Network Working Group
Request for Comments: 2559
Updates: 1778
Category: Standards Track

S. Boeyen
Entrust
T. Howes
Netscape
P. Richard
Xcert
April 1999

Internet X.509 Public Key Infrastructure

Operational Protocols - LDAPv2

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

1. Abstract

The protocol described in this document is designed to satisfy some of the operational requirements within the Internet X.509 Public Key Infrastructure (IPKI). Specifically, this document addresses requirements to provide access to Public Key Infrastructure (PKI) repositories for the purposes of retrieving PKI information and managing that same information. The mechanism described in this document is based on the Lightweight Directory Access Protocol (LDAP) v2, defined in RFC 1777, defining a profile of that protocol for use within the IPKI and updates encodings for certificates and revocation lists from RFC 1778. Additional mechanisms addressing PKIX operational requirements are specified in separate documents.

The key words 'MUST', 'REQUIRED', 'SHOULD', 'RECOMMENDED', and 'MAY' in this document are to be interpreted as described in RFC 2119.

2. Introduction

This specification is part of a multi-part standard for development of a Public Key Infrastructure (PKI) for the Internet. This specification addresses requirements to provide retrieval of X.509 PKI information, including certificates and CRLs from a repository. This specification also addresses requirements to add, delete and modify PKI information in a repository. A profile based on the LDAP version 2 protocol is provided to satisfy these requirements.

3. Model

The PKI components, as defined in PKIX Part 1, which are involved in PKIX operational protocol interactions include:

      -  End Entities
      -  Certification Authorities (CA)
      -  Repository

End entities and CAs using LDAPv2, retrieve PKI information from the repository using a subset of the LDAPv2 protocol.

CAs populate the repository with PKI information using a subset of the LDAPv2 protocol.

4. Lightweight Directory Access Protocol (LDAP)

The following sections examine the retrieval of PKI information from a repository and management of PKI information in a repository. A profile of the LDAPv2 protocol is defined for providing these services.

Section 5 satisfies the requirement to retrieve PKI information (a certificate, CRL, or other information of interest) from an entry in the repository, where the retrieving entity (either an end entity or a CA) has knowledge of the name of the entry. This is termed "repository read".

Section 6 satisfies the same requirement as 5 for the situation where the name of the entry is not known, but some other related information which may optionally be used as a filter against candidate entries in the repository, is known. This is termed "repository search".

Section 7 satisfies the requirement of CAs to add, delete and modify PKI information information (a certificate, CRL, or other information of interest)in the repository. This is termed "repository modify".

The subset of LDAPv2 needed to support each of these functions is described below. Note that the repository search service is a superset of the repository read service in terms of the LDAPv2 functionality needed.

Note that all tags are implicit by default in the ASN.1 definitions that follow.

5. LDAP Repository Read

To retrieve information from an entry corresponding to the subject or issuer name of a certificate, requires a subset of the following three LDAP operations:

BindRequest (and BindResponse)
SearchRequest (and SearchResponse)
UnbindRequest

The subset of each REQUIRED operation is given below.

5.1. Bind

5.1.1. Bind Request

The full LDAP v2 Bind Request is defined in RFC 1777.

An application providing a LDAP repository read service MUST implement the following subset of this operation:

BindRequest ::=

[APPLICATION 0] SEQUENCE {

           version      INTEGER (2),
           name         LDAPDN, -- MUST accept NULL LDAPDN
           simpleauth [0] OCTET STRING  -- MUST accept NULL simple
           }

An application providing a LDAP repository read service MAY implement other aspects of the BindRequest as well.

Different services may have different security requirements. Some services may allow anonymous search, others may require authentication. Those services allowing anonymous search may choose only to allow search based on certain criteria and not others.

A LDAP repository read service SHOULD implement some level of anonymous search access. A LDAP repository read service MAY implement authenticated search access.

5.1.2. Bind Response

The full LDAPv2 BindResponse is described in RFC 1777.

An application providing a LDAP repository read service MUST implement this entire protocol element, though only the following error codes may be returned from a Bind operation:

       success                      (0),
       operationsError              (1),
       protocolError                (2),
       authMethodNotSupported       (7),
       noSuchObject                 (32),
       invalidDNSyntax              (34),
       inappropriateAuthentication  (48),
       invalidCredentials           (49),
       busy                         (51),
       unavailable                  (52),
       unwillingToPerform           (53),
       other                        (80)

5.2. Search

5.2.1. Search Request

The full LDAPv2 SearchRequest is defined in RFC 1777.

An application providing a LDAP repository read service MUST implement the following subset of the SearchRequest.

SearchRequest ::=

[APPLICATION 3] SEQUENCE {

           baseObject     LDAPDN,
           scope             ENUMERATED {
                             baseObject   (0),
                                        },
           derefAliases   ENUMERATED {
                             neverDerefAliases   (0),
                                     },
           sizeLimit      INTEGER (0),
           timeLimit      INTEGER (0),
           attrsOnly      BOOLEAN, -- FALSE only
           filter         Filter,
           attributes     SEQUENCE OF AttributeType
                               }

Filter ::=

CHOICE {

          present        [7] AttributeType, -- "objectclass" only
          
                 }

This subset of the LDAPv2 SearchRequest allows the LDAPv2 "read" operation: a base object search with a filter testing for the existence of the objectClass attribute.

