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Security Limited Additional Mechanisms for PKIX and SMIME 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. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Limited Additional Mechanisms for PKIX and SMIME Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction S/MIME (Secure/Multipurpose Internet Mail Extensions) v4.0, described in , 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 ("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 . This specification is compatible with the Cryptographic Message Syntax (CMS) 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 . The most significant changes revolve around changes in recommendations around the cryptographic algorithms used by the specification. More details can be found in . 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.

Definitions For the purposes of this document, the following definitions apply.

ASN.1:

Abstract Syntax Notation One, as defined in ITU-T 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 .

Certificate:

A type that binds an entity's name to a public key with a digital signature. This type is defined in . 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 . The CRL is signed by the issuer. The type intended by this specification is the one defined in .

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.

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 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.

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. The following reference conventions are used: [ESS] refers to and . [SMIMEv2] refers to , , , , and . [SMIMEv3] refers to , , , , , and . [SMIMEv3.1] refers to , , , , and . [SMIMEv3.2] refers to , , , , and . [SMIMEv4] refers to , , , , and .

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.

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.
• 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.

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 .

• 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.
• 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.

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 . 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.

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 .

Certificate Revocation Lists Receiving agents MUST support the CRL format defined in . If sending agents include CRLs in outgoing messages, the CRL format defined in 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 . All agents MUST perform revocation status checking in accordance with . Receiving agents MUST recognize CRLs in received S/MIME messages. Agents SHOULD store CRLs received in messages for use in processing later messages.

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

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 , 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.

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 .

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 . Internationalized email address names use the OID defined in 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 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. 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. 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.

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. 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 , 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. 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 . Agents MUST handle multiple valid CA certificates containing the same subject name and the same public keys but with overlapping validity intervals.

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 . All agents MUST perform revocation status checking in accordance with . Receiving agents MUST recognize CRLs in received S/MIME messages.

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 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. 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 for the CMS fields that reference the originator's and recipient's certificates.

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 . EdDSA is defined to generate this parameter deterministically. The following are the RSA and RSASSA-PSS key size requirements for
S/MIME receiving agents during certificate and CRL signature verification: 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 , and the signature algorithm definition is found in 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 , and the signature algorithm definition is found in . For RSASSA-PSS with SHA-256, see . For ECDSA, see and . 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 and . 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.

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 . 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. 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 , unless otherwise specified here.

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 ). As per , certificates MUST contain a basicConstraints extension in CA certificates and SHOULD NOT contain that extension in end-entity certificates.

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 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.

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 , 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 , Section 3. Since the SubjectAltName type is a SEQUENCE OF GeneralName, multiple email addresses MAY be present.

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 . 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.

IANA Considerations This document has no IANA actions.

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. 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 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 , 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 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 .

