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rfc:rfc8551

Internet Engineering Task Force (IETF) J. Schaad Request for Comments: 8551 August Cellars Obsoletes: 5751 B. Ramsdell Category: Standards Track Brute Squad Labs, Inc. ISSN: 2070-1721 S. Turner

                                                                 sn3rd
                                                            April 2019
 Secure/Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
                       Message Specification

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.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8551.

Schaad, et al. Standards Track [Page 1] RFC 8551 S/MIME 4.0 Message Specification April 2019

Copyright Notice

 Copyright (c) 2019 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
 (https://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.

Schaad, et al. Standards Track [Page 2] RFC 8551 S/MIME 4.0 Message Specification April 2019

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
   1.1.  Specification Overview  . . . . . . . . . . . . . . . . .   5
   1.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   6
   1.3.  Conventions Used in This Document . . . . . . . . . . . .   7
   1.4.  Compatibility with Prior Practice of S/MIME . . . . . . .   8
   1.5.  Changes from S/MIME v3 to S/MIME v3.1 . . . . . . . . . .   9
   1.6.  Changes from S/MIME v3.1 to S/MIME v3.2 . . . . . . . . .   9
   1.7.  Changes for S/MIME v4.0 . . . . . . . . . . . . . . . . .  11
 2.  CMS Options . . . . . . . . . . . . . . . . . . . . . . . . .  12
   2.1.  DigestAlgorithmIdentifier . . . . . . . . . . . . . . . .  12
   2.2.  SignatureAlgorithmIdentifier  . . . . . . . . . . . . . .  12
   2.3.  KeyEncryptionAlgorithmIdentifier  . . . . . . . . . . . .  13
   2.4.  General Syntax  . . . . . . . . . . . . . . . . . . . . .  13
     2.4.1.  Data Content Type . . . . . . . . . . . . . . . . . .  14
     2.4.2.  SignedData Content Type . . . . . . . . . . . . . . .  14
     2.4.3.  EnvelopedData Content Type  . . . . . . . . . . . . .  14
     2.4.4.  AuthEnvelopedData Content Type  . . . . . . . . . . .  14
     2.4.5.  CompressedData Content Type . . . . . . . . . . . . .  14
   2.5.  Attributes and the SignerInfo Type  . . . . . . . . . . .  15
     2.5.1.  Signing Time Attribute  . . . . . . . . . . . . . . .  15
     2.5.2.  SMIMECapabilities Attribute . . . . . . . . . . . . .  16
     2.5.3.  Encryption Key Preference Attribute . . . . . . . . .  17
   2.6.  SignerIdentifier SignerInfo Type  . . . . . . . . . . . .  19
   2.7.  ContentEncryptionAlgorithmIdentifier  . . . . . . . . . .  19
     2.7.1.  Deciding Which Encryption Method to Use . . . . . . .  19
     2.7.2.  Choosing Weak Encryption  . . . . . . . . . . . . . .  21
     2.7.3.  Multiple Recipients . . . . . . . . . . . . . . . . .  21
 3.  Creating S/MIME Messages  . . . . . . . . . . . . . . . . . .  21
   3.1.  Preparing the MIME Entity for Signing, Enveloping, or
         Compressing . . . . . . . . . . . . . . . . . . . . . . .  22
     3.1.1.  Canonicalization  . . . . . . . . . . . . . . . . . .  23
     3.1.2.  Transfer Encoding . . . . . . . . . . . . . . . . . .  24
     3.1.3.  Transfer Encoding for Signing Using multipart/signed   25
     3.1.4.  Sample Canonical MIME Entity  . . . . . . . . . . . .  25
   3.2.  The application/pkcs7-mime Media Type . . . . . . . . . .  26
     3.2.1.  The name and filename Parameters  . . . . . . . . . .  27
     3.2.2.  The smime-type Parameter  . . . . . . . . . . . . . .  28
   3.3.  Creating an Enveloped-Only Message  . . . . . . . . . . .  29
   3.4.  Creating an Authenticated Enveloped-Only Message  . . . .  30
   3.5.  Creating a Signed-Only Message  . . . . . . . . . . . . .  31
     3.5.1.  Choosing a Format for Signed-Only Messages  . . . . .  32
     3.5.2.  Signing Using application/pkcs7-mime with SignedData   32
     3.5.3.  Signing Using the multipart/signed Format . . . . . .  33
   3.6.  Creating a Compressed-Only Message  . . . . . . . . . . .  36
   3.7.  Multiple Operations . . . . . . . . . . . . . . . . . . .  37
   3.8.  Creating a Certificate Management Message . . . . . . . .  38

Schaad, et al. Standards Track [Page 3] RFC 8551 S/MIME 4.0 Message Specification April 2019

   3.9.  Registration Requests . . . . . . . . . . . . . . . . . .  38
   3.10. Identifying an S/MIME Message . . . . . . . . . . . . . .  39
 4.  Certificate Processing  . . . . . . . . . . . . . . . . . . .  39
   4.1.  Key Pair Generation . . . . . . . . . . . . . . . . . . .  40
   4.2.  Signature Generation  . . . . . . . . . . . . . . . . . .  40
   4.3.  Signature Verification  . . . . . . . . . . . . . . . . .  40
   4.4.  Encryption  . . . . . . . . . . . . . . . . . . . . . . .  41
   4.5.  Decryption  . . . . . . . . . . . . . . . . . . . . . . .  41
 5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  41
   5.1.  Media Type for application/pkcs7-mime . . . . . . . . . .  42
   5.2.  Media Type for application/pkcs7-signature  . . . . . . .  43
   5.3.  authEnveloped-data smime-type . . . . . . . . . . . . . .  44
   5.4.  Reference Updates . . . . . . . . . . . . . . . . . . . .  44
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .  44
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  48
   7.1.  Reference Conventions . . . . . . . . . . . . . . . . . .  48
   7.2.  Normative References  . . . . . . . . . . . . . . . . . .  49
   7.3.  Informative References  . . . . . . . . . . . . . . . . .  52
 Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  57
 Appendix B.  Historic Mail Considerations . . . . . . . . . . . .  59
   B.1.  DigestAlgorithmIdentifier . . . . . . . . . . . . . . . .  59
   B.2.  Signature Algorithms  . . . . . . . . . . . . . . . . . .  59
   B.3.  ContentEncryptionAlgorithmIdentifier  . . . . . . . . . .  61
   B.4.  KeyEncryptionAlgorithmIdentifier  . . . . . . . . . . . .  62
 Appendix C.  Moving S/MIME v2 Message Specification to Historic
              Status . . . . . . . . . . . . . . . . . . . . . . .  62
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  62
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  63

Schaad, et al. Standards Track [Page 4] RFC 8551 S/MIME 4.0 Message Specification April 2019

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 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.
 This document defines version 4.0 of the S/MIME Message
 Specification.  As such, this document obsoletes version 3.2 of the
 S/MIME Message Specification [RFC5751].
 This specification contains a number of references to documents that
 have been obsoleted or replaced.  This is intentional, as the updated
 documents often do not have the same information or protocol
 requirements in them.

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 applications based on content-type.
 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, which can be used to transport those body parts.

Schaad, et al. Standards Track [Page 5] RFC 8551 S/MIME 4.0 Message Specification April 2019

 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
 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 [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 follow the Robustness Principle (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].

