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rfc:rfc3851

Network Working Group B. Ramsdell, Editor Request for Comments: 3851 Sendmail, Inc. Obsoletes: 2633 July 2004 Category: Standards Track

 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 3.1
                       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 (2004).

Abstract

 This document defines Secure/Multipurpose Internet Mail Extensions
 (S/MIME) version 3.1.  S/MIME provides a consistent way to send and
 receive secure MIME data.  Digital signatures provide authentication,
 message integrity, and non-repudiation with proof of origin.
 Encryption provides data confidentiality.  Compression can be used to
 reduce data size.  This document obsoletes RFC 2633.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Specification Overview . . . . . . . . . . . . . . . . .  3
     1.2.  Terminology. . . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Definitions. . . . . . . . . . . . . . . . . . . . . . .  4
     1.4.  Compatibility with Prior Practice of S/MIME. . . . . . .  5
     1.5.  Changes Since S/MIME v3. . . . . . . . . . . . . . . . .  5
 2.  CMS Options. . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  DigestAlgorithmIdentifier. . . . . . . . . . . . . . . .  5
     2.2.  SignatureAlgorithmIdentifier . . . . . . . . . . . . . .  6
     2.3.  KeyEncryptionAlgorithmIdentifier . . . . . . . . . . . .  6
     2.4.  General Syntax . . . . . . . . . . . . . . . . . . . . .  6
     2.5.  Attributes and the SignerInfo Type . . . . . . . . . . .  7
     2.6.  SignerIdentifier SignerInfo Type . . . . . . . . . . . . 11
     2.7.  ContentEncryptionAlgorithmIdentifier . . . . . . . . . . 12
 3.  Creating S/MIME Messages . . . . . . . . . . . . . . . . . . . 14

Ramsdell Standards Track [Page 1] RFC 3851 S/MIME 3.1 Message Specification July 2004

     3.1.  Preparing the MIME Entity for Signing, Enveloping
           or Compressing . . . . . . . . . . . . . . . . . . . . . 14
     3.2.  The application/pkcs7-mime Type. . . . . . . . . . . . . 19
     3.3.  Creating an Enveloped-only Message . . . . . . . . . . . 21
     3.4.  Creating a Signed-only Message . . . . . . . . . . . . . 22
     3.5.  Creating an Compressed-only Message. . . . . . . . . . . 26
     3.6.  Multiple Operations. . . . . . . . . . . . . . . . . . . 27
     3.7.  Creating a Certificate Management Messagetoc . . . . . . 27
     3.8.  Registration Requests. . . . . . . . . . . . . . . . . . 28
     3.9.  Identifying an S/MIME Message. . . . . . . . . . . . . . 28
 4.  Certificate Processing . . . . . . . . . . . . . . . . . . . . 29
     4.1.  Key Pair Generation. . . . . . . . . . . . . . . . . . . 29
 5.  Security Considerations. . . . . . . . . . . . . . . . . . . . 29
 A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . . . . 31
 B.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     B.1.  Normative References . . . . . . . . . . . . . . . . . . 32
     B.2.  Informative References . . . . . . . . . . . . . . . . . 34
 C.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
 D.  Editor's Address . . . . . . . . . . . . . . . . . . . . . . . 35
     Full Copyright Statement . . . . . . . . . . . . . . . . . . . 36

1. Introduction

 S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a
 consistent way to send and receive secure MIME data.  Based on the
 popular Internet MIME standard, S/MIME provides the following
 cryptographic security services for electronic messaging
 applications:  authentication, message integrity and non-repudiation
 of origin (using digital signatures), and data confidentiality (using
 encryption).
 S/MIME can be used by traditional mail user agents (MUAs) to add
 cryptographic security services to mail that is sent, and to
 interpret cryptographic security services in mail that is received.
 However, S/MIME is not restricted to mail; it can be used with any
 transport mechanism that transports MIME data, such as HTTP.  As
 such, S/MIME takes advantage of the object-based features of MIME and
 allows secure messages to be exchanged in mixed-transport systems.
 Further, S/MIME can be used in automated message transfer agents that
 use cryptographic security services that do not require any human
 intervention, such as the signing of software-generated documents and
 the encryption of FAX messages sent over the Internet.

Ramsdell Standards Track [Page 2] RFC 3851 S/MIME 3.1 Message Specification July 2004

1.1. Specification Overview

 This document describes a protocol for adding cryptographic signature
 and encryption services to MIME data.  The MIME standard [MIME-SPEC]
 provides a general structure for the content type of Internet
 messages and allows extensions for new content type applications.
 This specification defines how to create a MIME body part that has
 been cryptographically enhanced according to CMS [CMS], which is
 derived from PKCS #7 [PKCS-7].  This specification also defines the
 application/pkcs7-mime MIME type that can be used to transport those
 body parts.
 This document also discusses how to use the multipart/signed MIME
 type defined in [MIME-SECURE] to transport S/MIME signed messages.
 multipart/signed is used in conjunction with the application/pkcs7-
 signature MIME type, which is used to transport a detached S/MIME
 signature.
 In order to create S/MIME messages, an S/MIME agent MUST follow the
 specifications in this document, as well as the specifications listed
 in the Cryptographic Message Syntax document [CMS] [CMSALG].
 Throughout this specification, there are requirements and
 recommendations made for how receiving agents handle incoming
 messages.  There are separate requirements and recommendations for
 how sending agents create outgoing messages.  In general, the best
 strategy is to "be liberal in what you receive and conservative in
 what you send".  Most of the requirements are placed on the handling
 of incoming messages while the recommendations are mostly on the
 creation of outgoing messages.
 The separation for requirements on receiving agents and sending
 agents also derives from the likelihood that there will be S/MIME
 systems that involve software other than traditional Internet mail
 clients.  S/MIME can be used with any system that transports MIME
 data.  An automated process that sends an encrypted message might not
 be able to receive an encrypted message at all, for example.  Thus,
 the requirements and recommendations for the two types of agents are
 listed separately when appropriate.

