< draft-sheffer-tls-bcp-01.txt		draft-sheffer-tls-bcp-02.txt >
	tls Y. Sheffer		UTA Y. Sheffer
	Internet-Draft Porticor		Internet-Draft Porticor
	Intended status: BCP R. Holz		Intended status: Best Current Practice R. Holz
	Expires: March 24, 2014 TUM		Expires: August 17, 2014 TUM
	September 20, 2013		P. Saint-Andre
			&yet
			February 13, 2014
	Recommendations for Secure Use of TLS and DTLS		Recommendations for Secure Use of TLS and DTLS
	draft-sheffer-tls-bcp-01		draft-sheffer-tls-bcp-02
	Abstract		Abstract
	Over the last few years there have been several serious attacks on		Transport Layer Security (TLS) and Datagram Transport Security Layer
	TLS, including attacks on its most commonly used ciphers and modes of		(DTLS) are widely used to protect data exchanged over application
	operation. This document offers recommendations on securely using		protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the
	the TLS and DTLS protocols, given existing standards and		last few years, several serious attacks on TLS have emerged,
	implementations.		including attacks on its most commonly used cipher suites and modes
			of operation. This document provides recommendations for improving
			the security of both software implementations and deployed services
			that use TLS and DTLS.
	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
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	This Internet-Draft will expire on March 24, 2014.		This Internet-Draft will expire on August 17, 2014.
	Copyright Notice		Copyright Notice
	Copyright (c) 2013 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.
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	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
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	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	1.1. Conventions used in this document . . . . . . . . . . 3		2. Conventions used in this document . . . . . . . . . . . . . . 3
	2. Attacks on TLS . . . . . . . . . . . . . . . . . . . . 3		3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 3
	2.1. BEAST . . . . . . . . . . . . . . . . . . . . . . . . 4		3.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 3
	2.2. Lucky Thirteen . . . . . . . . . . . . . . . . . . . . 4		3.2. Fallback to SSL . . . . . . . . . . . . . . . . . . . . . 4
	2.3. Attacks on RC4 . . . . . . . . . . . . . . . . . . . . 4		3.3. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . 4
	2.4. Compression Attacks: CRIME and BREACH . . . . . . . . 4		3.4. Public Key Length . . . . . . . . . . . . . . . . . . . . 6
	3. Selection Criteria . . . . . . . . . . . . . . . . . . 4		3.5. Compression . . . . . . . . . . . . . . . . . . . . . . . 6
	4. Recommendations . . . . . . . . . . . . . . . . . . . 5		3.6. Session Resumption . . . . . . . . . . . . . . . . . . . 6
	4.1. Summary . . . . . . . . . . . . . . . . . . . . . . . 5		4. Detailed Guidelines . . . . . . . . . . . . . . . . . . . . . 6
	4.2. Cipher Suite Negotiation Details . . . . . . . . . . . 6		4.1. Cipher Suite Negotiation Details . . . . . . . . . . . . 7
	4.3. Downgrade Attacks . . . . . . . . . . . . . . . . . . 6		4.2. Alternative Cipher Suites . . . . . . . . . . . . . . . . 7
	4.4. Alternatives . . . . . . . . . . . . . . . . . . . . . 6		5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
	5. Implementation Status . . . . . . . . . . . . . . . . 7		6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
	6. Security Considerations . . . . . . . . . . . . . . . 8		6.1. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 8
	6.1. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . 8		6.2. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 8
	6.2. Perfect Forward Secrecy (PFS) . . . . . . . . . . . . 8		7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
	6.3. Session Resumption . . . . . . . . . . . . . . . . . . 9		8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
	7. IANA Considerations . . . . . . . . . . . . . . . . . 9		8.1. Normative References . . . . . . . . . . . . . . . . . . 9
	8. Acknowledgements . . . . . . . . . . . . . . . . . . . 9		8.2. Informative References . . . . . . . . . . . . . . . . . 10
	9. References . . . . . . . . . . . . . . . . . . . . . . 9		Appendix A. Appendix: Change Log . . . . . . . . . . . . . . . . 11
	9.1. Normative References . . . . . . . . . . . . . . . . . 9		A.1. -02 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
	9.2. Informative References . . . . . . . . . . . . . . . . 10		A.2. -01 . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
	Appendix A. Appendix: Change Log . . . . . . . . . . . . . . . . . 12		A.3. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
	A.1. -01 . . . . . . . . . . . . . . . . . . . . . . . . . 12		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
	A.2. -00 . . . . . . . . . . . . . . . . . . . . . . . . . 12
	Authors' Addresses . . . . . . . . . . . . . . . . . . 12
	1. Introduction		1. Introduction
	Over the last few years there have been several major attacks on TLS		Transport Layer Security (TLS) and Datagram Transport Security Layer
	[RFC5246], including attacks on its most commonly used ciphers and		(DTLS) are widely used to protect data exchanged over application
	modes of operation. Details are given in Section 2, but suffice it		protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP. Over the
	to say that both AES-CBC and RC4, which together make up for most		last few years, several serious attacks on TLS have emerged,
	current usage, have been seriously attacked in the context of TLS.		including attacks on its most commonly used cipher suites and modes
			of operation. For instance, both AES-CBC and RC4, which together
	Given these issues, there is need for IETF guidance on how TLS can be		comprise most current usage, have been attacked in the context of
	used securely. Unlike most IETF documents, this is guidance for		TLS. A companion document [I-D.sheffer-uta-tls-attacks] provides
	deployers, as well as for implementers. In fact the recommendations		detailed information about these attacks.