Schaad, et al. Standards Track [Page 6] RFC 8551 S/MIME 4.0 Message Specification April 2019

 7-bit data:
    Text data with lines less than 998 characters long, where none of
    the characters have the 8th bit set, and there are no NULL
    characters.  <CR> and <LF> occur only as part of a <CR><LF>
    end-of-line delimiter.
 8-bit data:
    Text data with lines less than 998 characters, and where none of
    the characters are NULL characters.  <CR> and <LF> occur only as
    part of a <CR><LF> end-of-line delimiter.
 Binary data:
    Arbitrary data.
 Transfer encoding:
    A reversible transformation made on data so 8-bit or binary data
    can be sent via a channel that only transmits 7-bit data.
 Receiving agent:
    Software that interprets and processes S/MIME CMS objects, MIME
    body parts that contain CMS content types, or both.
 Sending agent:
    Software that creates S/MIME CMS content types, MIME body parts
    that contain CMS content types, or both.
 S/MIME agent:
    User software that is a receiving agent, a sending agent, or both.
 Data integrity service:
    A security service that protects against unauthorized changes to
    data by ensuring that changes to the data are detectable
    [RFC4949].
 Data confidentiality:
    The property that data is not disclosed to system entities unless
    they have been authorized to know the data [RFC4949].

1.3. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

Schaad, et al. Standards Track [Page 7] RFC 8551 S/MIME 4.0 Message Specification April 2019

 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.
 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 [RFC2313] 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.
  1. S/MIME version 2 is described in RFC 2311 through RFC 2315

inclusive [SMIMEv2].

  1. S/MIME version 3 is described in RFC 2630 through RFC 2634

inclusive and RFC 5035 [SMIMEv3].

  1. S/MIME version 3.1 is described in RFC 2634, RFC 3850, RFC 3851,

RFC 3852, and RFC 5035 [SMIMEv3.1].

  1. S/MIME version 3.2 is described in RFC 2634, RFC 5035, RFC 5652,

RFC 5750, and RFC 5751 [SMIMEv3.2].

  1. [RFC2311] also has historical information about the development of

S/MIME.

Schaad, et al. Standards Track [Page 8] RFC 8551 S/MIME 4.0 Message Specification April 2019

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

 This section describes the changes made between S/MIME v3 and
 S/MIME v3.1.  Note that the requirement levels indicated by the
 capitalized key words ("MUST", "SHOULD", etc.) may have changed in
 later versions of S/MIME.
  1. The RSA public key algorithm was changed to a MUST implement. The

key wrap algorithm and the Diffie-Hellman (DH) algorithm [RFC2631]

    were changed to a SHOULD implement.
  1. The AES symmetric encryption algorithm has been included as a

SHOULD implement.

  1. The RSA public key algorithm was changed to a MUST implement

signature algorithm.

  1. 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.
  1. The use of binary encoding for some MIME entities is now

explicitly discussed.

  1. Header protection through the use of the message/rfc822 media type

has been added.

  1. 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

 This section describes the changes made between S/MIME v3.1 and
 S/MIME v3.2.  Note that the requirement levels indicated by the
 capitalized key words ("MUST", "SHOULD", etc.) may have changed in
 later versions of S/MIME.  Note that the section numbers listed here
 (e.g., 3.4.3.2) are from [RFC5751].
  1. Made editorial changes, e.g., replaced "MIME type" with "media

type", "content-type" with "Content-Type".

  1. Moved "Conventions Used in This Document" to Section 1.3. Added

definitions for SHOULD+, SHOULD-, and MUST-.

  1. 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.

Schaad, et al. Standards Track [Page 9] RFC 8551 S/MIME 4.0 Message Specification April 2019

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

to X.690.

  1. Section 1.4: Added references to S/MIME v3.1 RFCs.
  1. Section 2.1 (digest algorithm): SHA-256 added as MUST, SHA-1 and

MD5 made SHOULD-.

  1. Section 2.2 (signature algorithms): RSA with SHA-256 added as

MUST; 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.
  1. Section 2.3 (key encryption): DH changed to SHOULD-, and RSAES-

OAEP added as SHOULD+. Elaborated on requirements for key wrap

    algorithm.
  1. Section 2.5.1: Added requirement that receiving agents MUST

support both GeneralizedTime and UTCTime.

  1. Section 2.5.2: Replaced reference "sha1WithRSAEncryption" with

"sha256WithRSAEncryption", replaced "DES-3EDE-CBC" with "AES-128

    CBC", and deleted the RC5 example.
  1. Section 2.5.2.1: Deleted entire section (discussed

deprecated RC2).

  1. Section 2.7, Section 2.7.1, and Appendix A: References to RC2/40

removed.

  1. Section 2.7 (content encryption): AES-128 CBC added as MUST,

AES-192 and AES-256 CBC SHOULD+, and tripleDES now SHOULD-.

  1. Section 2.7.1: Updated pointers from 2.7.2.1 through 2.7.2.4 to

2.7.1.1 and 2.7.1.2.

  1. Section 3.1.1: Removed text about MIME character sets.
  1. Sections 3.2.2 and 3.6: Replaced "encrypted" with "enveloped".

Updated OID example to use AES-128 CBC OID.

  1. Section 3.4.3.2: Replaced "micalg" parameter for "SHA-1" with

"sha-1".

  1. Section 4: Updated reference to CERT v3.2.

Schaad, et al. Standards Track [Page 10] RFC 8551 S/MIME 4.0 Message Specification April 2019

  1. Section 4.1: Updated RSA and DSA key size discussion. Moved last

four sentences to security considerations. Updated reference to

    randomness requirements for security.
  1. Section 5: Added IANA registration templates to update media type

registry to point to this document as opposed to RFC 2311.

  1. Section 6: Updated security considerations.
  1. Section 7: Moved references from Appendix B to this section.

Updated references. Added informative references to SMIMEv2,

    SMIMEv3, and SMIMEv3.1.
  1. Appendix B: Added Appendix B to move S/MIME v2 to Historic status.

1.7. Changes for S/MIME v4.0

 This section describes the changes made between S/MIME v3.2 and
 S/MIME v4.0.
  1. Added the use of AuthEnvelopedData, including defining and

registering an smime-type value (Sections 2.4.4 and 3.4).

  1. Updated the content-encryption algorithms (Sections 2.7 and

2.7.1.2): added AES-256 Galois/Counter Mode (GCM), added

    ChaCha20-Poly1305, removed mention of AES-192 Cipher Block
    Chaining (CBC), and marked tripleDES as historic.
  1. Updated the set of signature algorithms (Section 2.2): added the

Edwards-curve Digital Signature Algorithm (EdDSA), added the

    Elliptic Curve Digital Signature Algorithm (ECDSA), and marked DSA
    as historic.
  1. Updated the set of digest algorithms (Section 2.1): added SHA-512,

and marked SHA-1 as historic.

  1. Updated the size of keys to be used for RSA encryption and RSA

signing (Section 4).

  1. Created Appendix B, which discusses considerations for dealing

with historic email messages.

Schaad, et al. Standards Track [Page 11] RFC 8551 S/MIME 4.0 Message Specification April 2019

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] provide 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 of 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 strength to, or greater strength than, the signature
 algorithm.
 Sending and receiving agents:
  1. MUST support SHA-256.
  1. MUST support SHA-512.
 [RFC5754] provides the details for using these algorithms with
 S/MIME.

