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rfc:rfc8550

Internet Engineering Task Force (IETF) J. Schaad Request for Comments: 8550 August Cellars Obsoletes: 5750 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
                        Certificate Handling

Abstract

 This document specifies conventions for X.509 certificate usage by
 Secure/Multipurpose Internet Mail Extensions (S/MIME) v4.0 agents.
 S/MIME provides a method to send and receive secure MIME messages,
 and certificates are an integral part of S/MIME agent processing.
 S/MIME agents validate certificates as described in RFC 5280
 ("Internet X.509 Public Key Infrastructure Certificate and
 Certificate Revocation List (CRL) Profile").  S/MIME agents must meet
 the certificate-processing requirements in this document as well as
 those in RFC 5280.  This document obsoletes RFC 5750.

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/rfc8550.

Schaad, et al. Standards Track [Page 1] RFC 8550 S/MIME 4.0 Certificate Handling 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 8550 S/MIME 4.0 Certificate Handling April 2019

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   1.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   4
   1.2.  Conventions Used in This Document . . . . . . . . . . . .   5
   1.3.  Compatibility with Prior Practice of S/MIME . . . . . . .   6
   1.4.  Changes from S/MIME v3 to S/MIME v3.1 . . . . . . . . . .   6
   1.5.  Changes from S/MIME v3.1 to S/MIME v3.2 . . . . . . . . .   7
   1.6.  Changes since S/MIME 3.2  . . . . . . . . . . . . . . . .   8
 2.  CMS Options . . . . . . . . . . . . . . . . . . . . . . . . .   8
   2.1.  Certificate Revocation Lists  . . . . . . . . . . . . . .   9
   2.2.  Certificate Choices . . . . . . . . . . . . . . . . . . .   9
     2.2.1.  Historical Note about CMS Certificates  . . . . . . .   9
   2.3.  Included Certificates . . . . . . . . . . . . . . . . . .  10
 3.  Using Distinguished Names for Internet Mail . . . . . . . . .  11
 4.  Certificate Processing  . . . . . . . . . . . . . . . . . . .  12
   4.1.  Certificate Revocation Lists  . . . . . . . . . . . . . .  13
   4.2.  Certificate Path Validation . . . . . . . . . . . . . . .  13
   4.3.  Certificate and CRL Signing Algorithms, and Key Sizes . .  14
   4.4.  PKIX Certificate Extensions . . . . . . . . . . . . . . .  15
     4.4.1.  Basic Constraints . . . . . . . . . . . . . . . . . .  16
     4.4.2.  Key Usage Extension . . . . . . . . . . . . . . . . .  16
     4.4.3.  Subject Alternative Name  . . . . . . . . . . . . . .  17
     4.4.4.  Extended Key Usage Extension  . . . . . . . . . . . .  17
 5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
   7.1.  Reference Conventions . . . . . . . . . . . . . . . . . .  20
   7.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
   7.2.  Informative References  . . . . . . . . . . . . . . . . .  23
 Appendix A.  Historic Considerations  . . . . . . . . . . . . . .  26
   A.1.  Signature Algorithms and Key Sizes  . . . . . . . . . . .  26
 Appendix B.  Moving S/MIME v2 Certificate Handling to Historic
              Status . . . . . . . . . . . . . . . . . . . . . . .  27
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  28
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

Schaad, et al. Standards Track [Page 3] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

1. Introduction

 S/MIME (Secure/Multipurpose Internet Mail Extensions) v4.0, described
 in [RFC8551], provides a method to send and receive secure MIME
 messages.  Before using a public key to provide security services,
 the S/MIME agent MUST verify that the public key is valid.  S/MIME
 agents MUST use PKIX certificates to validate public keys as
 described in [RFC5280] ("Internet X.509 Public Key Infrastructure
 Certificate and Certificate Revocation List (CRL) Profile").  S/MIME
 agents MUST meet the certificate-processing requirements specified in
 this document in addition to those stated in [RFC5280].
 This specification is compatible with the Cryptographic Message
 Syntax (CMS) [RFC5652] in that it uses the data types defined by CMS.
 It also inherits all the varieties of architectures for certificate-
 based key management supported by CMS.
 This document obsoletes [RFC5750].  The most significant changes
 revolve around changes in recommendations around the cryptographic
 algorithms used by the specification.  More details can be found in
 Section 1.6.
 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. Definitions

 For the purposes of this document, the following definitions apply.
 ASN.1:
    Abstract Syntax Notation One, as defined in ITU-T X.680 [X.680].
 Attribute certificate (AC):
    An X.509 AC is a separate structure from a subject's public key
    X.509 certificate.  A subject may have multiple X.509 ACs
    associated with each of its public key X.509 certificates.  Each
    X.509 AC binds one or more attributes with one of the subject's
    public key X.509 certificates.  The X.509 AC syntax is defined in
    [RFC5755].

