Internet-Draft D.W.Chadwick PKIX WG University of Salford Intended Category: Standards Track Expires: 20 February 2000 20 August 1999 Internet X.509 Public Key Infrastructure Operational Protocols - LDAPv3 STATUS OF THIS MEMO This document is an Internet-Draft and is in full conformance with all the provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft expires on 20 February 2000. Comments and suggestions on this document are encouraged. Comments on this document should be sent to the PKIX working group discussion list: or directly to the author. ABSTRACT This document describes the features of the Lightweight Directory Access Protocol v3 that are needed in order to support a public key infrastructure based on X.509 certificates and CRLs. 1. Introduction RFC 2559 [1] specifies the subset of LDAPv2 [2] that is necessary to retrieve X.509 [9] certificates and CRLs from LDAP servers. However LDAPv2 has a number of deficiencies that may limit its usefulness in certain circumstances. The most notable of these are: - LDAPv2 distinguished names must be composed from the IA5 character set and cannot contain accented or non-latin characters, - LDAPv2 only has a limited number of supported authentication schemes for binding to the server, in particular the use of hashed passwords or TLS [3] are not supported, - LDAPv2 only supports a single directory server. It is the responsibility of the user to pre-configure his client with the required set of LDAP servers, and to choose the correct one for each certificate and CRL retrieval. It is for these reasons (and others not listed here) that the IETF have stopped the standardisation of the LDAPv2 protocol and have replaced it with the LDAPv3 protocol [4]. However the LDAPv3 protocol is much more complex than the LDAPv2 protocol and many of its features are not essential for simple PKIX use. This document describes the features of LDAPv3 that are essential, or not required, or are optional for servers to support a PKI based on X.509. 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 RFC 2119 [5]. 2. Features Of Ldapv3 That MUST Be Supported Attribute descriptions are a superset of attribute type definitions. They allow attribute subtyping to be specified in the LDAPv3 protocol. The ;binary option is an exception to this. This option allows certificates and CRLs to be asked for and returned as binary values encoded using the Basic Encoding Rules [11]. The mechanism described in RFC2559 (PKIX LDAPv2) [1] is fully compliant with the ;binary option of LDAPv3. The ;binary option of attribute descriptions MUST be supported by all implementations. When a client adds, deletes, retrieves or modifies values that are defined to have the binary syntax defined in RFC 2252 [14], the attribute type name MUST always be specified with the ;binary attribute option as described in RFC 2256 [13]. When the server returns such an attribute in a search result, the attribute type name ?SHOULD/MUST? include the ;binary option. Other attribute description options SHOULD NOT be generated by clients. Servers MAY choose to support them at their discretion. Other parameters of the Search operation for "read" or "search" type queries will usually be set as specified in RFC 2559. UTF8 encoding [12] allows the full ISO 10646 character set to be used in the creation of distinguished names. UTF8 encoding of distinguished names MUST be supported as specified in RFC2253 [6]. Multiple attribute value assertions (AVAs) within RDN components of distinguished names MUST be supported and the ordering of the AVAs is non-deterministic. For example cn=John + uid=123 is the same as uid=123 + cn=John. LDAPv3 has the concept of unsolicited notifications that can be sent from the server to the client. This is used to indicate when the server is going down, so that a client can distinguish between a server failure and a network failure. A client MUST be prepared to accept unsolicited notifications defined in RFC 2251 [4]. The altServer attribute is used by servers to point to alternative servers that may be contacted if this server is temporarily unavailable. This attribute MUST be stored in the root DSE of the server and MUST be available to clients for retrieval. (The access controls on this attribute MUST be the same or less than those on certificates and revocation lists.) If no alternative servers exist this attribute MUST point to the current server. Clients MAY make use of this feature but do not need to. Servers MAY store any other operational attributes in the root DSE, but do not need to, except where mandated in this profile. If the Certification Practice Statement (CPS) allows unauthenticated anonymous access to the server, then the server MUST allow a client to perform a Search operation (for a "read" or "search" type request) without issuing a prior Bind operation. The server MUST also allow the client to present a Bind request with the simple authentication choice and a zero-length OCTET STRING. 