2.2. SignatureAlgorithmIdentifier

 There are different sets of requirements placed on receiving and
 sending agents.  By having the different requirements, the maximum
 amount of interoperability is achieved, as it allows for specialized
 protection of private key material but maximum signature validation.
 Receiving agents:
  1. MUST support ECDSA with curve P-256 and SHA-256.
  1. MUST support EdDSA with curve25519 using PureEdDSA mode [RFC8419].
  1. MUST- support RSA PKCS #1 v1.5 with SHA-256.
  1. SHOULD support the RSA Probabilistic Signature Scheme (RSASSA-PSS)

with SHA-256.

Schaad, et al. Standards Track [Page 12] RFC 8551 S/MIME 4.0 Message Specification April 2019

 Sending agents:
  1. MUST support at least one of the following algorithms: ECDSA with

curve P-256 and SHA-256, or EdDSA with curve25519 using PureEdDSA

    mode.
  1. MUST- support RSA PKCS #1 v1.5 with SHA-256.
  1. SHOULD support RSASSA-PSS with SHA-256.
 See Section 4.1 for information on key size and algorithm references.

2.3. KeyEncryptionAlgorithmIdentifier

 Receiving and sending agents:
  1. MUST support Elliptic Curve Diffie-Hellman (ECDH) ephemeral-static

mode for P-256, as specified in [RFC5753].

  1. MUST support ECDH ephemeral-static mode for X25519 using HKDF-256

("HKDF" stands for "HMAC-based Key Derivation Function") for the

    KDF, as specified in [RFC8418].
  1. MUST- support RSA encryption, as specified in [RFC3370].
  1. SHOULD+ support RSA Encryption Scheme - Optimal Asymmetric

Encryption Padding (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 algorithms [RFC3370] [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-bit and
 256-bit AES modes are mandatory to implement 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.
 Recipients MAY enforce this but MUST use the weaker of the two as
 part of any cryptographic strength computations they might do.
 Appendix B provides information on algorithm support in older
 versions of S/MIME.

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.

Schaad, et al. Standards Track [Page 13] RFC 8551 S/MIME 4.0 Message Specification April 2019

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 (OID), 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 OID 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.
 In order to distribute the symmetric key, 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.  In order to distribute the
 symmetric key, 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; it is only used to reduce
 the message's size.
 See Section 3.7 for further guidance on the use of this type in
 conjunction with other CMS types.

Schaad, et al. Standards Track [Page 14] RFC 8551 S/MIME 4.0 Message Specification April 2019

2.5. Attributes and the SignerInfo Type

 The SignerInfo type allows the inclusion of unsigned and signed
 attributes along with a signature.  These attributes can be required
 for the processing of messages (e.g., message digest), information
 the signer supplied (e.g., SMIME capabilities) that should be
 processed, or attributes that are not relevant to the current
 situation (e.g., mlExpansionHistory [RFC2634] for mail viewers).
 Receiving agents MUST be able to handle zero or one instance of each
 of the signed attributes listed here.  Sending agents SHOULD generate
 one instance of each of the following signed attributes in each
 S/MIME message:
  1. Signing time (Section 2.5.1 in this document)
  1. SMIME capabilities (Section 2.5.2 in this document)
  1. Encryption key Preference (Section 2.5.3 in this document)
  1. Message digest (Section 11.2 in [RFC5652])
  1. 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], respectively.
 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 signingTime attribute is used to convey the time that a message
 was signed.  The time of signing will most likely be created by a
 signer and therefore is only as trustworthy as that signer.

Schaad, et al. Standards Track [Page 15] RFC 8551 S/MIME 4.0 Message Specification April 2019

 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 signingTime attributes that
 are encoded in either UTCTime or GeneralizedTime.

2.5.2. SMIMECapabilities 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 SMIMECapability attribute containing an algorithm
 implies that the sender can deal with the algorithm as well as
 understand 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 attribute was 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.  CMS defines SignedAttributes as a SET OF Attribute.
 The SignedAttributes in a signerInfo MUST include a single instance
 of the SMIMECapabilities attribute.  CMS defines the ASN.1 syntax for
 Attribute to include attrValues SET OF AttributeValue.  An
 SMIMECapabilities attribute MUST only include a single instance of
 AttributeValue.  If a signature is detected as violating these
 requirements, the signature SHOULD be treated as failing.
 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 it need not list all its capabilities so that the
 capabilities list doesn't get too long.  In an SMIMECapabilities
 attribute, the OIDs are listed in order of their preference but
 SHOULD be separated logically along the lines of their categories
 (signature algorithms, symmetric algorithms, key encipherment
 algorithms, etc.).

Schaad, et al. Standards Track [Page 16] RFC 8551 S/MIME 4.0 Message Specification April 2019

 The structure of the SMIMECapabilities attribute is to facilitate
 simple table lookups and binary comparisons in order to determine
 matches.  For instance, the encoding for the SMIMECapability for
 sha256WithRSAEncryption includes rather than omits the NULL
 parameter.  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 same OID that is used to identify an algorithm SHOULD also be
 used in the SMIMECapability for that algorithm.  There are cases
 where a single OID can correspond to multiple algorithms.  In these
 cases, a single algorithm MUST be assigned to the SMIMECapability
 using that OID.  Additional OIDs from the smimeCapabilities OID tree
 are then allocated for the other algorithms usages.  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 an 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.

Schaad, et al. Standards Track [Page 17] RFC 8551 S/MIME 4.0 Message Specification April 2019

 If present, the SMIMEEncryptionKeyPreference attribute MUST be a
 SignedAttribute.  CMS defines SignedAttributes as a SET OF Attribute.
 The SignedAttributes in a signerInfo MUST include a single instance
 of the SMIMEEncryptionKeyPreference attribute.  CMS defines the ASN.1
 syntax for Attribute to include attrValues SET OF AttributeValue.  An
 SMIMEEncryptionKeyPreference attribute MUST only include a single
 instance of AttributeValue.  If a signature is detected as violating
 these requirements, the signature SHOULD be treated as failing.
 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:
  1. If an SMIMEEncryptionKeyPreference attribute is found in a

SignedData object received from the desired recipient, this

    identifies the X.509 certificate that SHOULD be used as the X.509
    key management certificate for the recipient.
  1. If an SMIMEEncryptionKeyPreference attribute is not found in a

SignedData object received from the desired recipient, the set of

    X.509 certificates SHOULD be searched for an X.509 certificate
    with the same subject name as the signer of an X.509 certificate
    that can be used for key management.

Schaad, et al. Standards Track [Page 18] RFC 8551 S/MIME 4.0 Message Specification April 2019

  1. Or, use some other method of determining the user's key management

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

    encryption cannot be done with the signer of the message.  If
    multiple X.509 key management certificates are found, the S/MIME
    agent can make an arbitrary choice between them.

2.6. SignerIdentifier SignerInfo Type

 S/MIME 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:
  1. MUST support encryption and decryption with AES-128 GCM and

AES-256 GCM [RFC5084].

  1. MUST- support encryption and decryption with AES-128 CBC

[RFC3565].

  1. 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.

