MUST and SHOULD implement algorithms are included

in section A of this document.

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

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

ambiguous from the OID. For instance, in an earlier 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

AttributeValue.

The sending agent SHOULD include the referenced certificate in the

set of certificates included in the signed message if this attribute

is used. The certificate may be omitted if it has been previously

made available to the receiving agent. Sending agents SHOULD use

this attribute if the commonly used or preferred encryption

certificate is not the same as the certificate used to sign the

message.

Receiving agents SHOULD store the preference data if the signature on

the message is valid and the signing time is greater than the

currently stored value. (As with the SMIMECapabilities, the clock

skew should be checked and the data not used if the skew is too

great.) Receiving agents SHOULD respect the sender's encryption key

preference attribute if possible. This however represents only a

preference and the receiving agent may use any certificate in

replying to the sender that is valid.

2.5.3.1 Selection of Recipient Key Management Certificate

In order to determine the key management certificate to be used when

sending a future CMS envelopedData message for a particular

recipient, the following steps SHOULD be followed:

- If an SMIMEEncryptionKeyPreference attribute is found in a

signedData object received from the desired recipient, this

identifies the X.509 certificate that should be used as the X.509

key management certificate for the recipient.

- If an SMIMEEncryptionKeyPreference attribute is not found in a

signedData object received from the desired recipient, the set of

X.509 certificates should be searched for a X.509 certificate with

the same subject name as the signing X.509 certificate which can

be used for key management.

- Or use some other method of determining the user's key management

key. If a X.509 key management certificate is not found, then

encryption cannot be done with the signer of the message. If multiple

X.509 key management certificates are found, the S/MIME agent can

make an arbitrary choice between them.

2.6 SignerIdentifier SignerInfo Type

S/MIME v3 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

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 capabilities list in messages whose

signature could not be verified, MUST NOT be accepted.

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

messages.

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

willing to use weak encryption for the particular data in the

message. If the sending agent decides that weak encryption is

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

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

encryption overrides any other decision in this section about which

encryption algorithm to use.

Sections 2.7.2.1 through 2.7.2.4 describe the decisions a sending

agent SHOULD use in deciding which type of encryption should be

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

SHOULD make its decision in the order given.

2.7.1.1 Rule 1: Known Capabilities

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

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

sending agent SHOULD use that information by selecting the first

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

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

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

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

message.

2.7.1.2 Rule 2: Unknown Capabilities, 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.

2.7.1.3 Rule 3: Unknown Capabilities, Unknown Version of S/MIME

If:

- the sending agent has no knowledge of the encryption capabilities

of the recipient,

- and the sending agent has no knowledge of the version of S/MIME

of the recipient,

then the sending agent SHOULD use tripleDES because it is a stronger

algorithm and is required by S/MIME v3. If the sending agent chooses

not to use tripleDES in this step, it SHOULD use RC2/40.

2.7.2 Choosing Weak Encryption

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

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

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

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

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

encryption method such as tripleDES.

2.7.3 Multiple Recipients

If a sending agent is composing an encrypted message to a group of

recipients where the encryption capabilities of some of the

recipients do not overlap, the sending agent is forced to send more

than one message. It should be noted that if the sending agent

chooses to send a message encrypted with a strong algorithm, and then

send the same message encrypted with a weak algorithm, someone

watching the communications channel may be able to learn the contents

of the strongly-encrypted message simply by decrypting the weakly-

encrypted message.

3. Creating S/MIME Messages

This section describes the S/MIME message formats and how they are

created. S/MIME messages are a combination of MIME bodies and CMS

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

3.1 Preparing the MIME Entity for Signing or Enveloping

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

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

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

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

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

applications other than Internet mail.

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 elsewhere.

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

The same procedure is used here with some additional restrictions

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

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

exact procedure. This section also describes additional requirements.

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

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

steps are recommended to defend against known corruptions that can

occur during mail transport that are of particular importance for

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

that these additional steps be performed on enveloped messages, or

signed and enveloped messages in order that the message can be

forwarded to any environment without modification.

These steps are descriptive rather than prescriptive. The implementor

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

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

conventions

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

form

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

MIME entity

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

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

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

further decoded and presented to the user or receiving application.