References Normative References National Institute of Standards and Technology (NIST) National Institute of Standards and Technology (NIST) This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 4.0. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 5751. This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] This document specifies conventions for X.509 certificate usage by Secure/Multipurpose Internet Mail Extensions (S/MIME) v3.2 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, the Internet X.509 Public Key Infrastructure Certificate and 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 3850. [STANDARDS-TRACK] This specification defines a profile for the use of X.509 Attribute Certificates in Internet Protocols. Attribute certificates may be used in a wide range of applications and environments covering a broad spectrum of interoperability goals and a broader spectrum of operational and assurance requirements. The goal of this document is to establish a common baseline for generic applications requiring broad interoperability as well as limited special purpose requirements. The profile places emphasis on attribute certificate support for Internet electronic mail, IPsec, and WWW security applications. This document obsoletes RFC 3281. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document describes four optional security service extensions for S/MIME. [STANDARDS-TRACK] In the original Enhanced Security Services for S/MIME document (RFC 2634), a structure for cryptographically linking the certificate to be used in validation with the signature was introduced; this structure was hardwired to use SHA-1. This document allows for the structure to have algorithm agility and defines a new attribute for this purpose. [STANDARDS-TRACK] This document defines a new name form for inclusion in the otherName field of an X.509 Subject Alternative Name and Issuer Alternative Name extension that allows a certificate subject to be associated with an internationalized email address. This document updates RFC 5280. This memo represents a republication of PKCS #9 v2.0 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document, except for the security considerations section, is taken directly from that specification. This memo provides information for the Internet community. This document defines a deterministic digital signature generation procedure. Such signatures are compatible with standard Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA) digital signatures and can be processed with unmodified verifiers, which need not be aware of the procedure described therein. Deterministic signatures retain the cryptographic security features associated with digital signatures but can be more easily implemented in various environments, since they do not need access to a source of high-quality randomness. This document supplements RFC 3279. It describes the conventions for using the RSA Probabilistic Signature Scheme (RSASSA-PSS) signature algorithm, the RSA Encryption Scheme - Optimal Asymmetric Encryption Padding (RSAES-OAEP) key transport algorithm and additional one-way hash functions with the Public-Key Cryptography Standards (PKCS) #1 version 1.5 signature algorithm in the Internet X.509 Public Key Infrastructure (PKI). Encoding formats, algorithm identifiers, and parameter formats are specified. [STANDARDS-TRACK] This memo represents a republication of PKCS #1 v2.1 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document is taken directly from the PKCS #1 v2.1 document, with certain corrections made during the publication process. This memo provides information for the Internet community. This document specifies the conventions for using the RSASSA-PSS (RSA Probabilistic Signature Scheme) digital signature algorithm with the Cryptographic Message Syntax (CMS). [STANDARDS-TRACK] This document updates RFC 3279 to specify algorithm identifiers and ASN.1 encoding rules for the Digital Signature Algorithm (DSA) and Elliptic Curve Digital Signature Algorithm (ECDSA) digital signatures when using SHA-224, SHA-256, SHA-384, or SHA-512 as the hashing algorithm. This specification applies to the Internet X.509 Public Key infrastructure (PKI) when digital signatures are used to sign certificates and certificate revocation lists (CRLs). This document also identifies all four SHA2 hash algorithms for use in the Internet X.509 PKI. [STANDARDS-TRACK] This document specifies algorithm identifiers and ASN.1 encoding formats for digital signatures and subject public keys used in the Internet X.509 Public Key Infrastructure (PKI). Digital signatures are used to sign certificates and certificate revocation list (CRLs). Certificates include the public key of the named subject. [STANDARDS-TRACK] Informative References RSA Laboratories National Institute of Standards and Technology n.d. This document describes a protocol for adding cryptographic signature and encryption services to MIME data. This memo provides information for the Internet community. It does not specify an Internet standard of any kind. This memo describes the mechanisms S/MIME uses to create and validate keys using certificates. This memo provides information for the Internet community. It does not specify an Internet standard of any kind. This document describes a method for encrypting data using the RSA public-key cryptosystem. This memo provides information for the Internet community. It does not specify an Internet standard of any kind. This document describes a syntax for certification requests. This memo provides information for the Internet community. It does not specify an Internet standard of any kind. This document describes a general syntax for data that may have cryptography applied to it, such as digital signatures and digital envelopes. This memo provides information for the Internet community. It does not specify an Internet standard of any kind. This document describes the Cryptographic Message Syntax. This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary messages. [STANDARDS-TRACK] This document standardizes one particular Diffie-Hellman variant, based on the ANSI X9.42 draft, developed by the ANSI X9F1 working group. [STANDARDS-TRACK] S/MIME (Secure/Multipurpose Internet Mail Extensions), provides a method to send and receive secure MIME messages. Before using a public key to provide security services, the S/MIME agent MUST certify that the public key is valid. S/MIME agents MUST use PKIX certificates to validate public keys as described in the Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL Profile. [STANDARDS-TRACK] This document describes a protocol for adding cryptographic signature and encryption services to MIME data. [STANDARDS-TRACK] This document specifies conventions for X.509 certificate usage by Secure/Multipurpose Internet Mail Extensions (S/MIME) 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 3280, the Internet X.509 Public Key Infrastructure Certificate and CRL Profile. S/MIME agents must meet the certificate processing requirements in this document as well as those in RFC 3280. [STANDARDS-TRACK] This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 3.1. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 2633. [STANDARDS-TRACK] This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 3.2. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 3851. [STANDARDS-TRACK] 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. This document specifies algorithm identifiers and ASN.1 encoding formats for elliptic curve constructs using the curve25519 and curve448 curves. The signature algorithms covered are Ed25519 and Ed448. The key agreement algorithms covered are X25519 and X448. The encoding for public key, private key, and Edwards-curve Digital Signature Algorithm (EdDSA) structures is provided. This document discusses how company security policy for data classification can be mapped to the S/MIME security label. Actual policies from three companies provide worked examples. This memo provides information for the Internet community. This document describes how to use secure DNS to associate an S/MIME user's certificate with the intended domain name, similar to the way that DNS-Based Authentication of Named Entities (DANE), RFC 6698, does for TLS. This note describes the fundamental algorithms of Elliptic Curve Cryptography (ECC) as they were defined in some seminal references from 1994 and earlier. These descriptions may be useful for implementing the fundamental algorithms without using any of the specialized methods that were developed in following years. Only elliptic curves defined over fields of characteristic greater than three are in scope; these curves are those used in Suite B. This document is not an Internet Standards Track specification; it is published for informational purposes. This document describes elliptic curve signature scheme Edwards-curve Digital Signature Algorithm (EdDSA). The algorithm is instantiated with recommended parameters for the edwards25519 and edwards448 curves. An example implementation and test vectors are provided. This document updates the security considerations for the MD5 message digest algorithm. It also updates the security considerations for HMAC-MD5. This document is not an Internet Standards Track specification; it is published for informational purposes. This document includes security considerations for the SHA-0 and SHA-1 message digest algorithm. This document is not an Internet Standards Track specification; it is published for informational purposes.

Historic Considerations

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 .
• 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 , 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 ).

For 512-bit RSA with SHA-1, see and without Change Notice 1; for 512-bit RSA with SHA-256, see and 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 and without Change Notice 1; for 512-bit DSA with SHA-256, see and without Change Notice 1; for 1024-bit DSA with SHA-1, see and with Change Notice 1; and for 1024-bit through 3072-bit DSA with SHA-256, see and . The first reference provides the signature algorithm's OID, and the second provides the signature algorithm's definition.

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.

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 updates to the S/MIME documents was done under the auspices of the LAMPS Working Group.

Contributors August Cellars