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Network Working Group                               B. Ramsdell, EditorRequest for Comments: 2632                                    WorldtalkCategory: Standards Track                                     June 1999S/MIME Version 3 Certificate HandlingStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1999).  All Rights Reserved.1. Overview   S/MIME (Secure/Multipurpose Internet Mail Extensions), described in   [SMIME-MSG], 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 [KEYM]. S/MIME agents MUST meet the certificate   processing requirements documented in this document in addition to   those stated in [KEYM].   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.1.1 Definitions   For the purposes of this memo, the following definitions apply.   ASN.1: Abstract Syntax Notation One, as defined in ITU-T X.680-689.   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 [X.509]Ramsdell                    Standards Track                     [Page 1]

RFC 2632         S/MIME Version 3 Certificate Handling         June 1999   BER: Basic Encoding Rules for ASN.1, as defined in ITU-T X.690.   Certificate: A type that binds an entity's distinguished name to a   public key with a digital signature. This type is defined in the   Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL   Profile [KEYM]. 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 prematurely 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   [KEYM]. The CRL is signed by the issuer. The type intended by this   specification is the one defined in [KEYM].   DER: Distinguished Encoding Rules for ASN.1, as defined in ITU-T   X.690.   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 Compatibility with Prior Practice of S/MIME   S/MIME version 3 agents should attempt to have the greatest   interoperability possible with S/MIME version 2 agents. S/MIME   version 2 is described inRFC 2311 throughRFC 2315, inclusive.RFC2311 also has historical information about the development of S/MIME.1.3 Terminology   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [MUSTSHOULD].Ramsdell                    Standards Track                     [Page 2]

RFC 2632         S/MIME Version 3 Certificate Handling         June 19992. 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 [SMIME-MSG].2.1 CertificateRevocationLists   Receiving agents MUST support the Certificate Revocation List (CRL)   format defined in [KEYM]. If sending agents include CRLs in outgoing   messages, the CRL format defined in [KEYM] MUST be used.   All agents MUST be capable of performing revocation checks using CRLs   as specified in [KEYM]. All agents MUST perform revocation status   checking in accordance with [KEYM]. Receiving agents MUST recognize   CRLs in received S/MIME messages.   Agents SHOULD store CRLs received in messages for use in processing   later messages.   Agents MUST handle multiple valid Certificate Authority (CA)   certificates containing the same subject name and the same public   keys but with overlapping validity intervals.2.2 CertificateChoices   Receiving agents MUST support PKIX v1 and PKIX v3 certificates. See   [KEYM] for details about the profile for certificate formats. End   entity certificates MAY include an Internet mail address, as   described insection 3.1.   Receiving agents SHOULD support X.509 attribute certificates.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 and   X.509 Attribute Certificates. 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.   Thus, sending and receiving agents MUST NOT use PKCS #6 extended   certificates.Ramsdell                    Standards Track                     [Page 3]

RFC 2632         S/MIME Version 3 Certificate Handling         June 19992.3 CertificateSet   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 has 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 using a database or directory   lookup scheme.   A sending agent SHOULD include at least one chain of certificates up   to, but not including, a Certificate Authority (CA) that it believes   that the recipient may trust as authoritative. A receiving agent   SHOULD be able to handle an arbitrarily large number of certificates   and chains.   Agents MAY send CA certificates, that is, certificates that are   self-signed and can be considered the "root" of other chains. 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 in   the case of DSA certificates the parameters may be located in the   root certificate.  This would require that the recipient possess the   root certificate in order 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 but are not currently recommended.Ramsdell                    Standards Track                     [Page 4]

RFC 2632         S/MIME Version 3 Certificate Handling         June 1999   Receiving agents SHOULD support the decoding of X.509 attribute   certificates included in CMS objects. All other issues regarding the   generation and use of X.509 attribute certificates are outside of the   scope of this specification.3. Using Distinguished Names for Internet Mail   End-entity certificates MAY contain an Internet mail address as   described in [RFC-822]. The address must be an "addr-spec" as defined   inSection 6.1 of that specification.  The email address SHOULD be in   the subjectAltName extension, and SHOULD NOT be in the subject   distinguished name.   Receiving agents MUST recognize email addresses in the subjectAltName   field. Receiving agents MUST recognize email addresses in the   Distinguished Name field in the PKCS #9 emailAddress attribute.   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, which may be to   display a message that shows the recipient the addresses in the   certificate or other certificate details.   All subject and issuer names MUST be populated (i.e. not an empty   SEQUENCE) in S/MIME-compliant PKIX certificates, except that the   subject DN in a user's (i.e. end-entity) 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   A receiving agent needs 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 PKIX Working Group is   investigating other mechanisms such as directory servers. Another   method under consideration by the IETF is to provide certificate   retrieval services as part of the existing Domain Name System (DNS).   Until such mechanisms are widely used, their utility may be limited   by the small number of correspondent's certificates that can beRamsdell                    Standards Track                     [Page 5]