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

signature.

- The agent could return the message to the sender.

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

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

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

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

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

3.1.4 Sample Canonical MIME Entity

This example shows a multipart/mixed message with full transfer

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

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

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

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

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

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

Content-Type: multipart/mixed; boundary=bar

--bar

Content-Type: text/plain; charset=iso-8859-1

Content-Transfer-Encoding: quoted-printable

=A1Hola Michael!

How do you like the new S/MIME specification?

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

From=20 because some mail transport agents will insert a

greater-than (>) sign, thus invalidating the signature.

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

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

that the message signature is not invalidated when passing =20

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

--bar

Content-Type: image/jpeg

Content-Transfer-Encoding: base64

iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//

jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq

uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn

HOxEa44b+EI=

--bar--

3.2 The application/pkcs7-mime Type

The application/pkcs7-mime type is used to carry CMS 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

envelopedData)

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

S/MIME.

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

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

information across gateways. When a MIME entity of type

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

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

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

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

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

the receiving systems to determine the appropriate application to

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

processing application. Note that this mechanism is provided as a

convenience for implementations in certain environments. A proper

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

the file extensions.

3.2.2 The smime-type parameter

The application/pkcs7-mime content type defines the optional "smime-

type" parameter. The intent of this parameter is to convey details

about the security applied (signed or enveloped) along with

infomation about the contained content. This memo defines the

following smime-types.

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

parameter.

1. If both signing and encryption can be applied to the content, then

two values for smime-type SHOULD be assigned "signed-*" and

"encrypted-*". If one operation can be assigned then this may be

omitted. Thus since "certs-only" can only be signed, "signed-" is

omitted.

2. A common string for a content oid should be assigned. We use

"data" for the id-data content OID when MIME is the inner content.

3. If no common string is assigned. Then the common string of

"OID.<oid>" is recommended (for example, "OID.1.3.6.1.5.5.7.6.1"

would be DES40).

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 entity.

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

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

A sample message would be:

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

name=smime.p7m

Content-Transfer-Encoding: base64

Content-Disposition: attachment; filename=smime.p7m

rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6

7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H

f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4

0GhIGfHfQbnj756YT64V

3.4 Creating a Signed-only Message

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

application/pkcs7-mime with SignedData, and multipart/signed. In

general, the multipart/signed form is preferred for sending, and

receiving agents SHOULD be able to handle both.

3.4.1 Choosing a Format for Signed-only Messages

There are no hard-and-fast rules when a particular signed-only format

should be chosen because it depends on the capabilities of all the

receivers and the relative importance of receivers with S/MIME

facilities being able to verify the signature versus the importance

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

Messages signed using the multipart/signed format can always be

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

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

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

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

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

message might have.

Messages signed using the signedData format cannot be viewed by a

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

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

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

message is ".p7m".

A sample message would be:

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

name=smime.p7m

Content-Transfer-Encoding: base64

Content-Disposition: attachment; filename=smime.p7m

567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7

77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH

HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh

6YT64V0GhIGfHfQbnj75

3.4.3 Signing Using the multipart/signed Format

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

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

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

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

MIME entity that is signed; the second part contains the "detached

signature" CMS SignedData object in which the encapContentInfo

eContent field is absent.

3.4.3.1 The application/pkcs7-signature MIME Type

This MIME type always contains a single CMS object of type

signedData. The signedData encapContentInfo eContent field MUST be

absent. The signerInfos field contains the signatures for the MIME

entity.

The file extension for signed-only messages using application/pkcs7-

signature is ".p7s".

3.4.3.2 Creating a multipart/signed Message

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

3.1, taking special care for clear-signing.

Step 2. The MIME entity is presented to CMS processing in order to

obtain an object of type signedData in which the encapContentInfo

eContent field is absent.

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

multipart/signed message with no processing other than that described

in section 3.1.

Step 4. Transfer encoding is applied to the "detached signature" CMS

SignedData object and it is inserted into a MIME entity of type

application/pkcs7-signature.

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

inserted into the second part of the multipart/signed entity.