1.2. Terminology

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

Ramsdell Standards Track [Page 3] RFC 3851 S/MIME 3.1 Message Specification July 2004

1.3. Definitions

 For the purposes of this specification, the following definitions
 apply.
 ASN.1: Abstract Syntax Notation One, as defined in CCITT X.208
 [X.208-88].
 BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209
 [X.209-88].
 Certificate: A type that binds an entity's name to a public key with
 a digital signature.
 DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT
 X.509 [X.509-88].
 7-bit data: Text data with lines less than 998 characters long, where
 none of the characters have the 8th bit set, and there are no NULL
 characters.  <CR> and <LF> occur only as part of a <CR><LF> end of
 line delimiter.
 8-bit data: Text data with lines less than 998 characters, and where
 none of the characters are NULL characters. <CR> and <LF> occur only
 as part of a <CR><LF> end of line delimiter.
 Binary data: Arbitrary data.
 Transfer Encoding: A reversible transformation made on data so 8-bit
 or binary data can be sent via a channel that only transmits 7-bit
 data.
 Receiving agent: Software that interprets and processes S/MIME CMS
 objects, MIME body parts that contain CMS content types, or both.
 Sending agent: Software that creates S/MIME CMS content types, MIME
 body parts that contain CMS content types, or both.
 S/MIME agent: User software that is a receiving agent, a sending
 agent, or both.

Ramsdell Standards Track [Page 4] RFC 3851 S/MIME 3.1 Message Specification July 2004

1.4. Compatibility with Prior Practice of S/MIME

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

1.5. Changes Since S/MIME v3

 The RSA public key algorithm was changed to a MUST implement key
 wrapping algorithm, and the Diffie-Hellman algorithm changed to a
 SHOULD implement.
 The AES symmetric encryption algorithm has been included as a SHOULD
 implement.
 The RSA public key algorithm was changed to a MUST implement
 signature algorithm.
 Ambiguous language about the use of "empty" SignedData messages to
 transmit certificates was clarified to reflect that transmission of
 certificate revocation lists is also allowed.
 The use of binary encoding for some MIME entities is now explicitly
 discussed.
 Header protection through the use of the message/rfc822 MIME type has
 been added.
 Use of the CompressedData CMS type is allowed, along with required
 MIME type and file extension additions.

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. [CMSALG] provides
 additional details regarding the use of the cryptographic algorithms.

2.1. DigestAlgorithmIdentifier

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

Ramsdell Standards Track [Page 5] RFC 3851 S/MIME 3.1 Message Specification July 2004

2.2. SignatureAlgorithmIdentifier

 Receiving agents MUST support id-dsa-with-sha1 defined in [CMSALG].
 The algorithm parameters MUST be absent (not encoded as NULL).
 Receiving agents MUST support rsaEncryption, defined in [CMSALG].
 Sending agents MUST support either id-dsa-with-sha1 or rsaEncryption.
 If using rsaEncryption, sending and receiving agents MUST support the
 digest algorithms in section 2.1 as specified.
 Note that S/MIME v3 clients might only implement signing or signature
 verification using id-dsa-with-sha1, and might also use id-dsa as an
 AlgorithmIdentifier in this field.  Receiving clients SHOULD
 recognize id-dsa as equivalent to id-dsa-with-sha1, and sending
 clients MUST use id-dsa-with-sha1 if using that algorithm.  Also note
 that S/MIME v2 clients are only required to verify digital signatures
 using the rsaEncryption algorithm with SHA-1 or MD5, and might not
 implement id-dsa-with-sha1 or id-dsa at all.

2.3. KeyEncryptionAlgorithmIdentifier

 Sending and receiving agents MUST support rsaEncryption, defined in
 [CMSALG].
 Sending and receiving agents SHOULD support Diffie-Hellman defined in
 [CMSALG], using the ephemeral-static mode.
 Note that S/MIME v3 clients might only implement key encryption and
 decryption using the Diffie-Hellman algorithm.  Also note that S/MIME
 v2 clients are only capable of decrypting content-encryption keys
 using the rsaEncryption algorithm.

2.4. General Syntax

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

2.4.1. Data Content Type

 Sending agents MUST use the id-data content type identifier to
 identify the "inner" MIME message content.  For example, when
 applying a digital signature to MIME data, the CMS SignedData
 encapContentInfo eContentType MUST include the id-data object
 identifier and the MIME content MUST be stored in the SignedData
 encapContentInfo eContent OCTET STRING (unless the sending agent is
 using multipart/signed, in which case the eContent is absent, per

Ramsdell Standards Track [Page 6] RFC 3851 S/MIME 3.1 Message Specification July 2004

 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.  Applying a
 signature to a message provides authentication, message integrity,
 and non-repudiation of origin.

2.4.3. EnvelopedData Content Type

 This content type is used to apply data confidentiality to a message.
 A sender needs to have access to a public key for each intended
 message recipient to use this service.

2.4.4. CompressedData Content Type

 This content type is used to apply data compression to a message.
 This content type does not provide authentication, message integrity,
 non-repudiation, or data confidentiality, and is only used to reduce
 message size.
 See section 3.6 for further guidance on the use of this type in
 conjunction with other CMS types.

2.5. Attributes and the 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)
  4. id-messageDigest (section 11.2 in [CMS])
  5. id-contentType (section 11.1 in [CMS])

Ramsdell Standards Track [Page 7] RFC 3851 S/MIME 3.1 Message Specification July 2004

 Further, receiving agents SHOULD be able to handle zero or one
 instance in the signingCertificate signed attribute, as defined in
 section 5 of [ESS].
 Sending agents SHOULD generate one instance of the signingCertificate
 signed attribute in each SignerInfo structure.
 Additional attributes and values for these attributes might be
 defined in the future.  Receiving agents SHOULD handle attributes or
 values that it does not recognize in a graceful manner.
 Interactive sending agents that include signed attributes that are
 not listed here SHOULD display those attributes to the user, so that
 the user is aware of all of the data being signed.

2.5.1. Signing-Time Attribute

 The signing-time attribute is used to convey the time that a message
 was signed.  The time of signing will most likely be created by a
 message originator and therefore is only as trustworthy as the
 originator.
 Sending agents MUST encode signing time through the year 2049 as
 UTCTime; signing times in 2050 or later MUST be encoded as
 GeneralizedTime.  When the UTCTime CHOICE is used, S/MIME agents MUST
 interpret the year field (YY) as follows:
 if YY is greater than or equal to 50, the year is interpreted as
 19YY; if YY is less than 50, the year is interpreted as 20YY.