	below call for the use of widely implemented algorithms, which are
	not seeing widespread use today.
	Rather than standardizing new mechanisms in TLS, our goal is to		Because of these attacks, those who implement and deploy TLS and DTLS
	recommend a few already-specified mechanisms and cipher suites, and		need updated guidance on how TLS can be used securely. Note that
	to encourage the industry to use them in order to improve the overall		this document provides guidance for deployed services, as well as
	security of TLS-protected network traffic. When picking these		software implementations. In fact, this document calls for the
	mechanisms, we consider their security, their technical maturity and		deployment of algorithms that are widely implemented but not yet
	interoperability, as well as their prevalence at the time of writing.		widely deployed.
	This recommendation applies to both TLS and DTLS. TLS 1.3, when it		The recommendations herein take into consideration the security of
	is standardized and deployed in the field, should resolve the current		various mechanisms, their technical maturity and interoperability,
			and their prevalence in implementatios at the time of writing. These
			recommendations apply to both TLS and DTLS. TLS 1.3, when it is
			standardized and deployed in the field, should resolve the current
	vulnerabilities while providing significantly better functionality,		vulnerabilities while providing significantly better functionality,
	and will very likely obsolete the current document.		and will very likely obsolete the current document.
	Our knowledge about the strength of various algorithms and feasible		Community knowledge about the strength of various algorithms and
	attacks can change quickly, and experience shows that a crypto BCP is		feasible attacks can change quickly, and experience shows that a
	a point-in-time statement more than other BCPs. Readers are advised		security BCP is a point-in-time statement. Readers are advised to
	to seek out any errata or udpates that apply to this document.		seek out any errata or updates that apply to this document.
	1.1. Conventions used in this document
	[[Are we normative? Currently we're not and this section might go		2. Conventions used in this document
	away.]]
	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. Attacks on TLS		3. Recommendations
	This section lists the attacks that motivated the current		3.1. Protocol Versions
	recommendations. This is not intended to be an extensive survey of
	TLS's security.
	While there are widely deployed mitigations for some of the attacks		It is important both to stop using old, less secure versions of SSL/
	listed below, we believe that their root causes necessitate a more		TLS and to start using modern, more secure versions. Therefore:
	systemic solution.
	2.1. BEAST		o Implementations MUST NOT negotiate SSL version 2.
	The BEAST attack [BEAST] uses issues with the TLS 1.0 implementation		Rationale: SSLv2 has serious security vulnerabilities [RFC6176].
	of CBC (that is, predictable IV) to decrypt parts of a packet, and
	specifically shows how this can be used to decrypt HTTP cookies when
	run over TLS.
	2.2. Lucky Thirteen		o Implementations SHOULD NOT negotiate SSL version 3.
	A consequence of the MAC-then-encrypt design in all current versions		Rationale: SSLv3 [RFC6101] was an improvement over SSLv2 and
	of TLS is the existence of padding oracle attacks [Padding-Oracle].		plugged some significant security holes, but did not support
	A recent incarnation of these attacks is the Lucky Thirteen attack		strong cipher suites.
	[CBC-Attack], a timing side-channel attack that allows the attacker
	to decrypt arbitrary ciphertext.
	2.3. Attacks on RC4		o Implementations MAY negotiate TLS version 1.0 [RFC2246].
	The RC4 algorithm [RC4] has been used with TLS (and previously, SSL)		Rationale: TLS 1.0 (published in 1999) includes a way to downgrade
	for many years. Attacks have also been known for a long time, e.g.		the connection to SSLv3 and does not support more modern, strong
	[RC4-Attack-FMS]. But recent attacks ([RC4-Attack],		cipher suites.
