draft-josefsson-kerberos5-starttls-00.txt		draft-josefsson-kerberos5-starttls-01.txt
	Network Working Group S. Josefsson
	Expires: May 14, 2005		Network Working Group S. Josefsson
			Internet-Draft SJD
			Intended status: Standards Track October 4, 2006
			Expires: April 7, 2007
	Using Transport Layer Security (TLS) with Kerberos 5		Using Kerberos V5 over the Transport Layer Security (TLS) protocol
	draft-josefsson-kerberos5-starttls-00		draft-josefsson-kerberos5-starttls-01
	Status of this Memo		Status of this Memo
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	Copyright Notice		Copyright Notice
	Copyright (C) The Internet Society (2004).		Copyright (C) The Internet Society (2006).
	Abstract		Abstract
	This document specify how the Transport Layer Security (TLS) protocol		This document specify how the Kerberos V5 protocol can be transported
	is used in conjunction with the Kerberos 5 protocol.		over the Transport Layer Security (TLS) protocol, to provide
			additional security features.
	Table of Contents		Table of Contents
	1. Introduction and Background . . . . . . . . . . . . . . . . . 3		1. Introduction and Background . . . . . . . . . . . . . . . . . 3
	2. Extension Mechanism for TCP/IP transport . . . . . . . . . . . 4		2. Kerberos V5 STARTTLS Extension . . . . . . . . . . . . . . . . 5
	3. Kerberos 5 STARTTLS Extension . . . . . . . . . . . . . . . . 4		3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
	3.1 STARTTLS requested by client (extension 1) . . . . . . . . 4		4. STARTTLS aware KDC Discovery . . . . . . . . . . . . . . . . . 7
	3.2 STARTTLS request accepted by server (extension 2) . . . . 5		5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
	3.3 Proceeding after successful TLS negotiation . . . . . . . 5		6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
	3.4 Proceeding after failed TLS negotiation . . . . . . . . . 5		7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
	3.5 STARTTLS aware KDC Discovery . . . . . . . . . . . . . . . 5		7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
	3.6 Initial Authentication via TLS . . . . . . . . . . . . . . 5		7.2. Informative References . . . . . . . . . . . . . . . . . . 10
	4. Security Considerations . . . . . . . . . . . . . . . . . . . 6		Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
	5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6		Intellectual Property and Copyright Statements . . . . . . . . . . 12
	5.1 Normative References . . . . . . . . . . . . . . . . . . . . 6
	5.2 Informative References . . . . . . . . . . . . . . . . . . . 6
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7
	Intellectual Property and Copyright Statements . . . . . . . . 8
	1. Introduction and Background		1. Introduction and Background
	This document describe how Shishi, a Kerberos 5 [1] implementation,		This document describe how a Kerberos V5 [2] 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) [2] protocol.		(KDCs) to use the Transport Layer Security (TLS) [4] 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 [6], and		can be authentication using X.509 certificates, OpenPGP keys [7], and
	user name and passwords via SRP [5].		user name and passwords via SRP [6].
	An inconclusive list of the motivation for using TLS with Kerberos 5
	is given below.
	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).
	o Flexible authentication against KDC. Kerberos 5 assume the user		There are several reasons to use Kerberos V5 over TLS.
	knows a key (usually in the form of a password). Sometimes
	external factors make this hard to fulfill. 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 Kerberos exchanges are privacy protected. Part of many Kerberos		o Kerberos exchanges are privacy protected. Part of many Kerberos
	packets are transfered without privacy protection (i.e.,		packets are transfered without privacy protection (i.e.,
	encryption). That part contains information, such as the client		encryption). That part contains information, such as the client
	principal name, the server principal name, the encryption types		principal name, the server principal name, the encryption types
	supported by the client, the lifetime of tickets, etc. Revealing		supported by the client, the lifetime of tickets, etc. Revealing
	such information is, in some threat models, considered a problem.		such information is, in some threat models, considered a problem.
	o Prevents downgrade attacks affecting encryption types. The		o Prevents downgrade attacks affecting encryption types. The
	encryption type of the ticket in KDC-REQ are sent in the clear in		encryption type of the ticket in KDC-REQ are sent in the clear in
	Kerberos 5. This allows an attacker to replace the encryption		Kerberos 5. This allows an attacker to replace the encryption
	type with a compromised mechanisms, e.g. 56-bit DES. Since		type with a compromised mechanisms, e.g., 56-bit DES. Since
	clients in general cannot know the encryption types other servers		clients in general cannot know the encryption types other servers
	support, it is difficult for the client to detect if there was a		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		man-in-the-middle or if the remote server simply did not support a
	stronger mechanism. Clients could chose to refuse 56-bit DES		stronger mechanism. Clients could chose to refuse, e.g., 56-bit
	altogether, but in some environments this leads to operational		DES altogether, but in some environments this leads to operational
	difficulties.		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		o The TLS protocol has been studied by many parties. In some threat
	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
			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",		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 [4].		document are to be interpreted as described in RFC 2119 [1].