3. Features Of Ldapv3 That SHOULD Be Supported In a distributed directory with multiple servers, LDAPv3 supports referrals as the mechanism to allow one server that cannot fulfil a client's request, to refer the client to another server that might be better able to fulfil the request. Servers SHOULD be able to return referrals to clients. Clients SHOULD be able to receive referrals, although they are not required to automatically process them and support multiple asynchronous outgoing connections. As a minimum, clients SHOULD be able to ask the user if the referrals are to be cached locally and added to the set of servers known to the client. Partial Search results are returned when a server only has a subset of the certificates requested by the client. Referrals to other servers are embedded in the SearchResultReference field. Clients and servers SHOULD be able to handle SearchResultReferences in the same way as they handle referrals. However, the returned referrals SHOULD NOT specify new search filters, attributes to be returned or user credentials. Servers SHOULD only return the hostport, and DN components and MAY return the scope component. 4. Features Of Ldapv3 that are Not Used by this Profile A client following this profile need not send the ModifyDN, Compare and Abandon operations. The server MAY choose to support these operations at its discretion. (Note that a client wishing to abnormally terminate a search request may, instead of issuing an Abandon operation, close the TCP/IP connection.) Editors note. Some people have wondered if we should make the Abandon operation optional for the client and mandatory for the server. Comments on this issue are requested. The LDAPv3 protocol is infinitely extensible via two mechanisms: extended operations and controls on existing operations. The client does not need to generate any LDAPv3 protocol extensions (extended operations or controls), unless flexible searching for certificates is supported (see below). The server SHOULD NOT return any LDAPv3 protocol extensions (extended operations or controls) apart from supported controls which were marked as critical by the user. Operational attributes are attributes stored by the server that hold administrative information. Clients following this profile do not need any operational attributes from the server, other than the altServer and subschemaSubentry attributes of the root DSE. The server need not store any operational attributes other than these in the root DSE. 5. Features Of Ldapv3 That MAY Be Supported The default behaviour for LDAPv2 and LDAPv3 servers is that a user must retrieve all the attribute values from an attribute, or none of them (subject of course to having access rights to the values). If the user of the LDAPv3 server wishes to retrieve a limited number of userCertificates from a user's entry, specifically those that match certain criteria, then this MAY be achieved by using the LDAPv3 matchedValuesOnly control [15] and the certificateMatch matching rule. Servers that support flexible matching via the certificateMatch matching rule SHOULD support the matchedValuesOnly control. If the CPS allows weak password based authentication for "read" or "search" access to the server, the client and the server SHOULD support the DIGEST-MD5 mechanism [7] as specified in [8], and MAY support a simple password Bind sequence following the negotiation of a TLS ciphersuite to provide connection confidentiality, as specified in [10]. If the CPS requires strong authentication for access to the server then the client and the server SHOULD support certificate based authentication as specified in [10]. 6. Schema Issues RFC2587 [16] describes some of the subschema applicable to LDAPv2 servers, specifically the public key certificate related attribute types and object classes that MUST or MAY be supported. RFC2587 is equally applicable to LDAPv3 servers and MUST be supported by them. RFC2587 does not describe in detail the matching rules that should be supported by LDAP servers, nor does it describe how attribute value assertions for each matching rule should be encoded in filter items (in fact these AVA encodings are not currently described anywhere). Finally RFC2587 does not mention attributeCertificates, since these are a relatively new development. LDAPv3 allows the subschema supported by a server to be published in a subschema subentry. Clients following this profile which support the Search operation containing an extensible matching rule MAY use the subschemaSubentry attribute in the root DSE to find the subschemaSubentry, and MAY use the matchingRule and matchingRuleUse operational attributes in the subschema subentry in order to determine whether the server supports the various matching rules described below. Servers which support extensible matching SHOULD publish the matching rules they support in the matchingRule and matchingRuleUse