Schaad, et al. Standards Track [Page 19] RFC 8551 S/MIME 4.0 Message Specification April 2019

  1. If the receiving agent has not yet created a list of capabilities

for the sender's public key, then, after verifying the signature

    on the incoming message and checking the timestamp, the receiving
    agent SHOULD create a new list containing at least the signing
    time and the symmetric capabilities.
  1. If such a list already exists, the receiving agent SHOULD verify

that the signing time in the incoming message is greater than the

    signing time stored in the list and that the signature is valid.
    If so, the receiving agent SHOULD update both the signing time and
    capabilities in the list.  Values of the signing time that lie far
    in the future (that is, a greater discrepancy than any reasonable
    clock skew), or a capabilities list in messages whose signature
    could not be verified, MUST NOT be accepted.
 The list of capabilities SHOULD be stored for future use in creating
 messages.
 Before sending a message, the sending agent MUST decide whether it is
 willing to use weak encryption for the particular data in the
 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.
 Sections 2.7.1.1 and 2.7.1.2 describe the decisions a sending agent
 SHOULD use when choosing 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 SHOULD use
 AES-256 GCM, as AES-256 GCM is a stronger algorithm and is required
 by S/MIME v4.0:

Schaad, et al. Standards Track [Page 20] RFC 8551 S/MIME 4.0 Message Specification April 2019

  1. The sending agent has no knowledge of the encryption capabilities

of the recipient.

  1. The sending agent has no knowledge of the version of S/MIME used

or supported by the recipient.

 If the sending agent chooses not to use AES-256 GCM in this step,
 given the presumption is that a client implementing AES-GCM would do
 both AES-256 and AES-128, it SHOULD use AES-128 CBC.

2.7.2. Choosing Weak Encryption

 Algorithms such as RC2 are considered to be weak encryption
 algorithms.  Algorithms such as TripleDES are not state of the art
 and are considered to be weaker algorithms than AES.  A sending agent
 that is controlled by a human SHOULD allow a human sender to
 determine the risks of sending data using a weaker encryption
 algorithm before sending the data, and possibly allow the human to
 use a stronger encryption algorithm such as AES GCM or AES CBC even
 if there is a possibility that the recipient will not be able to
 process that algorithm.

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" documents [ESS] provide
 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.

Schaad, et al. Standards Track [Page 21] RFC 8551 S/MIME 4.0 Message Specification April 2019

 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.
 Anyone reading this section is expected to understand MIME as
 described in [MIME-SPEC] and [RFC1847].

3.1. Preparing the MIME Entity for Signing, Enveloping, or Compressing

 S/MIME is used to secure MIME entities.  A MIME message is composed
 of a MIME header and a MIME body.  A body can consist of a single
 MIME entity or a tree of MIME entities (rooted with a multipart).
 S/MIME can be used to secure either a single MIME entity or a tree of
 MIME entities.  These entities can be in locations other than the
 root.  S/MIME can be applied multiple times to different entities in
 a single message.  A MIME entity that is the whole message includes
 only the MIME message headers and MIME body and does not include the
 rfc822 header.  Note that S/MIME can also be used to secure MIME
 entities used in applications other than Internet mail.  For cases
 where protection of the rfc822 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 EnvelopedData,
 CompressedData, and AuthEnvelopedData content types is described in
 Sections 3.2 and 3.5.  Other documents define additional CMS content
 types; those documents should be consulted for processing those CMS
 content types.
 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 those
 documents for the exact procedures.  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
 messages can be forwarded to any environment without modification.

Schaad, et al. Standards Track [Page 22] RFC 8551 S/MIME 4.0 Message Specification April 2019

 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 local conventions.
 Step 2.  The leaf parts of the MIME entity are converted to
          canonical form.
 Step 3.  Appropriate transfer encoding is applied to the leaves
          of the MIME entity.
 When an S/MIME message is received, the security services on the
 message are processed, and the result is the MIME entity.  That MIME
 entity is typically passed to a MIME-capable user agent where it is
 further decoded and presented to the user or receiving application.
 In order to protect outer, non-content-related message 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.  Given
 the security difference between headers, it is RECOMMENDED that the
 receiving client provide a distinction between header fields,
 depending on where they are located.
 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.

Schaad, et al. Standards Track [Page 23] RFC 8551 S/MIME 4.0 Message Specification April 2019

 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.

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.
 As a rule, S/MIME implementations SHOULD use transfer encoding as
 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 is 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.
 In the case where S/MIME implementations can determine that all
 intended recipients are capable of handling inner (all but the
 outermost) binary MIME objects, implementations 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)
 unnecessarily expands the message size.  Implementations MAY
 determine that recipient implementations are capable of
 handling inner binary MIME entities by (1) interpreting the
 id-cap-preferBinaryInside SMIMECapabilities attribute, (2) prior
 agreement, or (3) other means.

Schaad, et al. Standards Track [Page 24] RFC 8551 S/MIME 4.0 Message Specification April 2019

 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 as
 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 already
 7-bit data need no transfer encoding.  Entities such as 8-bit text
 and binary data can be encoded with quoted-printable or base64
 transfer encoding.
 The primary reason for the 7-bit requirement is that the Internet
 mail transport infrastructure cannot guarantee transport of 8-bit or
 binary data.  Even though many segments of the transport
 infrastructure now handle 8-bit and even binary data, it is sometimes
 not possible to know whether the transport path is 8-bit clean.  If a
 mail message with 8-bit data were to encounter a message transfer
 agent that cannot transmit 8-bit or binary data, the agent has three
 options, none of which are acceptable for a clear-signed message:
  1. The agent could change the transfer encoding; this would

invalidate the signature.

  1. The agent could transmit the data anyway, which would most likely

result in the 8th bit being corrupted; this too would invalidate

    the signature.
  1. 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 encountered 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>.

Schaad, et al. Standards Track [Page 25] RFC 8551 S/MIME 4.0 Message Specification April 2019

 Note that this example is not an example of an S/MIME message.
 Content-Type: multipart/mixed; boundary=bar
  1. -bar

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

 Content-Transfer-Encoding: quoted-printable
 =A1Hola Michael!
 How do you like the new S/MIME specification?
 It's generally a good idea to encode lines that begin with
 From=20because some mail transport agents will insert a
 greater-than (>) sign, thus invalidating the signature.
 Also, in some cases it might be desirable to encode any =20
 trailing whitespace that occurs on lines in order to ensure =20
 that the message signature is not invalidated when passing =20
 a gateway that modifies such whitespace (like BITNET). =20
  1. -bar

Content-Type: image/jpeg

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

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 base64
 transfer encoding is appropriate -- in particular, when used with
 SMTP transport.  The transfer encoding used depends on the transport

Schaad, et al. Standards Track [Page 26] RFC 8551 S/MIME 4.0 Message Specification April 2019

 through which the object is to be sent and is not a characteristic of
 the media type.
 Note that this discussion refers to the transfer encoding of the CMS
 object or "outside" MIME entity.  It is completely distinct from, and
 unrelated to, the transfer encoding of the MIME entity secured by the
 CMS object -- the "inside" object, which is described in Section 3.1.
 Because there are several types of application/pkcs7-mime objects, a
 sending agent SHOULD do as much as possible to help a receiving agent
 know about the contents of the object without forcing the receiving
 agent to decode the ASN.1 for the object.  The 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 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 [RFC2183] with the "filename" parameter.
 If a sending agent emits the above parameters, the value of the
 parameters SHOULD be a filename with the appropriate extension:
                                                              File
 Media Type                                                Extension
 -------------------------------------------------------------------
 application/pkcs7-mime (SignedData, EnvelopedData,           .p7m
    AuthEnvelopedData)
 application/pkcs7-mime (degenerate SignedData certificate    .p7c
    management message)
 application/pkcs7-mime (CompressedData)                      .p7z
 application/pkcs7-signature (SignedData)                     .p7s
 In addition, the filename 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 filename 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

Schaad, et al. Standards Track [Page 27] RFC 8551 S/MIME 4.0 Message Specification April 2019

 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 standalone S/MIME
 processing application.  Note that this mechanism is provided as a
 convenience for implementations in certain environments.  A proper
 S/MIME implementation MUST use the 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.