	2. Extension Mechanism for TCP/IP transport		2. Kerberos V5 STARTTLS Extension
	Kerberos 5 require Key Distribution Centers (KDCs) to accept requests		The STARTTLS extension uses the Kerberos V5 TCP extension mechanism
	over TCP. Each request and response is prefixed by 4 octets,		[3]. The extension uses bit #TBD in the extension bitmask.
	encoding an integer in network byte order, that indicate the length
	of the packet. The high bit of the 4 octet length field was reserved
	for future expansion. Servers that do not understand how to
	interpret a set high bit are required to return a KRB-ERROR with the
	KRB_ERR_FIELD_TOOLONG error code, and to close the TCP stream.
	We will use the reserved bit to provide an extension mechanism. When		The protocol is as follows. After the server has sent the 4-octet
	the reserved high bit is set, the remaining 31 bits of the 4 octets		value 0x00000000 to indicate support of this extension, the stream
	are treated as an extensible typed hole, and thus form a 31 bit		will be controlled by the TLS protocol and its framing. The TLS
	integer enumerating various extensions. Each of the values indicate		protocol is initiated by the client.
	a specific extended operation mode, two of which are used and defined
	here, and the rest are left for others to use.
	If the KDC do not understand a requested extension, it MUST return a		Typically, the client initiate the TLS handshake protocol by sending
	KRB-ERROR with a KRB_ERR_FIELD_TOOLONG value (prefixed by the 4 octet		a client hello, and the server responds, and the handshake continues
	length integer, with the high bit clear, as usual) and close the TCP		until it either succeed or fails.
	stream.
	The following table specify the meaning of the 31 lower bits in the 4		If for any reason the handshake fails, the STARTTLS protocol will
	octet field, when the high bit is set:		also fail, and the TLS error is used as the error indication.
	0 RESERVED.		If the handshake succeeds, the Kerberos V5 authentication protocol is
	1 STARTTLS requested by client.		performed within the protected TLS channel, like a normal TCP
	2 STARTTLS request accepted by server.		Kerberos V5 exchange. In particular, this means that every Kerberos
	3...2147483647 AVAILABLE for registration (via [email protected]).		V5 packet will be prefixed by a 4-octet length field, that indicate
	2147483648 RESERVED.		the length of the Kerberos V5 packet.
	3. Kerberos 5 STARTTLS Extension		3. Examples
	3.1 STARTTLS requested by client (extension 1)		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.
	When this message is sent by the client, the client is requesting the		Client Server
	server to start TLS negotiation on the TCP stream. The client MUST
	NOT start TLS negotiation immediately. Instead, the client wait for
	either a KRB-ERROR (sent normally, prefixed by a 4 octet length
	integer) indicating the server do not understand the set high bit, or
	4 octets which is to be interpreted as an integer in network byte
	order, where the high bit is set and the remaining 31 bit are
	interpreted as an integer specifying ``STARTTLS request accepted by
	server'' (extension 2). In the first case, the client infer that the
	server do not understand (or wish to support) STARTTLS, and can
	re-try using normal TCP, if unprotected Kerberos 5 exchanges are
	acceptable to the client policy. In the latter case, it should
	invoke TLS negotiation on the stream. If any other data is received,
	the client MUST close the TCP stream.
	3.2 STARTTLS request accepted by server (extension 2)		[ Kerberos V5 TCP extension mechanism negotiation starts ]
	This message should be sent by the server when it has received the		[0x70000000 & STARTTLS-bit] -------->
	extension 1 message. The message is an acknowledgment of the		[0x00000000]
	client's request to initiate STARTTLS on the channel. The server		<--------
	MUST then invoke a TLS negotiation.
	3.3 Proceeding after successful TLS negotiation		[ TLS negotiation starts ]
	If the TLS negotiation ended successfully, possibly also considering		ClientHello -------->
	client or server policies, the exchange within the TLS protected		ServerHello
	stream is performed like normal UDP Kerberos 5 exchanges, i.e., there		Certificate*
	is no TCP 4 octet length field before each packet. Instead each		ServerKeyExchange*
	Kerberos packet MUST be sent within one TLS record, so the		CertificateRequest*
	application can use the TLS record length as the Kerberos 5 packet		<-------- ServerHelloDone
	length.		Certificate*
			ClientKeyExchange
			CertificateVerify*
			[ChangeCipherSpec]
			Finished -------->
			[ChangeCipherSpec]
			<-------- Finished
	3.4 Proceeding after failed TLS negotiation		[ Kerberos V5 negotiation starts ]
	If the TLS negotiation fails, possibly due to client or server policy		Kerberos V5 AS-REQ -------->
	(e.g., inadequate support of encryption types in TLS, or lack of		Kerberos V5 AS-REP
	client or server authentication) the entity that detect the failure		<--------
	MUST disconnected the connection. It is expected that any error
	messages that explain the error condition is transfered by TLS.
