Movatterモバイル変換

Internet Engineering Task Force (IETF)                      S. JosefssonRequest for Comments: 6251                                        SJD ABCategory: Informational                                         May 2011ISSN: 2070-1721Using Kerberos Version 5over the Transport Layer Security (TLS) ProtocolAbstract   This document specifies how the Kerberos V5 protocol can be   transported over the Transport Layer Security (TLS) protocol in order   to provide additional security features.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6251.Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Josefsson                     Informational                     [Page 1]

RFC 6251             Protecting Kerberos V5 with TLS            May 2011   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Table of Contents1. Introduction and Background .....................................22. Kerberos V5 STARTTLS Extension ..................................33. Examples ........................................................44. STARTTLS-Aware KDC Discovery ....................................55. Server Certificates .............................................66. IANA Considerations .............................................77. Acknowledgements ................................................78. Security Considerations .........................................79. References ......................................................89.1. Normative References .......................................89.2. Informative References .....................................81.  Introduction and Background   This document describes how a Kerberos V5 [RFC4120] implementation   may upgrade communication between clients and Key Distribution   Centers (KDCs) to use the Transport Layer Security (TLS) [RFC5246]   protocol.   The TLS protocol offers integrity- and privacy-protected exchanges   that can be authenticated using X.509 certificates, OpenPGP keys   [RFC6091], and usernames and passwords via Secure Remote Password   (SRP) [RFC5054].   There are several reasons to use Kerberos V5 over TLS.   o  It prevents downgrade attacks affecting, e.g., encryption types      and pre-auth data negotiation.  The encryption type field in      KDC-REQ and the METHOD-DATA field with the requested pre-auth      types from the server in KDC_ERR_PREAUTH_REQUIRED errors in      KDC-REP are sent without integrity or privacy protection in      Kerberos V5.  This allows an active attacker to replace the      encryption type with a compromised encryption type, e.g., 56-bit      DES, or request that clients should use a broken pre-auth type.Josefsson                     Informational                     [Page 2]

RFC 6251             Protecting Kerberos V5 with TLS            May 2011      Since clients in general cannot know the encryption types other      servers support, or the pre-auth types servers prefer or require,      it is difficult for the client to detect if there was a man in the      middle or if the remote server simply did not support a stronger      encryption type or preferred another pre-auth type.   o  Kerberos exchanges are privacy protected.  Parts of many Kerberos      packets are transferred without privacy protection (i.e.,      encryption).  That part contains information, such as the client      principal name, the server principal name, the encryption types      supported by the client, the lifetime of tickets, etc.  Revealing      such information is, in some threat models, considered a problem.   o  It provides additional authentication against the KDC.  In some      situations, users are equipped with smart cards with an RSA      authentication key.  In others, users have an OpenPGP client on      their desktop, with a public OpenPGP key known to the server.   o  It provides explicit server authentication of the KDC to the      client.  In traditional Kerberos V5, authentication of the KDC is      proved as a side effect that the KDC knows your encryption key      (i.e., your password).   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 inRFC 2119 [RFC2119].2.  Kerberos V5 STARTTLS Extension   The STARTTLS extension uses the Kerberos V5 TCP extension mechanism   [RFC5021].  The extension uses bit 0 in the extension bitmask.   The protocol is as follows.  The client requests the extension by   setting the STARTTLS bit in the TCP extension mechanism bitmask.   (How to deal with extension negotiation failures at this point is   described in [RFC5021].)  After the server has sent the 4-octet value   0x00000000 to indicate support of this extension, the stream will be   controlled by the TLS protocol and its framing.  The TLS protocol is   initiated by the client.   Typically, the client initiates the TLS handshake protocol by sending   a client hello, the server responds, and the handshake continues   until it either succeeds or fails.   If, for any reason, the handshake fails, the STARTTLS protocol will   also fail, and the TLS error is used as the error indication.  In   this case, no further messages can be exchanged over the same TCP   session.Josefsson                     Informational                     [Page 3]

RFC 6251             Protecting Kerberos V5 with TLS            May 2011   If the handshake succeeds, the Kerberos V5 authentication protocol is   performed within the protected TLS channel, like a normal TCP   Kerberos V5 exchange.  In particular, this means that every Kerberos   V5 packet will be prefixed by a 4-octet length field that indicates   the length of the Kerberos V5 packet.   When no further Kerberos V5 messages need to be transferred in the   TLS session, the TLS session MUST be shut down properly using the   close_notify alert.  When the TLS session is shut down, the TCP   connection cannot be re-used to send any further data and MUST be   closed.3.  Examples   A complete packet flow for a successful AS-REQ/REP exchange protected   by this mechanism will be as follows.  The "STARTTLS-bit" is a   4-octet value with only the bit allocated for this extension set, and   | is the binary OR operation.Josefsson                     Informational                     [Page 4]

RFC 6251             Protecting Kerberos V5 with TLS            May 2011       Client                                               Server        [ Kerberos V5 TCP extension mechanism negotiation starts ]       0x80000000 | STARTTLS-bit    -------->                                                       0x00000000                                    <--------                            [ TLS negotiation starts ]       ClientHello                  -------->                                                       ServerHello                                                      Certificate*                                                ServerKeyExchange*                                               CertificateRequest*                                    <--------      ServerHelloDone       Certificate*       ClientKeyExchange       CertificateVerify*       [ChangeCipherSpec]       Finished                     -------->                                                [ChangeCipherSpec]                                    <--------             Finished                       [ Kerberos V5 negotiation starts ]       4-octet length field       Kerberos V5 AS-REQ           -------->                                                4-octet length field                                                Kerberos V5 AS-REP                                    <--------       * Indicates optional or situation-dependent messages that are not         always sent4.  STARTTLS-Aware KDC DiscoverySection 7.2.3 of Kerberos V5 [RFC4120] describes how Domain Name   System (DNS) SRV records [RFC2782] can be used to find the address of   a KDC.  We define a new Service of "kerberos-tls" to indicate that   the particular KDC is intended to support this STARTTLS extension.   The Proto (tcp), Realm, TTL, Class, SRV, Priority, Weight, Port, and   Target have the same meaning as inRFC 4120.Josefsson                     Informational                     [Page 5]

