Network Working Group S. Josefsson Internet-Draft SJD Intended status: Informational December 5, 2008 Expires: June 8, 2009 Using Kerberos V5 over the Transport Layer Security (TLS) protocol draft-josefsson-kerberos5-starttls-04 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 will expire on June 8, 2009. Josefsson Expires June 8, 2009 [Page 1]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 Abstract This document specify how the Kerberos V5 protocol can be transported over the Transport Layer Security (TLS) protocol, to provide additional security features. This document updates RFC 4120. Table of Contents 1. Introduction and Background . . . . . . . . . . . . . . . . . 3 2. Kerberos V5 STARTTLS Extension . . . . . . . . . . . . . . . . 5 3. Channel Binding Pre-Authentication Data . . . . . . . . . . . 6 4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5. STARTTLS aware KDC Discovery . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10 8. Security Considerations . . . . . . . . . . . . . . . . . . . 11 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 9.1. Normative References . . . . . . . . . . . . . . . . . . . 12 9.2. Informative References . . . . . . . . . . . . . . . . . . 12 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13 Intellectual Property and Copyright Statements . . . . . . . . . . 14 Josefsson Expires June 8, 2009 [Page 2]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 1. Introduction and Background This document describe 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 offer integrity and privacy protected exchanges that can be authentication using X.509 certificates, OpenPGP keys [RFC5081], and user name and passwords via SRP [RFC5054]. There are several reasons to use Kerberos V5 over TLS. o 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 5. This allows an 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. 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. Part of many Kerberos packets are transfered 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 Additional authentication against the KDC. In some situations, users are equipped with smart cards with a RSA authentication key. In others, users have a OpenPGP client on their desktop, with a public OpenPGP key known to the server. o The TLS protocol has been studied by many parties. In some threat models, the designer prefer to reduce the number of protocols that can hurt the overall system security if they are compromised. o Explicit server authentication of the KDC to the client. In traditional Kerberos 5, 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", Josefsson Expires June 8, 2009 [Page 3]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. Josefsson Expires June 8, 2009 [Page 4]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 2. Kerberos V5 STARTTLS Extension The STARTTLS extension uses the Kerberos V5 TCP extension mechanism [RFC5021]. The extension uses bit #TBD in the extension bitmask. The protocol is as follows. 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 initiate the TLS handshake protocol by sending a client hello, and the server responds, and the handshake continues until it either succeed 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. 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 indicate the length of the Kerberos V5 packet. However, to conform with this specification, any KDC-REQ (AS-REQ or TGS-REQ) message MUST contain the "pa-channel-binding" pre-authentication data. When no further Kerberos V5 messages needs 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 furhter data and MUST be closed. Josefsson Expires June 8, 2009 [Page 5]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 3. Channel Binding Pre-Authentication Data The pre-authentication structure is defined in RFC 4120 as: PA-DATA ::= SEQUENCE { -- NOTE: first tag is [1], not [0] padata-type [1] Int32, padata-value [2] OCTET STRING -- might be encoded AP-REQ } Here we define a new pre-authentication data, called "pa-channel- binding". It has a padata-type integer value of #TBD. The contents of the padata-value field is the channel binding data, as discussed in [RFC5056]. Josefsson Expires June 8, 2009 [Page 6]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 4. 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. Client Server [ Kerberos V5 TCP extension mechanism negotiation starts ] [0x70000000 & 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 sent. Josefsson Expires June 8, 2009 [Page 7]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 5. STARTTLS aware KDC Discovery Section 7.2.3 of Kerberos V5 [RFC4120] describe how Domain Name System (DNS) SRV records [RFC2782] can be used to find the address of an KDC. We define a new Proto of "tls" to indicate that the particular KDC is intended to support this STARTTLS extension. The Service, Realm, TTL, Class, SRV, Priority, Weight, Port and Target have the same meaning as in RFC 4120. For example: _kerberos._tls.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com. _kerberos._tls.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com. Josefsson Expires June 8, 2009 [Page 8]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 6. IANA Considerations The IANA is requested to allocate a bit in the "Kerberos TCP Extensions" registry for the extension described in this document, as per [RFC5021]. Josefsson Expires June 8, 2009 [Page 9]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 7. Acknowledgements Jeffrey Hutzelman provided comments that improved the protocol and document. Josefsson Expires June 8, 2009 [Page 10]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 8. Security Considerations The security considerations in Kerberos V5, TLS, and the extension mechanism framework are inherited. Note that TLS does not protect against Man-In-The-Middle (MITM) attacks unless clients verify the KDC's credentials (X.509 certificate, OpenPGP key, etc) correctly. 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 Expires June 8, 2009 [Page 11]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 9. References 9.1. 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. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC5021] Josefsson, S., "Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges over TCP", RFC 5021, August 2007. [RFC5056] Williams, N., "On the Use of Channel Bindings to Secure Channels", RFC 5056, November 2007. 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. [RFC5081] Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport Layer Security (TLS) Authentication", RFC 5081, November 2007. Josefsson Expires June 8, 2009 [Page 12]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 Author's Address Simon Josefsson Simon Josefsson Datakonsult AB Hagagatan 24 Stockholm 113 47 Sweden Email: simon@josefsson.org URI: http://josefsson.org/ Josefsson Expires June 8, 2009 [Page 13]
Internet-Draft Protecting Kerberos V5 with TLS December 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Josefsson Expires June 8, 2009 [Page 14]