operational attributes. X.509(1997) [9] supports both equality and flexible certificate matching rules by the server, via the certificateExactMatch and certificateMatch MATCHING-RULEs respectively. (For example, a client may flexibly search for certificates with a particular validity time, key usage, policy or other field.) LDAPv3 servers MUST support the certificateExactMatch matching rule. Clients MAY support certificateExactMatch values for equalityMatch filters. LDAPv3 servers MAY support the certificateMatch matching rule. If the server does support flexible matching (either via certificateMatch or some other matching rule), then the extensibleMatch filter of the Search request MUST be supported. Clients MAY support the extensibleMatch filter and one or more of the optional elements of certificateMatch. However, neither of the above matching rules are mentioned in the LDAPv3 standards [13 or 14], and only the equality matching rule is mentioned in [16], but nowhere is it defined for LDAP servers. It is expected that future revisions of the LDAPv3 documents will include these definitions, which are tentatively proposed below. ( 2.5.13.34 NAME 'certificateExactMatch' SYNTAX 1.3.6.1.4.1.1466.115.121.1.54 ) The syntax definition is: ( 1.3.6.1.4.1.1466.115.121.1.54 DESC 'Certificate Serial Number and Issuer' ) Values in this syntax are encoded as an integer, a dollar ($) separator and a string encoding of the distinguished name of the issuing CA. The string description of the certificateMatch matching rule is proposed here as: ( 2.5.13.35 NAME 'certificateMatch' SYNTAX 1.3.6.1.4.1.1466.115.121.1.55 ) The syntax definition is: ( 1.3.6.1.4.1.1466.115.121.1.55 DESC 'Certificate Assertion' ) The ASN.1 for certificateAssertion is defined in 12.7.2 of [9]. The LDAP string encoding of this is t.b.d. (but it will be similar to userCertificate syntax defined in RFC 1778). X.509(1997) [9] supports both equality and flexible matching rules of CRLs by the server, via the certificateListExactMatch and certificateListMatch MATCHING-RULEs respectively. LDAPv3 servers MUST (or should it be MAY?) support the certificateListExactMatch matching rule. Clients MAY support certificateListExactMatch values for equalityMatch filters. LDAPv3 servers MAY support the certificateListMatch matching rule. If the server does support flexible matching (either via certificateListMatch or some other matching rule), then the extensibleMatch filter of the Search request MUST be supported. Clients MAY support the extensibleMatch filter and one or more of the optional elements of certificateListMatch. Neither of the above matching rules are mentioned in the LDAPv3 standards [13 or 14], and only the certificateListExactMatch matching rule is mentioned in [16], but nowhere is it defined for LDAP servers. It is expected that future revisions of the LDAPv3 documents will include these definitions, which are tentatively proposed below. ( 2.5.13.38 NAME 'certificateListExactMatch' SYNTAX 1.3.6.1.4.1.1466.115.121.1.56 ) The syntax definition is: ( 1.3.6.1.4.1.1466.115.121.1.56 DESC 'Issuer name, time and distribution point name' ) Values in this syntax are encoded as a string encoding of a distinguished name, a dollar ($) separator, a string representation of generalised time, a dollar separator and an optional string encoding of the distinguished name of the distribution point. (Note that the latter DN encoding for a distribution point name is a subset of the allowed variants, which can be a generalName or an RDN. Should this simplification be allowed?) The string description of the certificateListMatch matching rule is proposed here as: ( 2.5.13.39 NAME 'certificateListMatch' SYNTAX 1.3.6.1.4.1.1466.115.121.1.57 ) The syntax definition is: ( 1.3.6.1.4.1.1466.115.121.1.57 DESC 'Certificate List Assertion' ) The ASN.1 for certificateListAssertion is defined in 12.7.6 of [9]. The LDAP string encoding of this is t.b.d. Editors Notes. 1. We need to decide whether searching for cross certificates should be supported by this LDAPv3 profile or not. If we decide that this should be supported, then we will need to define the matching rules to be supported and the string encodings for the assertion syntaxes (in fact this is not too difficult since they are similar to certificate matching rules and AVAs). 