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Versions: (draft-schaad-rfc5751-bis) 00 01 02 03 04 05 06 Draft is active
In: AD_Evaluation
LAMPS                                                          J. Schaad
Internet-Draft                                            August Cellars
Obsoletes: 5751 (if approved)                                B. Ramsdell
Intended status: Standards Track                  Brute Squad Labs, Inc.
Expires: October 16, 2017                                      S. Turner
                                                                   sn3rd
                                                          April 14, 2017


   Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
                         Message Specification
                    draft-ietf-lamps-rfc5751-bis-06

Abstract

   This document defines Secure/Multipurpose Internet Mail Extensions
   (S/MIME) version 4.0.  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 5751.

Contributing to this document

   The source for this draft is being maintained in GitHub.  Suggested
   changes should be submitted as pull requests at <https://github.com/
   lamps-wg/smime>.  Instructions are on that page as well.  Editorial
   changes can be managed in GitHub, but any substantial issues need to
   be discussed on the LAMPS mailing list.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 16, 2017.





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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Specification Overview  . . . . . . . . . . . . . . . . .   4
     1.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Conventions Used in This Document . . . . . . . . . . . .   6
     1.4.  Compatibility with Prior Practice of S/MIME . . . . . . .   7
     1.5.  Changes from S/MIME v3 to S/MIME v3.1 . . . . . . . . . .   7
     1.6.  Changes from S/MIME v3.1 to S/MIME v3.2 . . . . . . . . .   8
     1.7.  Changes for S/MIME v4.0 . . . . . . . . . . . . . . . . .   9
   2.  CMS Options . . . . . . . . . . . . . . . . . . . . . . . . .  10
     2.1.  DigestAlgorithmIdentifier . . . . . . . . . . . . . . . .  10
     2.2.  SignatureAlgorithmIdentifier  . . . . . . . . . . . . . .  10
     2.3.  KeyEncryptionAlgorithmIdentifier  . . . . . . . . . . . .  11
     2.4.  General Syntax  . . . . . . . . . . . . . . . . . . . . .  12
       2.4.1.  Data Content Type . . . . . . . . . . . . . . . . . .  12
       2.4.2.  SignedData Content Type . . . . . . . . . . . . . . .  12
       2.4.3.  EnvelopedData Content Type  . . . . . . . . . . . . .  12
       2.4.4.  AuthEnvelopedData Content Type  . . . . . . . . . . .  12
       2.4.5.  CompressedData Content Type . . . . . . . . . . . . .  12
     2.5.  Attributes and the SignerInfo Type  . . . . . . . . . . .  13



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       2.5.1.  Signing Time Attribute  . . . . . . . . . . . . . . .  13
       2.5.2.  SMIME Capabilities Attribute  . . . . . . . . . . . .  14
       2.5.3.  Encryption Key Preference Attribute . . . . . . . . .  15
     2.6.  SignerIdentifier SignerInfo Type  . . . . . . . . . . . .  17
     2.7.  ContentEncryptionAlgorithmIdentifier  . . . . . . . . . .  17
       2.7.1.  Deciding Which Encryption Method to Use . . . . . . .  17
       2.7.2.  Choosing Weak Encryption  . . . . . . . . . . . . . .  19
       2.7.3.  Multiple Recipients . . . . . . . . . . . . . . . . .  19
   3.  Creating S/MIME Messages  . . . . . . . . . . . . . . . . . .  19
     3.1.  Preparing the MIME Entity for Signing, Enveloping, or
           Compressing . . . . . . . . . . . . . . . . . . . . . . .  20
       3.1.1.  Canonicalization  . . . . . . . . . . . . . . . . . .  21
       3.1.2.  Transfer Encoding . . . . . . . . . . . . . . . . . .  22
       3.1.3.  Transfer Encoding for Signing Using multipart/signed   22
       3.1.4.  Sample Canonical MIME Entity  . . . . . . . . . . . .  23
     3.2.  The application/pkcs7-mime Media Type . . . . . . . . . .  24
       3.2.1.  The name and filename Parameters  . . . . . . . . . .  25
       3.2.2.  The smime-type Parameter  . . . . . . . . . . . . . .  26
     3.3.  Creating an Enveloped-Only Message  . . . . . . . . . . .  27
     3.4.  Creating an Authenticated Enveloped-Only Message  . . . .  27
     3.5.  Creating a Signed-Only Message  . . . . . . . . . . . . .  29
       3.5.1.  Choosing a Format for Signed-Only Messages  . . . . .  29
       3.5.2.  Signing Using application/pkcs7-mime with SignedData   30
       3.5.3.  Signing Using the multipart/signed Format . . . . . .  31
     3.6.  Creating a Compressed-Only Message  . . . . . . . . . . .  34
     3.7.  Multiple Operations . . . . . . . . . . . . . . . . . . .  34
     3.8.  Creating a Certificate Management Message . . . . . . . .  35
     3.9.  Registration Requests . . . . . . . . . . . . . . . . . .  36
     3.10. Identifying an S/MIME Message . . . . . . . . . . . . . .  36
   4.  Certificate Processing  . . . . . . . . . . . . . . . . . . .  36
     4.1.  Key Pair Generation . . . . . . . . . . . . . . . . . . .  37
     4.2.  Signature Generation  . . . . . . . . . . . . . . . . . .  37
     4.3.  Signature Verification  . . . . . . . . . . . . . . . . .  37
     4.4.  Encryption  . . . . . . . . . . . . . . . . . . . . . . .  38
     4.5.  Decryption  . . . . . . . . . . . . . . . . . . . . . . .  38
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  38
     5.1.  Media Type for application/pkcs7-mime . . . . . . . . . .  38
     5.2.  Media Type for application/pkcs7-signature  . . . . . . .  39
     5.3.  Register authEnveloped-data smime-type  . . . . . . . . .  40
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  40
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  44
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  44
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  48
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  51
   Appendix B.  Historic Mail Considerations . . . . . . . . . . . .  53
     B.1.  DigestAlgorithmIdentifier . . . . . . . . . . . . . . . .  53
     B.2.  Signature Algorithms  . . . . . . . . . . . . . . . . . .  54
     B.3.  ContentEncryptionAlgorithmIdentifier  . . . . . . . . . .  56



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     B.4.  KeyEncryptionAlgorithmIdentifier  . . . . . . . . . . . .  56
   Appendix C.  Moving S/MIME v2 Message Specification to Historic
                Status . . . . . . . . . . . . . . . . . . . . . . .  56
   Appendix D.  Acknowledgments  . . . . . . . . . . . . . . . . . .  57
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  57

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).  As a supplementary service, S/MIME provides for message
   compression.

   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 or SIP.
   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 of Internet messages and
   allows extensions for new content-type-based applications.

   This specification defines how to create a MIME body part that has
   been cryptographically enhanced according to the Cryptographic
   Message Syntax (CMS) [CMS], which is derived from PKCS #7 [RFC2315].
   This specification also defines the application/pkcs7-mime media type
   that can be used to transport those body parts.

   This document also discusses how to use the multipart/signed media
   type defined in [RFC1847] to transport S/MIME signed messages.
   multipart/signed is used in conjunction with the




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   application/pkcs7-signature media 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], [RFC3370],
   [RFC4056], [RFC3560], and [RFC5754].

   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.  Definitions

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

   ASN.1:             Abstract Syntax Notation One, as defined in ITU-T
                      Recommendations X.680, X.681, X.682 and X.683
                      [ASN.1].

   BER:               Basic Encoding Rules for ASN.1, as defined in ITU-
                      T Recommendation X.690 [X.690].

   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 ITU-T Recommendation X.690 [X.690].

   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>




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                      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.

   Data Integrity Service:  A security service that protects againist
                      unauthorized changes to data by ensuring that
                      changes to the data are detectable.  [RFC4949]

   Data Confidentiality:  The property that data is not discolsed to
                      system entities unless they have been authorized
                      to know the data.  [RFC4949]

   Data Origination:  The corroboration that the source of the data
                      received is as claimed.  [RFC4949].

1.3.  Conventions Used in This Document

   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 [RFC2119].

   We define the additional requirement levels:

   SHOULD+   This term means the same as SHOULD.  However, the authors
             expect that a requirement marked as SHOULD+ will be
             promoted at some future time to be a MUST.




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   SHOULD-   This term means the same as SHOULD.  However, the authors
             expect that a requirement marked as SHOULD- will be demoted
             to a MAY in a future version of this document.

   MUST-     This term means the same as MUST.  However, the authors
             expect that this requirement will no longer be a MUST in a
             future document.  Although its status will be determined at
             a later time, it is reasonable to expect that if a future
             revision of a document alters the status of a MUST-
             requirement, it will remain at least a SHOULD or a SHOULD-.

