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samba-mirror/third_party/heimdal/doc/standardisation/draft-josefsson-kerberos5-starttls-02.txt
Stefan Metzmacher 7055827b8f HEIMDAL: move code from source4/heimdal* to third_party/heimdal* 
This makes it clearer that we always want to do heimdal changes
via the lorikeet-heimdal repository.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Joseph Sutton <josephsutton@catalyst.net.nz>

Autobuild-User(master): Joseph Sutton <jsutton@samba.org>
Autobuild-Date(master): Wed Jan 19 21:41:59 UTC 2022 on sn-devel-184
2022-01-19 21:41:59 +00:00

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Network Working Group S. Josefsson
Internet-Draft SJD
Intended status: Standards Track October 21, 2006
Expires: April 24, 2007
Using Kerberos V5 over the Transport Layer Security (TLS) protocol
draft-josefsson-kerberos5-starttls-02
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
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Internet-Drafts are draft documents valid for a maximum of six months
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The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on April 24, 2007.
Copyright Notice
Copyright (C) The Internet Society (2006).
Josefsson Expires April 24, 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 24, 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 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
Josefsson Expires April 24, 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 24, 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 24, 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 ]
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 April 24, 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 24, 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 24, 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 24, 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 24, 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 24, 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
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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Acknowledgment
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Josefsson Expires April 24, 2007 [Page 12]
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