2.5.2. SMIMECapabilities Attribute

 The SMIMECapabilities attribute includes signature algorithms (such
 as "sha1WithRSAEncryption"), symmetric algorithms (such as "DES-
 EDE3-CBC"), and key encipherment algorithms (such as
 "rsaEncryption").  There are also several identifiers which indicate
 support for other optional features such as binary encoding and
 compression.  The SMIMECapabilities were designed to be flexible and
 extensible so that, in the future, a means of identifying other
 capabilities and preferences such as certificates can be added in a
 way that will not cause current clients to break.
 If present, the SMIMECapabilities attribute MUST be a
 SignedAttribute; it MUST NOT be an UnsignedAttribute.  CMS defines
 SignedAttributes as a SET OF Attribute.  The SignedAttributes in a
 signerInfo MUST NOT include multiple instances of the
 SMIMECapabilities attribute.  CMS defines the ASN.1 syntax for
 Attribute to include attrValues SET OF AttributeValue.  A

Ramsdell Standards Track [Page 8] RFC 3851 S/MIME 3.1 Message Specification July 2004

 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 SMIMECapabilities attribute specify a partial
 list as to what the client announcing the SMIMECapabilities can
 support.  A client does not have to list every capability it
 supports, and need not list all its capabilities so that the
 capabilities list doesn't get too long.  In an SMIMECapabilities
 attribute, the object identifiers (OIDs) are listed in order of their
 preference, but SHOULD be separated logically along the lines of
 their categories (signature algorithms, symmetric algorithms, key
 encipherment algorithms, etc.)
 The structure of the SMIMECapabilities attribute is to facilitate
 simple table lookups and binary comparisons in order to determine
 matches.  For instance, the DER-encoding for the SMIMECapability for
 DES EDE3 CBC MUST be identically encoded regardless of the
 implementation.  Because of the requirement for identical encoding,
 individuals documenting algorithms to be used in the
 SMIMECapabilities attribute SHOULD explicitly document the correct
 byte sequence for the common cases.
 For any capability, the associated parameters for the OID MUST
 specify all of the parameters necessary to differentiate between two
 instances of the same algorithm.  For instance, the number of rounds
 and the block size for RC5 needs to be specified in addition to the
 key length.
 The OIDs that correspond to algorithms SHOULD use the same OID as the
 actual algorithm, except in the case where the algorithm usage is
 ambiguous from the OID.  For instance, in an earlier specification,
 rsaEncryption was ambiguous because it could refer to either a
 signature algorithm or a key encipherment algorithm.  In the event
 that an OID is ambiguous, it needs to be arbitrated by the maintainer
 of the registered SMIMECapabilities list as to which type of
 algorithm will use the OID, and a new OID MUST be allocated under the
 smimeCapabilities OID to satisfy the other use of the OID.
 The registered SMIMECapabilities list specifies the parameters for
 OIDs that need them, most notably key lengths in the case of
 variable-length symmetric ciphers.  In the event that there are no
 differentiating parameters for a particular OID, the parameters MUST
 be omitted, and MUST NOT be encoded as NULL.

Ramsdell Standards Track [Page 9] RFC 3851 S/MIME 3.1 Message Specification July 2004

 Additional values for the SMIMECapabilities attribute might be
 defined in the future.  Receiving agents MUST handle a
 SMIMECapabilities object that has values that it does not recognize
 in a graceful manner.
 Section 2.7.1 explains a strategy for caching capabilities.

2.5.2.1. SMIMECapability For the RC2 Algorithm

 For the RC2 algorithm preference SMIMECapability, the capabilityID
 MUST be set to the value rc2-cbc as defined in [CMSALG].  The
 parameters field MUST contain SMIMECapabilitiesParametersForRC2CBC
 (see appendix A).
 Please note that the SMIMECapabilitiesParametersForRC2CBC is a single
 INTEGER which contains the effective key length (NOT the
 corresponding RC2 parameter version value).  So, for example, for RC2
 with a 128-bit effective key length, the parameter would be encoded
 as the INTEGER value 128, NOT the corresponding parameter version of
 58.

2.5.3. Encryption Key Preference Attribute

 The encryption key preference attribute allows the signer to
 unambiguously describe which of the signer's certificates has the
 signer's preferred encryption key.  This attribute is designed to
 enhance behavior for interoperating with those clients that use
 separate keys for encryption and signing.  This attribute is used to
 convey to anyone viewing the attribute which of the listed
 certificates is appropriate 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

Ramsdell Standards Track [Page 10] RFC 3851 S/MIME 3.1 Message Specification July 2004

 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 can use any certificate in
 replying to the sender that is valid.
 Section 2.7.1 explains a strategy for caching preference data.

2.5.3.1. Selection of Recipient Key Management Certificate

 In order to determine the key management certificate to be used when
 sending a future CMS EnvelopedData message for a particular
 recipient, the following steps SHOULD be followed:
  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 of a X.509 certificate which
    can be used for key management.
  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 v3.1 implementations MUST support both issuerAndSerialNumber
 as well as subjectKeyIdentifier.  Messages that use the
 subjectKeyIdentifier choice cannot be read by S/MIME v2 clients.
 It is important to understand that some certificates use a value for
 subjectKeyIdentifier that is not suitable for uniquely identifying a
 certificate.  Implementations MUST be prepared for multiple
 certificates for potentially different entities to have the same
 value for subjectKeyIdentifier, and MUST be prepared to try each

Ramsdell Standards Track [Page 11] RFC 3851 S/MIME 3.1 Message Specification July 2004

 matching certificate during signature verification before indicating
 an error condition.

2.7. ContentEncryptionAlgorithmIdentifier

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

2.7.1. Deciding Which Encryption Method To Use

 When a sending agent creates an encrypted message, it has to decide
 which type of encryption to use.  The decision process involves using
 information garnered from the capabilities lists included in messages
 received from the recipient, as well as out-of-band information such
 as private agreements, user preferences, legal restrictions, and so
 on.
 Section 2.5.2 defines a method by which a sending agent can
 optionally announce, among other things, its decrypting capabilities
 in its order of preference.  The following method for processing and
 remembering the encryption capabilities attribute in incoming signed
 messages SHOULD be used.
  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.
 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

Ramsdell Standards Track [Page 12] RFC 3851 S/MIME 3.1 Message Specification July 2004

 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 will 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) that the sending agent knows how to encrypt.  The
 sending agent SHOULD use one of the capabilities in the list if the
 agent reasonably expects the recipient to be able to decrypt the
 message.