   The term RSA in this document almost always refers to the PKCS#1 v1.5
   RSA signature or encryption algorithms even when not qualified as
   such.  There are a couple of places where it refers to the general
   RSA cryptographic operation, these can be determined from the context
   where it is used.

1.4.  Compatibility with Prior Practice of S/MIME

   S/MIME version 4.0 agents ought to 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
   [SMIMEv2], S/MIME version 3 is described in RFC 2630 through RFC 2634
   inclusive and RFC 5035 [SMIMEv3], S/MIME version 3.1 is described in
   RFC 3850, RFC 3851, RFC 3852, RFC 2634, and RFC 5035 [SMIMEv3.1], and
   S/MIME version 3.2 is described in [SMIMEv3.2].  [RFC2311] also has
   historical information about the development of S/MIME.

1.5.  Changes from S/MIME v3 to S/MIME v3.1

   The RSA public key algorithm was changed to a MUST implement key
   wrapping algorithm, and the Diffie-Hellman (DH) 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.





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   Header protection through the use of the message/rfc822 media type
   has been added.

   Use of the CompressedData CMS type is allowed, along with required
   media type and file extension additions.

1.6.  Changes from S/MIME v3.1 to S/MIME v3.2

   Editorial changes, e.g., replaced "MIME type" with "media type",
   content-type with Content-Type.

   Moved "Conventions Used in This Document" to Section 1.3.  Added
   definitions for SHOULD+, SHOULD-, and MUST-.

   Section 1.1 and Appendix A: Added references to RFCs for RSASSA-PSS,
   RSAES-OAEP, and SHA2 CMS algorithms.  Added CMS Multiple Signers
   Clarification to CMS reference.

   Section 1.2: Updated references to ASN.1 to X.680 and BER and DER to
   X.690.

   Section 1.4: Added references to S/MIME MSG 3.1 RFCs.

   Section 2.1 (digest algorithm): SHA-256 added as MUST, SHA-1 and MD5
   made SHOULD-.

   Section 2.2 (signature algorithms): RSA with SHA-256 added as MUST,
   and DSA with SHA-256 added as SHOULD+, RSA with SHA-1, DSA with
   SHA-1, and RSA with MD5 changed to SHOULD-, and RSASSA-PSS with
   SHA-256 added as SHOULD+.  Also added note about what S/MIME v3.1
   clients support.

   Section 2.3 (key encryption): DH changed to SHOULD-, and RSAES-OAEP
   added as SHOULD+.  Elaborated requirements for key wrap algorithm.

   Section 2.5.1: Added requirement that receiving agents MUST support
   both GeneralizedTime and UTCTime.

   Section 2.5.2: Replaced reference "sha1WithRSAEncryption" with
   "sha256WithRSAEncryption", "DES-3EDE-CBC" with "AES-128 CBC", and
   deleted the RC5 example.

   Section 2.5.2.1: Deleted entire section (discussed deprecated RC2).

   Section 2.7, 2.7.1, Appendix A: references to RC2/40 removed.

   Section 2.7 (content encryption): AES-128 CBC added as MUST, AES-192
   and AES-256 CBC SHOULD+, tripleDES now SHOULD-.



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   Section 2.7.1: Updated pointers from 2.7.2.1 through 2.7.2.4 to
   2.7.1.1 to 2.7.1.2.

   Section 3.1.1: Removed text about MIME character sets.

   Section 3.2.2 and 3.6: Replaced "encrypted" with "enveloped".  Update
   OID example to use AES-128 CBC oid.

   Section 3.4.3.2: Replace micalg parameter for SHA-1 with sha-1.

   Section 4: Updated reference to CERT v3.2.

   Section 4.1: Updated RSA and DSA key size discussion.  Moved last
   four sentences to security considerations.  Updated reference to
   randomness requirements for security.

   Section 5: Added IANA registration templates to update media type
   registry to point to this document as opposed to RFC 2311.

   Section 6: Updated security considerations.

   Section 7: Moved references from Appendix B to this section.  Updated
   references.  Added informational references to SMIMEv2, SMIMEv3, and
   SMIMEv3.1.

   Appendix C: Added Appendix C to move S/MIME v2 to Historic status.

1.7.  Changes for S/MIME v4.0

   -  Add the use of AuthEnvelopedData, including defining and
      registering an smime-type value (Section 2.4.4 and Section 3.4).

   -  Update the content encryption algorithms (Section 2.7 and
      Section 2.7.1.2): Add AES-256 GCM, add ChaCha200-Poly1305, remove
      AES-192 CBC, mark tripleDES as historic.

   -  Update the set of signature algorithms (Section 2.2): Add EdDSA
      and ECDSA, mark DSA as historic

   -  Update the set of digest algorithms (Section 2.1): Add SHA-512,
      mark SHA-1 as historic.

   -  Update the size of keys to be used for RSA encryption and RSA
      signing (Section 4).

   -  Create Appendix B which deals with considerations for dealing with
      historic email messages.




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2.  CMS Options

   CMS allows for a wide variety of options in content, attributes, 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.  [RFC3370] and
   [RFC5754] provides additional details regarding the use of the
   cryptographic algorithms.  [ESS] provides additional details
   regarding the use of additional attributes.

2.1.  DigestAlgorithmIdentifier

   The algorithms here are used for digesting the body of the message
   and are not the same as the digest algorithms used as part the
   signature algorithms.  The result of this is placed in the message-
   digest attribute of the signed attributes.  It is RECOMMENDED that
   the algorithm used for digesting the body of the message be of
   similar or greater strength than the signature algorithm.

   Sending and Receiving agents:

   -  MUST support SHA-256.

   -  MUST support SHA-512.

   [RFC5754] provides the details for using these algorithms with
   S/MIME.

2.2.  SignatureAlgorithmIdentifier

   Receiving agents:

   -  MUST support ECDSA with curve P-256 and SHA-256.

   -  MUST support EdDSA with curve 25519 using PureEdDSA mode.

   -  MUST- support RSA with SHA-256.

   -  SHOULD support RSASSA-PSS with SHA-256.

   -  MUST NOT support EdDSA using the pre-hash mode.

   Sending agents:

   -  MUST support at least one of the following algorithms: ECDSA with
      curve P-256 and SHA-256, or EdDSA with curve 25519 using PureEdDSA
      mode.




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   -  MUST- support RSA with SHA-256.

   -  SHOULD support RSASSA-PSS with SHA-256.

   -  MUST NOT support EdDSA using the pre-hash mode.

   Both ECDSA and EdDSA are included in the list of required algorithms
   for political reasons.  NIST is unable to provide the seeds that were
   used to create their standardized curves, this means that there is a
   section of the community which believes that there might be a
   backdoor to these curves.  The EdDSA curves were, in part, created in
   response to this feeling.  However, there are still significant
   sections of the industry which need to have NIST approved algorithms.
   For this reason, both sets of curves are represented in the recieving
   agent list, but there is only a requirement for curve in the sending
   agent list.

   See Section 4.1 for information on key size and algorithm references.

2.3.  KeyEncryptionAlgorithmIdentifier

   Receiving and sending agents:

   -  MUST support ECDH ephemeral-static mode for P-256, as specified in
      [RFC5753].

   -  MUST support ECDH ephemeral-static mode for X25519 using HKDF-256
      for the KDF, as specified in
      [I-D.ietf-curdle-cms-ecdh-new-curves].

   -  MUST- support RSA Encryption, as specified in [RFC3370].

   -  SHOULD+ support RSAES-OAEP, as specified in [RFC3560].

   When ECDH ephemeral-static is used, a key wrap algorithm is also
   specified in the KeyEncryptionAlgorithmIdentifier [RFC5652].  The
   underlying encryption functions for the key wrap and content
   encryption algorithm ([RFC3370] and [RFC3565]) and the key sizes for
   the two algorithms MUST be the same (e.g., AES-128 key wrap algorithm
   with AES-128 content encryption algorithm).  As both 128 and 256 bit
   AES modes are mandatory-to-implment as content encryption algorithms
   (Section 2.7), both the AES-128 and AES-256 key wrap algorithms MUST
   be supported when ECDH ephemeral-static is used.

   Appendix B provides information on algorithms support in older
   versions of S/MIME.





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2.4.  General Syntax

   There are several CMS content types.  Of these, only the Data,
   SignedData, EnvelopedData, AuthEnvelopedData, 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 media type 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 3.5.3 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.  AuthEnvelopedData Content Type

   This content type is used to apply data confidentiality and message
   integrity to a message.  This content type does not provide
   authentication or non-repudiation.  A sender needs to have access to
   a public key for each intended message recipient to use this service.

