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samba-mirror/third_party/heimdal/doc/standardisation/draft-josefsson-kerberos5-starttls-07.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 AB
Intended status: Informational July 31, 2009
Expires: February 1, 2010
Using Kerberos V5 over the Transport Layer Security (TLS) protocol
draft-josefsson-kerberos5-starttls-07
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material
from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from
the person(s) controlling the copyright in such materials, this
document may not be modified outside the IETF Standards Process, and
derivative works of it may not be created outside the IETF Standards
Process, except to format it for publication as an RFC or to
translate it into languages other than English.
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
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The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on February 1, 2010.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
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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.
Table of Contents
1. Introduction and Background . . . . . . . . . . . . . . . . . 4
2. Kerberos V5 STARTTLS Extension . . . . . . . . . . . . . . . . 6
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. STARTTLS aware KDC Discovery . . . . . . . . . . . . . . . . . 8
5. Server Certificates . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction and Background
This document describe how a Kerberos V5 [RFC4120] implementation may
upgrade communication between clients and Key Distribution Centers
(KDCs) to use the Transport Layer Security (TLS) [RFC5246] protocol.
The TLS protocol offer integrity and privacy protected exchanges that
can be authentication using X.509 certificates, OpenPGP keys
[RFC5081], and user name and passwords via SRP [RFC5054].
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 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.
o 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.
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
password).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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2. Kerberos V5 STARTTLS Extension
The STARTTLS extension uses the Kerberos V5 TCP extension mechanism
[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.
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.
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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 ]
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.
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4. STARTTLS aware KDC Discovery
Section 7.2.3 of Kerberos V5 [RFC4120] describe how Domain Name
System (DNS) SRV records [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.
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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.
Requiring server certificates to be used at all times would enable
attacks in those situations.
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 need to be able to 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 map realm names 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] 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.
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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].
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7. Acknowledgements
Jeffrey Hutzelman and Sam Hartman provided comments that improved the
protocol and document.
Josefsson Expires February 1, 2010 [Page 11]
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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.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5021] Josefsson, S., "Extended Kerberos Version 5 Key
Distribution Center (KDC) Exchanges over TCP", RFC 5021,
August 2007.
[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", RFC 5280, May 2008.
9.2. Informative References
[RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
"Using the Secure Remote Password (SRP) Protocol for TLS
Authentication", RFC 5054, November 2007.
[RFC5081] Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport
Layer Security (TLS) Authentication", RFC 5081,
November 2007.
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Author's Address
Simon Josefsson
Simon Josefsson Datakonsult AB
Hagagatan 24
Stockholm 113 47
Sweden
Email: simon@josefsson.org
URI: http://josefsson.org/
Josefsson Expires February 1, 2010 [Page 14]
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