< draft-ietf-tls-downgrade-scsv-00.txt		draft-ietf-tls-downgrade-scsv-01.txt >
	Network Working Group B. Moeller		Network Working Group B. Moeller
	Internet-Draft A. Langley		Internet-Draft A. Langley
	Updates: 2246, 4346, 5246 Google		Updates: 2246, 4346, 4347, 5246, 6347 Google
	(if approved) July 4, 2014		(if approved) November 10, 2014
	Intended status: Standards Track		Intended status: Standards Track
	Expires: January 5, 2015		Expires: May 14, 2015
	TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol		TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol
	Downgrade Attacks		Downgrade Attacks
	draft-ietf-tls-downgrade-scsv-00		draft-ietf-tls-downgrade-scsv-01
	Abstract		Abstract
	This document defines a Signaling Cipher Suite Value (SCSV) that		This document defines a Signaling Cipher Suite Value (SCSV) that
	prevents protocol downgrade attacks on the Transport Layer Security		prevents protocol downgrade attacks on the Transport Layer Security
	(TLS) protocol. It updates RFC 2246, RFC 4346, and RFC 5246.		(TLS) protocol. It updates RFC 2246, RFC 4346, and RFC 5246.
	Status of this Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on January 5, 2015.		This Internet-Draft will expire on May 14, 2015.
	Copyright Notice		Copyright Notice
	Copyright (c) 2014 IETF Trust and the persons identified as the		Copyright (c) 2014 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	2. Protocol values . . . . . . . . . . . . . . . . . . . . . . . 4		2. Protocol values . . . . . . . . . . . . . . . . . . . . . . . 3
	3. Server behavior . . . . . . . . . . . . . . . . . . . . . . . 5		3. Server behavior . . . . . . . . . . . . . . . . . . . . . . . 3
	4. Client behavior . . . . . . . . . . . . . . . . . . . . . . . 6		4. Client behavior . . . . . . . . . . . . . . . . . . . . . . . 4
	5. Security Considerations . . . . . . . . . . . . . . . . . . . 7		5. Operational Considerations . . . . . . . . . . . . . . . . . 5
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8		6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
	7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9		7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
	7.1. Normative References . . . . . . . . . . . . . . . . . . . 9		8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
	7.2. Informal References . . . . . . . . . . . . . . . . . . . 9		8.1. Normative References . . . . . . . . . . . . . . . . . . 7
	Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 10		8.2. Informative References . . . . . . . . . . . . . . . . . 7
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11		Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 7
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
	1. Introduction		1. Introduction
	To work around interoperability problems with legacy servers, many		To work around interoperability problems with legacy servers, many
	TLS client implementations do not rely on the TLS protocol version		TLS client implementations do not rely on the TLS protocol version
	negotiation mechanism alone, but will intentionally reconnect using a		negotiation mechanism alone, but will intentionally reconnect using a
	downgraded protocol if initial handshake attempts fail. Such clients		downgraded protocol if initial handshake attempts fail. Such clients
	may fall back to connections in which they announce a version as low		may fall back to connections in which they announce a version as low
	as TLS 1.0 (or even its predecessor, SSL 3.0) as the highest		as TLS 1.0 (or even its predecessor, SSL 3.0) as the highest
	supported version.		supported version.
	While such protocol downgrades can be a useful last resort for		While such fallback retries can be a useful last resort for
	connections to actual legacy servers, there's a risk that active		connections to actual legacy servers, there's a risk that active
	attackers could exploit the downgrade strategy to weaken the		attackers could exploit the downgrade strategy to weaken the
	cryptographic security of connections. Also, handshake errors due to		cryptographic security of connections. Also, handshake errors due to
	network glitches could similarly be misinterpreted as interaction		network glitches could similarly be misinterpreted as interaction
	with a legacy server and result in a protocol downgrade.		with a legacy server and result in a protocol downgrade.
	All unnecessary protocol downgrades are undesirable (e.g., from TLS		All unnecessary protocol downgrades are undesirable (e.g., from TLS
	1.2 to TLS 1.1 if both the client and the server actually do support		1.2 to TLS 1.1 if both the client and the server actually do support
	TLS 1.2); they can be particularly critical if they mean losing the		TLS 1.2); they can be particularly critical if they mean losing the
	TLS extension feature (when downgrading to SSL 3.0). This document		TLS extension feature (when downgrading to SSL 3.0). This document
	defines a Signaling Cipher Suite Value (SCSV) that can be employed to		defines a Signaling Cipher Suite Value (SCSV) that can be employed to
	prevent unintended protocol downgrades between clients and servers		prevent unintended protocol downgrades between clients and servers
	that comply to this document, by having the client indicate that the		that comply with this document, by having the client indicate that
	current connection attempt is merely a fallback.		the current connection attempt is merely a fallback, and by having
			the server return a fatal alert if it detects an inappropriate
			fallback. (The alert does not necessarily indicate an intentional
			downgrade attack, since network glitches too could result in
			inappropriate fallback retries.)