2.4.5.  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
   the message's size.



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   See Section 3.7 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 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:

   -  Signing Time (Section 2.5.1 in this document)

   -  SMIME Capabilities (Section 2.5.2 in this document)

   -  Encryption Key Preference (Section 2.5.3 in this document)

   -  Message Digest (Section 11.2 in [RFC5652])

   -  Content Type (Section 11.1 in [RFC5652])

   Further, receiving agents SHOULD be able to handle zero or one
   instance of the signingCertificate and signingCertificatev2 signed
   attributes, as defined in Section 5 of RFC 2634 [ESS] and Section 3
   of RFC 5035 [ESS].

   Sending agents SHOULD generate one instance of the signingCertificate
   or signingCertificatev2 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 they do 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.





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   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.

   Receiving agents MUST be able to process signing-time attributes that
   are encoded in either UTCTime or GeneralizedTime.

2.5.2.  SMIME Capabilities Attribute

   The SMIMECapabilities attribute includes signature algorithms (such
   as "sha256WithRSAEncryption"), symmetric algorithms (such as "AES-128
   CBC"), authenticated symmetric algorithms (such as "AES-128 GCM") and
   key encipherment algorithms (such as "rsaEncryption").  The presence
   of an algorthm based SMIME Capability attribute in this sequence
   implies that the sender can deal with the algorithm as well as
   undertanding the ASN.1 structures associated with that algorithm.
   There are also several identifiers that 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.).





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   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
   AES-128 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.

   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.

2.5.3.  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



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   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 signer of a X.509 certificate that
      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




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      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 v4.0 implementations MUST support both issuerAndSerialNumber
   and 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 AES-128 GCM and
      AES-256 GCM [RFC5084].

   -  MUST- support encryption and decryption with AES-128 CBC
      [RFC3565].

   -  SHOULD+ support encryption and decryption with ChaCha20-Poly1305
      [RFC7905].

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



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      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.  The decision to use or not use weak encryption
   overrides any other decision in this section about which encryption
   algorithm to use.

   Section 2.7.1.1 and Section 2.7.1.2 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,



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   then the sending agent SHOULD use AES-256 GCM because it is a
   stronger algorithm and is required by S/MIME v4.0.  If the sending
   agent chooses not to use AES-256 GCM in this step, it SHOULD use
   AES-128 CBC.

2.7.2.  Choosing Weak Encryption

   All algorithms that use 112-bit keys are 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 a weak encryption algorithm before sending the data, and
   possibly allow the human to use a stronger encryption method such as
   AES GCM or AES CBC.

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 media 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 that
   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 [RFC1847].



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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 message headers and MIME body, and does not include the RFC-822
   header.  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 header is required, the use of the message/rfc822 media 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, Section 3.5, 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.  The description of the procedures from [MIME-SPEC] is
   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



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   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 header fields
   (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 header
   fields.  It is up to the receiving client to decide how to present
   this "inner" header along with the unprotected "outer" header.

   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 media type
   and subtype of an entity, and are not described here.  Instead, the
   standard for the particular media 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 that 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].

   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.





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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 field 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 that "know" that all intended recipients 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



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   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 cannot 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.

   [RFC1847] prohibits an agent from changing the transfer encoding of
   the first part of a multipart/signed message.  If a compliant agent
   that cannot 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 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
   the transfer encoded parts, all MUST be <CR><LF>.

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



















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   Content-Type: multipart/mixed; boundary=bar

   --bar
   Content-Type: text/plain; charset=iso-8859-1
   Content-Transfer-Encoding: quoted-printable

   =A1Hola Michael!

   How do you like the new S/MIME specification?

   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 Media Type

   The application/pkcs7-mime media type is used to carry CMS content
   types including EnvelopedData, SignedData, and CompressedData.  The
   details of constructing these entities are described in subsequent
   sections.  This section describes the general characteristics of the
   application/pkcs7-mime media 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 media type.



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   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 Content-Type header
   field 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 [RFC2138] 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:

   Media Type                                                    File
                                                              Extension
   application/pkcs7-mime (SignedData, EnvelopedData)            .p7m
   application/pkcs7-mime (degenerate SignedData certificate     .p7c
   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 media 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



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   S/MIME implementation MUST use the media 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        id-data
            compressed-data    CompressedData    id-data
            authEnveloped-data AuthEnvelopedData 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 three values for smime-type SHOULD be assigned "signed-*",
       "authEnv-*", and "enveloped-*".  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.2.16.840.1.101.3.4.1.2" would be AES-128 CBC).

   It is explicitly intended that this field be a suitable hint for mail
   client applications to indicate whether a message is "signed",
   "authEnveloped" or "enveloped" without having to tunnel into the CMS
   payload.

   A registry for additional smime-type parameter values has been
   defined in [RFC7114].








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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
            [RFC5652], 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; name=smime.p7m;
           smime-type=enveloped-data
   Content-Transfer-Encoding: base64
   Content-Disposition: attachment; filename=smime.p7m

   MIIBHgYJKoZIhvcNAQcDoIIBDzCCAQsCAQAxgcAwgb0CAQAwJjASMRAwDgYDVQQDEw
   dDYXJsUlNBAhBGNGvHgABWvBHTbi7NXXHQMA0GCSqGSIb3DQEBAQUABIGAC3EN5nGI
   iJi2lsGPcP2iJ97a4e8kbKQz36zg6Z2i0yx6zYC4mZ7mX7FBs3IWg+f6KgCLx3M1eC
   bWx8+MDFbbpXadCDgO8/nUkUNYeNxJtuzubGgzoyEd8Ch4H/dd9gdzTd+taTEgS0ip
   dSJuNnkVY4/M652jKKHRLFf02hosdR8wQwYJKoZIhvcNAQcBMBQGCCqGSIb3DQMHBA
   gtaMXpRwZRNYAgDsiSf8Z9P43LrY4OxUk660cu1lXeCSFOSOpOJ7FuVyU=

3.4.  Creating an Authenticated Enveloped-Only Message

   This section describes the format for enveloping a MIME entity
   without signing it.  Authenticated enveloped messages provide
   confidentiality and data integrity.  It is important to note that
   sending authenticated enveloped messages does not provide for
   authentication when using S/MIME.  It is possible to replace



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   ciphertext in such a way that the processed message will still be
   valid, but the meaning can be altered.  However this is substantially
   more difficult than it is for an enveloped-only message as the

   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 AuthEnvelopedData.  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
            AuthEnvelopedData (see [RFC5083]).

   Step 3.  The AuthEnvelopedData 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 authenticated enveloped-only messages is
   "authEnveloped-data".  The file extension for this type of message is
   ".p7m".

   A sample message would be:


























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   Content-Type: application/pkcs7-mime; smime-type=authEnveloped-data;
         name=smime.p7m
   Content-Transfer-Encoding: base64
   Content-Disposition: attachment; filename=smime.p7m

   MIIDWQYLKoZIhvcNAQkQARegggNIMIIDRAIBADGBvjCBuwIBADAmMBIxEDAO
   BgNVBAMTB0NhcmxSU0ECEEY0a8eAAFa8EdNuLs1dcdAwCwYJKoZIhvcNAQEB
   BIGAgyZJo0ERTxA4xdTri5P5tVMyh0RARepTUCORZvlUbcUlaI8IpJZH3/J1
   Fv6MxTRS4O/K+ZcTlQmYeWLQvwdltQdOIP3mhpqXzTnOYhTK1IDtF2zx75Lg
   vE+ilpcLIzXfJB4RCBPtBWaHAof4Wb+VMQvLkk9OolX4mRSH1LPktgAwggJq
   BgkqhkiG9w0BBwEwGwYJYIZIAWUDBAEGMA4EDGPizioC9OHSsnNx4oCCAj7Y
   Cb8rOy8+55106newEJohC/aDgWbJhrMKzSOwa7JraXOV3HXD3NvKbl665dRx
   vmDwSCNaLCRU5q8/AxQx2SvnAbM+JKcEfC/VFdd4SiHNiUECAApLku2rMi5B
   WrhW/FXmx9d+cjum2BRwB3wj0q1wajdB0/kVRbQwg697dnlYyUog4vpJERjr
   7KAkawZx1RMHaM18wgZjUNpCBXFS3chQi9mTBp2i2Hf5iZ8OOtTx+rCQUmI6
   Jhy03vdcPCCARBjn3v0d3upZYDZddMA41CB9fKnnWFjadV1KpYwv80tqsEfx
   Vo0lJQ5VtJ8MHJiBpLVKadRIZ4iH2ULC0JtN5mXE1SrFKh7cqbJ4+7nqSRL3
   oBTud3rX41DGshOjpqcYHT4sqYlgZkc6dp0g1+hF1p3cGmjHdpysV2NVSUev
   ghHbvSqhIsXFzRSWKiZOigmlkv3R5LnjpYyP4brM62Jl7y0qborvV4dNMz7m
   D+5YxSlH0KAe8z6TT3LHuQdN7QCkFoiUSCaNhpAFaakkGIpqcqLhpOK4lXxt
   kptCG93eUwNCcTxtx6bXufPR5TUHohvZvfeqMp42kL37FJC/A8ZHoOxXy8+X
   X5QYxCQNuofWlvnIWv0Nr8w65x6lgVjPYmd/cHwzQKBTBMXN6pBud/PZL5zF
   tw3QHlQkBR+UflMWZKeN9L0KdQ27mQlCo5gQS85aifxoiiA2v9+0hxZw91rP
   IW4D+GS7oMMoKj8ZNyCJJsyf5smRZ+WxeBoolb3+TiGcBBCsRnfe6noLZiFO
   6Zeu2ZwE

3.5.  Creating a Signed-Only Message

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

   -  application/pkcs7-mime with SignedData.