2. We need to decide if userSMIMECertificates should also be supported as part of this profile or not. LDAPv3 servers MAY store attributeCertificates, and clients MAY request then to be returned by adding them to the Search Request AttributeDescriptionList (either explicitly or implicity via requesting all attributes). LDAPv3 servers MAY support searching for entries containing specific attribute certificates, via either the attributeCertificateExactMatch or attributeCertificateMatch matching rules. For the convenience of the reader, some of the subchema definitions to support attribute certificates are produced below, but it is anticipated that these will be moved to a subsequent revision of the LDAPv3 standard. attributeCertificate ATTRIBUTE ::= { WITH SYNTAX AttributeCertificate EQUALITY MATCHING RULE attributeCertificateExactMatch ID joint-iso-ccitt(2) ds(5) attributeType(4) attributeCertificate(58) } pmiUser OBJECT-CLASS ::= { -- a PMI user (i.e., a "claimant") SUBCLASS OF {top} KIND auxiliary MAY CONTAIN {attributeCertificate} ID joint-iso-ccitt(2) ds(5) objectClass(6) pmiUser (24)} attributeCertificateExactMatch MATCHING-RULE ::= { SYNTAX AttributeCertificateExactAssertion ID joint-iso-ccitt(2) ds(5) mr (13) attributeCertificateExactMatch (45) } AttributeCertificateExactAssertion ::= SEQUENCE { serialNumber CertificateSerialNumber, issuer IssuerSerial } attributeCertificateMatch MATCHING-RULE ::= { SYNTAX AttributeCertificateAssertion ID joint-iso-ccitt(2) ds(5) mr (13) attributeCertificateMatch (??) } AttributeCertificateAssertion ::= SEQUENCE { subject [0] CHOICE { baseCertificateID [0] IssuerSerial, subjectName [1] GeneralNames} OPTIONAL, issuer [1] GeneralNames OPTIONAL, attCertValidity [2] GeneralizedTime OPTIONAL, attType [3] SET OF AttributeType OPTIONAL} --At least one component of the sequence must be present The LDAP definitions and syntaxes for the above matching rules are suggested to be: ( 2.5.13.45 NAME 'attributeCertificateExactMatch' SYNTAX 1.3.6.1.4.1.1466.115.121.1.58 ) The syntax definition is: ( 1.3.6.1.4.1.1466.115.121.1.58 DESC 'Attribute certificate serial number and public key issuer and serial number' ) Values in this syntax are encoded as a string encoding of an integer (the serial number of the attribute certificate), a dollar ($) separator, a string representation of the distinguished name of the CA of the issuer (note this is a subset of the allowed GeneralName), a dollar separator and a string encoding of an integer (the serial number of the issuer's public key certificate). The LDAP definition of the attributeCertificateMatch matching rule is proposed here as: ( 2.5.13.?? NAME 'attributeCertificateMatch' SYNTAX 1.3.6.1.4.1.1466.115.121.1.59 ) The syntax definition is: ( 1.3.6.1.4.1.1466.115.121.1.59 DESC 'Attribute Certificate Assertion' ) The LDAP string encoding of this is t.b.d. 7. Security Considerations The PKI information to be retrieved from LDAPv3 servers (certificates and CRLs) is digitally signed and therefore additional integrity services are NOT REQUIRED. The CPS will specify whether the information should be publicly available or not. If publicly available, privacy services will NOT be REQUIRED for retrieval requests. If not publicly available, privacy services MAY be REQUIRED and these can be provided by a TLS ciphersuite as specified in clause 5. For update of the information by CAs either strong authentication or weaker password based authentication MUST be supported as specified in clause 5. Additional access controls SHOULD be provided. Organizations are NOT REQUIRED to provide external CAs or users with access to their directories. 7 Copyright Copyright (C) The Internet Society (date). 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. 8. References [1] S.Boeyen, T. Howes, P. Richard "Internet X.509 Public Key Infrastructure Operational Protocols - LDAPv2", RFC 2559, April 1999 [2] Yeong, W., Howes, T., and Kille, S. "Lightweight Directory Access Protocol", RFC 1777, March 1995. [3] T. Dierks, C. Allen. "The TLS Protocol Version 1.0", RFC 2246, January 1999. [4] M. Wahl, T. Howes, S. Kille, "Lightweight Directory Access Protocol (v3)", Dec. 1997, RFC 2251 [5] S.Bradner. "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [6] M. Wahl, S. Kille, T. Howes. "Lightweight Directory Access Protocol (v3): UTF-8 String Representation of Distinguished Names", RFC2253, December 1997. [7] R. Rivest, "The MD5 Message-Digest Algorithm", RFC 1321, April 1992 [8] P. Leach, C. Newman, "Using Digest Authentication as a SASL Mechanism", INTERNET DRAFT , November 1998. [9] ITU-T Rec. X.509(97) The Directory: Authentication Framework [10] M. Wahl, H. Alvestrand, J. Hodges, RL "Bob" Morgan. "Authentication Methods for LDAP" , November 1998 [11] ITU-T Rec. X.690, "Specification of ASN.1 encoding rules: Basic, Canonical, and Distinguished Encoding Rules", 1994. [12] F. Yergeau. "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998. [13] M.Wahl. "A Summary of the X.500(96) User Schema for use with LDAPv3" RFC 2256, Dec 1997 [14] M. Wahl, A. Coulbeck, T. Howes, S. Kille, "Lightweight Directory Access Protocol (v3): Attribute Syntax Definitions", RFC 2252, Dec 1997 [15] D.Chadwick, "Returning Matched Values with LDAPv3", Internet Draft , Aug 1999 [16] S.Boeyen, T. Howes, P. Richard "Internet X.509 Public Key Infrastructure, LDAPv2 Schema", RFC 2587, June 1999 10 Authors Address David Chadwick IS Institute University of Salford Salford England M5 4WT Email: d.w.chadwick@salford.ac.uk Internet-Draft PKIX Operational Protocols - LDAPv3 20 August 1999 4