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rfc:rfc7525

Internet Engineering Task Force (IETF) Y. Sheffer Request for Comments: 7525 Intuit BCP: 195 R. Holz Category: Best Current Practice NICTA ISSN: 2070-1721 P. Saint-Andre

                                                                  &yet
                                                              May 2015
  Recommendations for Secure Use of Transport Layer Security (TLS)
            and Datagram Transport Layer Security (DTLS)

Abstract

 Transport Layer Security (TLS) and Datagram Transport Layer Security
 (DTLS) are widely used to protect data exchanged over application
 protocols such as HTTP, SMTP, IMAP, POP, SIP, and XMPP.  Over the
 last few years, several serious attacks on TLS have emerged,
 including attacks on its most commonly used cipher suites and their
 modes of operation.  This document provides recommendations for
 improving the security of deployed services that use TLS and DTLS.
 The recommendations are applicable to the majority of use cases.

Status of This Memo

 This memo documents an Internet Best Current Practice.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 BCPs is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7525.

Sheffer, et al. Best Current Practice [Page 1] RFC 7525 TLS Recommendations May 2015

Copyright Notice

 Copyright (c) 2015 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Sheffer, et al. Best Current Practice [Page 2] RFC 7525 TLS Recommendations May 2015

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
 2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
 3.  General Recommendations . . . . . . . . . . . . . . . . . . .   5
   3.1.  Protocol Versions . . . . . . . . . . . . . . . . . . . .   5
     3.1.1.  SSL/TLS Protocol Versions . . . . . . . . . . . . . .   5
     3.1.2.  DTLS Protocol Versions  . . . . . . . . . . . . . . .   6
     3.1.3.  Fallback to Lower Versions  . . . . . . . . . . . . .   7
   3.2.  Strict TLS  . . . . . . . . . . . . . . . . . . . . . . .   7
   3.3.  Compression . . . . . . . . . . . . . . . . . . . . . . .   8
   3.4.  TLS Session Resumption  . . . . . . . . . . . . . . . . .   8
   3.5.  TLS Renegotiation . . . . . . . . . . . . . . . . . . . .   9
   3.6.  Server Name Indication  . . . . . . . . . . . . . . . . .   9
 4.  Recommendations: Cipher Suites  . . . . . . . . . . . . . . .   9
   4.1.  General Guidelines  . . . . . . . . . . . . . . . . . . .   9
   4.2.  Recommended Cipher Suites . . . . . . . . . . . . . . . .  11
     4.2.1.  Implementation Details  . . . . . . . . . . . . . . .  12
   4.3.  Public Key Length . . . . . . . . . . . . . . . . . . . .  12
   4.4.  Modular Exponential vs. Elliptic Curve DH Cipher Suites .  13
   4.5.  Truncated HMAC  . . . . . . . . . . . . . . . . . . . . .  14
 5.  Applicability Statement . . . . . . . . . . . . . . . . . . .  15
   5.1.  Security Services . . . . . . . . . . . . . . . . . . . .  15
   5.2.  Opportunistic Security  . . . . . . . . . . . . . . . . .  16
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   6.1.  Host Name Validation  . . . . . . . . . . . . . . . . . .  17
   6.2.  AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . .  18
   6.3.  Forward Secrecy . . . . . . . . . . . . . . . . . . . . .  18
   6.4.  Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . .  19
   6.5.  Certificate Revocation  . . . . . . . . . . . . . . . . .  19
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
   7.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
   7.2.  Informative References  . . . . . . . . . . . . . . . . .  22
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  26
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

Sheffer, et al. Best Current Practice [Page 3] RFC 7525 TLS Recommendations May 2015