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

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

2.7.2. Choosing Weak Encryption

 Like all algorithms that use 40 bit keys, RC2/40 is considered by
 many to be weak encryption.  A sending agent that is controlled by a
 human SHOULD allow a human sender to determine the risks of sending
 data using RC2/40 or a similarly weak encryption algorithm before
 sending the data, and possibly allow the human to use a stronger
 encryption method such as tripleDES.

2.7.3. Multiple Recipients

 If a sending agent is composing an encrypted message to a group of
 recipients where the encryption capabilities of some of the
 recipients do not overlap, the sending agent is forced to send more
 than one message.  Please note that if the sending agent chooses to

Ramsdell Standards Track [Page 13] RFC 3851 S/MIME 3.1 Message Specification July 2004

 send a message encrypted with a strong algorithm, and then send the
 same message encrypted with a weak algorithm, someone watching the
 communications channel could learn the contents of the strongly-
 encrypted message simply by decrypting the weakly-encrypted message.

3. Creating S/MIME Messages

 This section describes the S/MIME message formats and how they are
 created.  S/MIME messages are a combination of MIME bodies and CMS
 content types.  Several MIME types as well as several CMS content
 types are used.  The data to be secured is always a canonical MIME
 entity.  The MIME entity and other data, such as certificates and
 algorithm identifiers, are given to CMS processing facilities which
 produce a CMS object.  Finally, the CMS object is wrapped in MIME.
 The Enhanced Security Services for S/MIME [ESS] document provides
 descriptions of how nested, secured S/MIME messages are formatted.
 ESS provides a description 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, Enveloping or Compressing

 S/MIME is used to secure MIME entities.  A MIME entity can be a sub-
 part, sub-parts of a message, or the whole message with all its sub-
 parts.  A MIME entity that is the whole message includes only the
 MIME headers and MIME body, and does not include the RFC-822 headers.
 Note that S/MIME can also be used to secure MIME entities used in
 applications other than Internet mail.  If protection of the RFC-822
 headers is required, the use of the message/rfc822 MIME type is
 explained later in this section.
 The MIME entity that is secured and described in this section can be
 thought of as the "inside" MIME entity.  That is, it is the
 "innermost" object in what is possibly a larger MIME message.
 Processing "outside" MIME entities into CMS content types is
 described in Section 3.2, 3.4, and elsewhere.
 The procedure for preparing a MIME entity is given in [MIME-SPEC].
 The same procedure is used here with some additional restrictions

Ramsdell Standards Track [Page 14] RFC 3851 S/MIME 3.1 Message Specification July 2004

 when signing.  Description of the procedures from [MIME-SPEC] are
 repeated here, but it is suggested that the reader 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
 have any combination of signing, enveloping, and compressing applied.
 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, so that the
 message can be forwarded to any environment without modification.
 These steps are descriptive rather than prescriptive.  The
 implementer is free to use any procedure as long as the result is the
 same.
 Step 1.  The MIME entity is prepared according to the local
 conventions.
 Step 2.  The leaf parts of the MIME entity are converted to canonical
 form.
 Step 3.  Appropriate transfer encoding is applied to the leaves of
 the MIME entity.
 When an S/MIME message is received, the security services on the
 message are processed, and the result is the MIME entity.  That MIME
 entity is typically passed to a MIME-capable user agent where, it is
 further decoded and presented to the user or receiving application.
 In order to protect outer, non-content related message headers (for
 instance, the "Subject", "To", "From" and "CC" fields), the sending
 client MAY wrap a full MIME message in a message/rfc822 wrapper in
 order to apply S/MIME security services to these headers.  It is up
 to the receiving client to decide how to present these "inner"
 headers along with the unprotected "outer" headers.
 When an S/MIME message is received, if the top-level protected MIME
 entity has a Content-Type of message/rfc822, it can be assumed that
 the intent was to provide header protection.  This entity SHOULD be
 presented as the top-level message, taking into account header
 merging issues as previously discussed.

Ramsdell Standards Track [Page 15] RFC 3851 S/MIME 3.1 Message Specification July 2004

3.1.1. Canonicalization

 Each MIME entity MUST be converted to a canonical form that is
 uniquely and unambiguously representable in the environment where the
 signature is created and the environment where the signature will be
 verified.  MIME entities MUST be canonicalized for enveloping and
 compressing as well as signing.
 The exact details of canonicalization depend on the actual MIME type
 and subtype of an entity, and are not described here.  Instead, the
 standard for the particular MIME type SHOULD be consulted.  For
 example, canonicalization of type text/plain is different from
 canonicalization of audio/basic.  Other than text types, most types
 have only one representation regardless of computing platform or
 environment which can be considered their canonical representation.
 In general, canonicalization will be performed by the non-security
 part of the sending agent rather than the S/MIME implementation.
 The most common and important canonicalization is for text, which is
 often represented differently in different environments.  MIME
 entities of major type "text" MUST have both their line endings and
 character set canonicalized.  The line ending MUST be the pair of
 characters <CR><LF>, and the charset SHOULD be a registered charset
 [CHARSETS].  The details of the canonicalization are specified in
 [MIME-SPEC].  The chosen charset SHOULD be named in the charset
 parameter so that the receiving agent can unambiguously determine the
 charset used.
 Note that some charsets such as ISO-2022 have multiple
 representations for the same characters.  When preparing such text
 for signing, the canonical representation specified for the charset
 MUST be used.

