    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.

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    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

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    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].

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       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.

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       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.

       -  S/MIME version 2 is described in RFC 2311 through RFC 2315
          inclusive [SMIMEv2].

       -  S/MIME version 3 is described in RFC 2630 through RFC 2634
          inclusive and RFC 5035 [SMIMEv3].

       -  S/MIME version 3.1 is described in RFC 2634, RFC 3850, RFC 3851,
          RFC 3852, and RFC 5035 [SMIMEv3.1].

       -  S/MIME version 3.2 is described in RFC 2634, RFC 5035, RFC 5652,
          RFC 5750, and RFC 5751 [SMIMEv3.2].

       -  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.

       -  Version 1 and version 2 CRLs MUST be supported.

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    RFC 8550             S/MIME 4.0 Certificate Handling          April 2019

       -  Multiple certification authority (CA) certificates with the same
          subject and public key, but with overlapping validity periods,
          MUST be supported.

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

       -  The use of the MD2 digest algorithm for certificate signatures is
          discouraged, and security language was added.

       -  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.

       -  Receiving agents SHOULD display certificate information when
          displaying the results of signature verification.

       -  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.

       -  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].

       -  Moved "Conventions Used in This Document" to Section 1.2.  Added
          definitions for SHOULD+, SHOULD-, and MUST-.

       -  Section 1.1: Updated ASN.1 definition and reference.

       -  Section 1.3: Added text about v3.1 RFCs.

       -  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.

       -  Section 4.2: Added text to indicate how S/MIME agents locate the
          correct user certificate.

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       -  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.

       -  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.

       -  Section 5: Updated security considerations.

       -  Section 6: Moved references from Appendix A of RFC 3850 to this
          section.  Updated the references.

       -  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.

       -  Section 3: Support for internationalized email addresses is
          required.

       -  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.

       -  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].

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    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.

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    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].

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    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.

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       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.

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       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.

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       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:

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

       -  MUST support EdDSA with curve25519 using PureEdDSA mode.

       -  MUST- support RSA PKCS #1 v1.5 with SHA-256.

       -  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.

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       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.

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       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

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       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]

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    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:

       -  no Internet mail addresses in a certificate match the sender of a
          message, if the certificate contains at least one mail address

       -  no certificate chain leads to a trusted CA

       -  no ability to check the CRL for a certificate is implemented

       -  an invalid CRL was received

       -  the CRL being checked is expired

       -  the certificate is expired

       -  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]

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       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:

       -  The time the message was received is stored in a secure manner and
          is used at a later time to validate the message.

       -  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]

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       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]

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       [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]

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       [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]

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       [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]

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       [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]

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    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:

       -  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.

       -  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.

       -  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:

       -  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].

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       -  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.

       -  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.

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    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

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