Schaad, et al. Standards Track [Page 4] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 Certificate:
    A type that binds an entity's name to a public key with a digital
    signature.  This type is defined in [RFC5280].  This type also
    contains the distinguished name of the certificate issuer (the
    signer), an issuer-specific serial number, the issuer's signature
    algorithm identifier, a validity period, and extensions also
    defined in that document.
 Certificate Revocation List (CRL):
    A type that contains information about certificates whose validity
    an issuer has revoked.  The information consists of an issuer
    name, the time of issue, the next scheduled time of issue, a list
    of certificate serial numbers and their associated revocation
    times, and extensions as defined in [RFC5280].  The CRL is signed
    by the issuer.  The type intended by this specification is the one
    defined in [RFC5280].
 Receiving agent:
    Software that interprets and processes S/MIME CMS objects, MIME
    body parts that contain CMS objects, or both.
 Sending agent:
    Software that creates S/MIME CMS objects, MIME body parts that
    contain CMS objects, or both.
 S/MIME agent:
    User software that is a receiving agent, a sending agent, or both.

1.2. 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.
 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.

Schaad, et al. Standards Track [Page 5] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 MUST-     This term means the same as MUST.  However, the authors
           expect that this requirement will no longer be a MUST in a
           future document.  Although its status will be determined at
           a later time, it is reasonable to expect that if a future
           revision of a document alters the status of a MUST-
           requirement, it will remain at least a SHOULD or a SHOULD-.
 The term "RSA" in this document almost always refers to the
 PKCS #1 v1.5 RSA signature algorithm 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.

1.3. 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. RFC 2311 also has historical information about the development of

S/MIME.

 Appendix A contains information about algorithms that were used for
 prior versions of S/MIME but are no longer considered to meet modern
 security standards.  Support of these algorithms may be needed to
 support historic S/MIME artifacts such as messages or files but
 SHOULD NOT be used for new artifacts.

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

 This section reflects the changes that were made when S/MIME v3.1 was
 released.  The language of RFC 2119 ("MUST", "SHOULD", etc.) used for
 S/MIME v3 may have been superseded in later versions.
  1. Version 1 and version 2 CRLs MUST be supported.

Schaad, et al. Standards Track [Page 6] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

  1. Multiple certification authority (CA) certificates with the same

subject and public key, but with overlapping validity periods,

    MUST be supported.
  1. Version 2 ACs SHOULD be supported, and version 1 ACs MUST NOT be

used.

  1. The use of the MD2 digest algorithm for certificate signatures is

discouraged, and security language was added.

  1. Clarified email address use in certificates. Certificates that do

not contain an email address have no requirements for verifying

    the email address associated with the certificate.
  1. Receiving agents SHOULD display certificate information when

displaying the results of signature verification.

  1. Receiving agents MUST NOT accept a signature made with a

certificate that does not have at least one of the

    digitalSignature or nonRepudiation bits set.
  1. Added clarifications for the interpretation of the key usage and

extended key usage extensions.

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

 This section reflects the changes that were made when S/MIME v3.2 was
 released.  The language of RFC 2119 ("MUST", "SHOULD", etc.) used for
 S/MIME v3.1 may have been superseded in later versions.
 Note that the section numbers listed here (e.g., "Section 6") are
 from [RFC5750].
  1. Moved "Conventions Used in This Document" to Section 1.2. Added

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

  1. Section 1.1: Updated ASN.1 definition and reference.
  1. Section 1.3: Added text about v3.1 RFCs.
  1. Section 3: Aligned email address text with RFC 5280. Updated note

to indicate that the emailAddress IA5String upper bound is

    255 characters.  Added text about matching email addresses.
  1. Section 4.2: Added text to indicate how S/MIME agents locate the

correct user certificate.

Schaad, et al. Standards Track [Page 7] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

  1. Section 4.3: RSA with SHA-256 (PKCS #1 v1.5) 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+.  Updated key sizes and changed pointer to PKIX RFCs.
  1. Section 4.4.1: Aligned with PKIX on the use of a basicConstraints

extension in CA certificates. Clarified which extension is used

    to constrain end entities from using their keys to perform
    issuing-authority operations.
  1. Section 5: Updated security considerations.
  1. Section 6: Moved references from Appendix A of RFC 3850 to this

section. Updated the references.

  1. Appendix A: Added Appendix A to move S/MIME v2 Certificate

Handling to Historic status.

1.6. Changes since S/MIME 3.2

 This section reflects the changes that were made when S/MIME v4.0 was
 released.  The language of RFC 2119 ("MUST", "SHOULD", etc.) used for
 S/MIME v3.2 may have been superseded by S/MIME v4.0 and may be
 superseded by future versions.
  1. Section 3: Support for internationalized email addresses is

required.

  1. Section 4.3: Mandated support for the Elliptic Curve Digital

Signature Algorithm (ECDSA) with P-256 and the Edwards-curve

    Digital Signature Algorithm (EdDSA) with curve25519 [RFC8410].
    SHA-1 and MD5 algorithms are marked as historical, as they are no
    longer considered secure.  As the Digital Signature Algorithm
    (DSA) has been replaced by elliptic curve versions, support for
    DSA is now considered historical.  Increased lower bounds on RSA
    key sizes.
  1. Appendix A: Added Appendix A for algorithms that are now

considered to be historical.

2. CMS Options

 The CMS message format allows for a wide variety of options in
 content and algorithm support.  This section puts forth a number of
 support requirements and recommendations in order to achieve a base
 level of interoperability among all S/MIME implementations.  Most of
 the CMS format for S/MIME messages is defined in [RFC8551].