	3.5 STARTTLS aware KDC Discovery		* Indicates optional or situation-dependent messages that are not
			always sent.
			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.
	Section 7.2.3 of Kerberos 5 [1] describe how Domain Name System (DNS)
	SRV records [3] can be used to find the address of an KDC. To locate
	a KDC that support the STARTTLS extension, we use the "_tls" domain.
	For example:		For example:
	_kerberos._tls._tcp.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.		_kerberos._tls.EXAMPLE.COM. IN SRV 0 0 88 kdc1.example.com.
	_kerberos._tls._tcp.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.		_kerberos._tls.EXAMPLE.COM. IN SRV 1 0 88 kdc2.example.com.
	3.6 Initial Authentication via TLS		5. IANA Considerations
	The server MAY consider the authentication performed by the TLS		The IANA is requested to allocate a bit in the "Kerberos TCP
	exchange as sufficient to issue Kerberos 5 tickets to the client,		Extensions" registry for the extension described in this document, as
	without requiring, e.g., pre-authentication. However, it is not an		per [3].
	error to require or use pre-authentication as well.
	The client may also indicate that it wishes to use TLS both for		6. Security Considerations
	authentication and data protection by using the NULL encryption type
	in its request. The server can decide from its local policy whether
	or not issuing tickets based solely on TLS authentication, and
	whether NULL encryption within TLS, is acceptable or not.
	4. Security Considerations		The security considerations in Kerberos V5, TLS, and the extension
			mechanism framework are inherited.
	Because the initial token is not protected, it is possible for an		To protect against the inherent downgrade attack in the extension
	active attacker to make it appear to the client that the server do		framework, it is suggested that implementations offer a policy to
	not support this extension. It is up to client configuration to		require that this extension is successfully negotiated. For
	disallow non-TLS connections, if that vulnerability is deemed		interoperability with implementations that do not support this
	unacceptable. For interoperability, we suggest the default behaviour		extension, it is suggested that the policy is disabled by default.
	should be to allow automatic fall back to TCP or UDP.
	The security considerations of both TLS and Kerberos 5 are inherited.		7. References
	Using TLS for authentication and/or data protection together with
	Kerberos alter the authentication logic fundamentally. Thus, it may
	be that even if the TLS and Kerberos 5 protocols and implementations
	were secure, the combination of TLS and Kerberos 5 described here
	could be insecure.
	No channel bindings are provided in the Kerberos messages. It is an		7.1. Normative References
	open question whether, and how, this could be solved. One idea for
	solving this may be to specify a new encryption algorithm in Kerberos
	5 that is similar to the NULL encryption algorithm, but also include
	the TLS session identifier.
	5. References		[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
			Levels", BCP 14, RFC 2119, March 1997.
	5.1 Normative References		[2] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos
			Network Authentication Service (V5)", RFC 4120, July 2005.
	[1] Neuman, C., "The Kerberos Network Authentication Service (V5)",		[3] Josefsson, S., "Extended Kerberos Version 5 Key Distribution
	draft-ietf-krb-wg-kerberos-clarifications-07 (work in progress),		Center (KDC) Exchanges Over TCP",
	September 2004.		draft-ietf-krb-wg-tcp-expansion-01 (work in progress),
			September 2006.
	[2] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC		[4] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
	2246, January 1999.		Protocol Version 1.1", RFC 4346, April 2006.
	[3] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for		[5] 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.
	5.2 Informative References		7.2. Informative References
	[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
	Levels", BCP 14, RFC 2119, March 1997.
	[5] Taylor, D., "Using SRP for TLS Authentication",		[6] Taylor, D., "Using SRP for TLS Authentication",
	draft-ietf-tls-srp-08 (work in progress), August 2004.		draft-ietf-tls-srp-12 (work in progress), June 2006.
	[6] Mavroyanopoulos, N., "Using OpenPGP keys for TLS		[7] Mavroyanopoulos, N., "Using OpenPGP keys for TLS
	authentication", draft-ietf-tls-openpgp-keys-05 (work in		authentication", draft-ietf-tls-openpgp-keys-10 (work in
	progress), April 2004.		progress), June 2006.
	Author's Address		Author's Address
	Simon Josefsson		Simon Josefsson
			SJD
	EMail: [email protected]		Email: [email protected]
	Intellectual Property Statement		Full Copyright Statement
			Copyright (C) The Internet Society (2006).
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