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rfc:rfc2633

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.

Ramsdell Standards Track [Page 1] RFC 2633 S/MIME Version 3 Message Specification June 1999

 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.

Ramsdell Standards Track [Page 2] RFC 2633 S/MIME Version 3 Message Specification June 1999

 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 RFC 2311 through RFC 2315, 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.

Ramsdell Standards Track [Page 3] RFC 2633 S/MIME Version 3 Message Specification June 1999

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

Ramsdell Standards Track [Page 4] RFC 2633 S/MIME Version 3 Message Specification June 1999

 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:
  1. signingTime (section 2.5.1 in this document)
  2. sMIMECapabilities (section 2.5.2 in this document)
  3. 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.

Ramsdell Standards Track [Page 5] RFC 2633 S/MIME Version 3 Message Specification June 1999

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.)

Ramsdell Standards Track [Page 6] RFC 2633 S/MIME Version 3 Message Specification June 1999

 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

Ramsdell Standards Track [Page 7] RFC 2633 S/MIME Version 3 Message Specification June 1999

 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:
  1. 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.
  1. 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.

Ramsdell Standards Track [Page 8] RFC 2633 S/MIME Version 3 Message Specification June 1999

  1. 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.
  1. 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.
  1. 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.

Ramsdell Standards Track [Page 9] RFC 2633 S/MIME Version 3 Message Specification June 1999

 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.

Ramsdell Standards Track [Page 10] RFC 2633 S/MIME Version 3 Message Specification June 1999

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

 If:
  1. the sending agent has no knowledge of the encryption capabilities

of the recipient,

  1. 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.

Ramsdell Standards Track [Page 11] RFC 2633 S/MIME Version 3 Message Specification June 1999

 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

Ramsdell Standards Track [Page 12] RFC 2633 S/MIME Version 3 Message Specification June 1999

 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.

Ramsdell Standards Track [Page 13] RFC 2633 S/MIME Version 3 Message Specification June 1999

 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:
  1. The agent could change the transfer encoding; this would invalidate

the signature.

  1. 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.

Ramsdell Standards Track [Page 14] RFC 2633 S/MIME Version 3 Message Specification June 1999

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
  1. -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
  1. -bar

Content-Type: image/jpeg

   Content-Transfer-Encoding: base64
   iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
   jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
   uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
   HOxEa44b+EI=
  1. -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.

Ramsdell Standards Track [Page 15] RFC 2633 S/MIME Version 3 Message Specification June 1999

 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.

Ramsdell Standards Track [Page 16] RFC 2633 S/MIME Version 3 Message Specification June 1999

 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).

Ramsdell Standards Track [Page 17] RFC 2633 S/MIME Version 3 Message Specification June 1999

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.

Ramsdell Standards Track [Page 18] RFC 2633 S/MIME Version 3 Message Specification June 1999

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

Ramsdell Standards Track [Page 19] RFC 2633 S/MIME Version 3 Message Specification June 1999

     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.

Ramsdell Standards Track [Page 20] RFC 2633 S/MIME Version 3 Message Specification June 1999

 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
  1. -boundary42

Content-Type: text/plain

     This is a clear-signed message.
  1. -boundary42

Content-Type: application/pkcs7-signature; name=smime.p7s

     Content-Transfer-Encoding: base64
     Content-Disposition: attachment; filename=smime.p7s
     ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
     4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
     n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
     7GhIGfHfYT64VQbnj756
  1. -boundary42–

Ramsdell Standards Track [Page 21] RFC 2633 S/MIME Version 3 Message Specification June 1999

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".

Ramsdell Standards Track [Page 22] RFC 2633 S/MIME Version 3 Message Specification June 1999

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

Ramsdell Standards Track [Page 23] RFC 2633 S/MIME Version 3 Message Specification June 1999

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.

Ramsdell Standards Track [Page 24] RFC 2633 S/MIME Version 3 Message Specification June 1999

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.

Ramsdell Standards Track [Page 25] RFC 2633 S/MIME Version 3 Message Specification June 1999

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

}

Ramsdell Standards Track [Page 26] RFC 2633 S/MIME Version 3 Message Specification June 1999

– 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} –

Ramsdell Standards Track [Page 27] RFC 2633 S/MIME Version 3 Message Specification June 1999

– 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

Ramsdell Standards Track [Page 28] RFC 2633 S/MIME Version 3 Message Specification June 1999

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", RFC 2632, 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", RFC 2630,
                June 1999.
 [CONTDISP]     Troost, R., Dorner, S. and K. Moore, "Communicating
                Presentation Information in Internet Messages: The
                Content-Disposition Header Field", RFC 2183, 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",
                RFC 2631, June 1999.
 [DSS]          NIST FIPS PUB 186, "Digital Signature Standard", 18
                May 1994.
 [ESS]          Hoffman, P., Editor "Enhanced Security Services for
                S/MIME", RFC 2634, 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", RFC 2045; "MIME Part 2:
                Media Types", RFC 2046; "MIME Part 3: Message Header
                Extensions for Non-ASCII Text", RFC 2047; "MIME Part
                4: Registration Procedures", RFC 2048; "MIME Part 5:
                Conformance Criteria and Examples", RFC 2049,
                September 1993.
 [MIME-SECURE]  Galvin, J., Murphy, S., Crocker, S. and N. Freed,
                "Security Multiparts for MIME: Multipart/Signed and
                Multipart/Encrypted", RFC 1847, October 1995.

Ramsdell Standards Track [Page 29] RFC 2633 S/MIME Version 3 Message Specification June 1999

 [MUSTSHOULD]   Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP14, RFC 2119, March 1997.
 [PKCS-1]       Kaliski, B., "PKCS #1: RSA Encryption Version 2.0",
                RFC 2437, October 1998.
 [PKCS-7]       Kaliski, B., "PKCS #7: Cryptographic Message Syntax
                Version 1.5", RFC 2315, March 1998.
 [RANDOM]       Eastlake, 3rd, D., Crocker, S. and J. Schiller,
                "Randomness Recommendations for Security", RFC 1750,
                December 1994.
 [RC2]          Rivest, R., "A Description of the RC2 (r) Encryption
                Algorithm", RFC 2268, 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", RFC 2311, March 1998.

Ramsdell Standards Track [Page 30] RFC 2633 S/MIME Version 3 Message Specification June 1999

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

Ramsdell Standards Track [Page 31] RFC 2633 S/MIME Version 3 Message Specification June 1999

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Ramsdell Standards Track [Page 32]

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