LDAPExt Working Group Jeff Hodges, Stanford INTERNET-DRAFT RL "Bob" Morgan, Stanford Category: Standards Track Mark Wahl, Innosoft August, 1998 Lightweight Directory Access Protocol (v3): Extension for Transport Layer Security Status of this Document This document is an Internet-Draft. Internet-Drafts are working docu- ments 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.'' To view the entire list of current Internet-Drafts, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast). Comments and suggestions on this document are encouraged. Comments on this document should be sent to the LDAPEXT working group discussion list: ietf-ldapext@netscape.com This document expires in February 1999. 1. Abstract This document defines the "Start Transport Layer Security (TLS) Opera- tion" for LDAP [LDAPv3, TLS]. This operation provides for TLS establish- ment in an LDAP association and is defined in terms of an LDAP extended request. 2. Conventions Used in this Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", Hodges, Morgan, Wahl [Page 1] I-D LDAPv3: Extension for Transport Layer Security June 1998 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [ReqsKeywords]. 3. The Start TLS Request This section describes the Start TLS extended request and extended response themselves: how to form the request, the form of the response, and enumerates the various result codes the client MUST be prepared to handle. The section following this one then describes how to sequence an overall Start TLS Operation. 3.1. Requesting TLS Establishment A client may perform a Start TLS operation by transmitting an LDAP PDU containing an ExtendedRequest [LDAPv3] specifying the OID for the Start TLS operation: 1.3.6.1.4.1.1466.20037 An LDAP ExtendedRequest is defined as follows: ExtendedRequest ::= [APPLICATION 23] SEQUENCE { requestName [0] LDAPOID, requestValue [1] OCTET STRING OPTIONAL } A Start TLS extended request is formed by setting the requestName field to the OID string given above. The requestValue field is absent. The client MUST NOT send any PDUs on this connection following this request until it receives a Start TLS extended response. When a Start TLS extended request is made, the server MUST return an LDAP PDU containing a Start TLS extended response. An LDAP Exten- dedResponse is defined as follows: ExtendedResponse ::= [APPLICATION 24] SEQUENCE { COMPONENTS OF LDAPResult, responseName [10] LDAPOID OPTIONAL, response [11] OCTET STRING OPTIONAL } A Start TLS extended response MUST contain a responseName field which MUST be set to the same string as that in the esponseName field present in the Start TLS extended request. The response field is absent. The server MUST set the resultCode field to either success or one of the other values outlined in section 3.3. Hodges, Morgan, Wahl [Page 2] I-D LDAPv3: Extension for Transport Layer Security June 1998 3.2. "Success" Response If the ExtendedResponse contains a resultCode of success, this indicates that the server is willing and able to negotiate TLS. Refer to section 4, below, for details. 3.3. Response other than "success" If the ExtendedResponse contains a resultCode other than success, this indicates that the server is unwilling or unable to negotiate TLS. If the Start TLS extended request was not successful, the resultCode will be one of: operationsError (operations sequencing incorrect; e.g. TLS already established) protocolError (TLS not supported or incorrect PDU structure) referral (this server doesn't do TLS, try this one) unavailable (e.g. some major problem with TLS, or server is shutting down) The server MUST return operationsError if the client violates any of the Start TLS extended operation sequencing requirements described in sec- tion 4, below. If the server does not support TLS (whether by design or by current con- figuration), it MUST set the resultCode to protocolError (see section 4.1.1 of [LDAPv3]), or to referral. The server MUST include an actual referral value in the LDAP Result if it returns a resultCode of refer- ral. The client's current session is unaffected if the server does not support TLS. The client MAY proceed with any LDAP operation, or it MAY close the connection. The server MUST return unavailable if it supports TLS but cannot estab- lish a TLS connection for some reason, e.g. the certificate server not responding, it cannot contact its TLS implementation, or if the server is in process of shutting down. The client MAY retry the StartTLS opera- tion, or it MAY proceed with any other LDAP operation, or it MAY close the connection. 