   -  multipart/signed.

   In general, the multipart/signed form is preferred for sending, and
   receiving agents MUST be able to handle both.

3.5.1.  Choosing a Format for Signed-Only Messages

   There are no hard-and-fast rules as to when a particular signed-only
   format is chosen.  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 or not they have S/MIME software.
   They can also be viewed whether they are using a MIME-native user



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   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 that would break the signature.

3.5.2.  Signing Using application/pkcs7-mime with SignedData

   This signing format uses the application/pkcs7-mime media 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 are 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:



















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   Content-Type: application/pkcs7-mime; smime-type=signed-data;
       name=smime.p7m
   Content-Transfer-Encoding: base64
   Content-Disposition: attachment; filename=smime.p7m

   MIIDmQYJKoZIhvcNAQcCoIIDijCCA4YCAQExCTAHBgUrDgMCGjAtBgkqhkiG9w0BBw
   GgIAQeDQpUaGlzIGlzIHNvbWUgc2FtcGxlIGNvbnRlbnQuoIIC4DCCAtwwggKboAMC
   AQICAgDIMAkGByqGSM44BAMwEjEQMA4GA1UEAxMHQ2FybERTUzAeFw05OTA4MTcwMT
   EwNDlaFw0zOTEyMzEyMzU5NTlaMBMxETAPBgNVBAMTCEFsaWNlRFNTMIIBtjCCASsG
   ByqGSM44BAEwggEeAoGBAIGNze2D6gqeOT7CSCij5EeT3Q7XqA7sU8WrhAhP/5Thc0
   h+DNbzREjR/p+vpKGJL+HZMMg23j+bv7dM3F9piuR10DcMkQiVm96nXvn89J8v3UOo
   i1TxP7AHCEdNXYjDw7Wz41UIddU5dhDEeL3/nbCElzfy5FEbteQJllzzflvbAhUA4k
   emGkVmuBPG2o+4NyErYov3k80CgYAmONAUiTKqOfs+bdlLWWpMdiM5BAI1XPLLGjDD
   HlBd3ZtZ4s2qBT1YwHuiNrhuB699ikIlp/R1z0oIXks+kPht6pzJIYo7dhTpzi5dow
   fNI4W4LzABfG1JiRGJNkS9+MiVSlNWteL5c+waYTYfEX/Cve3RUP+YdMLRgUpgObo2
   OQOBhAACgYBc47ladRSWC6l63eM/qeysXty9txMRNKYWiSgRI9k0hmd1dRMSPUNbb+
   VRv/qJ8qIbPiR9PQeNW2PIu0WloErjhdbOBoA/6CN+GvIkq1MauCcNHu8Iv2YUgFxi
   rGX6FYvxuzTU0pY39mFHssQyhPB+QUD9RqdjTjPypeL08oPluKOBgTB/MAwGA1UdEw
   EB/wQCMAAwDgYDVR0PAQH/BAQDAgbAMB8GA1UdIwQYMBaAFHBEPoIub4feStN14z0g
   vEMrk/EfMB0GA1UdDgQWBBS+bKGz48H37UNwpM4TAeL945f+zTAfBgNVHREEGDAWgR
   RBbGljZURTU0BleGFtcGxlLmNvbTAJBgcqhkjOOAQDAzAAMC0CFFUMpBkfQiuJcSIz
   jYNqtT1na79FAhUAn2FTUlQLXLLd2ud2HeIQUltDXr0xYzBhAgEBMBgwEjEQMA4GA1
   UEAxMHQ2FybERTUwICAMgwBwYFKw4DAhowCQYHKoZIzjgEAwQuMCwCFD1cSW6LIUFz
   eXle3YI5SKSBer/sAhQmCq7s/CTFHOEjgASeUjbMpx5g6A==

3.5.3.  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 media type described in [RFC1847].  The
   multipart/signed media 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.

3.5.3.1.  The application/pkcs7-signature Media Type

   This media 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".







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3.5.3.2.  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 [RFC1847].
   The values to be placed in the micalg parameter SHOULD be from the
   following:

    Algorithm Value Used
    MD5       md5
    SHA-1     sha-1
    SHA-224   sha-224
    SHA-256   sha-256
    SHA-384   sha-384
    SHA-512   sha-512
    Any other (defined separately in algorithm profile or "unknown" if
              not defined)





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   (Historical note: some early implementations of S/MIME emitted and
   expected "rsa-md5", "rsa-sha1", and "sha1" for the micalg parameter.)
   Receiving agents SHOULD be able to recover gracefully from a micalg
   parameter value that they do not recognize.  Future names for this
   parameter will be consistent with the IANA "Hash Function Textual
   Names" registry.

3.5.3.3.  Sample multipart/signed Message

   Content-Type: multipart/signed;
       micalg=SHA1;
       boundary="----=_NextBoundry____Fri,_06_Sep_2002_00:25:21";
       protocol="application/pkcs7-signature"

   This is a multi-part message in MIME format.

   ------=_NextBoundry____Fri,_06_Sep_2002_00:25:21

   This is some sample content.
   ------=_NextBoundry____Fri,_06_Sep_2002_00:25:21
   Content-Type: application/pkcs7-signature; name=smime.p7s
   Content-Transfer-Encoding: base64
   Content-Disposition: attachment; filename=smime.p7s

   MIIDdwYJKoZIhvcNAQcCoIIDaDCCA2QCAQExCTAHBgUrDgMCGjALBgkqhkiG9w0BBw
   GgggLgMIIC3DCCApugAwIBAgICAMgwCQYHKoZIzjgEAzASMRAwDgYDVQQDEwdDYXJs
   RFNTMB4XDTk5MDgxNzAxMTA0OVoXDTM5MTIzMTIzNTk1OVowEzERMA8GA1UEAxMIQW
   xpY2VEU1MwggG2MIIBKwYHKoZIzjgEATCCAR4CgYEAgY3N7YPqCp45PsJIKKPkR5Pd
   DteoDuxTxauECE//lOFzSH4M1vNESNH+n6+koYkv4dkwyDbeP5u/t0zcX2mK5HXQNw
   yRCJWb3qde+fz0ny/dQ6iLVPE/sAcIR01diMPDtbPjVQh11Tl2EMR4vf+dsISXN/Lk
   URu15AmWXPN+W9sCFQDiR6YaRWa4E8baj7g3IStii/eTzQKBgCY40BSJMqo5+z5t2U
   tZakx2IzkEAjVc8ssaMMMeUF3dm1nizaoFPVjAe6I2uG4Hr32KQiWn9HXPSgheSz6Q
   +G3qnMkhijt2FOnOLl2jB80jhbgvMAF8bUmJEYk2RL34yJVKU1a14vlz7BphNh8Rf8
   K97dFQ/5h0wtGBSmA5ujY5A4GEAAKBgFzjuVp1FJYLqXrd4z+p7Kxe3L23ExE0phaJ
   KBEj2TSGZ3V1ExI9Q1tv5VG/+onyohs+JH09B41bY8i7RaWgSuOF1s4GgD/oI34a8i
   SrUxq4Jw0e7wi/ZhSAXGKsZfoVi/G7NNTSljf2YUeyxDKE8H5BQP1Gp2NOM/Kl4vTy
   g+W4o4GBMH8wDAYDVR0TAQH/BAIwADAOBgNVHQ8BAf8EBAMCBsAwHwYDVR0jBBgwFo
   AUcEQ+gi5vh95K03XjPSC8QyuT8R8wHQYDVR0OBBYEFL5sobPjwfftQ3CkzhMB4v3j
   l/7NMB8GA1UdEQQYMBaBFEFsaWNlRFNTQGV4YW1wbGUuY29tMAkGByqGSM44BAMDMA
   AwLQIUVQykGR9CK4lxIjONg2q1PWdrv0UCFQCfYVNSVAtcst3a53Yd4hBSW0NevTFj
   MGECAQEwGDASMRAwDgYDVQQDEwdDYXJsRFNTAgIAyDAHBgUrDgMCGjAJBgcqhkjOOA
   QDBC4wLAIUM/mGf6gkgp9Z0XtRdGimJeB/BxUCFGFFJqwYRt1WYcIOQoGiaowqGzVI