1. Introduction

 Transport Layer Security (TLS) [RFC5246] and Datagram Transport
 Security Layer (DTLS) [RFC6347] are widely used to protect data
 exchanged over application protocols such as HTTP, SMTP, IMAP, POP,
 SIP, and XMPP.  Over the last few years, several serious attacks on
 TLS have emerged, including attacks on its most commonly used cipher
 suites and their modes of operation.  For instance, both the AES-CBC
 [RFC3602] and RC4 [RFC7465] encryption algorithms, which together
 have been the most widely deployed ciphers, have been attacked in the
 context of TLS.  A companion document [RFC7457] provides detailed
 information about these attacks and will help the reader understand
 the rationale behind the recommendations provided here.
 Because of these attacks, those who implement and deploy TLS and DTLS
 need updated guidance on how TLS can be used securely.  This document
 provides guidance for deployed services as well as for software
 implementations, assuming the implementer expects his or her code to
 be deployed in environments defined in Section 5.  In fact, this
 document calls for the deployment of algorithms that are widely
 implemented but not yet widely deployed.  Concerning deployment, this
 document targets a wide audience -- namely, all deployers who wish to
 add authentication (be it one-way only or mutual), confidentiality,
 and data integrity protection to their communications.
 The recommendations herein take into consideration the security of
 various mechanisms, their technical maturity and interoperability,
 and their prevalence in implementations at the time of writing.
 Unless it is explicitly called out that a recommendation applies to
 TLS alone or to DTLS alone, each recommendation applies to both TLS
 and DTLS.
 It is expected that the TLS 1.3 specification will resolve many of
 the vulnerabilities listed in this document.  A system that deploys
 TLS 1.3 should have fewer vulnerabilities than TLS 1.2 or below.
 This document is likely to be updated after TLS 1.3 gets noticeable
 deployment.
 These are minimum recommendations for the use of TLS in the vast
 majority of implementation and deployment scenarios, with the
 exception of unauthenticated TLS (see Section 5).  Other
 specifications that reference this document can have stricter
 requirements related to one or more aspects of the protocol, based on
 their particular circumstances (e.g., for use with a particular
 application protocol); when that is the case, implementers are
 advised to adhere to those stricter requirements.  Furthermore, this

Sheffer, et al. Best Current Practice [Page 4] RFC 7525 TLS Recommendations May 2015

 document provides a floor, not a ceiling, so stronger options are
 always allowed (e.g., depending on differing evaluations of the
 importance of cryptographic strength vs. computational load).
 Community knowledge about the strength of various algorithms and
 feasible attacks can change quickly, and experience shows that a Best
 Current Practice (BCP) document about security is a point-in-time
 statement.  Readers are advised to seek out any errata or updates
 that apply to this document.

2. Terminology

 A number of security-related terms in this document are used in the
 sense defined in [RFC4949].
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

3. General Recommendations

 This section provides general recommendations on the secure use of
 TLS.  Recommendations related to cipher suites are discussed in the
 following section.

3.1. Protocol Versions

3.1.1. SSL/TLS Protocol Versions

 It is important both to stop using old, less secure versions of SSL/
 TLS and to start using modern, more secure versions; therefore, the
 following are the recommendations concerning TLS/SSL protocol
 versions:
 o  Implementations MUST NOT negotiate SSL version 2.
    Rationale: Today, SSLv2 is considered insecure [RFC6176].
 o  Implementations MUST NOT negotiate SSL version 3.
    Rationale: SSLv3 [RFC6101] was an improvement over SSLv2 and
    plugged some significant security holes but did not support strong
    cipher suites.  SSLv3 does not support TLS extensions, some of
    which (e.g., renegotiation_info [RFC5746]) are security-critical.
    In addition, with the emergence of the POODLE attack [POODLE],
    SSLv3 is now widely recognized as fundamentally insecure.  See
    [DEP-SSLv3] for further details.

Sheffer, et al. Best Current Practice [Page 5] RFC 7525 TLS Recommendations May 2015

 o  Implementations SHOULD NOT negotiate TLS version 1.0 [RFC2246];
    the only exception is when no higher version is available in the
    negotiation.
    Rationale: TLS 1.0 (published in 1999) does not support many
    modern, strong cipher suites.  In addition, TLS 1.0 lacks a per-
    record Initialization Vector (IV) for CBC-based cipher suites and
    does not warn against common padding errors.
 o  Implementations SHOULD NOT negotiate TLS version 1.1 [RFC4346];
    the only exception is when no higher version is available in the
    negotiation.
    Rationale: TLS 1.1 (published in 2006) is a security improvement
    over TLS 1.0 but still does not support certain stronger cipher
    suites.
 o  Implementations MUST support TLS 1.2 [RFC5246] and MUST prefer to
    negotiate TLS version 1.2 over earlier versions of TLS.
    Rationale: Several stronger cipher suites are available only with
    TLS 1.2 (published in 2008).  In fact, the cipher suites
    recommended by this document (Section 4.2 below) are only
    available in TLS 1.2.
 This BCP applies to TLS 1.2 and also to earlier versions.  It is not
 safe for readers to assume that the recommendations in this BCP apply
 to any future version of TLS.