Schaad, et al. Standards Track [Page 28] RFC 8551 S/MIME 4.0 Message Specification April 2019

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

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.  The
 "enveloped-only" structure does not support authenticated symmetric
 algorithms.  Use the "authenticated enveloped" structure for these
 algorithms.  Thus, 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 are processed into a
          CMS object of type EnvelopedData.  In addition to encrypting
          a copy of the content-encryption key (CEK) for each
          recipient, a copy of the CEK 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=

Schaad, et al. Standards Track [Page 29] RFC 8551 S/MIME 4.0 Message Specification April 2019

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 proof
 of origination when using S/MIME.  It is possible for a third party
 to replace 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 algorithm does provide a level of authentication.
 Any recipient for whom the message is encrypted can replace it
 without detection.
 Step 1.  The MIME entity to be enveloped is prepared according to
          Section 3.1.
 Step 2.  The MIME entity and other required data are processed into a
          CMS object of type AuthEnvelopedData.  In addition to
          encrypting a copy of the CEK for each recipient, a copy of
          the CEK 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".

Schaad, et al. Standards Track [Page 30] RFC 8551 S/MIME 4.0 Message Specification April 2019

 A sample message would be:
 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:
  1. application/pkcs7-mime with SignedData.
  1. multipart/signed.
 In general, the multipart/signed form is preferred for sending, and
 receiving agents MUST be able to handle both.

Schaad, et al. Standards Track [Page 31] RFC 8551 S/MIME 4.0 Message Specification April 2019

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
 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 as follows:
 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".

Schaad, et al. Standards Track [Page 32] RFC 8551 S/MIME 4.0 Message Specification April 2019

 A sample message would be:
 Content-Type: application/pkcs7-mime; smime-type=signed-data;
    name=smime.p7m
 Content-Transfer-Encoding: base64
 Content-Disposition: attachment; filename=smime.p7m
 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".

Schaad, et al. Standards Track [Page 33] RFC 8551 S/MIME 4.0 Message Specification April 2019

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.

Schaad, et al. Standards Track [Page 34] RFC 8551 S/MIME 4.0 Message Specification April 2019

 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 the algorithm profile
                      or "unknown" if not defined)
  • Note: MD5 and SHA-1 are historical and no longer considered secure.

See Appendix B for details.

 (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 values for this
 parameter will be taken from the IANA "Hash Function Textual Names"
 registry.

Schaad, et al. Standards Track [Page 35] RFC 8551 S/MIME 4.0 Message Specification April 2019

3.5.3.3. Sample multipart/signed Message

 Content-Type: multipart/signed;
     micalg=sha-256;
     boundary="----=_NextBoundary____Fri,_06_Sep_2002_00:25:21";
     protocol="application/pkcs7-signature"
 This is a multipart message in MIME format.
  1. —–=_NextBoundaryFri,_06_Sep_2002_00:25:21
 This is some sample content.
 ------=_NextBoundary____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
 MIIBJgYJKoZIhvcNAQcCoIIBFzCCARMCAQExADALBgkqhkiG9w0BBwExgf4w
 gfsCAQIwJjASMRAwDgYDVQQDEwdDYXJsUlNBAhBGNGvHgABWvBHTbi7EELOw
 MAsGCWCGSAFlAwQCAaAxMC8GCSqGSIb3DQEJBDEiBCCxwpZGNZzTSsugsn+f
 lEidzQK4mf/ozKqfmbxhcIkKqjALBgkqhkiG9w0BAQsEgYB0XJV7fjPa5Nuh
 oth5msDfP8A5urYUMjhNpWgXG8ae3XpppqVrPi2nVO41onHnkByjkeD/wc31
 A9WH8MzFQgSTsrJ65JvffTTXkOpRPxsSHn3wJFwP/atWHkh8YK/jR9bULhUl
 Mv5jQEDiwVX5DRasxu6Ld8zv9u5/TsdBNiufGw==
  1. —–=_NextBoundaryFri,_06_Sep_2002_00:25:21–
 The content that is digested (the first part of the multipart/signed)
 consists of the bytes:
 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.

Schaad, et al. Standards Track [Page 36] RFC 8551 S/MIME 4.0 Message Specification April 2019

 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
 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 related 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 and 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.

Schaad, et al. Standards Track [Page 37] RFC 8551 S/MIME 4.0 Message Specification April 2019

 When using compression, keep the following guidelines in mind:
  1. Compression of encrypted data that is transferred as binary data

is discouraged, since it will not yield significant compression.

 Encrypted data that is transferred as base64-encoded data could
 benefit as well.
  1. 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 (CRLs),
 such as in response to a registration request.
 Step 1.  The certificates and/or CRLs 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 the signerInfos field MUST be empty.
 Step 2.  The SignedData object is wrapped in a CMS ContentInfo
          object.
 Step 3.  The ContentInfo object is enclosed in an
          application/pkcs7-mime MIME entity.
 The smime-type parameter for a certificate management message is
 "certs-only".  The file extension for this type of message is ".p7c".

3.9. 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.

Schaad, et al. Standards Track [Page 38] RFC 8551 S/MIME 4.0 Message Specification April 2019

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 in the
 Content-Disposition header field.  The MIME parameters that carry the
 file suffix are not listed below.
 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
 "store and protect" certificates for correspondents in such a way as
 to guarantee their later retrieval.

Schaad, et al. Standards Track [Page 39] RFC 8551 S/MIME 4.0 Message Specification April 2019

4.1. Key Pair Generation

 All 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
 OID, 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 when
 generating RSA and RSASSA-PSS signatures:
         key size <= 2047 : SHOULD NOT (Note 2)
 2048 <= key size <= 4096 : SHOULD     (Note 1)
 4096 <  key size         : MAY        (Note 1)
 Note 1: See Security Considerations in Section 6.
 Note 2: See Historical Mail 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
 verification of RSA and RSASSA-PSS signatures:
         key size <= 2047 : SHOULD NOT (Note 2)
 2048 <= key size <= 4096 : MUST       (Note 1)
 4096 <  key size         : MAY        (Note 1)
 Note 1: See Security Considerations in Section 6.
 Note 2: See Historical Mail Considerations in Appendix B.
 Key sizes for ECDSA and EdDSA are fixed by the curve.

Schaad, et al. Standards Track [Page 40] RFC 8551 S/MIME 4.0 Message Specification April 2019

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 2)
 2048 <= key size <= 4096 : SHOULD     (Note 1)
 4096 <  key size         : MAY        (Note 1)
 Note 1: See Security Considerations in Section 6.
 Note 2: See Historical Mail 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 2)
 2048 <= key size <= 4096 : MUST       (Note 1)
 4096 <  key size         : MAY        (Note 1)
 Note 1: See Security Considerations in Section 6.
 Note 2: See Historical Mail Considerations in Appendix B.
 Key sizes for ECDH are fixed by the curve.

5. IANA Considerations

 This section (1) updates the media type registrations for
 application/pkcs7-mime and application/pkcs7-signature to refer to
 this document as opposed to RFC 5751, (2) adds authEnveloped-data to
 the list of values for smime-type, and (3) updates references from
 RFC 5751 to this document in general.
 Note that other documents can define additional media types for
 S/MIME.