			The fallback SCSV defined in this document is not suitable substitute
			for proper TLS version negotiation. TLS implementations need to
			properly handle TLS version negotiation and extensibility mechanisms
			to avoid the security issues and connection delays associated with
			fallback retries."
	This specification applies to implementations of TLS 1.0 [RFC2246],		This specification applies to implementations of TLS 1.0 [RFC2246],
	TLS 1.1 [RFC4346], and TLS 1.2 [RFC5246]. (It is particularly		TLS 1.1 [RFC4346], and TLS 1.2 [RFC5246], and to implementations of
	relevant if such implementations also include support for predecessor		DTLS 1.0 [RFC4347] and DTLS 1.2 [RFC6347]. (It is particularly
			relevant if TLS implementations also include support for predecessor
	protocol SSL 3.0 [RFC6101].) It can be applied similarly to later		protocol SSL 3.0 [RFC6101].) It can be applied similarly to later
	protocol versions.		protocol versions.
	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 [RFC2119].		document are to be interpreted as described in [RFC2119].
	2. Protocol values		2. Protocol values
	This document defines a new TLS cipher suite value:		This document defines a new TLS cipher suite value:
	TLS_FALLBACK_SCSV {0x56, 0x00}		TLS_FALLBACK_SCSV {0x56, 0x00}
	This is a signaling cipher suite value (SCSV), i.e., it does not		This is a signaling cipher suite value (SCSV), i.e., it does not
	actually correspond to a suite of cryptosystems, and it can never be		actually correspond to a suite of cryptosystems, and it can never be
	selected by the server in the handshake; rather, its presence in the		selected by the server in the handshake; rather, its presence in the
	client hello message serves as a backwards-compatible signal from the		Client Hello message serves as a backwards-compatible signal from the
	client to the server.		client to the server.
	This document also allocates a new alert value in the TLS Alert		This document also allocates a new alert value in the TLS Alert
	Registry [RFC5246]:		Registry [RFC5246]:
	enum {		enum {
	/* ... */		/* ... */
	inappropriate_fallback(86),		inappropriate_fallback(86),
	/* ... */		/* ... */
	(255)		(255)
	skipping to change at page 5, line 14 ¶		skipping to change at page 4, line 8 ¶
	3. Server behavior		3. Server behavior
	This section specifies server behavior when receiving the		This section specifies server behavior when receiving the
	TLS_FALLBACK_SCSV cipher suite from a client in		TLS_FALLBACK_SCSV cipher suite from a client in
	ClientHello.cipher_suites.		ClientHello.cipher_suites.
	o If TLS_FALLBACK_SCSV appears in ClientHello.cipher_suites and the		o If TLS_FALLBACK_SCSV appears in ClientHello.cipher_suites and the
	highest protocol version supported by the server is higher than		highest protocol version supported by the server is higher than
	the version indicated in ClientHello.client_version, the server		the version indicated in ClientHello.client_version, the server
	MUST respond with an inappropriate_fallback alert.		MUST respond with a fatal inappropriate_fallback alert.
	Otherwise (either TLS_FALLBACK_SCSV does not appear, or it appears		o Otherwise (either TLS_FALLBACK_SCSV does not appear, or it appears
	and the client's protocol version is at least the highest protocol		and the client's protocol version is at least the highest protocol
	version supported by the server), the server proceeds with the		version supported by the server), the server proceeds with the
	handshake as usual.		handshake as usual.
			(A protocol version is supported by the server if, in response to
			appropriate Client Hello messages, the server would use it for
			ServerHello.server_version. If a particular protocol version is
			implemented but completely disabled by server settings, it is not
			considered supported. For example, if the implementation's highest
			protocol version is TLS 1.2 but the server operator has disabled this
			version, a TLS 1.1 Client Hello with TLS_FALLBACK_SCSV does not
			warrant responding with an inappropriate_fallback alert.)