3.1.2. DTLS Protocol Versions

 DTLS, an adaptation of TLS for UDP datagrams, was introduced when TLS
 1.1 was published.  The following are the recommendations with
 respect to DTLS:
 o  Implementations SHOULD NOT negotiate DTLS version 1.0 [RFC4347].
    Version 1.0 of DTLS correlates to version 1.1 of TLS (see above).
 o  Implementations MUST support and MUST prefer to negotiate DTLS
    version 1.2 [RFC6347].
    Version 1.2 of DTLS correlates to version 1.2 of TLS (see above).
    (There is no version 1.1 of DTLS.)

Sheffer, et al. Best Current Practice [Page 6] RFC 7525 TLS Recommendations May 2015

3.1.3. Fallback to Lower Versions

 Clients that "fall back" to lower versions of the protocol after the
 server rejects higher versions of the protocol MUST NOT fall back to
 SSLv3 or earlier.
 Rationale: Some client implementations revert to lower versions of
 TLS or even to SSLv3 if the server rejected higher versions of the
 protocol.  This fallback can be forced by a man-in-the-middle (MITM)
 attacker.  TLS 1.0 and SSLv3 are significantly less secure than TLS
 1.2, the version recommended by this document.  While TLS 1.0-only
 servers are still quite common, IP scans show that SSLv3-only servers
 amount to only about 3% of the current Web server population.  (At
 the time of this writing, an explicit method for preventing downgrade
 attacks has been defined recently in [RFC7507].)

3.2. Strict TLS

 The following recommendations are provided to help prevent SSL
 Stripping (an attack that is summarized in Section 2.1 of [RFC7457]):
 o  In cases where an application protocol allows implementations or
    deployments a choice between strict TLS configuration and dynamic
    upgrade from unencrypted to TLS-protected traffic (such as
    STARTTLS), clients and servers SHOULD prefer strict TLS
    configuration.
 o  Application protocols typically provide a way for the server to
    offer TLS during an initial protocol exchange, and sometimes also
    provide a way for the server to advertise support for TLS (e.g.,
    through a flag indicating that TLS is required); unfortunately,
    these indications are sent before the communication channel is
    encrypted.  A client SHOULD attempt to negotiate TLS even if these
    indications are not communicated by the server.
 o  HTTP client and server implementations MUST support the HTTP
    Strict Transport Security (HSTS) header [RFC6797], in order to
    allow Web servers to advertise that they are willing to accept
    TLS-only clients.
 o  Web servers SHOULD use HSTS to indicate that they are willing to
    accept TLS-only clients, unless they are deployed in such a way
    that using HSTS would in fact weaken overall security (e.g., it
    can be problematic to use HSTS with self-signed certificates, as
    described in Section 11.3 of [RFC6797]).

Sheffer, et al. Best Current Practice [Page 7] RFC 7525 TLS Recommendations May 2015

 Rationale: Combining unprotected and TLS-protected communication
 opens the way to SSL Stripping and similar attacks, since an initial
 part of the communication is not integrity protected and therefore
 can be manipulated by an attacker whose goal is to keep the
 communication in the clear.

3.3. Compression

 In order to help prevent compression-related attacks (summarized in
 Section 2.6 of [RFC7457]), implementations and deployments SHOULD
 disable TLS-level compression (Section 6.2.2 of [RFC5246]), unless
 the application protocol in question has been shown not to be open to
 such attacks.
 Rationale: TLS compression has been subject to security attacks, such
 as the CRIME attack.
 Implementers should note that compression at higher protocol levels
 can allow an active attacker to extract cleartext information from
 the connection.  The BREACH attack is one such case.  These issues
 can only be mitigated outside of TLS and are thus outside the scope
 of this document.  See Section 2.6 of [RFC7457] for further details.