Schaad, et al. Standards Track [Page 8] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

2.1. Certificate Revocation Lists

 Receiving agents MUST support the CRL format defined in [RFC5280].
 If sending agents include CRLs in outgoing messages, the CRL format
 defined in [RFC5280] MUST be used.  Receiving agents MUST support
 both v1 and v2 CRLs.
 All agents MUST be capable of performing revocation checks using CRLs
 as specified in [RFC5280].  All agents MUST perform revocation status
 checking in accordance with [RFC5280].  Receiving agents MUST
 recognize CRLs in received S/MIME messages.
 Agents SHOULD store CRLs received in messages for use in processing
 later messages.

2.2. Certificate Choices

 Receiving agents MUST support v1 X.509 and v3 X.509 certificates as
 profiled in [RFC5280].  End-entity certificates MAY include an
 Internet mail address, as described in Section 3.
 Receiving agents SHOULD support X.509 version 2 ACs.  See [RFC5755]
 for details about the profile for ACs.

2.2.1. Historical Note about CMS Certificates

 The CMS message format supports a choice of certificate formats for
 public key content types: PKIX, PKCS #6 extended certificates
 [PKCS6], and PKIX ACs.
 The PKCS #6 format is not in widespread use.  In addition, PKIX
 certificate extensions address much of the same functionality and
 flexibility as was intended in the PKCS #6 certificate extensions.
 Thus, sending and receiving agents MUST NOT use PKCS #6 extended
 certificates.  Receiving agents MUST be able to parse and process a
 message containing PKCS #6 extended certificates, although ignoring
 those certificates is expected behavior.
 X.509 version 1 ACs are also not widely implemented and have
 been superseded by version 2 ACs.  Sending agents MUST NOT send
 version 1 ACs.

Schaad, et al. Standards Track [Page 9] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

2.3. Included Certificates

 Receiving agents MUST be able to handle an arbitrary number of
 certificates of arbitrary relationship to the message sender and to
 each other in arbitrary order.  In many cases, the certificates
 included in a signed message may represent a chain of certification
 from the sender to a particular root.  There may be, however,
 situations where the certificates in a signed message may be
 unrelated and included for convenience.
 Sending agents SHOULD include any certificates for the user's public
 key(s) and associated issuer certificates.  This increases the
 likelihood that the intended recipient can establish trust in the
 originator's public key(s).  This is especially important when
 sending a message to recipients that may not have access to the
 sender's public key through any other means or when sending a signed
 message to a new recipient.  The inclusion of certificates in
 outgoing messages can be omitted if S/MIME objects are sent within a
 group of correspondents that have established access to each other's
 certificates by some other means such as a shared directory or manual
 certificate distribution.  Receiving S/MIME agents SHOULD be able to
 handle messages without certificates by using a database or directory
 lookup scheme to find them.
 A sending agent SHOULD include at least one chain of certificates up
 to, but not including, a CA that it believes that the recipient may
 trust as authoritative.  A receiving agent MUST be able to handle an
 arbitrarily large number of certificates and chains.
 Agents MAY send CA certificates -- that is, cross-certificates,
 self-issued certificates, and self-signed certificates.  Note that
 receiving agents SHOULD NOT simply trust any self-signed certificates
 as valid CAs but SHOULD use some other mechanism to determine if this
 is a CA that should be trusted.  Also note that when certificates
 contain DSA public keys the parameters may be located in the root
 certificate.  This would require that the recipient possess both the
 end-entity certificate and the root certificate to perform a
 signature verification, and is a valid example of a case where
 transmitting the root certificate may be required.
 Receiving agents MUST support chaining based on the distinguished
 name fields.  Other methods of building certificate chains MAY be
 supported.
 Receiving agents SHOULD support the decoding of X.509 ACs included in
 CMS objects.  All other issues regarding the generation and use of
 X.509 ACs are outside the scope of this specification.  One
 specification that addresses AC use is defined in [RFC3114].

Schaad, et al. Standards Track [Page 10] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

3. Using Distinguished Names for Internet Mail

 End-entity certificates MAY contain an Internet mail address.
 Email addresses restricted to 7-bit ASCII characters use the
 pkcs-9-at-emailAddress object identifier (OID) (see below) and are
 encoded as described in Section 4.2.1.6 of [RFC5280].
 Internationalized email address names use the OID defined in
 [RFC8398] and are encoded as described therein.  The email address
 SHOULD be in the subjectAltName extension and SHOULD NOT be in the
 subject distinguished name.
 Receiving agents MUST recognize and accept certificates that contain
 no email address.  Agents are allowed to provide an alternative
 mechanism for associating an email address with a certificate that
 does not contain an email address, such as through the use of the
 agent's address book, if available.  Receiving agents MUST recognize
 both ASCII and internationalized email addresses in the
 subjectAltName extension.  Receiving agents MUST recognize email
 addresses in the distinguished name field in the PKCS #9 [RFC2985]
 emailAddress attribute:
 pkcs-9-at-emailAddress OBJECT IDENTIFIER ::=
  { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1 }
 Note that this attribute MUST be encoded as IA5String and has an
 upper bound of 255 characters.  The comparing of email addresses is
 fraught with peril.  [RFC8398] defines the procedure for doing the
 comparison of internationalized email addresses.  For ASCII email
 addresses, the domain component (right-hand side of the '@') MUST be
 compared using a case-insensitive function.  The local name
 component (left-hand side of the '@') SHOULD be compared using a
 case-insensitive function.  Some localities may perform other
 transformations on the local name component before doing the
 comparison; however, an S/MIME client cannot know what specific
 localities do.
 Sending agents SHOULD make the address in the From or Sender header
 in a mail message match an Internet mail address in the signer's
 certificate.  Receiving agents MUST check that the address in the
 From or Sender header of a mail message matches an Internet mail
 address in the signer's certificate, if mail addresses are present in
 the certificate.  A receiving agent SHOULD provide some explicit
 alternate processing of the message if this comparison fails; this
 might be done by displaying or logging a message that shows the
 recipient the mail addresses in the certificate or other certificate
 details.