	[RC4-Attack-AlF]) have weakened this algorithm even more. See
	[I-D.popov-tls-prohibiting-rc4] for more details.
	2.4. Compression Attacks: CRIME and BREACH		o Implementations MAY negotiate TLS version 1.1 [RFC4346].
	The CRIME attack [CRIME] allows an active attacker to decrypt		Rationale: TLS 1.1 (published in 2006) prevents downgrade attacks
	cyphertext (specifically, cookies) when TLS is used with protocol-		to SSL, but does not support certain stronger cipher suites.
	level compression.
	The BREACH attack [BREACH] makes similar use of HAdded TTP-level		o Implementations MUST support, and prefer to negotiate, TLS version
	compression, which is much more prevalent than compression at the TLS		1.2 [RFC5246].
	level, to decrypt secret data passed in the HTTP response.
	The former attack can be mitigated by disabling TLS compression, as		Rationale: Several stronger cipher suites are available only with
	recommended below. We are not aware of mitigations at the protocol		TLS 1.2 (published in 2008).
	level to the latter attack, and so application-level mitigations are
	needed (see [BREACH]). For example, implementations of HTTP that use
	CSRF tokens will need to randomize them even when the recommendations
	of the current document are adopted.
	3. Selection Criteria		As of the date of this writing, the latest version of TLS is 1.2.
			When TLS is updated to a newer version, this document will be updated
			to recommend support for the latest version. If this document is not
			updated in a timely manner, it can be assumed that support for the
			latest version of TLS is recommended.
	Given the above attacks, we are proposing that deployers opt for a		3.2. Fallback to SSL
	specific cipher suite when negotiating TLS. We have used the
	following criteria when framing our recommendations:
	o The cipher suite must be secure in default use, and should not		Some client implementations revert to SSLv3 if the server rejected
	require any additional security measures beyond those defined in		higher versions of SSL/TLS. This fallback can be forced by a MITM
	the standard.		attacker. Moreover, IP scans [[reference?]] show that SSLv3-only
	o The cipher suite must be widely implemented, i.e. available in a		servers amount to only about 3% of the current web server population.
	large percentage of popular cryptographic libraries.		Therefore, by default clients SHOULD NOT fall back from TLS to SSLv3.
	o The cipher suite must have undergone a significant amount of
	analysis, and the algorithm and mode of operation must both be
	standardized by relevant organizations.
	o We prefer cipher suites that provide client-side privacy and
	perfect forward secrecy, i.e. those that use ephemeral Diffie-
	Hellman. See Section 6.2 for more details.
	o As currently specified and implemented, elliptic curve groups are
	preferable over modular DH groups: they are easier and safer to
	use within TLS.
	o When there are multiple key sizes available, we have chosen the
	current industry standard, 128 bits of strength. Of course
	deployers are free to opt for a stronger cipher suite.
	4. Recommendations		3.3. Cipher Suites
	Following are recommendations for people implementing and deploying		It is important both to stop using old, insecure cipher suites and to
	client and server-side TLS.		start using modern, more secure cipher suites. Therefore:
	4.1. Summary		o Implementations MUST NOT negotiate the NULL cipher suites.
	Based on the criteria above, we recommend using as a preferred cipher		Rationale: The NULL cipher suites offer no encryption whatsoever
	suite the following:		and thus are completely insecure.
	o TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5829]		o Implementations MUST NOT negotiate RC4 cipher suites
	It is noted that the above cipher suite is an authenticated		Rationale: The RC4 stream cipher has a variety of cryptographic
	encryption (AEAD) algorithm [RFC5116], and therefore requires the use		weaknesses, as documented in [I-D.popov-tls-prohibiting-rc4].
	of TLS 1.2.
	We recommend using 2048-bit server certificates, with a SHA-256		o Implementations MUST NOT negotiate cipher suites offering only so-
	fingerprint. See [CAB-Baseline] for more details.		called "export-level" encryption (including algorithms with 40
			bits or 56 bits of security).
			Rationale: These cipher suites are deliberately "dumbed down" and
			are very easy to break.
			o Implementations SHOULD NOT negotiate cipher suites that use
			algorithms offering less than 128 bits of security (even if they
			advertise more bits, such as the 168-bit 3DES cipher suites).
			Rationale: Although these cipher suites are not actively subject
			to breakage, their useful life is short enough that stronger
			cipher suites are desirable.
			o Implementations SHOULD prefer cipher suites that use algorithms
			with at least 128 (and, if possible, 256) bits of security.