3.4. TLS Session Resumption

 If TLS session resumption is used, care ought to be taken to do so
 safely.  In particular, when using session tickets [RFC5077], the
 resumption information MUST be authenticated and encrypted to prevent
 modification or eavesdropping by an attacker.  Further
 recommendations apply to session tickets:
 o  A strong cipher suite MUST be used when encrypting the ticket (as
    least as strong as the main TLS cipher suite).
 o  Ticket keys MUST be changed regularly, e.g., once every week, so
    as not to negate the benefits of forward secrecy (see Section 6.3
    for details on forward secrecy).
 o  For similar reasons, session ticket validity SHOULD be limited to
    a reasonable duration (e.g., half as long as ticket key validity).
 Rationale: session resumption is another kind of TLS handshake, and
 therefore must be as secure as the initial handshake.  This document
 (Section 4) recommends the use of cipher suites that provide forward
 secrecy, i.e. that prevent an attacker who gains momentary access to
 the TLS endpoint (either client or server) and its secrets from
 reading either past or future communication.  The tickets must be
 managed so as not to negate this security property.

Sheffer, et al. Best Current Practice [Page 8] RFC 7525 TLS Recommendations May 2015

3.5. TLS Renegotiation

 Where handshake renegotiation is implemented, both clients and
 servers MUST implement the renegotiation_info extension, as defined
 in [RFC5746].
 The most secure option for countering the Triple Handshake attack is
 to refuse any change of certificates during renegotiation.  In
 addition, TLS clients SHOULD apply the same validation policy for all
 certificates received over a connection.  The [triple-handshake]
 document suggests several other possible countermeasures, such as
 binding the master secret to the full handshake (see [SESSION-HASH])
 and binding the abbreviated session resumption handshake to the
 original full handshake.  Although the latter two techniques are
 still under development and thus do not qualify as current practices,
 those who implement and deploy TLS are advised to watch for further
 development of appropriate countermeasures.

3.6. Server Name Indication

 TLS implementations MUST support the Server Name Indication (SNI)
 extension defined in Section 3 of [RFC6066] for those higher-level
 protocols that would benefit from it, including HTTPS.  However, the
 actual use of SNI in particular circumstances is a matter of local
 policy.
 Rationale: SNI supports deployment of multiple TLS-protected virtual
 servers on a single address, and therefore enables fine-grained
 security for these virtual servers, by allowing each one to have its
 own certificate.

4. Recommendations: Cipher Suites

 TLS and its implementations provide considerable flexibility in the
 selection of cipher suites.  Unfortunately, some available cipher
 suites are insecure, some do not provide the targeted security
 services, and some no longer provide enough security.  Incorrectly
 configuring a server leads to no or reduced security.  This section
 includes recommendations on the selection and negotiation of cipher
 suites.

4.1. General Guidelines

 Cryptographic algorithms weaken over time as cryptanalysis improves:
 algorithms that were once considered strong become weak.  Such
 algorithms need to be phased out over time and replaced with more
 secure cipher suites.  This helps to ensure that the desired security
 properties still hold.  SSL/TLS has been in existence for almost 20

Sheffer, et al. Best Current Practice [Page 9] RFC 7525 TLS Recommendations May 2015

 years and many of the cipher suites that have been recommended in
 various versions of SSL/TLS are now considered weak or at least not
 as strong as desired.  Therefore, this section modernizes the
 recommendations concerning cipher suite selection.
 o  Implementations MUST NOT negotiate the cipher suites with NULL
    encryption.
    Rationale: The NULL cipher suites do not encrypt traffic and so
    provide no confidentiality services.  Any entity in the network
    with access to the connection can view the plaintext of contents
    being exchanged by the client and server.  (Nevertheless, this
    document does not discourage software from implementing NULL
    cipher suites, since they can be useful for testing and
    debugging.)
 o  Implementations MUST NOT negotiate RC4 cipher suites.
    Rationale: The RC4 stream cipher has a variety of cryptographic
    weaknesses, as documented in [RFC7465].  Note that DTLS
    specifically forbids the use of RC4 already.
 o  Implementations MUST NOT negotiate cipher suites offering less
    than 112 bits of security, including so-called "export-level"
    encryption (which provide 40 or 56 bits of security).
    Rationale: Based on [RFC3766], at least 112 bits of security is
    needed.  40-bit and 56-bit security are considered insecure today.
    TLS 1.1 and 1.2 never negotiate 40-bit or 56-bit export ciphers.
 o  Implementations SHOULD NOT negotiate cipher suites that use
    algorithms offering less than 128 bits of security.
    Rationale: Cipher suites that offer between 112-bits and 128-bits
    of security are not considered weak at this time; however, it is
    expected that their useful lifespan is short enough to justify
    supporting stronger cipher suites at this time.  128-bit ciphers
    are expected to remain secure for at least several years, and
    256-bit ciphers until the next fundamental technology
    breakthrough.  Note that, because of so-called "meet-in-the-
    middle" attacks [Multiple-Encryption], some legacy cipher suites
    (e.g., 168-bit 3DES) have an effective key length that is smaller
    than their nominal key length (112 bits in the case of 3DES).
    Such cipher suites should be evaluated according to their
    effective key length.