Schaad, et al. Standards Track [Page 11] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 A receiving agent SHOULD display a subject name or other certificate
 details when displaying an indication of successful or unsuccessful
 signature verification.
 All subject and issuer names MUST be populated (i.e., not an empty
 SEQUENCE) in S/MIME-compliant X.509 certificates, except that the
 subject distinguished name in a user's (i.e., an end entity's)
 certificate MAY be an empty SEQUENCE, in which case the
 subjectAltName extension will include the subject's identifier and
 MUST be marked as critical.

4. Certificate Processing

 S/MIME agents need to provide some certificate retrieval mechanism in
 order to gain access to certificates for recipients of digital
 envelopes.  There are many ways to implement certificate retrieval
 mechanisms.  [X.500] directory service is an excellent example of a
 certificate retrieval-only mechanism that is compatible with classic
 X.500 distinguished names.  The IETF has published [RFC8162], which
 describes an experimental protocol to retrieve certificates from the
 Domain Name System (DNS).  Until such mechanisms are widely used,
 their utility may be limited by the small number of the
 correspondent's certificates that can be retrieved.  At a minimum,
 for initial S/MIME deployment, a user agent could automatically
 generate a message to an intended recipient requesting the
 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.  In many environments,
 it may be desirable to link the certificate retrieval/storage
 mechanisms together in some sort of certificate database.  In its
 simplest form, a certificate database would be local to a particular
 user and would function in a way similar to an "address book" that
 stores a user's frequent correspondents.  In this way, the
 certificate retrieval mechanism would be limited to the certificates
 that a user has stored (presumably from incoming messages).  A
 comprehensive certificate retrieval/storage solution might combine
 two or more mechanisms to allow the greatest flexibility and utility
 to the user.  For instance, a secure Internet mail agent might resort
 to checking a centralized certificate retrieval mechanism for a
 certificate if it cannot be found in a user's local certificate
 storage/retrieval database.

Schaad, et al. Standards Track [Page 12] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 Receiving and sending agents SHOULD provide a mechanism for the
 import and export of certificates, using a CMS certs-only message.
 This allows for import and export of full certificate chains as
 opposed to just a single certificate.  This is described in
 [RFC8551].
 Agents MUST handle multiple valid CA certificates containing the same
 subject name and the same public keys but with overlapping validity
 intervals.

4.1. Certificate Revocation Lists

 In general, it is always better to get the latest CRL information
 from a CA than to get information stored in an incoming message.  A
 receiving agent SHOULD have access to some CRL retrieval mechanism in
 order to gain access to certificate revocation information when
 validating certification paths.  A receiving or sending agent SHOULD
 also provide a mechanism to allow a user to store incoming
 certificate revocation information for correspondents in such a way
 as to guarantee its later retrieval.
 Receiving and sending agents SHOULD retrieve and utilize CRL
 information every time a certificate is verified as part of a
 certification path validation even if the certificate was already
 verified in the past.  However, in many instances (such as off-line
 verification), access to the latest CRL information may be difficult
 or impossible.  The use of CRL information, therefore, may be
 dictated by the value of the information that is protected.  The
 value of the CRL information in a particular context is beyond the
 scope of this specification but may be governed by the policies
 associated with particular certification paths.
 All agents MUST be capable of performing revocation checks using CRLs
 as specified in [RFC5280].  All agents MUST perform revocation status
 checking in accordance with [RFC5280].  Receiving agents MUST
 recognize CRLs in received S/MIME messages.

4.2. Certificate Path Validation

 In creating a user agent for secure messaging, certificate, CRL, and
 certification path validation should be highly automated while still
 acting in the best interests of the user.  Certificate, CRL, and path
 validation MUST be performed as per [RFC5280] when validating a
 correspondent's public key.  This is necessary before using a public
 key to provide security services such as verifying a signature,
 encrypting a content-encryption key (e.g., RSA), or forming a
 pairwise symmetric key (e.g., Diffie-Hellman) to be used to encrypt
 or decrypt a content-encryption key.