4. Sequencing of the Start TLS Operation This section describes the overall procedures clients and servers MUST follow for TLS establishment. These procedures take into consideration various aspects of the overall security of the LDAP association Hodges, Morgan, Wahl [Page 3] I-D LDAPv3: Extension for Transport Layer Security June 1998 including discovery of resultant security level and assertion of the client's authorization identity. Note that the precise effects, on a client's authorzation identity, of establishing TLS on an LDAP association are described in detail in sec- tion 7. 4.1. Requesting to Start TLS on an LDAP Association The client MAY send the Start TLS extended request at any time after establishing an LDAP association, except that in the following cases the client MUST NOT send a Start TLS extended request: - if TLS is currently established on the connection, or - during a multi-stage SASL negotiation, or - if there are any LDAP operations outstanding on the connection. The result of violating any of these requirements is a resultCode of operationsError, as described above in section 3.3. The client MAY have already perfomed a Bind operation when it sends a Start TLS request, or the client might have not yet bound. If the client did not establish a TLS connection before sending any other requests, and the server requires the client to establish a TLS connection before performing a particular request, the server MUST reject that request with a confidentialityRequired or strongAuthRequired result. The client MAY send a Start TLS extended request, or it MAY choose to close the connection. 4.2. Starting TLS The server will return an extended response with the resultCode of suc- cess if it is willing and able to negotiate TLS. It will return other resultCodes, documented above, if it is unable. In the successful case, the client, which has ceased to transfer LDAP requests on the connection, MUST either begin a TLS negotiation or close the connection. The client will send PDUs in the TLS Record Protocol directly over the underlying transport connection to the server to ini- tiate TLS negotiation [TLS]. 4.3. TLS Version Negotiation Negotiating the version of TLS or SSL to be used is a part of the TLS Handshake Protocol, as documented in [TLS]. Please refer to that docu- ment for details. Hodges, Morgan, Wahl [Page 4] I-D LDAPv3: Extension for Transport Layer Security June 1998 4.4. Discovery of Resultant Security Level After a TLS connection is established on an LDAP association, both par- ties MUST individually decide whether or not to continue based on the privacy level achieved. Ascertaining the TLS connection's privacy level is implementation dependent, and accomplished by communicating with one's respective local TLS implementation. If the client or server decides that the level of authentication or privacy is not high enough for it to continue, it SHOULD gracefully close the TLS connection immediately after the TLS negotiation has com- pleted (see sections 5 and 7.2, below). The client MAY attempt to Start TLS again, or MAY send an unbind request, or send any other LDAP request. 4.5. Assertion of Client's Authorization Identity The client MAY, upon receipt of a Start TLS extended response indicating success, assert that a specific authorization identity be utilized in determining the client's authorization status. The client accomplishes this via an LDAP Bind request specifying a SASL mechanism of "EXTERNAL" [SASL]. See section 7, below. 4.6. Server Identity Check The client SHOULD check its understanding of the server's hostname against the server's identity as presented in the server's Certificate message, in order to prevent man-in-the-middle attacks. If a subjectAltName extension of type dNSName is present, it SHOULD be used as the source of the server's identity. Matching is performed according to these rules: - The client MUST use the server hostname it used to open the LDAP connection as the value to compare against the server name as expressed in the server's certificate. The client MUST NOT use the server's canonical DNS name or any other derived form of name. - If a subjectAltName extension of type dNSName is present in the certificate, it SHOULD be used as the source of the server's identity. - Matching is case-insensitive. - The "*" wildcard character is allowed. Hodges, Morgan, Wahl [Page 5] I-D LDAPv3: Extension for Transport Layer Security June 1998 - If present, it applies only to the left-most name component. E.g. *.bar.com would match a.bar.com, b.bar.com, etc. but not bar.com. If more than one identity of a given type is present in the certificate (e.g. more than one dNSName name), a match in any one of the set is con- sidered acceptable. If the hostname does not match the dNSName-based identity in the certi- ficate per the above check, user-oriented clients SHOULD either notify the user (clients MAY give the user the opportunity to continue with the connection in any case) or terminate the connection and indicate that the server's identity is suspect. Automated clients SHOULD close the connection, returning and/or logging an error indicating hat the server's identity is suspect. 