3.1.2. Transfer Encoding

 When generating any of the secured MIME entities below, except the
 signing using the multipart/signed format, no transfer encoding is
 required at all.  S/MIME implementations MUST be able to deal with
 binary MIME objects.  If no Content-Transfer-Encoding header is
 present, the transfer encoding is presumed to be 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

Ramsdell Standards Track [Page 16] RFC 3851 S/MIME 3.1 Message Specification July 2004

 recipient so that they can verify the signatures directly.  If the
 transport internal to the site is not 8-bit clean, such as on a
 wide-area network with a single mail gateway, verifying the signature
 will not be possible unless the original MIME entity was only 7-bit
 data.
 S/MIME implementations which "know" that all intended recipient(s)
 are capable of handling inner (all but the outermost) binary MIME
 objects SHOULD use binary encoding as opposed to a 7-bit-safe
 transfer encoding for the inner entities.  The use of a 7-bit-safe
 encoding (such as base64) would unnecessarily expand the message
 size.  Implementations MAY "know" that recipient implementations are
 capable of handling inner binary MIME entities either by interpreting
 the id-cap-preferBinaryInside sMIMECapabilities attribute, by prior
 agreement, or by other means.
 If one or more intended recipients are unable to handle inner binary
 MIME objects, or if this capability is unknown for any of the
 intended recipients, S/MIME implementations SHOULD use transfer
 encoding described in section 3.1.3 for all MIME entities they
 secure.

3.1.3. Transfer Encoding for Signing Using multipart/signed

 If a multipart/signed entity is ever to be transmitted over the
 standard Internet SMTP infrastructure or other transport that is
 constrained to 7-bit text, it MUST have transfer encoding applied so
 that it is represented as 7-bit text.  MIME entities that are 7-bit
 data already need no transfer encoding.  Entities such as 8-bit text
 and binary data can be encoded with quoted-printable or base-64
 transfer encoding.
 The primary reason for the 7-bit requirement is that the Internet
 mail transport infrastructure cannot guarantee transport of 8-bit or
 binary data.  Even though many segments of the transport
 infrastructure now handle 8-bit and even binary data, it is sometimes
 not possible to know whether the transport path is 8-bit clean.  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.

Ramsdell Standards Track [Page 17] RFC 3851 S/MIME 3.1 Message Specification July 2004

 [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 need to 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?
     It's generally a good idea to encode lines that begin with
     From=20because some mail transport agents will insert a greater-
     than (>) sign, thus invalidating the signature.
     Also, in some cases it might be desirable to encode any   =20
     trailing whitespace that occurs on lines in order to ensure  =20
     that the message signature is not invalidated when passing =20
     a gateway that modifies such whitespace (like BITNET). =20
  1. -bar

Content-Type: image/jpeg

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

Ramsdell Standards Track [Page 18] RFC 3851 S/MIME 3.1 Message Specification July 2004

3.2. The application/pkcs7-mime Type

 The application/pkcs7-mime type is used to carry CMS content types
 including EnvelopedData, SignedData, and CompressedData.  The details
 of constructing these entities is described in subsequent sections.
 This section describes the general characteristics of the
 application/pkcs7-mime type.
 The carried CMS object always contains a MIME entity that is prepared
 as described in section 3.1 if the eContentType is id-data.  Other
 contents MAY be carried when the eContentType contains different
 values.  See [ESS] for an example of this with signed receipts.
 Since CMS content types are binary data, in most cases base-64
 transfer encoding is appropriate, in particular, when used with SMTP
 transport.  The transfer encoding used depends on the transport
 through which the object is to be sent, and is not a characteristic
 of the MIME type.
 Note that this discussion refers to the transfer encoding of the CMS
 object or "outside" MIME entity.  It is completely distinct from, and
 unrelated to, the transfer encoding of the MIME entity secured by the
 CMS object, the "inside" object, which is described in section 3.1.
 Because there are several types of application/pkcs7-mime objects, a
 sending agent SHOULD do as much as possible to help a receiving agent
 know about the contents of the object without forcing the receiving
 agent to decode the ASN.1 for the object.  The MIME headers of all
 application/pkcs7-mime objects SHOULD include the optional "smime-
 type" parameter, as described in the following sections.

3.2.1. The name and filename Parameters

 For the application/pkcs7-mime, sending agents SHOULD emit the
 optional "name" parameter to the Content-Type field for compatibility
 with older systems.  Sending agents SHOULD also emit the optional
 Content-Disposition field [CONTDISP] with the "filename" parameter.
 If a sending agent emits the above parameters, the value of the
 parameters SHOULD be a file name with the appropriate extension:

Ramsdell Standards Track [Page 19] RFC 3851 S/MIME 3.1 Message Specification July 2004

 MIME Type                                            File Extension
 application/pkcs7-mime (SignedData, EnvelopedData)      .p7m
 application/pkcs7-mime (degenerate SignedData           .p7c
   certificate management message)
 application/pkcs7-mime (CompressedData)                 .p7z
 application/pkcs7-signature (SignedData)                .p7s
 In addition, the file name SHOULD be limited to eight characters
 followed by a three letter extension.  The eight character filename
 base can be any distinct name; the use of the filename base "smime"
 SHOULD be used to indicate that the MIME entity is associated with
 S/MIME.
 Including a file name serves two purposes.  It facilitates easier use
 of S/MIME objects as files on disk.  It also can convey type
 information across gateways.  When a MIME entity of type
 application/pkcs7-mime (for example) arrives at a gateway that has no
 special knowledge of S/MIME, it will default the entity's MIME type
 to application/octet-stream and treat it as a generic attachment,
 thus losing the type information.  However, the suggested filename
 for an attachment is often carried across a gateway.  This often
 allows the receiving systems to determine the appropriate application
 to hand the attachment off to, in this case, a stand-alone S/MIME
 processing application.  Note that this mechanism is provided as a
 convenience for implementations in certain environments.  A proper
 S/MIME implementation MUST use the MIME types and MUST NOT rely on
 the file extensions.

3.2.2. The smime-type parameter

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

Ramsdell Standards Track [Page 20] RFC 3851 S/MIME 3.1 Message Specification July 2004

 Name                   CMS type                Inner Content
 enveloped-data         EnvelopedData           id-data
 signed-data            SignedData              id-data
 certs-only             SignedData              none
 compressed-data        CompressedData          id-data
 In order for consistency to be obtained with future specifications,
 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 can be
 omitted.  Thus since "certs-only" can only be signed, "signed-" is
 omitted.
 2. A common string for a content OID SHOULD be assigned.  We use
 "data" for the id-data content OID when MIME is the inner content.
 3. If no common string is assigned.  Then the common string of
 "OID.<oid>" is recommended (for example, "OID.1.3.6.1.5.5.7.6.1"
 would be DES40).
 It is explicitly intended that this field be a suitable hint for mail
 client applications to indicate whether a message is "signed" or
 "encrypted" without having to tunnel into the CMS payload.