	4. Client behavior		4. Client behavior
	The TLS_FALLBACK_SCSV cipher suite value is meant for use by clients		The TLS_FALLBACK_SCSV cipher suite value is meant for use by clients
	that repeat a connection attempt with a downgraded protocol in order		that repeat a connection attempt with a downgraded protocol (perform
	to avoid interoperability problems with legacy servers. This section		a "fallback retry") in order to work around interoperability problems
	specifies when to send it.		with legacy servers.
	o If a client sends a ClientHello.client_version containing a lower		o If a client sends a ClientHello.client_version containing a lower
	value than the latest (highest-valued) version supported by the		value than the latest (highest-valued) version supported by the
	client, it SHOULD include the TLS_FALLBACK_SCSV cipher suite value		client, it SHOULD include the TLS_FALLBACK_SCSV cipher suite value
	in ClientHello.cipher_suites. (Since the cipher suite list in the		in ClientHello.cipher_suites; see Section 6 for security
	ClientHello is ordered by preference, with the client's favorite		considerations for this recommendation. (Since the cipher suite
	choice first, signaling cipher suite values will generally appear		list in the ClientHello is ordered by preference, with the
	after all cipher suites that the client actually intends to		client's favorite choice first, signaling cipher suite values will
	negotiate.)		generally appear after all cipher suites that the client actually
			intends to negotiate.)
	However, as an exception to the above, when the client intends to		o As an exception to the above, when a client intends to resume a
	perform an abbreviated handshake to resume a previously negotiated
	session and sets ClientHello.client_version to the protocol		session and sets ClientHello.client_version to the protocol
	version negotiated for that session, the client MUST NOT include		version negotiated for that session, it MUST NOT include
	TLS_FALLBACK_SCSV in ClientHello.cipher_suites.		TLS_FALLBACK_SCSV in ClientHello.cipher_suites. (In this case, it
			is assumed that the client already knows the highest protocol
			version supported by the server: see [RFC5246], Appendix E.1.)
	Note that in the above, a protocol version is not considered		o If a client sets ClientHello.client_version to its highest
	supported by the client if it has been disabled by any applicable		supported protocol version, it MUST NOT include TLS_FALLBACK_SCSV
	system or user settings: it is about the highest protocol version		in ClientHello.cipher_suites.
	that the client would attempt using in a handshake, not about the
	highest protocol version implemented if its use is not actually
	enabled. (For example, if the implementation supports TLS 1.2 but
	the user has disabled this protocol version, a TLS 1.1 handshake is
	expected and does not warrant sending TLS_FALLBACK_SCSV.)
	5. Security Considerations		(A protocol version is supported by the client if the client normally
			attempts to use it in handshakes. If a particular protocol version
			is implemented but completely disabled by client settings, it is not
			considered supported. For example, if the implementation's highest
			protocol version is TLS 1.2 but the user has disabled this version, a
			TLS 1.1 handshake is expected and does not warrant sending
			TLS_FALLBACK_SCSV.)
			Fallback retries could be caused by events such as network glitches,
			and a client including TLS_FALLBACK_SCSV in ClientHello.cipher_suites
			may receive an inappropriate_fallback alert in response, indicating
			that the server supports a higher protocol version. Thus, if a
			client intends to use retries to work around network glitches, it
			should then retry with the highest version it supports.
			If a client keeps track of the highest protocol version apparently
			supported by a particular server for use in
			ClientHello.client_version later, then if the client receives an
			inappropriate_fallback alert from that server, it MUST clear the
			memorized highest supported protocol version. (Without the alert, it
			is a good idea -- but outside of the scope of this document -- for
			clients to clear that state after a time-out, since the server's
			highest protocol version could change over time.)
			For clients that use client-side TLS False Start [false-start], it is
			important to note that the TLS_FALLBACK_SCSV mechanism cannot protect
			the first round of application data sent by the client: refer to the
			Security Considerations in [false-start], Section 6.
			5. Operational Considerations
			Updating legacy server clusters to simultaneously add support for
			newer protocol versions and support for TLS_FALLBACK_SCSV can have
			complications, if the legacy server implementation is not "version-
			tolerant" (cannot properly handle Client Hello messages for newer
			protocol versions): fallback retries required for interoperability
			with old server nodes might be rejected by updated server nodes.
			Updating the server cluster in two consecutive steps makes this safe:
			first, update the server software but leave the highest supported
			version unchanged (by disabling newer versions in server settings);
			then, after all legacy (version-intolerant) implementations have been
			removed, change server settings to allow new protocol versions.
