Using Kerberos V5 over the Transport Layer Security (TLS) protocol
draft-josefsson-kerberos5-starttls-08

Abstract

This document specify how the Kerberos V5 protocol can be transported over the Transport Layer Security (TLS) protocol, to provide additional security features.

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Table of Contents

1.  Introduction and Background
2.  Kerberos V5 STARTTLS Extension
3.  Examples
4.  STARTTLS aware KDC Discovery
5.  Server Certificates
6.  IANA Considerations
7.  Acknowledgements
8.  Security Considerations
9.  References
    9.1.  Normative References
    9.2.  Informative References
§  Author's Address

1.  Introduction and Background

This document describe how a Kerberos V5 (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) [RFC4120] implementation may upgrade communication between clients and Key Distribution Centers (KDCs) to use the Transport Layer Security (TLS) (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) [RFC5246] protocol.

The TLS protocol offer integrity and privacy protected exchanges that can be authentication using X.509 certificates, OpenPGP keys (Mavrogiannopoulos, N., “Using OpenPGP Keys for Transport Layer Security (TLS) Authentication,” November 2007.) [RFC5081], and user name and passwords via Secure Remote Password (SRP) (Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin, “Using the Secure Remote Password (SRP) Protocol for TLS Authentication,” November 2007.) [RFC5054].

There are several reasons to use Kerberos V5 over TLS.

• 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 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. 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.
• Kerberos exchanges are privacy protected. Part 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.
• 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.
• 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

2.  Kerberos V5 STARTTLS Extension

The STARTTLS extension uses the Kerberos V5 TCP extension mechanism (Josefsson, S., “Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges over TCP,” August 2007.) [RFC5021]. The extension uses bit #TBD 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] (Josefsson, S., “Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges over TCP,” August 2007.).) 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.

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 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.

    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 sent.

4.  STARTTLS aware KDC Discovery

Section 7.2.3 of Kerberos V5 (Neuman, C., Yu, T., Hartman, S., and K. Raeburn, “The Kerberos Network Authentication Service (V5),” July 2005.) [RFC4120] describe how Domain Name System (DNS) SRV records (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.) [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.

5.  Server Certificates

The TLS protocol may be used in a mode that provides server authentication using, for example, X.509 and OpenPGP.

The Kerberos V5 STARTTLS protocol do not require clients to verify the server certificate. The goal is that support for TLS in Kerberos V5 clients should be as easy to implement and deploy as support for UDP/TCP. Use of TLS, even without server certificate validation, protects against some attacks that Kerberos V5 over UDP/TCP do not. (For example, passive network sniffing between the user and the KDC to track which Kerberos services are used by the user.) To require server certificates to be validated at all times would lead to disabling of TLS when clients are unable to validate server certificates, and this may have worse security properties than using TLS and not validate the server certificate would have.

Many client environments do not have secure long-term storage, which is required to validate certificates. This makes it impossible to use server certificate validation on a large number of client systems.

When clients have the ability, they MUST validate the server certificate. For this reason, if a KDC presents a 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, the server certificate could be a X.509 certificate or an OpenPGP key. In this mode, the client need no processing related to id-krb5starttls-san.

When the server presents a X.509 server certificate, clients MAY use "Certification Path Validation" as described in [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,” May 2008.) 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

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, S., “Extended Kerberos Version 5 Key Distribution Center (KDC) Exchanges over TCP,” August 2007.).

7.  Acknowledgements

Miguel A. Garcia, Jeffrey Hutzelman, Sam Hartman, and Magnus Nyström (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 (MITM) attacks unless clients verify the KDC's credentials (X.509 certificate, OpenPGP key, etc) correctly.

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.

9.  References

9.1. Normative References

9.2. Informative References

Author's Address