Sheffer, et al. Best Current Practice [Page 10] RFC 7525 TLS Recommendations May 2015

 o  Implementations SHOULD NOT negotiate cipher suites based on RSA
    key transport, a.k.a. "static RSA".
    Rationale: These cipher suites, which have assigned values
    starting with the string "TLS_RSA_WITH_*", have several drawbacks,
    especially the fact that they do not support forward secrecy.
 o  Implementations MUST support and prefer to negotiate cipher suites
    offering forward secrecy, such as those in the Ephemeral Diffie-
    Hellman and Elliptic Curve Ephemeral Diffie-Hellman ("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.  See Section 6.3 for a detailed
    discussion.

4.2. Recommended Cipher Suites

 Given the foregoing considerations, implementation and deployment of
 the following cipher suites is RECOMMENDED:
 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
 These cipher suites are supported only in TLS 1.2 because they are
 authenticated encryption (AEAD) algorithms [RFC5116].
 Typically, in order to prefer these suites, the order of suites needs
 to be explicitly configured in server software.  (See [BETTERCRYPTO]
 for helpful deployment guidelines, but note that its recommendations
 differ from the current document in some details.)  It would be ideal
 if server software implementations were to prefer these suites by
 default.
 Some devices have hardware support for AES-CCM but not AES-GCM, so
 they are unable to follow the foregoing recommendations regarding
 cipher suites.  There are even devices that do not support public key
 cryptography at all, but they are out of scope entirely.

Sheffer, et al. Best Current Practice [Page 11] RFC 7525 TLS Recommendations May 2015

4.2.1. Implementation 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 MUST prefer this cipher suite over weaker cipher suites
 whenever it is proposed, even if it is not the first proposal.
 Clients are of course free to offer stronger cipher suites, e.g.,
 using AES-256; when they do, the server SHOULD prefer the stronger
 cipher suite unless there are compelling reasons (e.g., seriously
 degraded performance) to choose otherwise.
 This document does not change the mandatory-to-implement TLS cipher
 suite(s) prescribed by TLS.  To maximize interoperability, RFC 5246
 mandates implementation of the TLS_RSA_WITH_AES_128_CBC_SHA cipher
 suite, which is significantly weaker than the cipher suites
 recommended here.  (The GCM mode does not suffer from the same
 weakness, caused by the order of MAC-then-Encrypt in TLS
 [Krawczyk2001], since it uses an AEAD mode of operation.)
 Implementers should consider the interoperability gain against the
 loss in security when deploying the TLS_RSA_WITH_AES_128_CBC_SHA
 cipher suite.  Other application protocols specify other cipher
 suites as mandatory to implement (MTI).
 Note that some profiles of TLS 1.2 use different cipher suites.  For
 example, [RFC6460] defines a profile that uses the
 TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
 TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites.
 [RFC4492] allows clients and servers to negotiate ECDH parameters
 (curves).  Both clients and servers SHOULD include the "Supported
 Elliptic Curves" extension [RFC4492].  For interoperability, clients
 and servers SHOULD support the NIST P-256 (secp256r1) curve
 [RFC4492].  In addition, clients SHOULD send an ec_point_formats
 extension with a single element, "uncompressed".

4.3. Public Key Length

 When using the cipher suites recommended in this document, two public
 keys are normally used in the TLS handshake: one for the Diffie-
 Hellman key agreement and one for server authentication.  Where a
 client certificate is used, a third public key is added.
 With a key exchange based on modular exponential (MODP) Diffie-
 Hellman groups ("DHE" cipher suites), DH key lengths of at least 2048
 bits are RECOMMENDED.

