draft-josefsson-kerberos5-starttls-03.txt		draft-josefsson-kerberos5-starttls-04.txt
	Network Working Group S. Josefsson		Network Working Group S. Josefsson
	Internet-Draft SJD		Internet-Draft SJD
	Intended status: Standards Track December 3, 2007		Intended status: Informational December 5, 2008
	Expires: June 5, 2008		Expires: June 8, 2009
	Using Kerberos V5 over the Transport Layer Security (TLS) protocol		Using Kerberos V5 over the Transport Layer Security (TLS) protocol
	draft-josefsson-kerberos5-starttls-03		draft-josefsson-kerberos5-starttls-04
	Status of this Memo		Status of this Memo
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	Copyright Notice
	Copyright (C) The IETF Trust (2007).
	Abstract		Abstract
	This document specify how the Kerberos V5 protocol can be transported		This document specify how the Kerberos V5 protocol can be transported
	over the Transport Layer Security (TLS) protocol, to provide		over the Transport Layer Security (TLS) protocol, to provide
	additional security features.		additional security features. This document updates RFC 4120.
	Table of Contents		Table of Contents
	1. Introduction and Background . . . . . . . . . . . . . . . . . 3		1. Introduction and Background . . . . . . . . . . . . . . . . . 3
	2. Kerberos V5 STARTTLS Extension . . . . . . . . . . . . . . . . 5		2. Kerberos V5 STARTTLS Extension . . . . . . . . . . . . . . . . 5
	3. Channel Binding Pre-Authentication Data . . . . . . . . . . . 6		3. Channel Binding Pre-Authentication Data . . . . . . . . . . . 6
	4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 7		4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
	5. STARTTLS aware KDC Discovery . . . . . . . . . . . . . . . . . 8		5. STARTTLS aware KDC Discovery . . . . . . . . . . . . . . . . . 8
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9		6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 10		7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
	8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11		8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
	8.1. Normative References . . . . . . . . . . . . . . . . . . . 11		9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
	8.2. Informative References . . . . . . . . . . . . . . . . . . 11		9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12		9.2. Informative References . . . . . . . . . . . . . . . . . . 12
	Intellectual Property and Copyright Statements . . . . . . . . . . 13		Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
			Intellectual Property and Copyright Statements . . . . . . . . . . 14
	1. Introduction and Background		1. Introduction and Background
	This document describe how a Kerberos V5 [3] implementation may		This document describe how a Kerberos V5 [RFC4120] implementation may
	upgrade communication between clients and Key Distribution Centers		upgrade communication between clients and Key Distribution Centers
	(KDCs) to use the Transport Layer Security (TLS) [4] protocol.		(KDCs) to use the Transport Layer Security (TLS) [RFC5246] protocol.
	The TLS protocol offer integrity and privacy protected exchanges that		The TLS protocol offer integrity and privacy protected exchanges that
	can be authentication using X.509 certificates, OpenPGP keys [8], and		can be authentication using X.509 certificates, OpenPGP keys
	user name and passwords via SRP [7].		[RFC5081], and user name and passwords via SRP [RFC5054].
	There are several reasons to use Kerberos V5 over TLS.		There are several reasons to use Kerberos V5 over TLS.
	o Prevents downgrade attacks affecting, e.g., encryption types and		o Prevents downgrade attacks affecting, e.g., encryption types and
	pre-auth data negotiation. The encryption type field in KDC-REQ,		pre-auth data negotiation. The encryption type field in KDC-REQ,
	and the METHOD-DATA field with the requested pre-auth types from		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		the server in KDC_ERR_PREAUTH_REQUIRED errors in KDC-REP, are sent
	without integrity or privacy protection in Kerberos 5. This		without integrity or privacy protection in Kerberos 5. This
	allows an attacker to replace the encryption type with a		allows an attacker to replace the encryption type with a
	compromised encryption type, e.g., 56-bit DES, or request that		compromised encryption type, e.g., 56-bit DES, or request that
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	models, the designer prefer to reduce the number of protocols that		models, the designer prefer to reduce the number of protocols that
	can hurt the overall system security if they are compromised.		can hurt the overall system security if they are compromised.
	o Explicit server authentication of the KDC to the client. In		o Explicit server authentication of the KDC to the client. In
	traditional Kerberos 5, authentication of the KDC is proved as a		traditional Kerberos 5, authentication of the KDC is proved as a
	side effect that the KDC knows your encryption key (i.e., your		side effect that the KDC knows your encryption key (i.e., your
	password).		password).