Schaad, et al. Standards Track [Page 13] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 Certificates and CRLs are made available to the path validation
 procedure in two ways: a) incoming messages and b) certificate and
 CRL retrieval mechanisms.  Certificates and CRLs in incoming messages
 are not required to be in any particular order, nor are they required
 to be in any way related to the sender or recipient of the message
 (although in most cases they will be related to the sender).
 Incoming certificates and CRLs SHOULD be cached for use in path
 validation and optionally stored for later use.  This temporary
 certificate and CRL cache SHOULD be used to augment any other
 certificate and CRL retrieval mechanisms for path validation on
 incoming signed messages.
 When verifying a signature and the certificates that are included in
 the message, if a signingCertificate attribute from RFC 2634 [ESS] or
 a signingCertificateV2 attribute from RFC 5035 [ESS] is found in an
 S/MIME message, it SHALL be used to identify the signer's
 certificate.  Otherwise, the certificate is identified in an S/MIME
 message, using either (1) the issuerAndSerialNumber, which identifies
 the signer's certificate by the issuer's distinguished name and the
 certificate serial number or (2) the subjectKeyIdentifier, which
 identifies the signer's certificate by a key identifier.
 When decrypting an encrypted message, if an
 SMIMEEncryptionKeyPreference attribute is found in an encapsulating
 SignedData, it SHALL be used to identify the originator's certificate
 found in OriginatorInfo.  See [RFC5652] for the CMS fields that
 reference the originator's and recipient's certificates.

4.3. Certificate and CRL Signing Algorithms, and Key Sizes

 Certificates and CRLs are signed by the certificate issuer.
 Receiving agents:
  1. MUST support ECDSA with curve P-256 with SHA-256.
  1. MUST support EdDSA with curve25519 using PureEdDSA mode.
  1. MUST- support RSA PKCS #1 v1.5 with SHA-256.
  1. SHOULD support the RSA Probabilistic Signature Scheme (RSASSA-PSS)

with SHA-256.

 Implementations SHOULD use deterministic generation for the parameter
 'k' for ECDSA as outlined in [RFC6979].  EdDSA is defined to generate
 this parameter deterministically.

Schaad, et al. Standards Track [Page 14] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 The following are the RSA and RSASSA-PSS key size requirements for
 S/MIME receiving agents during certificate and CRL signature
 verification:
         key size <= 2047 : SHOULD NOT (see Appendix A)
 2048 <= key size <= 4096 : MUST (see Security Considerations)
 4096 <  key size         : MAY  (see Security Considerations)
 The signature algorithm OIDs for RSA PKCS #1 v1.5 and RSASSA-PSS with
 SHA-256 using 1024-bit through 3072-bit public keys are specified in
 [RFC4055], and the signature algorithm definition is found in
 [FIPS186-2] with Change Notice 1.
 The signature algorithm OIDs for RSA PKCS #1 v1.5 and RSASSA-PSS with
 SHA-256 using 4096-bit public keys are specified in [RFC4055], and
 the signature algorithm definition is found in [RFC3447].
 For RSASSA-PSS with SHA-256, see [RFC4056].
 For ECDSA, see [RFC5758] and [RFC6090].  The first reference provides
 the signature algorithm's OID, and the second provides the signature
 algorithm's definition.  Curves other than curve P-256 MAY be used as
 well.
 For EdDSA, see [RFC8032] and [RFC8410].  The first reference provides
 the signature algorithm's OID, and the second provides the signature
 algorithm's definition.  Curves other than curve25519 MAY be used as
 well.

4.4. PKIX Certificate Extensions

 PKIX describes an extensible framework in which the basic certificate
 information can be extended and describes how such extensions can be
 used to control the process of issuing and validating certificates.
 The LAMPS Working Group has ongoing efforts to identify and create
 extensions that have value in particular certification environments.
 Further, there are active efforts underway to issue PKIX certificates
 for business purposes.  This document identifies the minimum required
 set of certificate extensions that have the greatest value in the
 S/MIME environment.  The syntax and semantics of all the identified
 extensions are defined in [RFC5280].
 Sending and receiving agents MUST correctly handle the basic
 constraints, key usage, authority key identifier, subject key
 identifier, and subject alternative name certificate extensions when
 they appear in end-entity and CA certificates.  Some mechanism SHOULD
 exist to gracefully handle other certificate extensions when they
 appear in end-entity or CA certificates.

Schaad, et al. Standards Track [Page 15] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 Certificates issued for the S/MIME environment SHOULD NOT contain any
 critical extensions (extensions that have the critical field set to
 TRUE) other than those listed here.  These extensions SHOULD be
 marked as non-critical, unless the proper handling of the extension
 is deemed critical to the correct interpretation of the associated
 certificate.  Other extensions may be included, but those extensions
 SHOULD NOT be marked as critical.
 Interpretation and syntax for all extensions MUST follow [RFC5280],
 unless otherwise specified here.

4.4.1. Basic Constraints

 The basicConstraints extension serves to delimit the role and
 position that an issuing-authority or end-entity certificate plays in
 a certification path.
 For example, certificates issued to CAs and subordinate CAs contain a
 basicConstraints extension that identifies them as issuing-authority
 certificates.  End-entity certificates contain the key usage
 extension, which restrains end entities from using the key when
 performing issuing-authority operations (see Section 4.4.2).
 As per [RFC5280], certificates MUST contain a basicConstraints
 extension in CA certificates and SHOULD NOT contain that extension in
 end-entity certificates.