An application providing a LDAP repository read service MAY implement other aspects of the SearchRequest as well.

5.2.2.

The full LDAPv2 SearchResponse is defined in RFC 1777.

An application providing a LDAP repository read service over LDAPv2 MUST implement the full SearchResponse.

Note that in the case of multivalued attributes such as userCertificate a SearchResponse containing this attribute will include all values, assuming the requester has sufficient access permissions. The application/relying party may need to select an appropriate value to be used. Also note that retrieval of a certificate from a named entry does not guarantee that the certificate will include that same Distinguished Name (DN) and in some cases the subject DN in the certificate may be NULL.

5.3. Unbind

The full LDAPv2 UnbindRequest is defined in RFC 1777.

An application providing a LDAP repository read service MUST implement the full UnbindRequest.

6. LDAP Repository Search

To search, using arbitrary criteria, for an entry in a repository containing a certificate, CRL, or other information of interest, requires a subset of the following three LDAP operations:

BindRequest (and BindResponse)
SearchRequest (and SearchResponse)
UnbindRequest

The subset of each operation REQUIRED is given below.

6.1. Bind

The BindRequest and BindResponse subsets needed are the same as those described in Section 5.1.

The full LDAP v2 Bind Request is defined in RFC 1777.

6.2. Search

6.2.1. Search Request

The full LDAPv2 SearchRequest is defined in RFC 1777.

An application providing a LDAP repository search service MUST implement the following subset of the SearchRequest protocol unit.

SearchRequest ::=

[APPLICATION 3] SEQUENCE {

           baseObject     LDAPDN,
           scope          ENUMERATED {
                               baseObject     (0),
                               singleLevel    (1),
                               wholeSubtree   (2)
                                     },
           derefAliases   ENUMERATED {
                               neverDerefAliases     (0),
                                     },
           sizeLimit      INTEGER (0 .. maxInt),
           timeLimit      INTEGER (0 .. maxInt),
           attrsOnly      BOOLEAN,  -- FALSE only
           filter         Filter,
           attributes     SEQUENCE OF AttributeType
                                }

All aspects of the SearchRequest MUST be supported, except for the following:

Only the neverDerefAliases value of derefAliases needs to be supported

Only the FALSE value for attrsOnly needs to be supported

This subset provides a more general search capability. It is a superset of the SearchRequest subset defined in Section 5.2.1. The elements added to this service are:

singleLevel and wholeSubtree scope needs to be supported

sizeLimit is included

timeLimit is included

Enhanced filter capability

An application providing a LDAP repository search service MAY implement other aspects of the SearchRequest as well.

6.2.2. Search Response

The full LDAPv2 SearchResponse is defined in RFC 1777.

An application providing a LDAP repository search service over LDAPv2 MUST implement the full SearchResponse.

6.3. Unbind

An application providing a LDAP repository search service MUST implement the full UnbindRequest.

7. LDAP Repository Modify

   To add, delete and modify PKI information in a repository  requires a
   subset of the following LDAP operations:

BindRequest (and BindResponse)
ModifyRequest (and ModifyResponse)
AddRequest (and AddResponse)
DelRequest (and DelResponse
UnbindRequest

The subset of each operation REQUIRED is given below.

7.1. Bind

The full LDAP v2 Bind Request is defined in RFC 1777.

An application providing a LDAP repository modify service MUST implement the following subset of this operation:

BindRequest ::=

[APPLICATION 0] SEQUENCE {

           version      INTEGER (2),
           name         LDAPDN,
           simpleauth [0] OCTET STRING
           }

A LDAP repository modify service MUST implement authenticated access.

The BindResponse subsets needed are the same as those described in Section 5.1.2.

7.2. Modify

7.2.1. Modify Request

The full LDAPv2 ModifyRequest is defined in RFC 1777.

An application providing a LDAP repository modify service MUST implement the following subset of the ModifyRequest protocol unit.

ModifyRequest ::=

        [APPLICATION 6] SEQUENCE {
       object         LDAPDN,
       modification   SEQUENCE OF SEQUENCE {
                        operation     ENUMERATED {
                                        add     (0),
                                        delete  (1)
                                      },
                        modification  SEQUENCE {
                                      type   AttributeType,
                                      values SET OF
                                             AttributeValue
                                      }
                      }
        }

All aspects of the ModifyRequest MUST be supported, except for the following:

Only the add and delete values of operation need to be supported

7.2.2. Modify Response

The full LDAPv2 ModifyResponse is defined in RFC 1777.

An application providing a LDAP repository modify service MUST implement the full ModifyResponse.

7.3. Add

7.3.1. Add Request

The full LDAPv2 AddRequest is defined in RFC 1777.