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in RFC 2119 [1].		document are to be interpreted as described in RFC 2119 [RFC2119].
	2. Kerberos V5 STARTTLS Extension		2. Kerberos V5 STARTTLS Extension
	The STARTTLS extension uses the Kerberos V5 TCP extension mechanism		The STARTTLS extension uses the Kerberos V5 TCP extension mechanism
	[5]. The extension uses bit #TBD in the extension bitmask.		[RFC5021]. The extension uses bit #TBD in the extension bitmask.
	The protocol is as follows. After the server has sent the 4-octet		The protocol is as follows. After the server has sent the 4-octet
	value 0x00000000 to indicate support of this extension, the stream		value 0x00000000 to indicate support of this extension, the stream
	will be controlled by the TLS protocol and its framing. The TLS		will be controlled by the TLS protocol and its framing. The TLS
	protocol is initiated by the client.		protocol is initiated by the client.
	Typically, the client initiate the TLS handshake protocol by sending		Typically, the client initiate the TLS handshake protocol by sending
	a client hello, and the server responds, and the handshake continues		a client hello, and the server responds, and the handshake continues
	until it either succeed or fails.		until it either succeed or fails.
	If for any reason the handshake fails, the STARTTLS protocol will		If for any reason the handshake fails, the STARTTLS protocol will
	also fail, and the TLS error is used as the error indication.		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		If the handshake succeeds, the Kerberos V5 authentication protocol is
	performed within the protected TLS channel, like a normal TCP		performed within the protected TLS channel, like a normal TCP
	Kerberos V5 exchange. In particular, this means that every Kerberos		Kerberos V5 exchange. In particular, this means that every Kerberos
	V5 packet will be prefixed by a 4-octet length field, that indicate		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		the length of the Kerberos V5 packet. However, to conform with this
	specification, any KDC-REQ (AS-REQ or TGS-REQ) message MUST contain		specification, any KDC-REQ (AS-REQ or TGS-REQ) message MUST contain
	the "pa-channel-binding" pre-authentication data.		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		3. Channel Binding Pre-Authentication Data
	The pre-authentication structure is defined in RFC 4120 as:		The pre-authentication structure is defined in RFC 4120 as:
	PA-DATA ::= SEQUENCE {		PA-DATA ::= SEQUENCE {
	-- NOTE: first tag is [1], not [0]		-- NOTE: first tag is [1], not [0]
	padata-type [1] Int32,		padata-type [1] Int32,
	padata-value [2] OCTET STRING -- might be encoded AP-REQ		padata-value [2] OCTET STRING -- might be encoded AP-REQ
	}		}
	Here we define a new pre-authentication data, called "pa-channel-		Here we define a new pre-authentication data, called "pa-channel-
	binding". It has a padata-type integer value of #TBD. The contents		binding". It has a padata-type integer value of #TBD. The contents
	of the padata-value field is the channel binding data, as discussed		of the padata-value field is the channel binding data, as discussed
	in [6].		in [RFC5056].
	4. Examples		4. Examples
	A complete packet flow for a successful AS-REQ/REP exchange protected		A complete packet flow for a successful AS-REQ/REP exchange protected
	by this mechanism will be as follows. The "STARTTLS-bit" is a		by this mechanism will be as follows. The "STARTTLS-bit" is a
	4-octet value with only the bit allocated for this extension set.		4-octet value with only the bit allocated for this extension set.
	Client Server		Client Server
	[ Kerberos V5 TCP extension mechanism negotiation starts ]		[ Kerberos V5 TCP extension mechanism negotiation starts ]
	skipping to change at page 8, line 7		skipping to change at page 8, line 7
	Kerberos V5 AS-REQ -------->		Kerberos V5 AS-REQ -------->
	4 octet length field		4 octet length field
	Kerberos V5 AS-REP		Kerberos V5 AS-REP
	<--------		<--------
	* Indicates optional or situation-dependent messages that are not		* Indicates optional or situation-dependent messages that are not
	always sent.		always sent.