Schaad, et al. Standards Track [Page 41] RFC 8551 S/MIME 4.0 Message Specification April 2019

5.1. Media Type for application/pkcs7-mime

 Type name: application
 Subtype Name: pkcs7-mime
 Required Parameters: NONE
 Optional Parameters: smime-type
                      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, RFC 5751,
                          and this document
 Applications that use this media type: Security applications
 Fragment identifier considerations: N/A
 Additional information:
     Deprecated alias names for this type: N/A
     Magic number(s): N/A
     File extensions(s): See Section 3.2.1 of this document
     Macintosh file type code(s): N/A
 Person & email address to contact for further information:
    The IESG <iesg@ietf.org>
 Intended usage: COMMON
 Restrictions on usage: NONE
 Author: Sean Turner
 Change Controller: LAMPS working group delegated from the IESG

Schaad, et al. Standards Track [Page 42] RFC 8551 S/MIME 4.0 Message Specification April 2019

5.2. Media Type for application/pkcs7-signature

 Type name: application
 Subtype Name: pkcs7-signature
 Required Parameters: N/A
 Optional Parameters: N/A
 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, RFC 5751,
                          and this document
 Applications that use this media type: Security applications
 Fragment identifier considerations: N/A
 Additional information:
     Deprecated alias names for this type: N/A
     Magic number(s): N/A
     File extensions(s): See Section 3.2.1 of this document
     Macintosh file type code(s): N/A
 Person & email address to contact for further information:
    The IESG <iesg@ietf.org>
 Intended usage: COMMON
 Restrictions on usage: N/A
 Author: Sean Turner
 Change Controller: LAMPS working group delegated from the IESG

Schaad, et al. Standards Track [Page 43] RFC 8551 S/MIME 4.0 Message Specification April 2019

5.3. authEnveloped-data smime-type

 IANA has registered the following value in the "Parameter Values for
 the smime-type Parameter" registry.
    smime-type value: authEnveloped-data
    Reference: RFC 8551, Section 3.2.2

5.4. Reference Updates

 IANA is to update all references to RFC 5751 to this document.  Known
 registries to be updated are "CoAP Content-Formats" and "media-
 types".

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:
  1. The Million Message Attack described in RFC 3218 [RFC3218].
  1. The Diffie-Hellman "small-subgroup" attacks described in RFC 2785

[RFC2785].

  1. 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's 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.

Schaad, et al. Standards Track [Page 44] RFC 8551 S/MIME 4.0 Message Specification April 2019

 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, as such keys 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.
 Some cryptographic algorithms such as RC2 offer little actual
 security over sending plaintext.  Other algorithms such as TripleDES
 provide security but are no longer considered to be state of the art.
 S/MIME requires the use of current state-of-the-art algorithms such
 as AES and provides the ability to announce cryptographic
 capabilities to parties with whom you communicate.  This allows the
 sender to create messages that can use the strongest common
 encryption algorithm.  Using algorithms such as RC2 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.

Schaad, et al. Standards Track [Page 45] RFC 8551 S/MIME 4.0 Message Specification April 2019

 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
 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 for the sender of the message to 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:
  1. 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.

Schaad, et al. Standards Track [Page 46] RFC 8551 S/MIME 4.0 Message Specification April 2019

  1. 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 the second recipient from
    the message.
  1. A direct path needs to exist from the starting key to the key used

as the CEK. That path needs to guarantee 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 (1) used directly as the CEK or (2) used
    to create a secret that is then 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.
 [RFC6278] defines how to use static-static key agreement with CMS, so
 the first precondition can be met.  Currently, all S/MIME key
 agreement methods derive a key-encryption key (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
 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 plaintext will always be known, as the
 ciphertext length and the plaintext 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.
 When compression is used with encryption, it has the potential to
 provide an additional layer of security.  However, care needs to be
 taken when designing a protocol that relies on using compression, so
 as not to create a compression oracle.  Compression oracle attacks
 require an adaptive input to the process and attack the unknown

Schaad, et al. Standards Track [Page 47] RFC 8551 S/MIME 4.0 Message Specification April 2019

 content of a message based on the length of the compressed output.
 This means that no attack on the encryption key is necessarily
 required.
 A recent paper on S/MIME and OpenPGP email security [Efail] has
 pointed out a number of problems with the current S/MIME
 specifications and how people have implemented mail clients.  Due to
 the nature of how CBC mode operates, the modes allow for malleability
 of plaintexts.  This malleability allows for attackers to make
 changes in the ciphertext and, if parts of the plaintext are known,
 create arbitrary blocks of plaintext.  These changes can be made
 without the weak integrity check in CBC mode being triggered.  This
 type of attack can be prevented by the use of an Authenticated
 Encryption with Associated Data (AEAD) algorithm with a more robust
 integrity check on the decryption process.  It is therefore
 recommended that mail systems migrate to using AES-GCM as quickly as
 possible and that the decrypted content not be acted on prior to
 finishing the integrity check.
 The other attack that is highlighted in [Efail] is due to an error in
 how mail clients deal with HTML and multipart/mixed messages.
 Clients MUST require that a text/html content type be a complete HTML
 document (per [RFC1866]).  Clients SHOULD treat each of the different
 pieces of the multipart/mixed construct as being of different
 origins.  Clients MUST treat each encrypted or signed piece of a MIME
 message as being of different origins both from unprotected content
 and from each other.

7. References

7.1. Reference Conventions

 [ASN.1] refers to [X.680], [X.681], [X.682], and [X.683].
 [CMS] refers to [RFC5083] and [RFC5652].
 [ESS] refers to [RFC2634] and [RFC5035].
 [MIME-SPEC] refers to [RFC2045], [RFC2046], [RFC2047], [RFC2049],
 [RFC6838], and [RFC4289].
 [SMIMEv2] refers to [RFC2311], [RFC2312], [RFC2313], [RFC2314], and
 [RFC2315].
 [SMIMEv3] refers to [RFC2630], [RFC2631], [RFC2632], [RFC2633],
 [RFC2634], and [RFC5035].

Schaad, et al. Standards Track [Page 48] RFC 8551 S/MIME 4.0 Message Specification April 2019

 [SMIMEv3.1] refers to [RFC2634], [RFC5035], [RFC5652], [RFC5750], and
 [RFC5751].
 [SMIMEv3.2] refers to [RFC2634], [RFC3850], [RFC3851], [RFC3852], and
 [RFC5035].
 [SMIMEv4] refers to [RFC2634], [RFC5035], [RFC5652], [RFC8550], and
 this document.

7.2. Normative References

 [CHARSETS] IANA, "Character sets assigned by IANA",
            <http://www.iana.org/assignments/character-sets>.
 [FIPS186-4]
            National Institute of Standards and Technology (NIST),
            "Digital Signature Standard (DSS)", Federal Information
            Processing Standards Publication 186-4,
            DOI 10.6028/NIST.FIPS.186-4, July 2013,
            <https://nvlpubs.nist.gov/nistpubs/fips/
            nist.fips.186-4.pdf>.
 [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, <https://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,
            <https://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,
            <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc2049>.