Sheffer, et al. Best Current Practice [Page 12] RFC 7525 TLS Recommendations May 2015

 Rationale: For various reasons, in practice, DH keys are typically
 generated in lengths that are powers of two (e.g., 2^10 = 1024 bits,
 2^11 = 2048 bits, 2^12 = 4096 bits).  Because a DH key of 1228 bits
 would be roughly equivalent to only an 80-bit symmetric key
 [RFC3766], it is better to use keys longer than that for the "DHE"
 family of cipher suites.  A DH key of 1926 bits would be roughly
 equivalent to a 100-bit symmetric key [RFC3766] and a DH key of 2048
 bits might be sufficient for at least the next 10 years
 [NIST.SP.800-56A].  See Section 4.4 for additional information on the
 use of MODP Diffie-Hellman in TLS.
 As noted in [RFC3766], correcting for the emergence of a TWIRL
 machine would imply that 1024-bit DH keys yield about 65 bits of
 equivalent strength and that a 2048-bit DH key would yield about 92
 bits of equivalent strength.
 With regard to ECDH keys, the IANA "EC Named Curve Registry" (within
 the "Transport Layer Security (TLS) Parameters" registry [IANA-TLS])
 contains 160-bit elliptic curves that are considered to be roughly
 equivalent to only an 80-bit symmetric key [ECRYPT-II].  Curves of
 less than 192 bits SHOULD NOT be used.
 When using RSA, servers SHOULD authenticate using certificates with
 at least a 2048-bit modulus for the public key.  In addition, the use
 of the SHA-256 hash algorithm is RECOMMENDED (see [CAB-Baseline] for
 more details).  Clients SHOULD indicate to servers that they request
 SHA-256, by using the "Signature Algorithms" extension defined in
 TLS 1.2.

4.4. Modular Exponential vs. Elliptic Curve DH Cipher Suites

 Not all TLS implementations support both modular exponential (MODP)
 and elliptic curve (EC) Diffie-Hellman groups, as required by
 Section 4.2.  Some implementations are severely limited in the length
 of DH values.  When such implementations need to be accommodated, the
 following are RECOMMENDED (in priority order):
 1.  Elliptic Curve DHE with appropriately negotiated parameters
     (e.g., the curve to be used) and a Message Authentication Code
     (MAC) algorithm stronger than HMAC-SHA1 [RFC5289]
 2.  TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 [RFC5288], with 2048-bit
     Diffie-Hellman parameters
 3.  TLS_DHE_RSA_WITH_AES_128_GCM_SHA256, with 1024-bit parameters

Sheffer, et al. Best Current Practice [Page 13] RFC 7525 TLS Recommendations May 2015

 Rationale: Although Elliptic Curve Cryptography is widely deployed,
 there are some communities where its adoption has been limited for
 several reasons, including its complexity compared to modular
 arithmetic and longstanding perceptions of IPR concerns (which, for
 the most part, have now been resolved [RFC6090]).  Note that ECDHE
 cipher suites exist for both RSA and ECDSA certificates, so moving to
 ECDHE cipher suites does not require moving away from RSA-based
 certificates.  On the other hand, there are two related issues
 hindering effective use of MODP Diffie-Hellman cipher suites in TLS:
 o  There are no standardized, widely implemented protocol mechanisms
    to negotiate the DH groups or parameter lengths supported by
    client and server.
 o  Many servers choose DH parameters of 1024 bits or fewer.
 o  There are widely deployed client implementations that reject
    received DH parameters if they are longer than 1024 bits.  In
    addition, several implementations do not perform appropriate
    validation of group parameters and are vulnerable to attacks
    referenced in Section 2.9 of [RFC7457].
 Note that with DHE and ECDHE cipher suites, the TLS master key only
 depends on the Diffie-Hellman parameters and not on the strength of
 the RSA certificate; moreover, 1024 bit MODP DH parameters are
 generally considered insufficient at this time.
 With MODP ephemeral DH, deployers ought to carefully evaluate
 interoperability vs. security considerations when configuring their
 TLS endpoints.

4.5. Truncated HMAC

 Implementations MUST NOT use the Truncated HMAC extension, defined in
 Section 7 of [RFC6066].
 Rationale: the extension does not apply to the AEAD cipher suites
 recommended above.  However it does apply to most other TLS cipher
 suites.  Its use has been shown to be insecure in [PatersonRS11].

