Network Working Group S. Josefsson Internet-Draft SJD Intended status: Standards Track October 4, 2006 Expires: April 7, 2007 Using Kerberos V5 over the Transport Layer Security (TLS) protocol draft-josefsson-kerberos5-starttls-01 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 April 7, 2007. Copyright Notice Copyright (C) The Internet Society (2006). Josefsson Expires April 7, 2007 [Page 1] Internet-Draft Protecting Kerberos V5 with TLS October 2006 Abstract This document specify how the Kerberos V5 protocol can be transported over the Transport Layer Security (TLS) protocol, to provide additional security features. Table of Contents 1. Introduction and Background . . . . . . . . . . . . . . . . . 3 2. Kerberos V5 STARTTLS Extension . . . . . . . . . . . . . . . . 5 3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. STARTTLS aware KDC Discovery . . . . . . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. Normative References . . . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . . . 12 Josefsson Expires April 7, 2007 [Page 2] Internet-Draft Protecting Kerberos V5 with TLS October 2006 1. Introduction and Background This document describe how a Kerberos V5 [2] implementation may upgrade communication between clients and Key Distribution Centers (KDCs) to use the Transport Layer Security (TLS) [4] protocol. The TLS protocol offer integrity and privacy protected exchanges that can be authentication using X.509 certificates, OpenPGP keys [7], and user name and passwords via SRP [6]. There are several reasons to use Kerberos V5 over TLS. 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 Prevents downgrade attacks affecting encryption types. The encryption type of the ticket in KDC-REQ are sent in the clear in Kerberos 5. This allows an attacker to replace the encryption type with a compromised mechanisms, e.g., 56-bit DES. Since clients in general cannot know the encryption types other servers support, 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 mechanism. Clients could chose to refuse, e.g., 56-bit DES altogether, but in some environments this leads to operational difficulties. 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. In some situations, the policy may be that password authentication may only be done through SRP. 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 Josefsson Expires April 7, 2007 [Page 3] Internet-Draft Protecting Kerberos V5 with TLS October 2006 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 in RFC 2119 [1]. Josefsson Expires April 7, 2007 [Page 4] Internet-Draft Protecting Kerberos V5 with TLS October 2006 2. Kerberos V5 STARTTLS Extension The STARTTLS extension uses the Kerberos V5 TCP extension mechanism [3]. 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. 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. Josefsson Expires April 7, 2007 [Page 5] Internet-Draft Protecting Kerberos V5 with TLS October 2006 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. 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 ] Kerberos V5 AS-REQ --------> Kerberos V5 AS-REP <-------- * Indicates optional or situation-dependent messages that are not always sent. Josefsson Expires April 7, 2007 [Page 6] Internet-Draft Protecting Kerberos V5 with TLS October 2006 4. STARTTLS aware KDC Discovery Section 7.2.3 of Kerberos V5 [2] describe how Domain Name System (DNS) SRV records [5] can be used to find the address of an KDC. Using the terminology of Section 7.2.3 of RFC 4120, 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 April 7, 2007 [Page 7] Internet-Draft Protecting Kerberos V5 with TLS October 2006 5. 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 [3]. Josefsson Expires April 7, 2007 [Page 8] Internet-Draft Protecting Kerberos V5 with TLS October 2006 6. Security Considerations The security considerations in Kerberos V5, TLS, and the extension mechanism framework are inherited. To protect against the inherent downgrade attack in the extension framework, it is suggested that implementations offer a policy to require that this extension is successfully negotiated. For interoperability with implementations that do not support this extension, it is suggested that the policy is disabled by default. Josefsson Expires April 7, 2007 [Page 9] Internet-Draft Protecting Kerberos V5 with TLS October 2006 7. References 7.1. Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005. [3] Josefsson, S., "Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges Over TCP", draft-ietf-krb-wg-tcp-expansion-01 (work in progress), September 2006. [4] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006. [5] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000. 7.2. Informative References [6] Taylor, D., "Using SRP for TLS Authentication", draft-ietf-tls-srp-12 (work in progress), June 2006. [7] Mavroyanopoulos, N., "Using OpenPGP keys for TLS authentication", draft-ietf-tls-openpgp-keys-10 (work in progress), June 2006. Josefsson Expires April 7, 2007 [Page 10] Internet-Draft Protecting Kerberos V5 with TLS October 2006 Author's Address Simon Josefsson SJD Email: simon@josefsson.org Josefsson Expires April 7, 2007 [Page 11] Internet-Draft Protecting Kerberos V5 with TLS October 2006 Full Copyright Statement Copyright (C) The Internet Society (2006). 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. 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