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

protocol parameter and the micalg parameter.

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

that quotation marks are required around the protocol parameter

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

MUST be quoted.

The micalg parameter allows for one-pass processing when the

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

dependent on the message digest algorithm(s) used in the calculation

of the Message Integrity Check. If multiple message digest algorithms

are used they MUST be separated by commas per [MIME-SECURE]. The

values to be placed in the micalg parameter SHOULD be from the

following:

Algorithm Value

used

MD5 md5

SHA-1 sha1

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

To achieve signing and enveloping, any of the signed-only and

encrypted-only formats may be nested. This is allowed because the

above formats are all MIME entities, and because they all secure MIME

entities.

An S/MIME implementation MUST be able to receive and process

arbitrarily nested S/MIME within reasonable resource limits of the

recipient computer.

It is possible to either sign a message first, or to envelope the

message first. It is up to the implementor and the user to choose.

When signing first, the signatories are then securely obscured by the

enveloping. When enveloping first the signatories are exposed, but it

is possible to verify signatures without removing the enveloping.

This may be useful in an environment were automatic signature

verification is desired, as no private key material is required to

verify a signature.

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 memo. 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 memo 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 memo discusses security. Security issues not covered in

other parts of the memo include:

40-bit encryption is considered weak by most cryptographers. Using

weak cryptography in S/MIME offers little actual security over

sending plaintext. However, other features of S/MIME, such as the

specification of tripleDES and the ability to announce stronger

cryptographic capabilities to parties with whom you communicate,

allow senders to create messages that use strong encryption. Using

weak cryptography is never recommended unless the only alternative is

no cryptography. When feasible, sending and receiving agents should

inform senders and recipients the relative cryptographic strength of

messages.

It is impossible for most software or people to estimate the value of

a message. Further, it is impossible for most software or people to

estimate the actual cost of decrypting a message that is encrypted

with a key of a particular size. Further, it is quite difficult to

determine the cost of a failed decryption if a recipient cannot

decode a message. Thus, choosing between different key sizes (or

choosing whether to just use plaintext) is also impossible. However,

decisions based on these criteria are made all the time, and

therefore this memo gives a framework for using those estimates in

choosing algorithms.

If a sending agent is sending the same message using different

strengths of cryptography, an attacker watching the communications

channel may be able to determine the contents of the strongly-

encrypted message by decrypting the weakly-encrypted version. In

other words, a sender should not send a copy of a message using

weaker cryptography than they would use for the original of the

message.

Modification of the ciphertext can go undetected if authentication is

not also used, which is the case when sending EnvelopedData without

wrapping it in SignedData or enclosing SignedData within it.

A. ASN.1 Module

SecureMimeMessageV3

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

pkcs(1) pkcs-9(9) smime(16) modules(0) smime(4) }

DEFINITIONS IMPLICIT TAGS ::=

BEGIN

IMPORTS

-- Cryptographic Message Syntax

SubjectKeyIdentifier, IssuerAndSerialNumber,

RecipientKeyIdentifier

FROM CryptographicMessageSyntax

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

pkcs(1) pkcs-9(9) smime(16) modules(0) cms(1) };

-- id-aa is the arc with all new authenticated and unauthenticated

-- attributes produced the by S/MIME Working Group

id-aa OBJECT IDENTIFIER ::= {iso(1) member-body(2) usa(840)

rsadsi(113549)

pkcs(1) pkcs-9(9) smime(16) attributes(2)}

-- S/MIME Capabilities provides a method of broadcasting the symetric

-- capabilities understood. Algorithms should be ordered by preference

-- and grouped by type

smimeCapabilities OBJECT IDENTIFIER ::=

{iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}

SMIMECapability ::= SEQUENCE {

capabilityID OBJECT IDENTIFIER,

parameters ANY DEFINED BY capabilityID OPTIONAL }

SMIMECapabilities ::= SEQUENCE OF SMIMECapability

-- Encryption Key Preference provides a method of broadcasting the

-- preferred 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

-- parameters

dES-EDE3-CBC OBJECT IDENTIFIER ::=

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

encryptionAlgorithm(3) 7}

CBCParameter ::= IV

IV ::= OCTET STRING (SIZE (8..8))

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

-- as the parameter

rC2-CBC OBJECT IDENTIFIER ::=

{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

-- rc2ParameterVersion values are 160, 120, 58 respectively.