4.4.2. Key Usage Extension

 The key usage extension serves to limit the technical purposes for
 which a public key listed in a valid certificate may be used.
 Issuing-authority certificates may contain a key usage extension that
 restricts the key to signing certificates, CRLs, and other data.
 For example, a CA may create subordinate issuer certificates that
 contain a key usage extension that specifies that the corresponding
 public key can be used to sign end-user certificates and CRLs.
 If a key usage extension is included in a PKIX certificate, then it
 MUST be marked as critical.
 S/MIME receiving agents MUST NOT accept the signature of a message if
 it was verified using a certificate that contains a key usage
 extension without at least one of the digitalSignature or
 nonRepudiation bits set.  Sometimes S/MIME is used as a secure
 message transport for applications beyond interpersonal messaging; in

Schaad, et al. Standards Track [Page 16] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 such cases, the S/MIME-enabled application can specify additional
 requirements concerning the digitalSignature or nonRepudiation bits
 within this extension.
 If the key usage extension is not specified, receiving clients MUST
 presume that both the digitalSignature and nonRepudiation bits
 are set.

4.4.3. Subject Alternative Name

 The subject alternative name extension is used in S/MIME as the
 preferred means to convey the email address or addresses that
 correspond to the entity for this certificate.  If the local portion
 of the email address is ASCII, it MUST be encoded using the
 rfc822Name CHOICE of the GeneralName type as described in [RFC5280],
 Section 4.2.1.6.  If the local portion of the email address is not
 ASCII, it MUST be encoded using the otherName CHOICE of the
 GeneralName type as described in [RFC8398], Section 3.  Since the
 SubjectAltName type is a SEQUENCE OF GeneralName, multiple email
 addresses MAY be present.

4.4.4. Extended Key Usage Extension

 The extended key usage extension also serves to limit the technical
 purposes for which a public key listed in a valid certificate may be
 used.  The set of technical purposes for the certificate therefore
 are the intersection of the uses indicated in the key usage and
 extended key usage extensions.
 For example, if the certificate contains a key usage extension
 indicating a digital signature and an extended key usage extension
 that includes the id-kp-emailProtection OID, then the certificate may
 be used for signing but not encrypting S/MIME messages.  If the
 certificate contains a key usage extension indicating a digital
 signature but no extended key usage extension, then the certificate
 may also be used to sign but not encrypt S/MIME messages.
 If the extended key usage extension is present in the certificate,
 then interpersonal-message S/MIME receiving agents MUST check that it
 contains either the id-kp-emailProtection OID or the
 anyExtendedKeyUsage OID as defined in [RFC5280].  S/MIME uses other
 than interpersonal messaging MAY require the explicit presence of the
 extended key usage extension, the presence of other OIDs in the
 extension, or both.

Schaad, et al. Standards Track [Page 17] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

5. IANA Considerations

 This document has no IANA actions.

6. Security Considerations

 All of the security issues faced by any cryptographic application
 must be faced by an S/MIME agent.  Among these issues are protecting
 the user's private key, preventing various attacks, and helping the
 user avoid mistakes such as inadvertently encrypting a message for
 the wrong recipient.  The entire list of security considerations is
 beyond the scope of this document, but some significant concerns are
 listed here.
 When processing certificates, there are many situations where the
 processing might fail.  Because the processing may be done by a user
 agent, a security gateway, or some other program, there is no single
 way to handle such failures.  Just because the methods to handle the
 failures have not been listed, however, the reader should not assume
 that they are not important.  The opposite is true: if a certificate
 is not provably valid and associated with the message, the processing
 software should take immediate and noticeable steps to inform the end
 user about it.
 Some of the many places where signature and certificate checking
 might fail include the following:
  1. no Internet mail addresses in a certificate match the sender of a

message, if the certificate contains at least one mail address

  1. no certificate chain leads to a trusted CA
  1. no ability to check the CRL for a certificate is implemented
  1. an invalid CRL was received
  1. the CRL being checked is expired
  1. the certificate is expired
  1. the certificate has been revoked
 There are certainly other instances where a certificate may be
 invalid, and it is the responsibility of the processing software to
 check them all thoroughly and decide what to do if the check fails.

Schaad, et al. Standards Track [Page 18] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 It is possible for there to be multiple unexpired CRLs for a CA.  If
 an agent is consulting CRLs for certificate validation, it SHOULD
 make sure that the most recently issued CRL for that CA is consulted,
 since an S/MIME message sender could deliberately include an older
 unexpired CRL in an S/MIME message.  This older CRL might not include
 recently revoked certificates; this scenario might lead an agent to
 accept a certificate that has been revoked in a subsequent CRL.
 When determining the time for a certificate validity check, agents
 have to be careful to use a reliable time.  In most cases, the time
 used SHOULD be the current time.  Some exceptions to this would be as
 follows:
  1. The time the message was received is stored in a secure manner and

is used at a later time to validate the message.

  1. The time in a SigningTime attribute is found in a countersignature

attribute [RFC5652] that has been successfully validated.