3.3. Creating an Enveloped-only Message

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

Ramsdell Standards Track [Page 21] RFC 3851 S/MIME 3.1 Message Specification July 2004

 Step 3.  The EnvelopedData object is wrapped in a CMS ContentInfo
 object.
 Step 4.  The ContentInfo object is inserted into an
 application/pkcs7-mime MIME entity.
 The smime-type parameter for enveloped-only messages is "enveloped-
 data".  The file extension for this type of message is ".p7m".
 A sample message would be:
     Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
          name=smime.p7m
     Content-Transfer-Encoding: base64
     Content-Disposition: attachment; filename=smime.p7m
     rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
     7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
     f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
     0GhIGfHfQbnj756YT64V

3.4. Creating a Signed-only Message

 There are two formats for signed messages defined for S/MIME:
 application/pkcs7-mime with SignedData, and multipart/signed.  In
 general, the multipart/signed form is preferred for sending, and
 receiving agents MUST 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
 is 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.

Ramsdell Standards Track [Page 22] RFC 3851 S/MIME 3.1 Message Specification July 2004

 Messages signed using the SignedData format cannot be viewed by a
 recipient unless they have S/MIME facilities.  However, the
 SignedData format protects the message content from being changed by
 benign intermediate agents.  Such agents might do line wrapping or
 content-transfer encoding changes which would break the signature.

3.4.2. Signing Using application/pkcs7-mime with SignedData

 This signing format uses the application/pkcs7-mime MIME type.  The
 steps to create this format are:
 Step 1.  The MIME entity is prepared according to section 3.1.
 Step 2.  The MIME entity and other required data is processed into a
 CMS object of type SignedData.
 Step 3.  The SignedData object is wrapped in a CMS ContentInfo
 object.
 Step 4.  The ContentInfo object is inserted into an
 application/pkcs7-mime MIME entity.
 The smime-type parameter for messages using application/pkcs7-mime
 with SignedData is "signed-data".  The file extension for this type
 of message is ".p7m".
 A sample message would be:
     Content-Type: application/pkcs7-mime; smime-type=signed-data;
          name=smime.p7m
     Content-Transfer-Encoding: base64
     Content-Disposition: attachment; filename=smime.p7m
     567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7
     77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH
     HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh
     6YT64V0GhIGfHfQbnj75

3.4.3. Signing Using the multipart/signed Format

 This format is a clear-signing format.  Recipients without any S/MIME
 or CMS processing facilities are able to view the message.  It makes
 use of the multipart/signed MIME type described in [MIME-SECURE].
 The multipart/signed MIME type has two parts.  The first part
 contains the MIME entity that is signed; the second part contains the
 "detached signature" CMS SignedData object in which the
 encapContentInfo eContent field is absent.

Ramsdell Standards Track [Page 23] RFC 3851 S/MIME 3.1 Message Specification July 2004

3.4.3.1. The application/pkcs7-signature MIME Type

 This MIME type always contains a CMS ContentInfo containing a single
 CMS object of type SignedData.  The SignedData encapContentInfo
 eContent field MUST be absent.  The signerInfos field contains the
 signatures for the MIME entity.
 The file extension for signed-only messages using application/pkcs7-
 signature is ".p7s".

3.4.3.2. Creating a multipart/signed Message

 Step 1.  The MIME entity to be signed is prepared according to
 section 3.1, taking special care for clear-signing.
 Step 2.  The MIME entity is presented to CMS processing in order to
 obtain an object of type SignedData in which the encapContentInfo
 eContent field is absent.
 Step 3.  The MIME entity is inserted into the first part of a
 multipart/signed message with no processing other than that described
 in section 3.1.
 Step 4.  Transfer encoding is applied to the "detached signature" CMS
 SignedData object and it is inserted into a MIME entity of type
 application/pkcs7-signature.
 Step 5.  The MIME entity of the application/pkcs7-signature is
 inserted into the second part of the multipart/signed entity.
 The multipart/signed Content type has two required parameters: the
 protocol parameter and the micalg parameter.
 The protocol parameter MUST be "application/pkcs7-signature".  Note
 that quotation marks are required around the protocol parameter
 because MIME requires that the "/" character in the parameter value
 MUST be quoted.
 The micalg parameter allows for one-pass processing when the
 signature is being verified.  The value of the micalg parameter is
 dependent on the message digest algorithm(s) used in the calculation
 of the Message Integrity Check.  If multiple message digest
 algorithms are used they MUST be separated by commas per [MIME-
 SECURE].  The values to be placed in the micalg parameter SHOULD be
 from the following:

Ramsdell Standards Track [Page 24] RFC 3851 S/MIME 3.1 Message Specification July 2004

 Algorithm   Value
 used
 MD5         md5
 SHA-1       sha1
 SHA-256     sha256
 SHA-384     sha384
 SHA-512     sha512
 Any other   (defined separately in algorithm profile or "unknown"
              if not defined)
 (Historical note: some early implementations of S/MIME emitted and
 expected "rsa-md5" and "rsa-sha1" for the micalg parameter.)
 Receiving agents SHOULD be able to recover gracefully from a micalg
 parameter value that they do not recognize.
 The SHA-256, SHA-384, and SHA-512 algorithms [FIPS180-2] are not
 currently recommended in S/MIME, and are included here for
 completeness.