RFC 2632         S/MIME Version 3 Certificate Handling         June 1999   retrieved. At a minimum, for initial S/MIME deployment, a user agent   could automatically generate a message to an intended recipient   requesting that recipient's certificate in a signed return message.   Receiving and sending agents SHOULD also provide a mechanism to allow   a user to "store and protect" certificates for correspondents in such   a way so 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 similar way as a "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 may combine two or more   mechanisms to allow the greatest flexibility and utility to the user.   For instance, a secure Internet mail agent may resort to checking a   centralized certificate retrieval mechanism for a certificate if it   can not 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 [SMIME-   MSG].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 away from incoming messages.   A receiving agent SHOULD have access to some certificate-revocation   list (CRL) retrieval mechanism in order to gain access to   certificate-revocation information when validating certificate   chains.  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 so 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   certificate chain 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 theRamsdell                    Standards Track                     [Page 6]

RFC 2632         S/MIME Version 3 Certificate Handling         June 1999   CRL information in a particular context is beyond the scope of this   memo but may be governed by the policies associated with particular   certificate hierarchies.   All agents MUST be capable of performing revocation checks using CRLs   as specified in [KEYM]. All agents MUST perform revocation status   checking in accordance with [KEYM]. Receiving agents MUST recognize   CRLs in received S/MIME messages.4.2 Certificate Chain Validation   In creating a user agent for secure messaging, certificate, CRL, and   certificate chain validation SHOULD be highly automated while still   acting in the best interests of the user. Certificate, CRL, and chain   validation MUST be performed as per [KEYM] 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 (ex: RSA); or forming a pairwise   symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a   content-encryption key.   Certificates and CRLs are made available to the chain 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 chain 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 chain validation on incoming signed   messages.4.3 Certificate and CRL Signing Algorithms   Certificates and Certificate-Revocation Lists (CRLs) are signed by   the certificate issuer. A receiving agent MUST be capable of   verifying the signatures on certificates and CRLs made with id-dsa-   with-sha1 [DSS].   A receiving agent SHOULD be capable of verifying the signatures on   certificates and CRLs made with md2WithRSAEncryption,   md5WithRSAEncryption and sha-1WithRSAEncryption signature algorithms   with key sizes from 512 bits to 2048 bits described in [PKCS#1V2].Ramsdell                    Standards Track                     [Page 7]

RFC 2632         S/MIME Version 3 Certificate Handling         June 19994.4 PKIX Certificate Extensions   PKIX describes an extensible framework in which the basic certificate   information can be extended and how such extensions can be used to   control the process of issuing and validating certificates. The PKIX   Working Group has ongoing efforts to identify and create extensions   which have value in particular certification environments. Further,   there are active efforts underway to issue PKIX certificates for   business purposes. This document identifies the minumum required set   of certificate extensions which have the greatest value in the S/MIME   environment. The syntax and semantics of all the identified   extensions are defined in [KEYM].   Sending and receiving agents MUST correctly handle the Basic   Constraints Certificate Extension, the Key Usage Certificate   Extension, authorityKeyID, subjectKeyID, and the subjectAltNames when   they appear in end-user certificates. Some mechanism SHOULD exist to   handle the defined certificate extensions when they appear in   intermediate 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 [KEYM],   unless otherwise specified here.4.4.1 Basic Constraints Certificate Extension   The basic constraints extension serves to delimit the role and   position of an issuing authority or end-entity certificate plays in a   chain of certificates.   For example, certificates issued to CAs and subordinate CAs contain a   basic constraint extension that identifies them as issuing authority   certificates. End-entity certificates contain an extension that   constrains the certificate from being an issuing authority   certificate.   Certificates SHOULD contain a basicConstraints extension in CA   certificates, and SHOULD NOT contain that extension in end entity   certificates.Ramsdell                    Standards Track                     [Page 8]