			6. Security Considerations
	Section 4 does not require client implementations to send		Section 4 does not require client implementations to send
	TLS_FALLBACK_SCSV in any particular case, it merely recommends it;		TLS_FALLBACK_SCSV in any particular case, it merely recommends it;
	behavior can be adapted according to the client's security needs.		behavior can be adapted according to the client's security needs. It
	For example, during the initial deployment of a new protocol version		is important to remember that omitting TLS_FALLBACK_SCSV enables
	(when some interoperability problems may have to be expected),		downgrade attacks, so implementors must take into account whether the
	smoothly falling back to the previous protocol version in case of		protocol version given by ClientHello.client_version still provides
	problems may be preferrable to potentially not being able to connect		an acceptable level of protection. For example, during the initial
	at all: so TLS_FALLBACK_SCSV could be omitted for this particular		deployment of a new protocol version (when some interoperability
	protocol downgrade step.		problems may have to be expected), smoothly falling back to the
			previous protocol version in case of problems may be preferable to
			potentially not being able to connect at all: so TLS_FALLBACK_SCSV
			could be omitted for this particular protocol downgrade step.
	However, it is strongly recommended to send TLS_FALLBACK_SCSV when		However, it is strongly recommended to send TLS_FALLBACK_SCSV when
	downgrading to SSL 3.0 as the CBC cipher suites in SSL 3.0 have		downgrading to SSL 3.0 as the CBC cipher suites in SSL 3.0 have
	weaknesses that cannot be addressed by implementation workarounds		weaknesses that cannot be addressed by implementation workarounds
	like the remaining weaknesses in later (TLS) protocol versions.		like the remaining weaknesses in later (TLS) protocol versions.
	6. IANA Considerations		7. IANA Considerations
	[[ NOTE IN DRAFT: The requested registry allocation requires		[[ TO BE REMOVED: The requested registry allocations require
	Standards Action, i.e., will only be official with the IESG's		Standards Action, i.e., will only be official with the IESG's
	Standards Track RFC approval. Since this document is currently an		Standards Track RFC approval. Since this document is currently an
	Internet-Draft, IANA so far has in fact not added the cipher suite		Internet-Draft, IANA so far has in fact not added the cipher suite
	number to the registry. ]]		number and alert number to the respective registries. The values as
			shown are used in early implementations.
			+-----------+-------------------+---------+-----------------+
			| Value | Description | DTLS-OK | Reference |
			+-----------+-------------------+---------+-----------------+
			| 0x56,0x00 | TLS_FALLBACK_SCSV | Y | (this document) |
			+-----------+-------------------+---------+-----------------+
			http://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml
			#tls-parameters-4
			+-------+------------------------+---------+-----------------+
			| Value | Description | DTLS-OK | Reference |
			+-------+------------------------+---------+-----------------+
			| 86 | inappropriate_fallback | Y | (this document) |
			+-------+------------------------+---------+-----------------+
			http://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml
			#tls-parameters-6
			]]
	IANA has added TLS cipher suite number 0x56,0x00 with name		IANA has added TLS cipher suite number 0x56,0x00 with name
	TLS_FALLBACK_SCSV to the TLS Cipher Suite registry.		TLS_FALLBACK_SCSV to the TLS Cipher Suite registry, and alert number
			86 with name inappropriate_fallback to the TLS Alert registry.
	7. References		8. References
	7.1. Normative References		8.1. Normative References
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119, March 1997.		Requirement Levels", BCP 14, RFC 2119, March 1997.
	[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",		[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
	RFC 2246, January 1999.		RFC 2246, January 1999.
	[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security		[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
	(TLS) Protocol Version 1.1", RFC 4346, April 2006.		(TLS) Protocol Version 1.1", RFC 4346, April 2006.
			[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
			Security", RFC 4347, April 2006.
	[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security		[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
	(TLS) Protocol Version 1.2", RFC 5246, August 2008.		(TLS) Protocol Version 1.2", RFC 5246, August 2008.
	7.2. Informal References		[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
			Security Version 1.2", RFC 6347, January 2012.
			8.2. Informative References
	[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure		[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
	Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,		Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
	August 2011.		August 2011.
			[false-start]
			Langley, A., Modadugu, N., and B. Moeller, "Transport
			Layer Security (TLS) False Start", Work in Progress,
			draft-bmoeller-tls-falsestart-01, November 2014.
	Appendix A. Acknowledgements		Appendix A. Acknowledgements
	This specification was inspired by an earlier proposal by Eric		This specification was inspired by an earlier proposal by Eric
	Rescorla.		Rescorla. We also thank Daniel Kahn Gillmor, Joe Saloway, Brian
			Smith, Martin Thomson, and others in the TLS Working Group for their
			feedback and suggestions.
	Authors' Addresses		Authors' Addresses
	Bodo Moeller		Bodo Moeller
	Google Switzerland GmbH		Google Switzerland GmbH
	Brandschenkestrasse 110		Brandschenkestrasse 110
	Zurich 8002		Zurich 8002
	Switzerland		Switzerland
	Email: bmoeller@acm.org		Email: bmoeller@acm.org
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