			Rationale: Although the useful life of such cipher suites is
			unknown, it is probably at least several years for the 128-bit
			ciphers and "until the next fundamental technology breakthrough"
			for 256-bit ciphers.
			o Implementations MUST support, and SHOULD prefer to negotiate,
			cipher suites offering forward secrecy, such as those in the
			"EDH", "DHE", and "ECDHE" families.
			Rationale: Forward secrecy (sometimes called "perfect forward
			secrecy") prevents the recovery of information that was encrypted
			with older session keys, thus limiting the amount of time during
			which attacks can be successful.
			Given the foregoing considerations, implementation of the following
			cipher suites is RECOMMENDED (see [RFC5289] for details):
			o TLS_DHE_RSA_WITH_AES_128_GCM_SHA256
			o TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
			o TLS_DHE_RSA_WITH_AES_256_GCM_SHA384
			o TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
			We suggest that TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 be preferred in
			general.
			Unfortunately, those cipher suites are supported only in TLS 1.2
			since they are authenticated encryption (AEAD) algorithms [RFC5116].
			A future version of this document might recommend cipher suites for
			earlier versions of TLS.
	[RFC4492] allows clients and servers to negotiate ECDH parameters		[RFC4492] allows clients and servers to negotiate ECDH parameters
	(curves). We recommend that clients and servers prefer verifiably		(curves). Clients and servers SHOULD prefer verifiably random curves
	random curves (specifically Brainpool P-256, brainpoolp256r1		(specifically Brainpool P-256, brainpoolp256r1 [RFC7027]), and fall
	[I-D.merkle-tls-brainpool]), and fall back to the commonly used NIST		back to the commonly used NIST P-256 (secp256r1) curve [RFC4492]. In
	P-256 (secp256r1) [RFC4492]. In addition, clients should send an		addition, clients SHOULD send an ec_point_formats extension with a
	ec_point_formats extension with a single element, "uncompressed".		single element, "uncompressed".
	We recommend to always disable TLS-level compression ([RFC5246], Sec.		3.4. Public Key Length
	6.2.2).		Because Diffie-Hellman keys of 1024 bits are estimated to be roughly
			equivalent to 80-bit symmetric keys, it is better to use longer keys
			for the "DH" family of cipher suites. Unfortunately, some existing
			software cannot handle (or cannot easily handle) key lengths greater
			than 1024 bits. The most common workaround for these systems is to
			prefer the "ECDHE" family of cipher suites instead of the "DH"
			family, then use longer keys. Key lengths of at least 2048 bits are
			RECOMMENDED, since they are estimated to be roughly equivalent to
			112-bit symmetric keys and might be sufficient for at least the next
			10 years. In addition to 2048-bit server certificates, the use of
			SHA-256 fingerprints is RECOMMENDED (see [CAB-Baseline] for more
			details).
	Finally, we recommend that clients disable fallback to SSLv3 (see		Note: The foregoing recommendations are preliminary and will likely
	Section 4.3).		be corrected and enhanced in a future version of this document.
	4.2. Cipher Suite Negotiation Details		3.5. Compression
	We recommend that clients include the above cipher suite as the first		Implementations and deployments SHOULD disable TLS-level compression
	proposal to any server, unless they have prior knowledge that the		([RFC5246], Sec. 6.2.2).
	server cannot respond to a TLS 1.2 client_hello message.
	We recommend that servers prefer this cipher suite (or a similar but		3.6. Session Resumption
	stronger one) whenever it is proposed, even if it is not the first
	proposal.
	Both clients and servers should include the "Supported Elliptic		If TLS session resumption is used, care ought to be taken to do so
			safely. In particular, the resumption information (either session
			IDs [RFC5246] or session tickets [RFC5077]) needs to be authenticated
			and encrypted to prevent modification or eavesdropping by an
			attacker. For session tickets, a strong cipher suite SHOULD be used
			when encrypting the ticket (as least as strong as the main TLS cipher
			suite); ticket keys MUST be changed regularly, e.g. once every week,
			so as not to negate the effect of forward secrecy. Session ticket
			validity SHOULD be limited to a reasonable duration (e.g. 1 day), so
			as not to negate the benefits of forward secrecy.
			4. Detailed Guidelines
			The following sections provide more detailed information about the
			recommendations listed above.
			4.1. Cipher Suite Negotiation Details
			Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the
			first proposal to any server, unless they have prior knowledge that
			the server cannot respond to a TLS 1.2 client_hello message.
			Servers SHOULD prefer this cipher suite (or a similar but stronger
			one) whenever it is proposed, even if it is not the first proposal.
			Both clients and servers SHOULD include the "Supported Elliptic
	Curves" extension [RFC4492].		Curves" extension [RFC4492].