   ------=_NextBoundry____Fri,_06_Sep_2002_00:25:21--

   The content that is digested (the first part of the multipart/signed)
   consists of the bytes:




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   54 68 69 73 20 69 73 20 73 6f 6d 65 20 73 61 6d 70 6c 65 20 63 6f 6e
   74 65 6e 74 2e 0d 0a

3.6.  Creating a Compressed-Only Message

   This section describes the format for compressing a MIME entity.
   Please note that versions of S/MIME prior to version 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 [RFC3274] 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 are 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

   eNoLycgsVgCi4vzcVIXixNyCnFSF5Py8ktS8Ej0AlCkKVA==

3.7.  Multiple Operations

   The signed-only, enveloped-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



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   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 where 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.

3.8.  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 that 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".






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3.9.  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 certification 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 v4.0 does not
   require a particular certificate request mechanism.

3.10.  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 field, or the "filename" parameter on the
   Content-Disposition header field.  These parameters that give the
   file suffix are not listed below as part of the parameter section.

    Media type               parameters                     file suffix
    application/pkcs7-mime   n/a                            n/a
    multipart/signed         protocol=                      n/a
                             "application/pkcs7-signature"
    application/octet-stream n/a                            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
   [RFC5750].

   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



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   "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 [RFC4086] and the private key MUST be
   protected in a secure fashion.

   An S/MIME user agent MUST NOT generate asymmetric keys less than 2048
   bits for use with an RSA signature algorithm.

   For 2048-bit through 4096-bit RSA with SHA-256 see [RFC5754] and
   [FIPS186-4].  The first reference provides the signature algorithm's
   object identifier, and the second provides the signature algorithm's
   definition.

   For RSASSA-PSS with SHA-256, see [RFC4056].  For RSAES-OAEP, see
   [RFC3560].

4.2.  Signature Generation

   The following are the requirements for an S/MIME agent generated RSA
   and RSASSA-PSS signatures:

           key size <= 2047 : SHOULD NOT (Note 1)
   2048 <= key size <= 4096 : SHOULD     (see Security Considerations)
   4096 <  key size         : MAY        (see Security Considerations)

   Note 1: see Historical Mail Considerations in Section 6.
   Note 2: see Security Considerations in Appendix B.

   Key sizes for ECDSA and EdDSA are fixed by the curve.

4.3.  Signature Verification

   The following are the requirements for S/MIME receiving agents during
   signature verification of RSA and RSASSA-PSS signatures:

           key size <= 2047 : SHOULD NOT (Note 1)
   2048 <= key size <= 4096 : MUST       (Note 2)
   4096 <  key size         : MAY        (Note 2)

   Note 1: see Historical Mail Considerations in Section 6.
   Note 2: see Security Considerations in Appendix B.

   Key sizes for ECDSA and EdDSA are fixed by the curve.




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4.4.  Encryption

   The following are the requirements for an S/MIME agent when
   establishing keys for content encryption using the RSA, and RSA-OAEP
   algorithms:

           key size <= 2047 : SHOULD NOT (Note 1)
   2048 <= key size <= 4096 : SHOULD     (Note 2)
   4096 <  key size         : MAY        (Note 2)

   Note 1: see Historical Mail Considerations in Section 6.
   Note 2: see Security Considerations in Appendix B.

   Key sizes for ECDH are fixed by the curve.

4.5.  Decryption

   The following are the requirements for an S/MIME agent when
   establishing keys for content decryption using the RSA and RSAES-OAEP
   algorithms:

           key size <= 2047 : MAY        (Note 1)
   2048 <= key size <= 4096 : MUST       (Note 2)
   4096 <  key size         : MAY        (Note 2)

   Note 1: see Historical Mail Considerations in Section 6.
   Note 2: see Security Considerations in Appendix B.

   Key sizes for ECDH are fixed by the curve.

5.  IANA Considerations

   The following information updates the media type registration for
   application/pkcs7-mime and application/pkcs7-signature to refer to
   this document as opposed to RFC 2311.

   Note that other documents can define additional MIME media types for
   S/MIME.

5.1.  Media Type for application/pkcs7-mime











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   Type name: application

   Subtype Name: pkcs7-mime

   Required Parameters: NONE

   Optional Parameters: smime-type/signed-data
                        smime-type/enveloped-data
                        smime-type/compressed-data
                        smime-type/certs-only
                        name

   Encoding Considerations: See Section 3 of this document

   Security Considerations: See Section 6 of this document

   Interoperability Considerations: See Sections 1-6 of this document

   Published Specification: RFC 2311, RFC 2633, and this document

   Applications that use this media type: Security applications

   Additional information: NONE

   Person & email to contact for further information: iesg@ietf.org

   Intended usage: COMMON

   Restrictions on usage: NONE

   Author: Sean Turner

   Change Controller: S/MIME working group delegated from the IESG

5.2.  Media Type for application/pkcs7-signature
















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   Type name: application

   Subtype Name: pkcs7-signature

   Required Parameters: NONE

   Optional Parameters: NONE

   Encoding Considerations: See Section 3 of this document

   Security Considerations: See Section 6 of this document

   Interoperability Considerations: See Sections 1-6 of this document

   Published Specification: RFC 2311, RFC 2633, and this document

   Applications that use this media type: Security applications

   Additional information: NONE

   Person & email to contact for further information: iesg@ietf.org

   Intended usage: COMMON

   Restrictions on usage: NONE

   Author: Sean Turner

   Change Controller: S/MIME working group delegated from the IESG

5.3.  Register authEnveloped-data smime-type

   IANA is required to register the following value in the "Parameter
   Values for the smime-type Parameter" registry.  The values to be
   registered are:

      smime-type value: authEnveloped-data

      Reference: [[This Document, Section 3.2.2]]

6.  Security Considerations

   Cryptographic algorithms will be broken or weakened over time.
   Implementers and users need to check that the cryptographic
   algorithms listed in this document continue to provide the expected
   level of security.  The IETF from time to time may issue documents
   dealing with the current state of the art.  For example:




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   -  The Million Message Attack described in RFC 3218 [RFC3218].

   -  The Diffie-Hellman "small-subgroup" attacks described in RFC 2785
      [RFC2785].

   -  The attacks against hash algorithms described in RFC 4270
      [RFC4270].

   This specification uses Public-Key Cryptography technologies.  It is
   assumed that the private key is protected to ensure that it is not
   accessed or altered by unauthorized parties.

   It is impossible for most people or software to estimate the value of
   a message's content.  Further, it is impossible for most people or
   software to estimate the actual cost of recovering an encrypted
   message content 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 process a message's content.  Thus,
   choosing between different key sizes (or choosing whether to just use
   plaintext) is also impossible for most people or software.  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.

   The choice of 2048 bits as an RSA asymmetric key size in this
   specification is based on the desire to provide at least 100 bits of
   security.  The key sizes that must be supported to conform to this
   specification seem appropriate for the Internet based on [RFC3766].
   Of course, there are environments, such as financial and medical
   systems, that may select different key sizes.  For this reason, an
   implementation MAY support key sizes beyond those recommended in this
   specification.

   Receiving agents that validate signatures and sending agents that
   encrypt messages need to be cautious of cryptographic processing
   usage when validating signatures and encrypting messages using keys
   larger than those mandated in this specification.  An attacker could
   send certificates with keys that would result in excessive
   cryptographic processing, for example, keys larger than those
   mandated in this specification, which could swamp the processing
   element.  Agents that use such keys without first validating the
   certificate to a trust anchor are advised to have some sort of
   cryptographic resource management system to prevent such attacks.

   Using weak cryptography in S/MIME offers little actual security over
   sending plaintext.  However, other features of S/MIME, such as the
   specification of AES and the ability to announce stronger
   cryptographic capabilities to parties with whom you communicate,



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   allow senders to create messages that use strong encryption.  Using
   weak cryptography is never recommended unless the only alternative is
   no cryptography.

