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.

   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

       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.

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].

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.

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.

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

   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:

   -  signingTime (section 2.5.1 in this document)
   -  sMIMECapabilities (section 2.5.2 in this document)
   -  sMIMEEncryptionKeyPreference (section 2.5.3 in this document)
   -  id-messageDigest (section 11.2 in [CMS])
   -  id-contentType (section 11.1 in [CMS])

   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

   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.

   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

   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:

   -  If an SMIMEEncryptionKeyPreference attribute is found in a
      SignedData object received from the desired recipient, this
      identifies the X.509 certificate that SHOULD be used as the X.509
      key management certificate for the recipient.

   -  If an SMIMEEncryptionKeyPreference attribute is not found in a
      SignedData object received from the desired recipient, the set of
      X.509 certificates SHOULD be searched for a X.509 certificate with
      the same subject name as the signing of a X.509 certificate which
      can be used for key management.

   -  Or use some other method of determining the user's key management
      key.  If a X.509 key management certificate is not found, then
      encryption cannot be done with the signer of the message.  If
      multiple X.509 key management certificates are found, the S/MIME
      agent can make an arbitrary choice between them.

2.6. SignerIdentifier SignerInfo Type

   S/MIME v3.1 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

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.

   -  If the receiving agent has not yet created a list of capabilities
      for the sender's public key, then, after verifying the signature
      on the incoming message and checking the timestamp, the receiving
      agent SHOULD create a new list containing at least the signing
      time and the symmetric capabilities.

   -  If such a list already exists, the receiving agent SHOULD verify
      that the signing time in the incoming message is greater than the
      signing time stored in the list and that the signature is valid.
      If so, the receiving agent SHOULD update both the signing time and
      capabilities in the list.  Values of the signing time that lie far
      in the future (that is, a greater discrepancy than any reasonable
      clock skew), or a capabilities list in messages whose signature
      could not be verified, MUST NOT be accepted.

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

   Before sending a message, the sending agent MUST decide whether it is
   willing to use weak encryption for the particular data in the

   message.  If the sending agent decides that weak encryption is
   unacceptable for this data, then the sending agent MUST NOT use a
   weak algorithm such as RC2/40.  The decision to use or not use weak
   encryption overrides any other decision in this section about which
   encryption algorithm to use.

   Sections 2.7.2.1 through 2.7.2.4 describe the decisions a sending
   agent D>SHOULD use in deciding which type of encryption will be applied
   to a message.  These rules are ordered, so the sending agent D>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

   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

   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.

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

   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:

   -  The agent could change the transfer encoding; this would
      invalidate the signature.
   -  The agent could transmit the data anyway, which would most likely
      result in the 8th bit being corrupted; this too would invalidate
      the signature.
   -  The agent could return the message to the sender.

   [MIME-SECURE] prohibits an agent from changing the transfer encoding
   of the first part of a multipart/signed message.  If a compliant
   agent that can not transmit 8-bit or binary data encounters a
   multipart/signed message with 8-bit or binary data in the first part,
   it would have to return the message to the sender as undeliverable.

3.1.4. Sample Canonical MIME Entity

   This example shows a multipart/mixed message with full transfer
   encoding.  This message contains a text part and an attachment.  The
   sample message text includes characters that are not US-ASCII and
   thus 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

       --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

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

       iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
       jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
       uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
       HOxEa44b+EI=

       --bar--

3.2. The application/pkcs7-mime Type

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

   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.

   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).

   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.

   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.

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:

   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

       --boundary42
       Content-Type: text/plain

       This is a clear-signed message.

       --boundary42
       Content-Type: application/pkcs7-signature; name=smime.p7s
       Content-Transfer-Encoding: base64
       Content-Disposition: attachment; filename=smime.p7s

       ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
       4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
       n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
       7GhIGfHfYT64VQbnj756

       --boundary42--

   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

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:

   -  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.

   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

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

   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].

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
}

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.

   [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.

   [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.

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

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