4.7. Refresh of Server Capabilities Information The client SHOULD refresh any cached server capabilities information (e.g. from the server's root DSE; see section 3.4 of [LDAPv3]) upon TLS session establishment. This is necessary to protect against active- intermediary attacks which may have altered any server capabilities information retrieved prior to TLS establishment. The server MAY adver- tise different capabilities after TLS establishment. 5. Closing a TLS Connection 5.1. Graceful Closure Either the client or server MAY terminate the TLS connection on an LDAP association by sending a TLS closure alert. This will leave the LDAP association intact. Before closing a TLS connection, the client MUST either wait for any outstanding LDAP operations to complete, or explicitly abandon them [LDAPv3]. After the initiator of a close has sent a closure alert, it MUST discard any TLS messages until it has received an alert from the other party. It will cease to send TLS Record Protocol PDUs, and following the reciept of the alert, MAY send and receive LDAP PDUs. The other party, if it receives a closure alert, MUST immediately transmit a TLS closure alert. It will subequently cease to send TLS Record Protocol PDUs, and MAY send and receive LDAP PDUs. 5.2. Abrupt Closure Either the client or server MAY abruptly close the entire LDAP Hodges, Morgan, Wahl [Page 6] I-D LDAPv3: Extension for Transport Layer Security June 1998 association and any TLS connection established on it by dropping the underlying TCP connection. A server MAY beforehand send the client a Notice of Disconnection [LDAPv3] in this case. 6. Effects of TLS on a Client's Authorization Identity This section describes the effects on a client's authorization identity brought about by establishing TLS on an LDAP association. The default effects are described first, and next the facilities for client asser- tion of authorization identity are discussed including error conditions. Lastly, the effects of closing the TLS connection are described. Authorization identities and related concepts are defined in [AuthMeth]. 6.1. TLS Connection Establishment Effects 6.1.1. Default Effects Upon establishment of the TLS connection onto the LDAP association, any previously established authentication and authorization identities MUST remain in force, including anonymous state. This holds even in the case where the server requests client authentication via TLS -- e.g. requests the client to supply its certificate during TLS negotiation (see [TLS]). 6.1.2. Client Assertion of Authorization Identity A client MAY either implicitly request that its LDAP authorization iden- tity be derived from its authenticated TLS credentials or it MAY expli- citly provide an authorization identity and assert that it be used in combination with its authenticated TLS credentials. The former is known as an implicit assertion, and the latter as an explicit assertion. 6.1.2.1. Implicit Assertion An implicit authorization identity assertion is accomplished after TLS establishment by invoking a Bind request of the SASL form using the "EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL NOT include the optional credentials octet string (found within the SaslCredentials sequence in the Bind Request). The server will derive the client's authorization identity from the authentication identity supplied in the client's TLS credentials (typically a public key certificate) according to local policy. The underlying mechanics of how this is accomplished are implementation specific. 6.1.2.2. Explicit Assertion An explicit authorization identity assertion is accomplished after TLS establishment by invoking a Bind request of the SASL form using the Hodges, Morgan, Wahl [Page 7] I-D LDAPv3: Extension for Transport Layer Security June 1998 "EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL include the creden- tials octet string. This string MUST be constructed as documented in section 11 of [AuthMeth]. 