	Clients are of course free to offer stronger cipher suites, e.g.		Clients are of course free to offer stronger cipher suites, e.g.
	using AES-256; when they do, the server should prefer the stronger		using AES-256; when they do, the server SHOULD prefer the stronger
	cipher suite unless there are reasons (e.g. performance) to choose		cipher suite unless there are compelling reasons (e.g., seriously
	otherwise.		degraded performance) to choose otherwise.
	Note that other profiles of TLS 1.2 exist that use different cipher		Note that other profiles of TLS 1.2 exist that use different cipher
	suites. For example, [RFC6460] defines a profile that uses the		suites. For example, [RFC6460] defines a profile that uses the
	TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and		TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
	TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites.		TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites.
	This document is not an application profile standard, in the sense of		This document is not an application profile standard, in the sense of
	Sec. 9 of [RFC5246]. As a result, clients and servers are still		Sec. 9 of [RFC5246]. As a result, clients and servers are still
	required to support the TLS mandatory cipher suite,		required to support the TLS mandatory cipher suite,
	TLS_RSA_WITH_AES_128_CBC_SHA.		TLS_RSA_WITH_AES_128_CBC_SHA.
	4.3. Downgrade Attacks		4.2. Alternative Cipher Suites
	Some client implementations revert to SSLv3 if the server rejected
	higher versions of SSL/TLS. This fallback can be forced by a MITM
	attacker. Moreover, IP scans [[reference?]] show that SSLv3-only
	servers amount to about 3% of the current server population. As a
	result, we recommend that by default, clients should avoid falling
	back to SSLv3.
	4.4. Alternatives
	Elliptic Curves Cryptography is not universally deployed for several		Elliptic Curves Cryptography is not universally deployed for several
	reasons, including its complexity compared to modular arithmetic and		reasons, including its complexity compared to modular arithmetic and
	longstanding IPR concerns. On the other hand, there are two related		longstanding IPR concerns. On the other hand, there are two related
	issues hindering effective use of modular Diffie-Hellman cipher		issues hindering effective use of modular Diffie-Hellman cipher
	suites in TLS:		suites in TLS:
	o There are no protocol mechanisms to negotiate the DH groups or		o There are no protocol mechanisms to negotiate the DH groups or
	parameter lengths supported by client and server.		parameter lengths supported by client and server.
	o There are widely deployed client implementations that reject		o There are widely deployed client implementations that reject
	received DH parameters, if they are longer than 1024 bits.		received DH parameters, if they are longer than 1024 bits.
	We note that with DHE and ECDHE cipher suites, the TLS master key		We note that with DHE and ECDHE cipher suites, the TLS master key
	only depends on the Diffie Hellman parameters and not on the strength		only depends on the Diffie Hellman parameters and not on the strength
	the the RSA certificate; moreover, 1024 bits DH parameters are		the the RSA certificate; moreover, 1024 bits DH parameters are
	generally considered insufficient at this time.		generally considered insufficient at this time.
	Because of the above, we recommend using (in priority order):		Because of the above, we recommend using (in priority order):
	1. Elliptic Curve DHE with negotiated parameters, as described in		1. Elliptic Curve DHE with negotiated parameters [RFC5289]
	Section 4.1.
	2. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288], with 2048-bit		2. TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288], with 2048-bit
	Diffie-Hellman parameters.		Diffie-Hellman parameters
	3. The same cipher suite, with 1024-bit parameters.		3. The same cipher suite, with 1024-bit parameters.
	With modular ephemeral DH, deployers should carefully evaluate		With modular ephemeral DH, deployers SHOULD carefully evaluate
	interoperability vs. security considerations when configuring their		interoperability vs. security considerations when configuring their
	TLS endpoints.		TLS endpoints.
	5. Implementation Status		5. IANA Considerations
	Since this document does not propose a new protocol or a new cipher
	suite, we do not provide a full implementation status, as per
	[RFC6982]. However it is useful to list some known existing
	implementations of the recommended cipher suite(s).