 The signingTime attribute could be deliberately set to a time where
 the receiving agent would (1) use a CRL that does not contain a
 revocation for the signing certificate or (2) use a certificate that
 has expired or is not yet valid.  This could be done by either
 (1) the sender of the message or (2) an attacker that has compromised
 the key of the sender.
 In addition to the security considerations identified in [RFC5280],
 caution should be taken when processing certificates that have not
 first been validated to a trust anchor.  Certificates could be
 manufactured by untrusted sources for the purpose of mounting denial-
 of-service attacks or other attacks.  For example, keys selected to
 require excessive cryptographic processing, or extensive lists of CRL
 Distribution Point (CDP) and/or Authority Information Access (AIA)
 addresses in the certificate, could be used to mount denial-of-
 service attacks.  Similarly, attacker-specified CDP and/or AIA
 addresses could be included in fake certificates to allow the
 originator to detect receipt of the message even if signature
 verification fails.
 RSA 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 sign certificates or CRLs.  Such keys
 were previously considered secure, so processing previously received
 signed and encrypted mail may require processing certificates or CRLs
 signed with 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 accepting certificates and CRLs with
 smaller key sizes (e.g., spoofed certificates) versus the costs of

Schaad, et al. Standards Track [Page 19] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 denial of service.  If an implementation supports verification of
 certificates or CRLs generated with RSA and DSA keys of less than
 2048 bits, it MUST warn the user.  Implementers should consider
 providing a stronger warning for weak signatures on certificates and
 CRLs associated with newly received messages than the one provided
 for certificates and CRLs associated with previously stored messages.
 Server implementations (e.g., secure mail list servers) where user
 warnings are not appropriate SHOULD reject messages with weak
 cryptography.
 If an implementation is concerned about compliance with National
 Institute of Standards and Technology (NIST) key size
 recommendations, then see [SP800-57].

7. References

7.1. Reference Conventions

  [ESS] refers to [RFC2634] and [RFC5035].
  [SMIMEv2] refers to [RFC2311], [RFC2312], [RFC2313], [RFC2314], and
  [RFC2315].
  [SMIMEv3] refers to [RFC2630], [RFC2631], [RFC2632], [RFC2633],
  [RFC2634], and [RFC5035].
  [SMIMEv3.1] refers to [RFC2634], [RFC3850], [RFC3851], [RFC3852],
  and [RFC5035].
  [SMIMEv3.2] refers to [RFC2634], [RFC5035], [RFC5652], [RFC5750],
  and [RFC5751].
  [SMIMEv4] refers to [RFC2634], [RFC5035], [RFC5652], [RFC8551], and
  this document.

7.2. Normative References

 [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>.

Schaad, et al. Standards Track [Page 20] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 [FIPS186-3]
            National Institute of Standards and Technology (NIST),
            "Digital Signature Standard (DSS)", Federal Information
            Processing Standards Publication 186-3, June 2009,
            <https://csrc.nist.gov/csrc/media/publications/fips/186/3/
            archive/2009-06-25/documents/fips_186-3.pdf>.
 [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>.
 [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>.
 [RFC2985]  Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
            Classes and Attribute Types Version 2.0", RFC 2985,
            DOI 10.17487/RFC2985, November 2000,
            <https://www.rfc-editor.org/info/rfc2985>.
 [RFC3279]  Bassham, L., Polk, W., and R. Housley, "Algorithms and
            Identifiers for the Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 3279, DOI 10.17487/RFC3279, April
            2002, <https://www.rfc-editor.org/info/rfc3279>.
 [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
            Standards (PKCS) #1: RSA Cryptography Specifications
            Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
            2003, <https://www.rfc-editor.org/info/rfc3447>.
 [RFC4055]  Schaad, J., Kaliski, B., and R. Housley, "Additional
            Algorithms and Identifiers for RSA Cryptography for use in
            the Internet X.509 Public Key Infrastructure Certificate
            and Certificate Revocation List (CRL) Profile", RFC 4055,
            DOI 10.17487/RFC4055, June 2005,
            <https://www.rfc-editor.org/info/rfc4055>.
 [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>.
 [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>.

Schaad, et al. Standards Track [Page 21] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
            Housley, R., and W. Polk, "Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
            <https://www.rfc-editor.org/info/rfc5280>.
 [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
            RFC 5652, DOI 10.17487/RFC5652, September 2009,
            <https://www.rfc-editor.org/info/rfc5652>.
 [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>.
 [RFC5755]  Farrell, S., Housley, R., and S. Turner, "An Internet
            Attribute Certificate Profile for Authorization",
            RFC 5755, DOI 10.17487/RFC5755, January 2010,
            <https://www.rfc-editor.org/info/rfc5755>.
 [RFC5758]  Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T.
            Polk, "Internet X.509 Public Key Infrastructure:
            Additional Algorithms and Identifiers for DSA and ECDSA",
            RFC 5758, DOI 10.17487/RFC5758, January 2010,
            <https://www.rfc-editor.org/info/rfc5758>.
 [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
            Algorithm (DSA) and Elliptic Curve Digital Signature
            Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
            2013, <https://www.rfc-editor.org/info/rfc6979>.
 [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>.
 [RFC8398]  Melnikov, A., Ed. and W. Chuang, Ed., "Internationalized
            Email Addresses in X.509 Certificates", RFC 8398,
            DOI 10.17487/RFC8398, May 2018,
            <https://www.rfc-editor.org/info/rfc8398>.
 [RFC8551]  Schaad, J., Ramsdell, B., and S. Turner,
            "Secure/Multipurpose Internet Mail Extensions (S/MIME)
            Version 4.0 Message Specification", RFC 8551,
            DOI 10.17487/RFC8551, April 2019,
            <https://www.rfc-editor.org/info/rfc8551>.