3.4.3.3. Sample multipart/signed Message

     Content-Type: multipart/signed;
        protocol="application/pkcs7-signature";
        micalg=sha1; boundary=boundary42
  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 25] RFC 3851 S/MIME 3.1 Message Specification July 2004

 The content that is digested (the first part of the multipart/signed)
 are the bytes:
 43 6f 6e 74 65 6e 74 2d 54 79 70 65 3a 20 74 65 78 74 2f 70 6c 61 69
 6e 0d 0a 0d 0a 54 68 69 73 20 69 73 20 61 20 63 6c 65 61 72 2d 73 69
 67 6e 65 64 20 6d 65 73 73 61 67 65 2e 0d 0a

3.5. Creating an Compressed-only Message

 This section describes the format for compressing a MIME entity.
 Please note that versions of S/MIME prior to 3.1 did not specify any
 use of CompressedData, and will not recognize it.  The use of a
 capability to indicate the ability to receive CompressedData is
 described in [CMSCOMPR] and is the preferred method for
 compatibility.
 Step 1.  The MIME entity to be compressed is prepared according to
 section 3.1.
 Step 2.  The MIME entity and other required data is processed into a
 CMS object of type CompressedData.
 Step 3.  The CompressedData object is wrapped in a CMS ContentInfo
 object.
 Step 4.  The ContentInfo object is inserted into an
 application/pkcs7-mime MIME entity.
 The smime-type parameter for compressed-only messages is
 "compressed-data".  The file extension for this type of message is
 ".p7z".
 A sample message would be:
     Content-Type: application/pkcs7-mime; smime-type=compressed-data;
          name=smime.p7z
     Content-Transfer-Encoding: base64
     Content-Disposition: attachment; filename=smime.p7z
     rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
     7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
     f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
     0GhIGfHfQbnj756YT64V

Ramsdell Standards Track [Page 26] RFC 3851 S/MIME 3.1 Message Specification July 2004

3.6. Multiple Operations

 The signed-only, encrypted-only, and compressed-only MIME formats can
 be nested.  This works because these formats are all MIME entities
 that encapsulate other MIME entities.
 An S/MIME implementation MUST be able to receive and process
 arbitrarily nested S/MIME within reasonable resource limits of the
 recipient computer.
 It is possible to apply any of the signing, encrypting, and
 compressing operations in any order.  It is up to the implementer 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 can 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 could have changed
 the unauthenticated portions of the encrypted message.
 When using compression, keep the following guidelines in mind:
  1. Compression of binary encoded encrypted data is discouraged, since

it will not yield significant compression. Base64 encrypted data

    could very well benefit, however.
 -  If a lossy compression algorithm is used with signing, you will
    need to compress first, then sign.

3.7. Creating a Certificate Management Message

 The certificate management message or MIME entity is used to
 transport certificates and/or certificate revocation lists, such as
 in response to a registration request.
 Step 1.  The certificates and/or certificate revocation lists are
 made available to the CMS generating process which creates a CMS
 object of type SignedData.  The SignedData encapContentInfo eContent
 field MUST be absent and signerInfos field MUST be empty.

Ramsdell Standards Track [Page 27] RFC 3851 S/MIME 3.1 Message Specification July 2004

 Step 2.  The SignedData object is wrapped in a CMS ContentInfo
 object.
 Step 3.  The ContentInfo object is enclosed in an application/pkcs7-
 mime MIME entity.
 The smime-type parameter for a certificate management message is
 "certs-only".  The file extension for this type of message is ".p7c".

3.8. Registration Requests

 A sending agent that signs messages MUST have a certificate for the
 signature so that a receiving agent can verify the signature.  There
 are many ways of getting certificates, such as through an exchange
 with a certificate authority, through a hardware token or diskette,
 and so on.
 S/MIME v2 [SMIMEV2] specified a method for "registering" public keys
 with certificate authorities using an application/pkcs10 body part.
 Since that time, the IETF PKIX Working Group has developed other
 methods for requesting certificates.  However, S/MIME v3.1 does not
 require a particular certificate request mechanism.

3.9. Identifying an S/MIME Message

 Because S/MIME takes into account interoperation in non-MIME
 environments, several different mechanisms are employed to carry the
 type information, and it becomes a bit difficult to identify S/MIME
 messages.  The following table lists criteria for determining whether
 or not a message is an S/MIME message.  A message is considered an
 S/MIME message if it matches any of the criteria listed below.
 The file suffix in the table below comes from the "name" parameter in
 the content-type header, or the "filename" parameter on the content-
 disposition header.  These parameters that give the file suffix are
 not listed below as part of the parameter section.
 MIME type:   application/pkcs7-mime
 parameters:  any
 file suffix: any
 MIME type:   multipart/signed
 parameters:  protocol="application/pkcs7-signature"
 file suffix: any
 MIME type:   application/octet-stream
 parameters:  any
 file suffix: p7m, p7s, p7c, p7z

Ramsdell Standards Track [Page 28] RFC 3851 S/MIME 3.1 Message Specification July 2004

4. Certificate Processing

 A receiving agent MUST provide some certificate retrieval mechanism
 in order to gain access to certificates for recipients of digital
 envelopes.  This specification does not cover how S/MIME agents
 handle certificates, only what they do after a certificate has been
 validated or rejected.  S/MIME certificate issues are covered in
 [CERT31].
 At a minimum, for initial S/MIME deployment, a user agent could
 automatically generate a message to an intended recipient requesting
 that recipient's certificate in a signed return message.  Receiving
 and sending agents SHOULD also provide a mechanism to allow a user to
 "store and protect" certificates for correspondents in such a way so
 as to guarantee their later retrieval.

4.1. Key Pair Generation

 All generated key pairs MUST be generated from a good source of non-
 deterministic random input [RANDOM] and the private key MUST be
 protected in a secure fashion.
 If an S/MIME agent needs to generate an RSA key pair, then the S/MIME
 agent or some related administrative utility or function SHOULD
 generate RSA key pairs using the following guidelines.  A user agent
 SHOULD generate RSA key pairs at a minimum key size of 768 bits.  A
 user agent MUST NOT generate RSA key pairs less than 512 bits long.
 Creating keys longer than 1024 bits can 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.
 Implementers SHOULD be aware that multiple (active) key pairs can be
 associated with a single individual.  For example, one key pair can
 be used to support confidentiality, while a different key pair can be
 used for authentication.