	+----------+--------------+---------------------+-------------------+		This document requests no actions of IANA.
	| Category | Software | As Of Version | Comment |
	+----------+--------------+---------------------+-------------------+
	| Library | OpenSSL | 1.0.1 | |
	| | GnuTLS | | |
	| | NSS | 3.11.1 | |
	| Browser | Internet | IE8 on Windows 7 | |
	| | Explorer | | |
	| | Firefox | TBD | |
	| | Chrome | TLS 1.2 and AES-GCM | |
	| | | expected in Chrome | |
	| | | 30 | |
	| | Safari | TBD | |
	| Web | Apache | ?? | |
	| server | (mod_gnutls) | | |
	| | Apache | ?? | |
	| | (mod_ssl) | | |
	| | Nginx | 1.0.9, 1.1.6 | With a recent |
	| | | | version of |
	| | | | OpenSSL |
	+----------+--------------+---------------------+-------------------+
	6. Security Considerations		6. Security Considerations
	6.1. AES-GCM		6.1. AES-GCM
	Please refer to [RFC5246], Sec. 11 for general security		Please refer to [RFC5246], Sec. 11 for general security
	considerations when using TLS 1.2, and to [RFC5288], Sec. 6 for		considerations when using TLS 1.2, and to [RFC5288], Sec. 6 for
	security considerations that apply specifically to AES-GCM when used		security considerations that apply specifically to AES-GCM when used
	with TLS.		with TLS.
	6.2. Perfect Forward Secrecy (PFS)		6.2. Forward Secrecy
	PFS is a defense against an attacker who records encrypted		Forward secrecy (also often called Perfect Forward Secrecy or "PFS")
	conversations where the session keys are only encrypted with the		is a defense against an attacker who records encrypted conversations
	communicating parties' long-term keys. Should the attacker be able		where the session keys are only encrypted with the communicating
	to obtain these long-term keys at some point later in the future, he		parties' long-term keys. Should the attacker be able to obtain these
	will be able to decrypt the session keys and thus the entire		long-term keys at some point later in the future, he will be able to
	conversation. In the context of TLS and DTLS, such compromise of		decrypt the session keys and thus the entire conversation. In the
	long-term keys is not entirely implausible. It can happen, for		context of TLS and DTLS, such compromise of long-term keys is not
	example, due to:		entirely implausible. It can happen, for example, due to:
	o A client or server being attacked by some other attack vector, and		o A client or server being attacked by some other attack vector, and
	the private key retrieved.		the private key retrieved.
	o A long-term key retrieved from a device that has been sold or		o A long-term key retrieved from a device that has been sold or
	otherwise decommissioned without prior wiping.		otherwise decommissioned without prior wiping.
	o A long-term key used on a device as a default key [Heninger2012].		o A long-term key used on a device as a default key [Heninger2012].
	o A key generated by a Trusted Third Party like a CA, and later		o A key generated by a Trusted Third Party like a CA, and later
	retrieved from it either by extortion or compromise		retrieved from it either by extortion or compromise
	[Soghoian2011].		[Soghoian2011].
	o A cryptographic break-through, or the use of asymmetric keys with		o A cryptographic break-through, or the use of asymmetric keys with
	insufficient length [Kleinjung2010].		insufficient length [Kleinjung2010].
	PFS ensures in such cases that the session keys cannot be determined		PFS ensures in such cases that the session keys cannot be determined
	even by an attacker who obtains the long-term keys some time after		even by an attacker who obtains the long-term keys some time after
	the conversation. It also protects against an attacker who is in		the conversation. It also protects against an attacker who is in
	possession of the long-term keys, but remains passive during the		possession of the long-term keys, but remains passive during the
	conversation.		conversation.
	PFS is generally achieved by using the Diffie-Hellman scheme to		PFS is generally achieved by using the Diffie-Hellman scheme to
	derive session keys. The Diffie-Hellman scheme has both parties		derive session keys. The Diffie-Hellman scheme has both parties
	maintain private secrets and send parameters over the network as		maintain private secrets and send parameters over the network as
	modular powers over certain cyclic groups. The properties of the so-		modular powers over certain cyclic groups. The properties of the so-
	called Discrete Logarithm Problem (DLP) allow to derive the session		called Discrete Logarithm Problem (DLP) allow to derive the session
	keys without an eavesdropper being able to do so. There is currently		keys without an eavesdropper being able to do so. There is currently
	no known attack against DLP if sufficiently large parameters are		no known attack against DLP if sufficiently large parameters are
	chosen.		chosen.
	Unfortunately, many TLS/DTLS cipher suites were defined that do not		Unfortunately, many TLS/DTLS cipher suites were defined that do not
	enable PFS, e.g. TLS_RSA_WITH_AES_256_CBC_SHA256. We thus advocate		enable PFS, e.g. TLS_RSA_WITH_AES_256_CBC_SHA256. We thus advocate
	strict use of PFS-only ciphers. These are listed in Section		strict use of PFS-only ciphers.
	Section 4.1.
	6.3. Session Resumption
	TBD, https://www.imperialviolet.org/2013/06/27/botchingpfs.html.
	7. IANA Considerations
	[Note to RFC Editor: please remove this section before publication.]
	This document requires no IANA actions.