Schaad, et al. Standards Track [Page 22] RFC 8550 S/MIME 4.0 Certificate Handling 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>.

7.3 Informative References

 [PKCS6]    RSA Laboratories, "PKCS #6: Extended-Certificate Syntax
            Standard", November 1993.
 [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>.

Schaad, et al. Standards Track [Page 23] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 [RFC3114]  Nicolls, W., "Implementing Company Classification Policy
            with the S/MIME Security Label", RFC 3114,
            DOI 10.17487/RFC3114, May 2002,
            <https://www.rfc-editor.org/info/rfc3114>.
 [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>.
 [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>.
 [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
            Curve Cryptography Algorithms", RFC 6090,
            DOI 10.17487/RFC6090, February 2011,
            <https://www.rfc-editor.org/info/rfc6090>.
 [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>.
 [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
            Signature Algorithm (EdDSA)", RFC 8032,
            DOI 10.17487/RFC8032, January 2017,
            <https://www.rfc-editor.org/info/rfc8032>.
 [RFC8162]  Hoffman, P. and J. Schlyter, "Using Secure DNS to
            Associate Certificates with Domain Names for S/MIME",
            RFC 8162, DOI 10.17487/RFC8162, May 2017,
            <https://www.rfc-editor.org/info/rfc8162>.

Schaad, et al. Standards Track [Page 24] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

 [RFC8410]  Josefsson, S. and J. Schaad, "Algorithm Identifiers for
            Ed25519, Ed448, X25519, and X448 for Use in the Internet
            X.509 Public Key Infrastructure", RFC 8410,
            DOI 10.17487/RFC8410, August 2018,
            <https://www.rfc-editor.org/info/rfc8410>.
 [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>.
 [X.500]    "Information technology - Open Systems Interconnection -
            The Directory - Part 1: Overview of concepts, models and
            services", ITU-T Recommendation X.500,
            ISO/IEC 9594-1:2017.

Schaad, et al. Standards Track [Page 25] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

Appendix A. Historic Considerations

A.1. Signature Algorithms and Key Sizes

 There are a number of problems with validating certificates on
 sufficiently historic messages.  For this reason, it is strongly
 suggested that user agents treat these certificates differently from
 those on current messages.  These problems include the following:
  1. CAs are not required to keep certificates on a CRL beyond one

update after a certificate has expired. This means that unless

    CRLs are cached as part of the message it is not always possible
    to check 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.
  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 certificates 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.
    [SMIMEv3.2] set the lower limit on suggested key sizes for
    creating and validation at 1024 bits.  [SMIMEv3.1] set the lower
    limit at 768 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 level of any signature that is created with these hash
    algorithms should also be considered as suspect.
 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].

Schaad, et al. Standards Track [Page 26] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

  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 support was dropped in [SMIMEv4]. DSA was

dropped as part of a general movement from finite fields to

    elliptic curves.  Issues related to dealing with non-deterministic
    generation of the parameter 'k' have come up (see [RFC6979]).
 For 512-bit RSA with SHA-1, see [RFC3279] and [FIPS186-2] without
 Change Notice 1; for 512-bit RSA with SHA-256, see [RFC4055] and
 [FIPS186-2] without 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 [RFC3279] and [FIPS186-2] without
 Change Notice 1; for 512-bit DSA with SHA-256, see [RFC5758] and
 [FIPS186-2] without Change Notice 1; for 1024-bit DSA with SHA-1, see
 [RFC3279] and [FIPS186-2] with Change Notice 1; and for 1024-bit
 through 3072-bit DSA with SHA-256, see [RFC5758] and [FIPS186-3].
 The first reference provides the signature algorithm's OID, and the
 second provides the signature algorithm's definition.

Appendix B. Moving S/MIME v2 Certificate Handling to Historic Status

 The S/MIME v3 [SMIMEv3], v3.1 [SMIMEv3.1], v3.2 [SMIMEv3.2], and v4.0
 (this document) specifications are backward compatible with the
 S/MIME v2 Certificate Handling Specification [SMIMEv2], with the
 exception of the algorithms (dropped RC2/40 requirement, and added
 DSA and RSASSA-PSS requirements).  Therefore, RFC 2312 [SMIMEv2] was
 moved to Historic status.

Schaad, et al. Standards Track [Page 27] RFC 8550 S/MIME 4.0 Certificate Handling April 2019

Acknowledgements

 Many thanks go out to the other authors of the S/MIME v2 Certificate
 Handling RFC: Steve Dusse, Paul Hoffman, and Jeff Weinstein.  Without
 v2, there wouldn't be a v3, v3.1, v3.2, or v4.0.
 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.
 Bill Flanigan, Trevor Freeman, Elliott Ginsburg, Alfred Hoenes, Paul
 Hoffman, Russ Housley, David P. Kemp, Michael Myers, John Pawling,
 and Denis Pinkas.
 The version 4 update to the S/MIME documents was done under the
 auspices of the LAMPS Working Group.

Authors' Addresses

 Jim Schaad
 August Cellars
 Email: ietf@augustcellars.com
 Blake Ramsdell
 Brute Squad Labs, Inc.
 Email: blaker@gmail.com
 Sean Turner
 sn3rd
 Email: sean@sn3rd.com

Schaad, et al. Standards Track [Page 28]

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