Schaad, et al. Standards Track [Page 49] RFC 8551 S/MIME 4.0 Message Specification April 2019

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC2183]  Troost, R., Dorner, S., and K. Moore, Ed., "Communicating
            Presentation Information in Internet Messages: The
            Content-Disposition Header Field", RFC 2183,
            DOI 10.17487/RFC2183, August 1997,
            <https://www.rfc-editor.org/info/rfc2183>.
 [RFC2634]  Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
            RFC 2634, DOI 10.17487/RFC2634, June 1999,
            <https://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,
            <https://www.rfc-editor.org/info/rfc3274>.
 [RFC3370]  Housley, R., "Cryptographic Message Syntax (CMS)
            Algorithms", RFC 3370, DOI 10.17487/RFC3370, August 2002,
            <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc3565>.
 [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,
            <https://www.rfc-editor.org/info/rfc4289>.
 [RFC4056]  Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in
            Cryptographic Message Syntax (CMS)", RFC 4056,
            DOI 10.17487/RFC4056, June 2005,
            <https://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,
            <https://www.rfc-editor.org/info/rfc4086>.

Schaad, et al. Standards Track [Page 50] RFC 8551 S/MIME 4.0 Message Specification April 2019

 [RFC5083]  Housley, R., "Cryptographic Message Syntax (CMS)
            Authenticated-Enveloped-Data Content Type", RFC 5083,
            DOI 10.17487/RFC5083, November 2007,
            <https://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,
            <https://www.rfc-editor.org/info/rfc5084>.
 [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
            RFC 5652, DOI 10.17487/RFC5652, September 2009,
            <https://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, <https://www.rfc-editor.org/info/rfc5753>.
 [RFC5754]  Turner, S., "Using SHA2 Algorithms with Cryptographic
            Message Syntax", RFC 5754, DOI 10.17487/RFC5754,
            January 2010, <https://www.rfc-editor.org/info/rfc5754>.
 [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
            Specifications and Registration Procedures", BCP 13,
            RFC 6838, DOI 10.17487/RFC6838, January 2013,
            <https://www.rfc-editor.org/info/rfc6838>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.
 [RFC8418]  Housley, R., "Use of the Elliptic Curve Diffie-Hellman Key
            Agreement Algorithm with X25519 and X448 in the
            Cryptographic Message Syntax (CMS)", RFC 8418,
            DOI 10.17487/RFC8418, August 2018,
            <https://www.rfc-editor.org/info/rfc8418>.
 [RFC8419]  Housley, R., "Use of Edwards-Curve Digital Signature
            Algorithm (EdDSA) Signatures in the Cryptographic Message
            Syntax (CMS)", RFC 8419, DOI 10.17487/RFC8419,
            August 2018, <https://www.rfc-editor.org/info/rfc8419>.
 [RFC8550]  Schaad, J., Ramsdell, B., and S. Turner,
            "Secure/Multipurpose Internet Mail Extensions (S/MIME)
            Version 4.0 Certificate Handling", RFC 8550,
            DOI 10.17487/RFC8550, April 2019,
            <https://www.rfc-editor.org/info/rfc8550>.

Schaad, et al. Standards Track [Page 51] RFC 8551 S/MIME 4.0 Message Specification April 2019

 [X.680]    "Information Technology - Abstract Syntax Notation One
            (ASN.1): Specification of basic notation", ITU-T
            Recommendation X.680, ISO/IEC 8824-1:2015, August 2015,
            <https://www.itu.int/rec/T-REC-X.680>.
 [X.681]    "Information Technology - Abstract Syntax Notation One
            (ASN.1): Information object specification", ITU-T
            Recommendation X.681, ISO/IEC 8824-2:2015, August 2015,
            <https://www.itu.int/rec/T-REC-X.681>.
 [X.682]    "Information Technology - Abstract Syntax Notation One
            (ASN.1): Constraint specification", ITU-T
            Recommendation X.682, ISO/IEC 8824-3:2015, August 2015,
            <https://www.itu.int/rec/T-REC-X.682>.
 [X.683]    "Information Technology - Abstract Syntax Notation One
            (ASN.1): Parameterization of ASN.1 specifications", ITU-T
            Recommendation X.683, ISO/IEC 8824-4:2015, August 2015,
            <https://www.itu.int/rec/T-REC-X.683>.
 [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 Recommendation X.690, ISO/IEC 8825-1:2015,
            August 2015, <https://www.itu.int/rec/T-REC-X.690>.

7.3. Informative References

 [Efail]    Poddebniak, D., Dresen, C., Muller, J., Ising, F.,
            Schinzel, S., Friedberger, S., Somorovsky, J., and J.
            Schwenk, "Efail: Breaking S/MIME and OpenPGP Email
            Encryption using Exfiltration Channels",
            UsenixSecurity 2018, August 2018,
            <https://www.usenix.org/system/files/conference/
            usenixsecurity18/sec18-poddebniak.pdf>.
 [FIPS186-2]
            National Institute of Standards and Technology (NIST),
            "Digital Signature Standard (DSS) (also with Change
            Notice 1)", Federal Information Processing Standards
            Publication 186-2, January 2000,
            <https://csrc.nist.gov/publications/detail/fips/186/2/
            archive/2000-01-27>.
 [RFC1866]  Berners-Lee, T. and D. Connolly, "Hypertext Markup
            Language - 2.0", RFC 1866, DOI 10.17487/RFC1866,
            November 1995, <https://www.rfc-editor.org/info/rfc1866>.

Schaad, et al. Standards Track [Page 52] RFC 8551 S/MIME 4.0 Message Specification April 2019

 [RFC2268]  Rivest, R., "A Description of the RC2(r) Encryption
            Algorithm", RFC 2268, DOI 10.17487/RFC2268, March 1998,
            <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc2312>.
 [RFC2313]  Kaliski, B., "PKCS #1: RSA Encryption Version 1.5",
            RFC 2313, DOI 10.17487/RFC2313, March 1998,
            <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc2315>.
 [RFC2630]  Housley, R., "Cryptographic Message Syntax", RFC 2630,
            DOI 10.17487/RFC2630, June 1999,
            <https://www.rfc-editor.org/info/rfc2630>.
 [RFC2631]  Rescorla, E., "Diffie-Hellman Key Agreement Method",
            RFC 2631, DOI 10.17487/RFC2631, June 1999,
            <https://www.rfc-editor.org/info/rfc2631>.
 [RFC2632]  Ramsdell, B., Ed., "S/MIME Version 3 Certificate
            Handling", RFC 2632, DOI 10.17487/RFC2632, June 1999,
            <https://www.rfc-editor.org/info/rfc2632>.
 [RFC2633]  Ramsdell, B., Ed., "S/MIME Version 3 Message
            Specification", RFC 2633, DOI 10.17487/RFC2633, June 1999,
            <https://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,
            <https://www.rfc-editor.org/info/rfc2785>.

Schaad, et al. Standards Track [Page 53] RFC 8551 S/MIME 4.0 Message Specification April 2019

 [RFC3218]  Rescorla, E., "Preventing the Million Message Attack on
            Cryptographic Message Syntax", RFC 3218,
            DOI 10.17487/RFC3218, January 2002,
            <https://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,
            <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc3851>.
 [RFC3852]  Housley, R., "Cryptographic Message Syntax (CMS)",
            RFC 3852, DOI 10.17487/RFC3852, July 2004,
            <https://www.rfc-editor.org/info/rfc3852>.
 [RFC4134]  Hoffman, P., Ed., "Examples of S/MIME Messages", RFC 4134,
            DOI 10.17487/RFC4134, July 2005,
            <https://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,
            <https://www.rfc-editor.org/info/rfc4270>.
 [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
            FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
            <https://www.rfc-editor.org/info/rfc4949>.
 [RFC5035]  Schaad, J., "Enhanced Security Services (ESS) Update:
            Adding CertID Algorithm Agility", RFC 5035, DOI
            10.17487/RFC5035, August 2007,
            <https://www.rfc-editor.org/info/rfc5035>.
 [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,
            <https://www.rfc-editor.org/info/rfc5750>.