	8. Acknowledgements		7. Acknowledgements
	We would like to thank Stephen Farrell, Simon Josefsson, Yoav Nir,		We would like to thank Stephen Farrell, Simon Josefsson, Yoav Nir,
	Kenny Paterson, Patrick Pelletier, and Rich Salz for their review.		Kenny Paterson, Patrick Pelletier, and Rich Salz for their review.
	Thanks to Brian Smith whose "browser cipher suites" page is a great		Thanks to Brian Smith whose "browser cipher suites" page is a great
	resource. Finally, Thanks to all others who commented on the TLS and		resource. Finally, thanks to all others who commented on the TLS and
	other lists and are not mentioned here by name.		other lists and are not mentioned here by name.
	The document was prepared using the lyx2rfc tool, created by Nico		8. References
	Williams.
	9. References
	9.1. 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.
	[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.		[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
	Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites		Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
	for Transport Layer Security (TLS)", RFC 4492, May 2006.		for Transport Layer Security (TLS)", RFC 4492, May 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.
	[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois		[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
	Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,		Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
	August 2008.		August 2008.
	[RFC5829] Brown, A., Clemm, G., and J. Reschke, "Link Relation Types		[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with
	for Simple Version Navigation between Web Resources",		SHA-256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
	RFC 5829, April 2010.		August 2008.
	[I-D.merkle-tls-brainpool]
	Merkle, J. and M. Lochter, "ECC Brainpool Curves for
	Transport Layer Security (TLS)",
	draft-merkle-tls-brainpool-04 (work in progress),
	July 2013.
	9.2. Informative References
	[I-D.popov-tls-prohibiting-rc4]
	Popov, A., "Prohibiting RC4 Cipher Suites",
	draft-popov-tls-prohibiting-rc4-00 (work in progress),
	August 2013.
	[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
	Encryption", RFC 5116, January 2008.
	[RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport
	Layer Security (TLS)", RFC 6460, January 2012.
	[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
	Code: The Implementation Status Section", RFC 6982,
	July 2013.
	[CBC-Attack]
	AlFardan, N. and K. Paterson, "Lucky Thirteen: Breaking
	the TLS and DTLS Record Protocols", IEEE Symposium on
	Security and Privacy , 2013.
	[BEAST] Rizzo, J. and T. Duong, "Browser Exploit Against SSL/TLS",
	2011, <http://packetstormsecurity.com/files/105499/
	Browser-Exploit-Against-SSL-TLS.html>.
	[CRIME] Rizzo, J. and T. Duong, "The CRIME Attack", EKOparty
	Security Conference 2012, 2012.
	[BREACH] Prado, A., Harris, N., and Y. Gluck, "The BREACH Attack",
	2013, <http://breachattack.com/>.
	[RC4] Schneier, B., "Applied Cryptography: Protocols,
	Algorithms, and Source Code in C, 2nd Ed.", 1996.
	[RC4-Attack-FMS]
	Fluhrer, S., Mantin, I., and A. Shamir, "Weaknesses in the
	Key Scheduling Algorithm of RC4", Selected Areas in
	Cryptography , 2001.
	[RC4-Attack]		[RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
	ISOBE, T., OHIGASHI, T., WATANABE, Y., and M. MORII, "Full		(SSL) Version 2.0", RFC 6176, March 2011.
	Plaintext Recovery Attack on Broadcast RC4", International
	Workshop on Fast Software Encryption , 2013.
	[RC4-Attack-AlF]		[RFC7027] Merkle, J. and M. Lochter, "Elliptic Curve Cryptography
	AlFardan, N., Bernstein, D., Paterson, K., Poettering, B.,		(ECC) Brainpool Curves for Transport Layer Security
	and J. Schuldt, "On the Security of RC4 in TLS", Usenix		(TLS)", RFC 7027, October 2013.
	Security Symposium 2013, 2013, <https://www.usenix.org/
	conference/usenixsecurity13/security-rc4-tls>.
	[Padding-Oracle]		8.2. Informative References
	Vaudenay, S., "Security Flaws Induced by CBC Padding
	Applications to SSL, IPSEC, WTLS...", EUROCRYPT 2002,
	2002, <http://www.iacr.org/cryptodb/archive/2002/
	EUROCRYPT/2850/2850.pdf>.
	[CAB-Baseline]		[CAB-Baseline]
	"Baseline Requirements for the Issuance and Management of		"Baseline Requirements for the Issuance and Management of
	Publicly-Trusted Certificates Version 1.1.6", 2013,		Publicly-Trusted Certificates Version 1.1.6", 2013,
	<https://www.cabforum.org/documents.html>.		<https://www.cabforum.org/documents.html>.