   RSA and DSA keys of less than 2048 bits are now considered by many
   experts to be cryptographically insecure (due to advances in
   computing power), and should no longer be used to protect messages.
   Such keys were previously considered secure, so processing previously
   received signed and encrypted mail will often result in the use of
   weak keys.  Implementations that wish to support previous versions of
   S/MIME or process old messages need to consider the security risks
   that result from smaller key sizes (e.g., spoofed messages) versus
   the costs of denial of service.  If an implementation supports
   verification of digital signatures generated with RSA and DSA keys of
   less than 1024 bits, it MUST warn the user.  Implementers should
   consider providing different warnings for newly received messages and
   previously stored messages.  Server implementations (e.g., secure
   mail list servers) where user warnings are not appropriate SHOULD
   reject messages with weak signatures.

   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 digital signatures.

   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 in EnvelopedData 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.  This is one of the reasons for moving from
   EnvelopedData to AuthEnvelopedData as the authenticated encryption
   algorithms provide the authentication without needing the SignedData
   layer.

   If an implementation is concerned about compliance with National
   Institute of Standards and Technology (NIST) key size
   recommendations, then see [SP800-57].

   If messaging environments make use of the fact that a message is
   signed to change the behavior of message processing (examples would
   be running rules or UI display hints), without first verifying that



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   the message is actually signed and knowing the state of the
   signature, this can lead to incorrect handling of the message.
   Visual indicators on messages may need to have the signature
   validation code checked periodically if the indicator is supposed to
   give information on the current status of a message.

   Many people assume that the use of an authenticated encryption
   algorithm is all that is needed to be in a situtation where the
   sender of the message will be authenticated.  In almost all cases
   this is not a correct statement.  There are a number of preconditions
   that need to hold for an authenticated encryption algorithm to
   provide this service:

   -  The starting key must be bound to a single entity.  The use of a
      group key only would allow for the statement that a message was
      sent by one of the entities that held the key but will not
      identify a specific entity.

   -  The message must have exactly one sender and one recipient.
      Having more than one recipient would allow for the second
      recipient to create a message that the first recipient would
      believe is from the sender by stripping them as a recipient from
      the message.

   -  A direct path needs to exist from the starting key to the key used
      as the content encryption key (CEK) which guarantees that no third
      party could have seen the resulting CEK.  This means that one
      needs to be using an algorithm that is called a "Direct
      Encryption" or a "Direct Key Agreement" algorithm in other
      contexts.  This means that the starting key is used directly as
      the CEK key, or that the starting key is used to create a secret
      which then is transformed into the CEK via a KDF step.

   S/MIME implementations almost universally use ephemeral-static rather
   than static-static key agreement and do not use a shared secret for
   encryption, this means that the first precondition is not met.  There
   is a document [RFC6278] which defined how to use static-static key
   agreement with CMS so that is readably doable.  Currently, all S/MIME
   key agreement methods derive a KEK and wrap a CEK.  This violates the
   third precondition above.  New key agreement algorithms that directly
   created the CEK without creating an intervening KEK would need to be
   defined.

   Even when all of the preconditions are met and origination of a
   message is established by the use of an authenticated encryption
   algorithm, users need to be aware that there is no way to prove this
   to a third party.  This is because either of the parties can
   successfully create the message (or just alter the content) based on



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   the fact that the CEK is going to be known to both parties.  Thus the
   origination is always built on a presumption that "I did not send
   this message to myself."

   All of the authenticated encryption algorithms in this document use
   counter mode for the encryption portion of the algorithm.  This means
   that the length of the plain text will always be known as the cipher
   text length and the plain text length are always the same.  This
   information can enable passive observers to infer information based
   solely on the length of the message.  Applications for which this is
   a concern need to provide some type of padding so that the length of
   the message does not provide this information.

7.  References

7.1.  Normative References

   [ASN.1]    "Information Technology - Abstract Syntax Notation
              (ASN.1)".

              ASN.1 syntax consists of the following references [X.680],
              [X.681], [X.682], and [X.683].

   [CHARSETS]
              "Character sets assigned by IANA.",
              <http://www.iana.org/assignments/character-sets.>.

   [CMS]      "Cryptograhic Message Syntax".

              This is the set of documents dealing with the
              cryptographic message syntax and refers to [RFC5652] and
              [RFC5083].

   [ESS]      "Enhanced Security Services for S/MIME".

              This is the set of documents dealing with enhanged
              security services and refers to [RFC2634] and [RFC5035].

   [FIPS186-4]
              National Institute of Standards and Technology (NIST),
              "Digital Signature Standard (DSS)", Federal Information
              Processing Standards Publication 186-4, July 2013.

   [I-D.ietf-curdle-cms-ecdh-new-curves]
              Housley, R., "Use of the Elliptic Curve Diffie-Hellamn Key
              Agreement Algorithm with X25519 and X448 in the
              Cryptographic Message Syntax (CMS)", draft-ietf-curdle-
              cms-ecdh-new-curves-02 (work in progress), March 2017.



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   [I-D.ietf-lamps-rfc5750-bis]
              Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/ MIME) Version
              4.0 Certificate Handling", draft-ietf-lamps-rfc5750-bis-03
              (work in progress), March 2017.

   [MIME-SPEC]
              "MIME Message Specifications".

              This is the set of documents that define how to use MIME.
              This set of documents is [RFC2045], [RFC2046], [RFC2047],
              [RFC2049], [RFC4288], and [RFC4289].

   [RFC1847]  Galvin, J., Murphy, S., Crocker, S., and N. Freed,
              "Security Multiparts for MIME: Multipart/Signed and
              Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847,
              October 1995, <http://www.rfc-editor.org/info/rfc1847>.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <http://www.rfc-editor.org/info/rfc2045>.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <http://www.rfc-editor.org/info/rfc2046>.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <http://www.rfc-editor.org/info/rfc2047>.

   [RFC2049]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Five: Conformance Criteria and
              Examples", RFC 2049, DOI 10.17487/RFC2049, November 1996,
              <http://www.rfc-editor.org/info/rfc2049>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2138]  Rigney, C., Rubens, A., Simpson, W., and S. Willens,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2138, DOI 10.17487/RFC2138, April 1997,
              <http://www.rfc-editor.org/info/rfc2138>.




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   [RFC2634]  Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
              RFC 2634, DOI 10.17487/RFC2634, June 1999,
              <http://www.rfc-editor.org/info/rfc2634>.

   [RFC3274]  Gutmann, P., "Compressed Data Content Type for
              Cryptographic Message Syntax (CMS)", RFC 3274,
              DOI 10.17487/RFC3274, June 2002,
              <http://www.rfc-editor.org/info/rfc3274>.

   [RFC3370]  Housley, R., "Cryptographic Message Syntax (CMS)
              Algorithms", RFC 3370, DOI 10.17487/RFC3370, August 2002,
              <http://www.rfc-editor.org/info/rfc3370>.

   [RFC3560]  Housley, R., "Use of the RSAES-OAEP Key Transport
              Algorithm in Cryptographic Message Syntax (CMS)",
              RFC 3560, DOI 10.17487/RFC3560, July 2003,
              <http://www.rfc-editor.org/info/rfc3560>.

   [RFC3565]  Schaad, J., "Use of the Advanced Encryption Standard (AES)
              Encryption Algorithm in Cryptographic Message Syntax
              (CMS)", RFC 3565, DOI 10.17487/RFC3565, July 2003,
              <http://www.rfc-editor.org/info/rfc3565>.

   [RFC4056]  Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in
              Cryptographic Message Syntax (CMS)", RFC 4056,
              DOI 10.17487/RFC4056, June 2005,
              <http://www.rfc-editor.org/info/rfc4056>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", RFC 4288, DOI 10.17487/RFC4288,
              December 2005, <http://www.rfc-editor.org/info/rfc4288>.

   [RFC4289]  Freed, N. and J. Klensin, "Multipurpose Internet Mail
              Extensions (MIME) Part Four: Registration Procedures",
              BCP 13, RFC 4289, DOI 10.17487/RFC4289, December 2005,
              <http://www.rfc-editor.org/info/rfc4289>.

   [RFC5035]  Schaad, J., "Enhanced Security Services (ESS) Update:
              Adding CertID Algorithm Agility", RFC 5035,
              DOI 10.17487/RFC5035, August 2007,
              <http://www.rfc-editor.org/info/rfc5035>.





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   [RFC5083]  Housley, R., "Cryptographic Message Syntax (CMS)
              Authenticated-Enveloped-Data Content Type", RFC 5083,
              DOI 10.17487/RFC5083, November 2007,
              <http://www.rfc-editor.org/info/rfc5083>.

   [RFC5084]  Housley, R., "Using AES-CCM and AES-GCM Authenticated
              Encryption in the Cryptographic Message Syntax (CMS)",
              RFC 5084, DOI 10.17487/RFC5084, November 2007,
              <http://www.rfc-editor.org/info/rfc5084>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <http://www.rfc-editor.org/info/rfc5652>.