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

-- Digest Algorithms:

-- md5 OBJECT IDENTIFIER ::=

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

-- digestAlgorithm(2) 5}

-- sha-1 OBJECT IDENTIFIER ::=

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

-- algorithm(2) 26}

-- Asymmetric Encryption Algorithms

--

-- rsaEncryption OBJECT IDENTIFIER ::=

-- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)

-- 1}

--

-- rsa OBJECT IDENTIFIER ::=

-- {joint-iso-ccitt(2) ds(5) algorithm(8) encryptionAlgorithm(1) 1}

--

-- id-dsa OBJECT IDENTIFIER ::=

-- {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)

-- 2}

--

-- md5WithRSAEncryption OBJECT IDENTIFIER ::=

-- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)

-- 4}

--

-- sha-1WithRSAEncryption OBJECT IDENTIFIER ::=

-- {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)

-- 5}

--

-- 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)

-- 5}

-- See [CMS] for a description of how to encode the attribute

-- value.

END

B. References

[3DES] ANSI X9.52-1998, "Triple Data Encryption Algorithm

Modes of Operation", American National Standards

Institute, 1998.

[CERT3] Ramsdell, B., Editor, "S/MIME Version 3 Certificate

Handling", RFC2632, June 1999.

[CHARSETS] Character sets assigned by IANA. See

<FTP://ftp.isi.edu/in-

notes/iana/assignments/character-sets>.

[CMS] Housley, R., "Cryptographic Message Syntax", RFC2630,

June 1999.

[CONTDISP] Troost, R., Dorner, S. and K. Moore, "Communicating

Presentation Information in Internet Messages: The

Content-Disposition Header Field", RFC2183, August

1997.

[DES] ANSI X3.106, "American National Standard for

Information Systems- Data Link Encryption," American

National Standards Institute, 1983.

[DH] Rescorla, E., "Diffie-Hellman Key Agreement Method",

RFC2631, June 1999.

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

May 1994.

[ESS] Hoffman, P., Editor "Enhanced Security Services for

S/MIME", RFC2634, June 1999.

[MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC

1321, April 1992.

[MIME-SPEC] The primary definition of MIME. "MIME Part 1: Format

of Internet Message Bodies", RFC2045; "MIME Part 2:

Media Types", RFC2046; "MIME Part 3: Message Header

Extensions for Non-ASCII Text", RFC2047; "MIME Part

4: Registration Procedures", RFC2048; "MIME Part 5:

Conformance Criteria and Examples", RFC2049,

September 1993.

[MIME-SECURE] Galvin, J., Murphy, S., Crocker, S. and N. Freed,

"Security Multiparts for MIME: Multipart/Signed and

Multipart/Encrypted", RFC1847, October 1995.

[MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP14, RFC2119, March 1997.

[PKCS-1] Kaliski, B., "PKCS #1: RSA Encryption Version 2.0",

RFC2437, October 1998.

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

Version 1.5", RFC2315, March 1998.

[RANDOM] Eastlake, 3rd, D., Crocker, S. and J. Schiller,

"Randomness Recommendations for Security", RFC1750,

December 1994.

[RC2] Rivest, R., "A Description of the RC2 (r) Encryption

Algorithm", RFC2268, January 1998.

[SHA1] NIST FIPS PUB 180-1, "Secure Hash Standard," National

Institute of Standards and Technology, U.S. Department

of Commerce, DRAFT, 31May 1994.

[SMIMEV2] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L.

and L. Repka, "S/MIME Version 2 Message

Specification", RFC2311, March 1998.

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

Editor's Address

Blake Ramsdell

Worldtalk

17720 NE 65th St Ste 201

Redmond, WA 98052

Phone: +1 425 376 0225

EMail: blaker@deming.com

Full Copyright Statement

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and distributed, in whole or in part, without restriction of any

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included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

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The limited permissions granted above are perpetual and will not be

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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

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Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.

 
 
 
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