	[TLS-IANA]
	"Transport Layer Security (TLS) Parameters - TLS Cipher
	Suite Registry", <https://www.iana.org/assignments/
	tls-parameters/tls-parameters.xhtml#tls-parameters-4>.
	[Heninger2012]		[Heninger2012]
	Heninger, N., Durumeric, Z., Wustrow, E., and J.		Heninger, N., Durumeric, Z., Wustrow, E., and J.
	Halderman, "Mining Your Ps and Qs: Detection of Widespread		Halderman, "Mining Your Ps and Qs: Detection of Widespread
	Weak Keys in Network Devices", Usenix Security		Weak Keys in Network Devices", Usenix Security Symposium
	Symposium 2012, 2012.		2012, 2012.
			[I-D.popov-tls-prohibiting-rc4]
			Popov, A., "Prohibiting RC4 Cipher Suites", draft-popov-
			tls-prohibiting-rc4-01 (work in progress), October 2013.
			[I-D.sheffer-uta-tls-attacks]
			Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
			Current Attacks on TLS and DTLS", draft-sheffer-uta-tls-
			attacks-00 (work in progress), February 2014.
	[Kleinjung2010]		[Kleinjung2010]
	Kleinjung, T., "Factorization of a 768-Bit RSA Modulus",		Kleinjung, T., "Factorization of a 768-Bit RSA Modulus",
	CRYPTO 10, 2010.		CRYPTO 10, 2010.
			[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
			RFC 2246, January 1999.
			[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
			(TLS) Protocol Version 1.1", RFC 4346, April 2006.
			[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
			"Transport Layer Security (TLS) Session Resumption without
			Server-Side State", RFC 5077, January 2008.
			[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
			Encryption", RFC 5116, January 2008.
			[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
			Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
			August 2011.
			[RFC6460] Salter, M. and R. Housley, "Suite B Profile for Transport
			Layer Security (TLS)", RFC 6460, January 2012.
	[Soghoian2011]		[Soghoian2011]
	Soghoian, C. and S. Stamm, "Certified lies: Detecting and		Soghoian, C. and S. Stamm, "Certified lies: Detecting and
	defeating government interception attacks against SSL.",		defeating government interception attacks against SSL.",
	Proc. 15th Int. Conf. Financial Cryptography and Data		Proc. 15th Int. Conf. Financial Cryptography and Data
	Security , 2011.		Security , 2011.
	Appendix A. Appendix: Change Log		Appendix A. Appendix: Change Log
	Note to RFC Editor: please remove this section before publication.		Note to RFC Editor: please remove this section before publication.
	A.1. -01		A.1. -02
			o Reorganized the content to focus on recommendations.
			o Moved description of attacks to a separate document (draft-
			sheffer-uta-tls-attacks).
			o Strengthened recommendations regarding session resumption.
			A.2. -01
	o Clarified our motivation in the introduction.		o Clarified our motivation in the introduction.
	o Added a section justifying the need for PFS.		o Added a section justifying the need for PFS.
	o Added recommendations for RSA and DH parameter lengths. Moved		o Added recommendations for RSA and DH parameter lengths. Moved
	from DHE to ECDHE, with a discussion on whether/when DHE is		from DHE to ECDHE, with a discussion on whether/when DHE is
	appropriate.		appropriate.
	o Recommendation to avoid fallback to SSLv3.		o Recommendation to avoid fallback to SSLv3.
	o Initial information about browser support - more still needed!		o Initial information about browser support - more still needed!
	o More clarity on compression.		o More clarity on compression.
	o Client can offer stronger cipher suites.		o Client can offer stronger cipher suites.
	o Discussion of the regular TLS mandatory cipher suite.		o Discussion of the regular TLS mandatory cipher suite.
	A.2. -00		A.3. -00
	o Initial version.		o Initial version.
	Authors' Addresses		Authors' Addresses
	Yaron Sheffer		Yaron Sheffer
	Porticor		Porticor
	29 HaHarash St.		29 HaHarash St.
	Hod HaSharon 4501303		Hod HaSharon 4501303
	Israel		Israel
	Email: yaronf.ietf@gmail.com		Email: yaronf.ietf@gmail.com
	Ralph Holz		Ralph Holz
	Technische Universitaet Muenchen		Technische Universitaet Muenchen
	Boltzmannstr. 3		Boltzmannstr. 3
	Garching 85748		Garching 85748
	Germany		Germany
	Email: holz@net.in.tum.de		Email: holz@net.in.tum.de
			Peter Saint-Andre
			&yet
			Email: ietf@stpeter.im
End of changes. 80 change blocks.
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