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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
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Internet-Drafts are working documents of the Internet Engineering
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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
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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].
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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.
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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.
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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.
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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].
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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.
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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]
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Author's Address
Simon Josefsson
SJD
Email: simon@josefsson.org
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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.
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 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
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Acknowledgment
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Josefsson Expires April 7, 2007 [Page 12]
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