6.1.2.3. Error Conditions For either form of assertion, the server MUST verify that the client's authentication identity as supplied in its TLS credentials is permitted to be mapped to the asserted authorization identity. The server MUST reject the Bind operation with an invalidCredentials resultCode in the Bind response if the client is not so authorized. The LDAP association's authentication identity and authorization identity (if any) which were in effect after TLS establishment but prior to making the Bind request, MUST remain in force. Additionally, with either form of assertion, if a TLS session has not been established between the client and server prior to making the SASL EXTERNAL Bind request and there is no other external source of authenti- cation credentials (e.g. IP-level security RFC 1825), or if, during the process of establishing the TLS session, the server did not request the client's authentication credentials, the SASL EXTERNAL bind MUST fail with a result code of inappropriateAuthentication. Any authentication identity and authorization identity, as well as TLS connection, which were in effect prior to making the Bind request, MUST remain in force. 6.2. TLS Connection Closure Effects Closure of the TLS connection MUST cause the LDAP association to move to an anonymous authentication and authorization state regardless of the state established over TLS and regardless of the authentication and authorization state prior to TLS connection establishment. 7. Security Considerations The goals of using the TLS protocol with LDAP are to ensure connection confidentiality and integrity, and to optionally provide for authentica- tion. TLS expressly provides these capabilities, as described in [TLS]. All security gained via use of the Start TLS operation is gained by the use of TLS itself. The Start TLS operation, on its own, does not provide any additional security. The use of TLS does not provide or ensure for confidentiality and/or non-repudiation of the data housed by an LDAP-based directory server. Nor does it secure the data from inspection by the server administra- tors. Once established, TLS only provides for and ensures confidential- ity and integrity of the operations and data in transit over the LDAP associa- tion, and only if the implementations on the client and server Hodges, Morgan, Wahl [Page 8] I-D LDAPv3: Extension for Transport Layer Security June 1998 support and negotiate it. The level of security provided though the use of TLS depends directly on both the quality of the TLS implementation used and the style of usage of that implementation. Additionally, an active-intermediary attacker can remove the Start TLS extended operation from the supportedExtension attribute of the root DSE. Therefore, both parties SHOULD independently ascertain and consent to the security level achieved once TLS is esta- blished and before begining use of the TLS connection. For example, the security level of the TLS connection might have been negotiated down to plaintext. Clients SHOULD either warn the user when the security level achieved does not provide confidentiality and/or integrity protection, or be con- figurable to refuse to proceed without an acceptable level of security. Client and server implementors SHOULD take measures to ensure proper protection of credentials and other confidential data where such meas- ures are not otherwise provided by the TLS implementation. Server implementors SHOULD allow for server administrators to elect whether and when connection confidentiality and/or integrity is required, as well as elect whether and when client authentication via TLS is required. 8. Acknowledgements The authors thank Tim Howes, Paul Hoffman, John Kristian, Shirish Rai, Jonathan Trostle, and Harald Alvestrand for their contributions to this document. 9. References [AuthMeth] M. Wahl, H. Alvestrand, J. Hodges, R. Morgan. "Authenti- cation Methods for LDAP". INTERNET-DRAFT, Work In Pro- gress. draft-ietf-ldapext-authmeth-03.txt [LDAPv3] M. Wahl, S. Kille and T. Howes. "Lightweight Directory Access Protocol (v3)". RFC 2251. [ReqsKeywords] Scott Bradner. "Key Words for use in RFCs to Indicate Requirement Levels". RFC 2119. [SASL] J. Myers. "Simple Authentication and Security Layer (SASL)". RFC 2222. [TLS] Tim Dierks, C. Allen. "The TLS Protocol Version 1.0". INTERNET-DRAFT, Work In Progress. draft-ietf-tls- Hodges, Morgan, Wahl [Page 9] I-D LDAPv3: Extension for Transport Layer Security June 1998 protocol-05.txt 10. Authors' Addresses Jeff Hodges Computing & Communication Services Stanford University Pine Hall 241 Panama Street Stanford, CA 94305-4122 USA Phone: +1-650-723-2452 EMail: Jeff.Hodges@Stanford.edu RL "Bob" Morgan Computing & Communication Services Stanford University Pine Hall 241 Panama Street Stanford, CA 94305-4122 USA Phone: +1-650-723-9711 EMail: Bob.Morgan@Stanford.edu Mark Wahl Innosoft International, Inc. 8911 Capital of Texas Hwy, Suite 4140 Austin, TX 78759 USA Phone: +1 626 919 3600 EMail: Mark.Wahl@innosoft.com ----------------------------------- Full Copyright Statement Copyright (C) The Internet Society (1998). All Rights Reserved. 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