Schaad, et al. Standards Track [Page 54] RFC 8551 S/MIME 4.0 Message Specification April 2019

 [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, <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc6194>.
 [RFC6268]  Schaad, J. and S. Turner, "Additional New ASN.1 Modules
            for the Cryptographic Message Syntax (CMS) and the Public
            Key Infrastructure Using X.509 (PKIX)", RFC 6268,
            DOI 10.17487/RFC6268, July 2011,
            <https://www.rfc-editor.org/info/rfc6268>.
 [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, <https://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, <https://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,
            <https://www.rfc-editor.org/info/rfc7905>.
 [SP800-56A]
            National Institute of Standards and Technology (NIST),
            "Recommendation for Pair-Wise Key Establishment Schemes
            Using Discrete Logarithm Cryptography", NIST Special
            Publication 800-56A Revision 2,
            DOI 10.6028/NIST.SP.800-56Ar2, May 2013,
            <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
            NIST.SP.800-56Ar2.pdf>.

Schaad, et al. Standards Track [Page 55] RFC 8551 S/MIME 4.0 Message Specification April 2019

 [SP800-57] National Institute of Standards and Technology (NIST),
            "Recommendation for Key Management - Part 1: General",
            NIST Special Publication 800-57 Revision 4,
            DOI 10.6028/NIST.SP.800-57pt1r4, January 2016,
            <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
            NIST.SP.800-57pt1r4.pdf>.
 [TripleDES]
            Tuchman, W., "Hellman Presents No Shortcut Solutions to
            the DES", IEEE Spectrum v. 16, n. 7, pp. 40-41,
            DOI 10.1109/MSPEC.1979.6368160, July 1979.

Schaad, et al. Standards Track [Page 56] RFC 8551 S/MIME 4.0 Message Specification April 2019

Appendix A. ASN.1 Module

 Note: The ASN.1 module contained herein is unchanged from RFC 5751
 [SMIMEv2] and RFC 3851 [SMIMEv3.1], with the exception of a change to
 the preferBinaryInside ASN.1 comment in RFC 3851 [SMIMEv3.1].  If a
 module is needed that is compatible with current ASN.1 standards, one
 can be found in [RFC6268].  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
 IMPORTS
  1. - 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) };
  1. - id-aa is the arc with all new authenticated and unauthenticated
  2. - 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)}
  1. - S/MIME Capabilities provides a method of broadcasting the
  2. - symmetric capabilities understood. Algorithms SHOULD be ordered
  3. - 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
  1. - Encryption Key Preference provides a method of broadcasting the
  2. - preferred encryption certificate.

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 id-aa-encrypKeyPref OBJECT IDENTIFIER ::= {id-aa 11}
 SMIMEEncryptionKeyPreference ::= CHOICE {
    issuerAndSerialNumber   [0] IssuerAndSerialNumber,
    receipentKeyId          [1] RecipientKeyIdentifier,
    subjectAltKeyIdentifier [2] SubjectKeyIdentifier
 }
  1. - "receipentKeyId" is spelled incorrectly but is kept for
  2. - historical reasons.
 id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
         rsadsi(113549) pkcs(1) pkcs-9(9) 16 }
 id-cap  OBJECT IDENTIFIER ::= { id-smime 11 }
  1. - The preferBinaryInside OID indicates an ability to receive
  2. - messages with binary encoding inside the CMS wrapper.
  3. - The preferBinaryInside attribute's value field is ABSENT.
 id-cap-preferBinaryInside  OBJECT IDENTIFIER ::= { id-cap 1 }
  1. - The following is a list of OIDs to be used with S/MIME v3.
  1. - Signature Algorithms Not Found in [RFC3370], [RFC5754], [RFC4056],
  2. - and [RFC3560]
  1. -
  2. - md2WithRSAEncryption OBJECT IDENTIFIER ::=
  3. - {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1)
  4. - 2}
  1. -
  2. - Other Signed Attributes
  3. -
  4. - signingTime OBJECT IDENTIFIER ::=
  5. - {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
  6. - 5}
  7. - See [CMS] for a description of how to encode the attribute
  8. - value.
 SMIMECapabilitiesParametersForRC2CBC ::= INTEGER
 --        (RC2 Key Length (number of bits))
 END

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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 documents have
 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 the updated
 documents often do not have the same information in them.

B.1. DigestAlgorithmIdentifier

 The following algorithms have been called out for some level of
 support by previous S/MIME specifications:
  1. SHA-1 was dropped in [SMIMEv4]. SHA-1 is no longer considered 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 recent advances.
  1. MD5 was dropped in [SMIMEv4]. 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
 sufficiently historic messages.  For this reason, it is strongly
 suggested that user agents treat these signatures differently from
 those on current messages.  These problems include the following:
  1. Certification authorities 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 to see if a certificate has been revoked.
    The same problems exist with Online Certificate Status Protocol
    (OCSP) responses, as they may be based on a CRL rather than on the
    certificate database.

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  1. 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
    the 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.
  1. Hash functions used to validate signatures on historic messages

may no 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 that they are no
    longer considered to be collision resistant implies that the
    security levels of the signatures are generally considered
    suspect.  If a message is known to be historic and it has been in
    the possession of the client for some time, then it might still be
    considered to be secure.
  1. 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.
 The following algorithms have been called out for some level of
 support by previous S/MIME specifications:
  1. RSA with MD5 was dropped in [SMIMEv4]. MD5 is no longer

considered to be secure, as it is no longer collision resistant.

    Details can be found in [RFC6151].
  1. RSA and DSA with SHA-1 were dropped in [SMIMEv4]. SHA-1 is no

longer considered 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 recent advances.
  1. DSA with SHA-256 was dropped in [SMIMEv4]. DSA has been replaced

by elliptic curve versions.

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 As requirements for "mandatory to implement" have changed over time,
 some issues have been created that can cause interoperability
 problems:
  1. 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.
  1. 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.
  1. 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.
 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
 OID, 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; and for 1024-bit and
 above DSA with SHA-256, see [RFC5754] and [FIPS186-4].  The first
 reference provides the signature algorithm's OID, 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:
  1. 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.
  1. DES EDE3 CBC [TripleDES], also known as "tripleDES", was dropped

in [SMIMEv4]. This algorithm is removed from the list of

    supported algorithms because (1) it has a 64-bit block size and
    (2) it offers less than 128 bits of security.  This algorithm
    should be supported only to decrypt existing email; it should not
    be used to encrypt new emails.

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B.4. KeyEncryptionAlgorithmIdentifier

 The following algorithms have been called out for some level of
 support by previous S/MIME specifications:
  1. DH ephemeral-static mode, as specified in [RFC3370] and

[SP800-57], was dropped in [SMIMEv4].

  1. 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]
 specifications are backward 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, RFC 2311 [SMIMEv2] was moved to Historic
 status.

Acknowledgements

 Many thanks go out to the other authors of the S/MIME version 2
 Message Specification RFC: Steve Dusse, Paul Hoffman, Laurence
 Lundblade, and Lisa Repka.  Without v2, there wouldn't be a v3, v3.1,
 v3.2, or v4.0.
 Some of the examples in this document were copied from [RFC4134].
 Thanks go to 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 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.

Schaad, et al. Standards Track [Page 62] RFC 8551 S/MIME 4.0 Message Specification April 2019

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

Schaad, et al. Standards Track [Page 63]

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