   [RFC5753]  Turner, S. and D. Brown, "Use of Elliptic Curve
              Cryptography (ECC) Algorithms in Cryptographic Message
              Syntax (CMS)", RFC 5753, DOI 10.17487/RFC5753, January
              2010, <http://www.rfc-editor.org/info/rfc5753>.

   [RFC5754]  Turner, S., "Using SHA2 Algorithms with Cryptographic
              Message Syntax", RFC 5754, DOI 10.17487/RFC5754, January
              2010, <http://www.rfc-editor.org/info/rfc5754>.

   [SMIMEv4.0]
              "S/MIME version 4.0".

              This group of documents represents S/MIME version 4.0.
              This set of documents are [RFC2634],
              [I-D.ietf-lamps-rfc5750-bis], [[This Document]],
              [RFC5652], and [RFC5035].

   [X.680]    "Information Technology - Abstract Syntax Notation One
              (ASN.1): Specification of basic notation.  ITU-T
              Recommendation X.680 (2002)", ITU-T X.680, ISO/
              IEC 8824-1:2008, November 2008.

   [X.681]    "Information Technology - Abstract Syntax Notation One
              (ASN.1): Information object specification", ITU-T X.681,
              ISO/IEC 8824-2:2008, November 2008.

   [X.682]    "Information Technology - Abstract Syntax Notation One
              (ASN.1): Constraint specification", ITU-T X.682, ISO/
              IEC 8824-3:2008, November 2008.

   [X.683]    "Information Technology - Abstract Syntax Notation One
              (ASN.1): Parameteriztion of ASN.1 specifications",
              ITU-T X.683, ISO/IEC 8824-4:2008, November 2008.




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   [X.690]    "Information Technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER).", ITU-T X.690, ISO/IEC 8825-1:2002, July 2002.

7.2.  Informative References

   [FIPS186-2]
              National Institute of Standards and Technology (NIST),
              "Digital Signature Standard (DSS) [With Change Notice 1]",
              Federal Information Processing Standards
              Publication 186-2, January 2000.

   [RFC2268]  Rivest, R., "A Description of the RC2(r) Encryption
              Algorithm", RFC 2268, DOI 10.17487/RFC2268, March 1998,
              <http://www.rfc-editor.org/info/rfc2268>.

   [RFC2311]  Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L., and
              L. Repka, "S/MIME Version 2 Message Specification",
              RFC 2311, DOI 10.17487/RFC2311, March 1998,
              <http://www.rfc-editor.org/info/rfc2311>.

   [RFC2312]  Dusse, S., Hoffman, P., Ramsdell, B., and J. Weinstein,
              "S/MIME Version 2 Certificate Handling", RFC 2312,
              DOI 10.17487/RFC2312, March 1998,
              <http://www.rfc-editor.org/info/rfc2312>.

   [RFC2313]  Kaliski, B., "PKCS #1: RSA Encryption Version 1.5",
              RFC 2313, DOI 10.17487/RFC2313, March 1998,
              <http://www.rfc-editor.org/info/rfc2313>.

   [RFC2314]  Kaliski, B., "PKCS #10: Certification Request Syntax
              Version 1.5", RFC 2314, DOI 10.17487/RFC2314, March 1998,
              <http://www.rfc-editor.org/info/rfc2314>.

   [RFC2315]  Kaliski, B., "PKCS #7: Cryptographic Message Syntax
              Version 1.5", RFC 2315, DOI 10.17487/RFC2315, March 1998,
              <http://www.rfc-editor.org/info/rfc2315>.

   [RFC2630]  Housley, R., "Cryptographic Message Syntax", RFC 2630,
              DOI 10.17487/RFC2630, June 1999,
              <http://www.rfc-editor.org/info/rfc2630>.

   [RFC2631]  Rescorla, E., "Diffie-Hellman Key Agreement Method",
              RFC 2631, DOI 10.17487/RFC2631, June 1999,
              <http://www.rfc-editor.org/info/rfc2631>.





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   [RFC2632]  Ramsdell, B., Ed., "S/MIME Version 3 Certificate
              Handling", RFC 2632, DOI 10.17487/RFC2632, June 1999,
              <http://www.rfc-editor.org/info/rfc2632>.

   [RFC2633]  Ramsdell, B., Ed., "S/MIME Version 3 Message
              Specification", RFC 2633, DOI 10.17487/RFC2633, June 1999,
              <http://www.rfc-editor.org/info/rfc2633>.

   [RFC2785]  Zuccherato, R., "Methods for Avoiding the "Small-Subgroup"
              Attacks on the Diffie-Hellman Key Agreement Method for S/
              MIME", RFC 2785, DOI 10.17487/RFC2785, March 2000,
              <http://www.rfc-editor.org/info/rfc2785>.

   [RFC3218]  Rescorla, E., "Preventing the Million Message Attack on
              Cryptographic Message Syntax", RFC 3218,
              DOI 10.17487/RFC3218, January 2002,
              <http://www.rfc-editor.org/info/rfc3218>.

   [RFC3766]  Orman, H. and P. Hoffman, "Determining Strengths For
              Public Keys Used For Exchanging Symmetric Keys", BCP 86,
              RFC 3766, DOI 10.17487/RFC3766, April 2004,
              <http://www.rfc-editor.org/info/rfc3766>.

   [RFC3850]  Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Certificate Handling",
              RFC 3850, DOI 10.17487/RFC3850, July 2004,
              <http://www.rfc-editor.org/info/rfc3850>.

   [RFC3851]  Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 3851, DOI 10.17487/RFC3851, July 2004,
              <http://www.rfc-editor.org/info/rfc3851>.

   [RFC3852]  Housley, R., "Cryptographic Message Syntax (CMS)",
              RFC 3852, DOI 10.17487/RFC3852, July 2004,
              <http://www.rfc-editor.org/info/rfc3852>.

   [RFC4134]  Hoffman, P., Ed., "Examples of S/MIME Messages", RFC 4134,
              DOI 10.17487/RFC4134, July 2005,
              <http://www.rfc-editor.org/info/rfc4134>.

   [RFC4270]  Hoffman, P. and B. Schneier, "Attacks on Cryptographic
              Hashes in Internet Protocols", RFC 4270,
              DOI 10.17487/RFC4270, November 2005,
              <http://www.rfc-editor.org/info/rfc4270>.






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   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <http://www.rfc-editor.org/info/rfc4949>.

   [RFC5750]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Certificate
              Handling", RFC 5750, DOI 10.17487/RFC5750, January 2010,
              <http://www.rfc-editor.org/info/rfc5750>.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, DOI 10.17487/RFC5751, January
              2010, <http://www.rfc-editor.org/info/rfc5751>.

   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations
              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
              RFC 6151, DOI 10.17487/RFC6151, March 2011,
              <http://www.rfc-editor.org/info/rfc6151>.

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
              <http://www.rfc-editor.org/info/rfc6194>.

   [RFC6278]  Herzog, J. and R. Khazan, "Use of Static-Static Elliptic
              Curve Diffie-Hellman Key Agreement in Cryptographic
              Message Syntax", RFC 6278, DOI 10.17487/RFC6278, June
              2011, <http://www.rfc-editor.org/info/rfc6278>.

   [RFC7114]  Leiba, B., "Creation of a Registry for smime-type
              Parameter Values", RFC 7114, DOI 10.17487/RFC7114, January
              2014, <http://www.rfc-editor.org/info/rfc7114>.

   [RFC7905]  Langley, A., Chang, W., Mavrogiannopoulos, N.,
              Strombergson, J., and S. Josefsson, "ChaCha20-Poly1305
              Cipher Suites for Transport Layer Security (TLS)",
              RFC 7905, DOI 10.17487/RFC7905, June 2016,
              <http://www.rfc-editor.org/info/rfc7905>.

   [SMIMEv2]  "S/MIME version v2".

              This group of documents represents S/MIME version 2.  This
              set of documents are [RFC2311], [RFC2312], [RFC2313],
              [RFC2314], and [RFC2315].

   [SMIMEv3]  "S/MIME version 3".





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              This group of documents represents S/MIME version 3.  This
              set of documents are [RFC2630], [RFC2631], [RFC2632],
              [RFC2633], [RFC2634], and [RFC5035].

   [SMIMEv3.1]
              "S/MIME version 3.1".

              This group of documents represents S/MIME version 3.1.
              This set of documents are [RFC2634], [RFC3850], [RFC3851],
              [RFC3852], and [RFC5035].

   [SMIMEv3.2]
              "S/MIME version 3.2".