5. Security Considerations

 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

Ramsdell Standards Track [Page 29] RFC 3851 S/MIME 3.1 Message Specification July 2004

 no cryptography.  When feasible, sending and receiving agents SHOULD
 inform senders and recipients of the relative cryptographic strength
 of messages.
 It is impossible for most software or people to estimate the value of
 a message.  Further, it is impossible for most software or people to
 estimate the actual cost of decrypting a message that is encrypted
 with a key of a particular size.  Further, it is quite difficult to
 determine the cost of a failed decryption if a recipient cannot
 decode a message.  Thus, choosing between different key sizes (or
 choosing whether to just use plaintext) is also impossible.  However,
 decisions based on these criteria are made all the time, and
 therefore this specification gives a framework for using those
 estimates in choosing algorithms.
 If a sending agent is sending the same message using different
 strengths of cryptography, an attacker watching the communications
 channel might be able to determine the contents of the strongly-
 encrypted message by decrypting the weakly-encrypted version.  In
 other words, a sender SHOULD NOT send a copy of a message using
 weaker cryptography than they would use for the original of the
 message.
 Modification of the ciphertext can go undetected if authentication is
 not also used, which is the case when sending EnvelopedData without
 wrapping it in SignedData or enclosing SignedData within it.
 See RFC 3218 [MMA] for more information about thwarting the adaptive
 chosen ciphertext vulnerability in PKCS #1 Version 1.5
 implementations.
 In some circumstances the use of the Diffie-Hellman key agreement
 scheme in a prime order subgroup of a large prime p is vulnerable to
 certain attacks known as "small-subgroup" attacks.  Methods exist,
 however, to prevent these attacks.  These methods are described in
 RFC 2785 [DHSUB].

Ramsdell Standards Track [Page 30] RFC 3851 S/MIME 3.1 Message Specification July 2004

A. ASN.1 Module

SecureMimeMessageV3dot1

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

DEFINITIONS IMPLICIT TAGS ::= BEGIN

IMPORTS – Cryptographic Message Syntax

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

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

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

      rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) attributes(2)}

– S/MIME Capabilities provides a method of broadcasting the symmetric – 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 31] RFC 3851 S/MIME 3.1 Message Specification July 2004

id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2)

 us(840) rsadsi(113549) pkcs(1) pkcs9(9) 16 }

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

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

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

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

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

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

END

B. References

B.1. Normative References

 [CERT31]      Ramsdell, B., Ed., "S/MIME Version 3.1 Certificate
               Handling", RFC 3850, July 2004.
 [CHARSETS]    Character sets assigned by IANA.  See
               http://www.iana.org/assignments/character-sets
 [CMS]         Housley, R., "Cryptographic Message Syntax (CMS)", RFC
               3852, July 2004.
 [CMSAES]      Schaad, J., "Use of the Advanced Encryption Standard
               (AES) Encryption Algorithm in Cryptographic Message
               Syntax (CMS)", RFC 3565, July 2003.

Ramsdell Standards Track [Page 32] RFC 3851 S/MIME 3.1 Message Specification July 2004

 [CMSALG]      Housley, R., "Cryptographic Message Syntax (CMS)
               Algorithms", RFC 3370, August 2002.
 [CMSCOMPR]    Gutmann, P., "Compressed Data Content Type for
               Cryptographic Message Syntax (CMS)", RFC 3274, June
               2002.
 [CONTDISP]    Troost, R., Dorner, S., and K. Moore, "Communicating
               Presentation Information in Internet Messages: The
               Content-Disposition Header Field", RFC 2183, August
               1997.
 [ESS]         Hoffman, P., "Enhanced Security Services for S/MIME",
               RFC 2634, June 1999.
 [FIPS180-2]   "Secure Hash Signature Standard (SHS)", National
               Institute of Standards and Technology (NIST).  FIPS
               Publication 180-2.
 [MIME-SPEC]   Freed, N. and N. Borenstein, "Multipurpose Internet
               Mail Extensions (MIME) Part One: Format of Internet
               Message Bodies", RFC 2045, November 1996.
               Freed, N. and N. Borenstein, "Multipurpose Internet
               Mail Extensions (MIME) Part Two: Media Types", RFC
               2046, November 1996.
               Moore, K., "MIME (Multipurpose Internet Mail
               Extensions) Part Three:  Message Header Extensions for
               Non-ASCII Text", RFC 2047, November 1996.
               Freed, N., Klensin, J., and J. Postel, "Multipurpose
               Internet Mail Extensions (MIME) Part Four: Registration
               Procedures", BCP 13, RFC 2048, November 1996.
               Freed, N. and N. Borenstein, "Multipurpose Internet
               Mail Extensions (MIME) Part Five: Conformance Criteria
               and Examples", RFC 2049, November 1996.
 [MIME-SECURE] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
               "Security Multiparts for MIME: Multipart/Signed and
               Multipart/Encrypted", RFC 1847, October 1995.
 [MUSTSHOULD]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
 [X.208-88]    CCITT.  Recommendation X.208: Specification of Abstract
               Syntax Notation One (ASN.1).  1988.

Ramsdell Standards Track [Page 33] RFC 3851 S/MIME 3.1 Message Specification July 2004

 [X.209-88]    CCITT.  Recommendation X.209: Specification of Basic
               Encoding Rules for Abstract Syntax Notation One
               (ASN.1).  1988.
 [X.509-88]    CCITT.  Recommendation X.509: The Directory -
               Authentication Framework.  1988.

B.2. Informative References

 [DHSUB]       Zuccherato, R., "Methods for Avoiding the "Small-
               Subgroup" Attacks on the Diffie-Hellman Key Agreement
               Method for S/MIME", RFC 2785, March 2000.
 [MMA]         Rescorla, E., "Preventing the Million Message Attack on
               Cryptographic Message Syntax", RFC 3218, January 2002.
 [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.
 [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 34] RFC 3851 S/MIME 3.1 Message Specification July 2004

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.
 A number of the members of the S/MIME Working Group have also worked
 very hard and contributed to this document.  Any list of people is
 doomed to omission, and for that I apologize.  In alphabetical order,
 the following people stand out in my mind due to the fact that they
 made direct contributions to this document.
 Tony Capel
 Piers Chivers
 Dave Crocker
 Bill Flanigan
 Peter Gutmann
 Paul Hoffman
 Russ Housley
 William Ottaway
 John Pawling
 Jim Schaad

D. Editor's Address

 Blake Ramsdell
 Sendmail, Inc.
 704 228th Ave NE #775
 Sammamish, WA  98074
 EMail: blake@sendmail.com

Ramsdell Standards Track [Page 35] RFC 3851 S/MIME 3.1 Message Specification July 2004

Full Copyright Statement

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 to the rights, licenses and restrictions contained in BCP 78, and
 except as set forth therein, the authors retain all their rights.
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Acknowledgement

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Ramsdell Standards Track [Page 36]

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