	5. STARTTLS aware KDC Discovery		5. STARTTLS aware KDC Discovery
	Section 7.2.3 of Kerberos V5 [3] describe how Domain Name System		Section 7.2.3 of Kerberos V5 [RFC4120] describe how Domain Name
	(DNS) SRV records [2] can be used to find the address of an KDC.		System (DNS) SRV records [RFC2782] can be used to find the address of
	Using the terminology of Section 7.2.3 of RFC 4120, we define a new		an KDC. We define a new Proto of "tls" to indicate that the
	Proto of "tls" to indicate that the particular KDC is intended to		particular KDC is intended to support this STARTTLS extension. The
	support this STARTTLS extension. The Service, Realm, TTL, Class,		Service, Realm, TTL, Class, SRV, Priority, Weight, Port and Target
	SRV, Priority, Weight, Port and Target have the same meaning as in		have the same meaning as in RFC 4120.
	RFC 4120.
	For example:		For example:
	_kerberos._tls.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.		_kerberos._tls.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.
	_kerberos._tls.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.		_kerberos._tls.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.
	6. IANA Considerations		6. IANA Considerations
	The IANA is requested to allocate a bit in the "Kerberos TCP		The IANA is requested to allocate a bit in the "Kerberos TCP
	Extensions" registry for the extension described in this document, as		Extensions" registry for the extension described in this document, as
	per [5].		per [RFC5021].
	7. Security Considerations		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		The security considerations in Kerberos V5, TLS, and the extension
	mechanism framework are inherited.		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		To protect against the inherent downgrade attack in the extension
	framework, it is suggested that implementations offer a policy to		framework, implementations SHOULD offer a policy mode that requires
	require that this extension is successfully negotiated. For		this extension to always be successfully negotiated, for a particular
	interoperability with implementations that do not support this		realm, or generally. For interoperability with implementations that
	extension, it is suggested that the policy is disabled by default.		do not support this extension, the policy mode SHOULD be disabled by
			default.
	8. References		9. References
	8.1. Normative References		9.1. Normative References
	[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Levels", BCP 14, RFC 2119, March 1997.		Requirement Levels", BCP 14, RFC 2119, March 1997.
	[2] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for		[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
	specifying the location of services (DNS SRV)", RFC 2782,		specifying the location of services (DNS SRV)", RFC 2782,
	February 2000.		February 2000.
	[3] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos		[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
	Network Authentication Service (V5)", RFC 4120, July 2005.		Kerberos Network Authentication Service (V5)", RFC 4120,
			July 2005.
	[4] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)		[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
	Protocol Version 1.1", RFC 4346, April 2006.		(TLS) Protocol Version 1.2", RFC 5246, August 2008.
	[5] Josefsson, S., "Extended Kerberos Version 5 Key Distribution		[RFC5021] Josefsson, S., "Extended Kerberos Version 5 Key
	Center (KDC) Exchanges over TCP", RFC 5021, August 2007.		Distribution Center (KDC) Exchanges over TCP", RFC 5021,
			August 2007.
	[6] Williams, N., "On the Use of Channel Bindings to Secure		[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
	Channels", RFC 5056, November 2007.		Channels", RFC 5056, November 2007.
	8.2. Informative References		9.2. Informative References
	[7] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin, "Using		[RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
	the Secure Remote Password (SRP) Protocol for TLS		"Using the Secure Remote Password (SRP) Protocol for TLS
	Authentication", RFC 5054, November 2007.		Authentication", RFC 5054, November 2007.
	[8] Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport Layer		[RFC5081] Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport
	Security (TLS) Authentication", RFC 5081, November 2007.		Layer Security (TLS) Authentication", RFC 5081,
			November 2007.
	Author's Address		Author's Address
	Simon Josefsson		Simon Josefsson
	SJD		Simon Josefsson Datakonsult AB
			Hagagatan 24
			Stockholm 113 47
			Sweden
	Email: simon@josefsson.org		Email: simon@josefsson.org
			URI: http://josefsson.org/
	Full Copyright Statement		Full Copyright Statement
	Copyright (C) The IETF Trust (2007).		Copyright (C) The IETF Trust (2008).
	This document is subject to the rights, licenses and restrictions		This document is subject to the rights, licenses and restrictions
	contained in BCP 78, and except as set forth therein, the authors		contained in BCP 78, and except as set forth therein, the authors
	retain all their rights.		retain all their rights.
	This document and the information contained herein are provided on an		This document and the information contained herein are provided on an
	"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS		"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
	OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND		OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
	THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS		THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
	OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF		OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
	skipping to change at page 13, line 44		skipping to change at line 323
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