              This group of documents represents S/MIME version 3.2.
              This set of documents are [RFC2634], [RFC5750], [RFC5751],
              [RFC5652], and [RFC5035].

   [SP800-56A]
              National Institute of Standards and Technology (NIST),
              "Special Publication 800-56A Revision 2: Recommendation
              Pair-Wise Key Establishment Schemes Using Discrete
              Logarithm Cryptography", May 2013.

   [SP800-57]
              National Institute of Standards and Technology (NIST),
              "Special Publication 800-57: Recommendation for Key
              Management", August 2005.

   [TripleDES]
              Tuchman, W., "Hellman Presents No Shortcut Solutions to
              DES"", IEEE Spectrum v. 16, n. 7, pp 40-41, July 1979.

Appendix A.  ASN.1 Module

   Note: The ASN.1 module contained herein is unchanged from RFC 3851
   [SMIMEv3.1] with the exception of a change to the prefersBinaryInside
   ASN.1 comment.  This module uses the 1988 version of ASN.1.

   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




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   IMPORTS

   -- Cryptographic Message Syntax [CMS]
      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 by the 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
   }

   -- receipentKeyId is spelt incorrectly, but kept for historical
   -- reasons.

   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 OID indicates an ability to receive



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   -- messages with binary encoding inside the CMS wrapper.
   -- The preferBinaryInside attribute's value field is ABSENT.

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

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

   -- Signature Algorithms Not Found in [RFC3370], [RFC5754], [RFC4056],
   -- and [RFC3560]

   --
   -- 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

Appendix B.  Historic Mail Considerations

   Over the course of updating the S/MIME specifications, the set of
   recommended algorithms has been modified each time the document has
   been updated.  This means that if a user has historic emails and
   their user agent has been updated to only support the current set of
   recommended algorithms some of those old emails will no longer be
   accessible.  It is strongly suggested that user agents implement some
   of the following algorithms for dealing with historic emails.

   This appendix contains a number of references to documents that have
   been obsoleted or replaced, this is intentional as frequently the
   updated documents do not have the same information in them.

B.1.  DigestAlgorithmIdentifier

   The following algorithms have been called our for some level of
   support by previous S/MIME specifications:




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   -  SHA-1 was dropped in [SMIMEv4.0].  SHA-1 is no longer considerd to
      be secure as it is no longer collision-resistant.  The IETF
      statement on SHA-1 can be found in [RFC6194] but it is out-of-date
      relative to the most recient advances.

   -  MD5 was dropped in [SMIMEv4.0].  MD5 is no longer considered to be
      secure as it is no longer collision-resistant.  Details can be
      found in [RFC6151].

B.2.  Signature Algorithms

   There are a number of problems with validating signatures on
   sufficently historic messages.  For this reason it is strongly
   suggested that UAs treat these signatures differently from those on
   current messages.  These problems include:

   -  CAs are not required to keep certificates on a CRL beyond one
      update after a certificate has expired.  This means that unless
      CRLs are cached as part of the message it is not always possible
      to check if a certificate has been revoked.  The same problems
      exist with OCSP responses as they may be based on a CRL rather
      than on the certificate database.

   -  RSA and DSA keys of less than 2048 bits are now considered by many
      experts to be cryptographically insecure (due to advances in
      computing power).  Such keys were previously considered secure, so
      processing of historic signed messages will often result in the
      use of weak keys.  Implementations that wish to support previous
      versions of S/MIME or process old messages need to consider the
      security risks that result from smaller key sizes (e.g., spoofed
      messages) versus the costs of denial of service.

      [SMIMEv3.1] set the lower limit on suggested key sizes for
      creating and validation at 1024 bits.  Prior to that the lower
      bound on key sizes was 512 bits.

   -  Hash functions used to validate signatures on historic messages
      may longer be considered to be secure.  (See below.)  While there
      are not currently any known practical pre-image or second pre-
      image attacks against MD5 or SHA-1, the fact they are no longer
      considered to be collision resistent the security levels of the
      signatures are generally considered suspect.

   -  The previous two issues apply to the certificates used to validate
      the binding of the public key to the identity that signed the
      message as well.





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   The following algorithms have been called out for some level of
   support by previous S/MIME specifications:

   -  RSA with MD5 was dropped in [SMIMEv4.0].  MD5 is no longer
      considered to be secure as it is no longer collision-resistant.
      Details can be found in [RFC6151].

   -  RSA and DSA with SHA-1 were dropped in [SMIMEv4.0].  SHA-1 is no
      longer considered to be secure as it is no longer collision-
      resistant.  The IETF statment on SHA-1 can be found in [RFC6194]
      but it is out-of-date relative to the most recent advances.

   -  DSA with SHA-256 was dropped in [SMIMEv4.0].  DSA has been
      replaced by elliptic curve versions.

   As requirements for manditory to implement has changed over time,
   some issues have been created that can cause interopatability
   problems:

   -  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.

   -  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.

   -  Note that S/MIME v3.1 clients support verifying id-dsa-with-sha1
      and rsaEncryption and might not implement sha256withRSAEncryption.

   NOTE: 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.

   For 512-bit RSA with SHA-1 see [RFC3370] and [FIPS186-2] without
   Change Notice 1, for 512-bit RSA with SHA-256 see [RFC5754] and
   [FIPS186-2] without Change Notice 1, and for 1024-bit through
   2048-bit RSA with SHA-256 see [RFC5754] and [FIPS186-2] with Change
   Notice 1.  The first reference provides the signature algorithm's
   object identifier, and the second provides the signature algorithm's
   definition.

   For 512-bit DSA with SHA-1 see [RFC3370] and [FIPS186-2] without
   Change Notice 1, for 512-bit DSA with SHA-256 see [RFC5754] and
   [FIPS186-2] without Change Notice 1, for 1024-bit DSA with SHA-1 see
   [RFC3370] and [FIPS186-2] with Change Notice 1, for 1024-bit and
   above DSA with SHA-256 see [RFC5754] and [FIPS186-4].  The first




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   reference provides the signature algorithm's object identifier and
   the second provides the signature algorithm's definition.

B.3.  ContentEncryptionAlgorithmIdentifier

   The following algorithms have been called out for some level of
   support by previous S/MIME specifications:

   -  RC2/40 [RFC2268] was dropped in [SMIMEv3.2].  The algorithm is
      known to be insecure and, if supported, should only be used to
      decrypt existing email.

   -  DES EDE3 CBC [TripleDES], also known as "tripleDES" is dropped in
      [SMIMEv4.0].  This algorithms is removed from the supported list
      due to the fact that it has a 64-bit block size and the fact that
      it offers less that 128-bits of security.  This algorithm should
      be supported only to decrypt existing email, it should not be used
      to encrypt new emails.

B.4.  KeyEncryptionAlgorithmIdentifier

   The following algorithms have been called out for some level of
   support by previous S/MIME specifications:

   -  DH ephemeral-static mode, as specified in [RFC3370] and
      [SP800-57], was dropped in [SMIMEv4.0].

   -  RSA key sizes have been increased over time.  Decrypting old mail
      with smaller key sizes is reasonable, however new mail should use
      the updated key sizes.

   For 1024-bit DH, see [RFC3370].  For 1024-bit and larger DH, see
   [SP800-56A]; regardless, use the KDF, which is from X9.42, specified
   in [RFC3370].

Appendix C.  Moving S/MIME v2 Message Specification to Historic Status

   The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], and v3.2 [SMIMEv3.2] are
   backwards compatible with the S/MIME v2 Message Specification
   [SMIMEv2], with the exception of the algorithms (dropped RC2/40
   requirement and added DSA and RSASSA-PSS requirements).  Therefore,
   it is recommended that RFC 2311 [SMIMEv2] be moved to Historic
   status.








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Appendix D.  Acknowledgments

   Many thanks go out to the other authors of the S/MIME version 2
   Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence
   Lundblade, and Lisa Repka.  Without v2, there wouldn't be a v3, v3.1,
   v3.2 or v4.0.

   Some of the examples in this document were stolen from [RFC4134].
   Thanks go the the people who wrote and verified the examples in that
   document.

   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 because they made direct
   contributions to various versions of this document:

   Tony Capel, Piers Chivers, Dave Crocker, Bill Flanigan, Peter
   Gutmann, Alfred Hoenes, Paul Hoffman, Russ Housley, William Ottaway,
   and John Pawling.

   The version 4 update to the S/MIME documents was done under the
   auspices of the LAMPS Working Group.

Authors' Addresses

   Jim Schaad
   August Cellars

   Email: ietf@augustcellars.com


   Blake Ramsdell
   Brute Squad Labs, Inc.

   Email: blaker@gmail.com


   Sean Turner
   sn3rd

   Email: sean@sn3rd.com









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