RFC 2632         S/MIME Version 3 Certificate Handling         June 19994.4.2 Key Usage Certificate 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, certificate revocation   lists and other data.   For example, a certification authority may create subordinate issuer   certificates which contain a keyUsage extension which specifies that   the corresponding public key can be used to sign end user certs and   sign CRLs.   If a key usage extension is included in a PKIX certificate, then it   MUST be marked as critical.4.4.2.1 Key Usage in Diffie-Hellman Key Exchange Certificates   For Diffie-Hellman key exchange certificates (certificates in which   the subject public key algorithm is dhpublicnumber), if the keyUsage   keyAgreement bit is set to 1 AND if the public key is to be used to   form a pairwise key to decrypt data, then the S/MIME agent MUST only   use the public key if the keyUsage encipherOnly bit is set to 0. If   the keyUsage keyAgreement bit is set to 1 AND if the key is to be   used to form a pairwise key to encrypt data, then the S/MIME agent   MUST only use the public key if the keyUsage decipherOnly bit is set   to 0.4.4.3 Subject Alternative Name Extension   The subject alternative name extension is used in S/MIME as the   preferred means to convey theRFC-822 email address(es) that   correspond to the entity for this certificate. AnyRFC-822 email   addresses present MUST be encoded using the rfc822Name CHOICE of the   GeneralName type. Since the SubjectAltName type is a SEQUENCE OF   GeneralName, multipleRFC-822 email addresses MAY be present.5. Security Considerations   All of the security issues faced by any cryptographic application   must be faced by a 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.Ramsdell                    Standards Track                     [Page 9]

RFC 2632         S/MIME Version 3 Certificate Handling         June 1999   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 other program, there is no single way   to handle such failures. Just because the methods to handle the   failures has 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 noticable steps to inform the end   user about it.   Some of the many places where signature and certificate checking   might fail include:   - no Internet mail addresses in a certificate match the sender     of a message   - no certificate chain leads to a trusted CA   - no ability to check the CRL for a certificate   - 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 to decide what to do if the check   fails.Ramsdell                    Standards Track                    [Page 10]

RFC 2632         S/MIME Version 3 Certificate Handling         June 1999A. References   [CERTV2]     Dusse, S., Hoffman, P. and B. Ramsdell,"S/MIME Version 2                Certificate Handling",RFC 2312, March 1998.   [CMS]        Housley, R., "Cryptographic Message Syntax",RFC 2630,                June 1999.   [DSS]        NIST FIPS PUB 186, "Digital Signature Standard", 18 May                1994.   [KEYM]       Housley, R., Ford, W., Polk, W. and D. Solo, "Internet                X.509 Public Key Infrastructure Certificate and CRL                Profile",RFC 2459, January 1999.   [MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate                Requirement Levels",BCP 14,RFC 2119, March 1997.   [PKCS#1V2]   Kaliski, B., "PKCS #1: RSA Cryptography Specifications                Version 2.0",RFC 2437, October 1998.   [RFC-822]    Crocker, D., "Standard For The Format Of ARPA Internet                Text Messages", STD 11,RFC 822, August 1982.   [SMIME-MSG]  Ramsdell, B., Editor, "S/MIME Version 3 Message                Specification",RFC 2633, June 1999.   [X.500]      ITU-T Recommendation X.500 (1997) | ISO/IEC 9594-1:1997,                Information technology - Open Systems Interconnection -                The Directory:  Overview of concepts, models and                services.   [X.501]      ITU-T Recommendation X.501 (1997) | ISO/IEC 9594-2:1997,                Information technology - Open Systems Interconnection -                The Directory:  Models.   [X.509]      ITU-T Recommendation X.509 (1997) | ISO/IEC 9594-8:1997,                Information technology - Open Systems Interconnection -                The Directory:  Authentication framework.   [X.520]      ITU-T Recommendation X.520 (1997) | ISO/IEC 9594-6:1997,                Information technology - Open Systems Interconnection -                The Directory:  Selected attribute types.Ramsdell                    Standards Track                    [Page 11]

RFC 2632         S/MIME Version 3 Certificate Handling         June 1999B. Acknowledgements   Many thanks go out to the other authors of the S/MIME v2 RFC:  Steve   Dusse, Paul Hoffman and Jeff Weinstein. Without v2, there wouldn't be   a v3.   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 due to the fact that they   made direct contributions to this document.   Bill Flanigan Elliott Ginsburg Paul Hoffman Russ Housley Michael   Myers John Pawling Denis Pinkas Jim SchaadEditor's Address   Blake Ramsdell   Worldtalk   17720 NE 65th St Ste 201   Redmond, WA 98052   Phone: +1 425 376 0225   EMail: blaker@deming.comRamsdell                    Standards Track                    [Page 12]

RFC 2632         S/MIME Version 3 Certificate Handling         June 1999Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Ramsdell                    Standards Track                    [Page 13]