Using Kerberos V5 over the Transport Layer Security (TLS) protocol
draft-josefsson-kerberos5-starttls-04

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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
2.  Kerberos V5 STARTTLS Extension
3.  Channel Binding Pre-Authentication Data
4.  Examples
5.  STARTTLS aware KDC Discovery
6.  IANA Considerations
7.  Acknowledgements
8.  Security Considerations
9.  References
    9.1.  Normative References
    9.2.  Informative References
§  Author's Address
§  Intellectual Property and Copyright Statements

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 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 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 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.
• 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.
• 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.
• 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. 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.

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] (Williams, N., “On the Use of Channel Bindings to Secure Channels,” November 2007.).

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.

5.  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.

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

Jeffrey Hutzelman provided comments that improved the protocol and document.

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.

9.  References

9.1. Normative References

9.2. Informative References

Author's Address

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