Network Working Group B. Ramsdell, Editor Request for Comments: 2632 Worldtalk Category: Standards Track June 1999 S/MIME Version 3 Certificate Handling Status 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. 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. X.509]
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. RFC 2311 through RFC 2315, inclusive. RFC 2311 also has historical information about the development of S/MIME. MUSTSHOULD].
SMIME-MSG]. 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. KEYM] for details about the profile for certificate formats. End entity certificates MAY include an Internet mail address, as described in section 3.1. Receiving agents SHOULD support X.509 attribute certificates.
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. RFC-822]. The address must be an "addr-spec" as defined in Section 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.
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].
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. 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. 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].
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.
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.
[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.
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