RFC 6251             Protecting Kerberos V5 with TLS            May 2011   For example:   _kerberos-tls._tcp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.   _kerberos-tls._tcp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.5.  Server Certificates   The TLS protocol may be used in a mode that provides server   authentication using, for example, X.509 and OpenPGP.   A goal for the protocol described in this memo is that it should be   as easy to implement and deploy on clients as support for UDP/TCP.   Since many client environments do not have access to long-term   storage, or to long-term storage that is sufficiently secure to   enable validation of server certificates, the Kerberos V5 STARTTLS   protocol does not require clients to verify server certificates.  If   server certification had been required, then environments with   constrained clients such as those mentioned would be forced to   disable TLS; this would arguably be worse than TLS without server   certificate validation, as the use of TLS, even without server   certificate validation, protects against some attacks that Kerberos   V5 over UDP/TCP does not.  For example, even without server   certificate validation, TLS does protect against passive network   sniffing aimed at tracking Kerberos service usage by a given client.   However, note that the use of TLS without server certificate   verification opens up a range of active attacks such as man in the   middle.   When clients have the ability, they MUST validate the server   certificate.  For this reason, if a KDC presents an X.509 server   certificate over TLS, it MUST contain an otherName Subject   Alternative Name (SAN) identified using a type-id of   id-krb5starttls-san.  The intention is to bind the server certificate   to the Kerberos realm for the purpose of using Kerberos V5 STARTTLS.   The value field of the otherName should contain the realm as the   "Realm" ASN.1 type.          id-krb5starttls-san OBJECT IDENTIFIER ::=            { iso(1) identified-organization(3) dod(6) internet(1)              private(4) enterprise(1) gnu(11591)              shishi(6) krb5starttls-san(1) }   To validate a server certificate, the client MAY use local   configuration (e.g., a list that maps the Kerberos realm to a copy of   the server's certificate) and compare that with the authentication   information provided from the server via TLS.  For illustration, theJosefsson                     Informational                     [Page 6]

RFC 6251             Protecting Kerberos V5 with TLS            May 2011   server certificate could be an X.509 certificate or an OpenPGP key.   In this mode, the client needs no processing related to id-   krb5starttls-san.   When the server presents an X.509 server certificate, clients MAY use   "Certification Path Validation" as described in [RFC5280] to validate   the KDC server certificate.  In addition, unless the client can   otherwise verify that the server certificate is bound to the KDC of   the target realm, the client MUST verify that the server certificate   contains the id-krb5starttls-san SAN and that the value is identical   to the intended Kerberos realm.6.  IANA Considerations   Per [RFC5021], the IANA has allocated a bit (value 0) in the   "Kerberos TCP Extensions" registry for Krb5 over TLS, the extension   described in this document.7.  Acknowledgements   Miguel A. Garcia, Sam Hartman, Jeffrey Hutzelman, Magnus Nystroem,   and Peter Saint-Andre (in alphabetical order) provided comments that   improved the protocol and document.8.  Security Considerations   The security considerations in Kerberos V5, TLS, and the Kerberos V5   TCP extension mechanism are inherited.   Note that TLS does not protect against man-in-the-middle attacks   unless clients verify the KDC's credentials (X.509 certificate,   OpenPGP key, etc.) correctly.  Although certificate validation adds   an extra layer of protection, that is not considered strictly   necessary to improve the security profile of Kerberos V5 as outlined   in this document.   If server authentication is used, some information about the server   (such as its name) is visible to passive attackers.   To protect against the inherent downgrade attack in the extension   framework, implementations SHOULD offer a policy mode that requires   this extension to always be successfully negotiated, for a particular   realm, or generally.  For interoperability with implementations that   do not support this extension, the policy mode SHOULD be disabled by   default.Josefsson                     Informational                     [Page 7]

RFC 6251             Protecting Kerberos V5 with TLS            May 20119.  References9.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for              specifying the location of services (DNS SRV)",RFC 2782,              February 2000.   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The              Kerberos Network Authentication Service (V5)",RFC 4120,              July 2005.   [RFC5021]  Josefsson, S., "Extended Kerberos Version 5 Key              Distribution Center (KDC) Exchanges over TCP",RFC 5021,              August 2007.   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security              (TLS) Protocol Version 1.2",RFC 5246, August 2008.   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,              Housley, R., and W. Polk, "Internet X.509 Public Key              Infrastructure Certificate and Certificate Revocation List              (CRL) Profile",RFC 5280, May 2008.9.2.  Informative References   [RFC5054]  Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,              "Using the Secure Remote Password (SRP) Protocol for TLS              Authentication",RFC 5054, November 2007.   [RFC6091]  Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys              for Transport Layer Security (TLS) Authentication",RFC 6091, February 2011.Author's Address   Simon Josefsson   Simon Josefsson Datakonsult AB   Hagagatan 24   Stockholm  113 47   Sweden   EMail: simon@josefsson.org   URI:http://josefsson.org/Josefsson                     Informational                     [Page 8]