An application providing a LDAP repository modify service MUST implement the full AddRequest.

7.3.2. Add Response

The full LDAPv2 AddResponse is defined in RFC 1777.

An application providing a LDAP repository modify service MUST implement the full AddResponse.

7.4. Delete

7.4.1. Delete Request

The full LDAPv2 DelRequest is defined in RFC 1777.

An application providing a LDAP repository modify service MUST implement the full DelRequest.

7.4.2. Delete Response

The full LDAPv2 DelResponse is defined in RFC 1777.

An application providing a LDAP repository modify service MUST implement the full DelResponse.

7.5. Unbind

An application providing a LDAP repository modify service MUST implement the full UnbindRequest.

8. Non-standard attribute value encodings

When conveyed in LDAP requests and results, attributes defined in X.500 are to be encoded using string representations defined in RFC 1778, The String Representation of Standard Attribute Syntaxes. These string encodings were based on the attribute definitions from X.500(1988). Thus, the string representations of the PKI information elements are for version 1 certificates and version 1 revocation lists. Since this specification uses version 3 certificates and version 2 revocation lists, as defined in X.509(1997), the RFC 1778 string encoding of these attributes is inappropriate.

For this reason, these attributes MUST be encoded using a syntax similar to the syntax "Undefined" from section 2.1 of RFC 1778: values of these attributes are encoded as if they were values of type "OCTET STRING", with the string value of the encoding being the DER- encoding of the value itself. For example, when writing a userCertificate to the repository, the CA generates a DER-encoding of the certificate and uses that encoding as the value of the userCertificate attribute in the LDAP Modify request.This encoding style is consistent with the encoding scheme proposed for LDAPv3, which is now being defined within the IETF.

Note that certificates and revocation lists will be transferred using this mechanism rather than the string encodings in RFC 1778 and client systems which do not understand this encoding may experience problems with these attributes.

9. Transport

An application providing a LDAP repository read service, LDAP repository search service, or LDAP repository modify service MUST support LDAPv2 transport over TCP, as defined in Section 3.1 of RFC 1777.

An application providing a LDAP repository read service, LDAP repository search service, or LDAP repository modify service MAY support LDAPv2 transport over other reliable transports as well.

10. Security Considerations

Since the elements of information which are key to the PKI service (certificates and CRLs) are both digitally signed pieces of information, additional integrity service is NOT REQUIRED. As neither information element need be kept secret and anonymous access to such information, for retrieval purposes is generally acceptable, privacy service is NOT REQUIRED for information retrieval requests.

CAs have additional requirements, including modification of PKI information. Simple authentication alone is not sufficient for these purposes. It is RECOMMENDED that some stronger means of authentication and/or (if simple authentication is used) some means of protecting the privacy of the password is used, (e.g. accept modifications only via physically secure networks, use IPsec, use SSH or TLS or SSL tunnel). Without such authentication, it is possible that a denial-of-service attack could occur where the attacker replaces valid certificates with bogus ones.

For the LDAP repository modify service, profiled in section 7, there are some specific security considerations with respect to access control. These controls apply to a repository which is under the same management control as the CA. Organizations operating directories are NOT REQUIRED to provide external CAs access permission to their directories.

The CA MUST have access control permissions allowing it to:

For CA entries:

- add, modify and delete all PKI attributes for its own

directory entry;

- add, modify and delete all values of these attributes.

For CRL distribution point entries (if used):

         - create, modify and delete entries of object class
           cRLDistributionPoint immediately subordinate to its own
           entry;
         - add, modify and delete all attributes, and all values of
           these attributes for these entries.

For subscriber (end-entity) entries:

add, modify and delete the attribute userCertificate and all values of that attribute, issued by this CA to/from these entries.

The CA is the ONLY entity with these permissions.

An application providing LDAP repository read, LDAP repository search, or LDAP repository modify service as defined in this specification is NOT REQUIRED to implement any additional security features other than those described herein, however an implementation SHOULD do so.

11. References

   [1]  Yeong, Y., Howes, T. and S. Kille, "Lightweight Directory Access
   
        Protocol", RFC 1777, March 1995.
   
   [2]  Howes, T., Kille, S., Yeong, W. and C. Robbins, "The String
        Representation of Standard Attribute Syntaxes", RFC 1778, March
        1995.
   
   [3]  Bradner, S., "Key Words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

12. Authors' Addresses

Sharon Boeyen
Entrust Technologies Limited
750 Heron Road
Ottawa, Ontario
Canada K1V 1A7

   EMail: sharon.boeyen@entrust.com

Tim Howes
Netscape Communications Corp.
501 E. Middlefield Rd.
Mountain View, CA 94043
USA

EMail:

          howes@netscape.com

Patrick Richard
Xcert Software Inc.
Suite 1001, 701 W. Georgia Street
P.O. Box 10145
Pacific Centre
Vancouver, B.C.
Canada V7Y 1C6

EMail:

          patr@xcert.com

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