Sheffer, et al. Best Current Practice [Page 14] RFC 7525 TLS Recommendations May 2015

5. Applicability Statement

 The recommendations of this document primarily apply to the
 implementation and deployment of application protocols that are most
 commonly used with TLS and DTLS on the Internet today.  Examples
 include, but are not limited to:
 o  Web software and services that wish to protect HTTP traffic with
    TLS.
 o  Email software and services that wish to protect IMAP, POP3, or
    SMTP traffic with TLS.
 o  Instant-messaging software and services that wish to protect
    Extensible Messaging and Presence Protocol (XMPP) or Internet
    Relay Chat (IRC) traffic with TLS.
 o  Realtime media software and services that wish to protect Secure
    Realtime Transport Protocol (SRTP) traffic with DTLS.
 This document does not modify the implementation and deployment
 recommendations (e.g., mandatory-to-implement cipher suites)
 prescribed by existing application protocols that employ TLS or DTLS.
 If the community that uses such an application protocol wishes to
 modernize its usage of TLS or DTLS to be consistent with the best
 practices recommended here, it needs to explicitly update the
 existing application protocol definition (one example is [TLS-XMPP],
 which updates [RFC6120]).
 Designers of new application protocols developed through the Internet
 Standards Process [RFC2026] are expected at minimum to conform to the
 best practices recommended here, unless they provide documentation of
 compelling reasons that would prevent such conformance (e.g.,
 widespread deployment on constrained devices that lack support for
 the necessary algorithms).

5.1. Security Services

 This document provides recommendations for an audience that wishes to
 secure their communication with TLS to achieve the following:
 o  Confidentiality: all application-layer communication is encrypted
    with the goal that no party should be able to decrypt it except
    the intended receiver.
 o  Data integrity: any changes made to the communication in transit
    are detectable by the receiver.

Sheffer, et al. Best Current Practice [Page 15] RFC 7525 TLS Recommendations May 2015

 o  Authentication: an endpoint of the TLS communication is
    authenticated as the intended entity to communicate with.
 With regard to authentication, TLS enables authentication of one or
 both endpoints in the communication.  In the context of opportunistic
 security [RFC7435], TLS is sometimes used without authentication.  As
 discussed in Section 5.2, considerations for opportunistic security
 are not in scope for this document.
 If deployers deviate from the recommendations given in this document,
 they need to be aware that they might lose access to one of the
 foregoing security services.
 This document applies only to environments where confidentiality is
 required.  It recommends algorithms and configuration options that
 enforce secrecy of the data in transit.
 This document also assumes that data integrity protection is always
 one of the goals of a deployment.  In cases where integrity is not
 required, it does not make sense to employ TLS in the first place.
 There are attacks against confidentiality-only protection that
 utilize the lack of integrity to also break confidentiality (see, for
 instance, [DegabrieleP07] in the context of IPsec).
 This document addresses itself to application protocols that are most
 commonly used on the Internet with TLS and DTLS.  Typically, all
 communication between TLS clients and TLS servers requires all three
 of the above security services.  This is particularly true where TLS
 clients are user agents like Web browsers or email software.
 This document does not address the rarer deployment scenarios where
 one of the above three properties is not desired, such as the use
 case described in Section 5.2 below.  As another scenario where
 confidentiality is not needed, consider a monitored network where the
 authorities in charge of the respective traffic domain require full
 access to unencrypted (plaintext) traffic, and where users
 collaborate and send their traffic in the clear.

5.2. Opportunistic Security

 There are several important scenarios in which the use of TLS is
 optional, i.e., the client decides dynamically ("opportunistically")
 whether to use TLS with a particular server or to connect in the
 clear.  This practice, often called "opportunistic security", is
 described at length in [RFC7435] and is often motivated by a desire
 for backward compatibility with legacy deployments.

Sheffer, et al. Best Current Practice [Page 16] RFC 7525 TLS Recommendations May 2015

 In these scenarios, some of the recommendations in this document
 might be too strict, since adhering to them could cause fallback to
 cleartext, a worse outcome than using TLS with an outdated protocol
 version or cipher suite.
 This document specifies best practices for TLS in general.  A
 separate document containing recommendations for the use of TLS with
 opportunistic security is to be completed in the future.

6. Security Considerations

 This entire document discusses the security practices directly
 affecting applications using the TLS protocol.  This section contains
 broader security considerations related to technologies used in
 conjunction with or by TLS.

6.1. Host Name Validation

 Application authors should take note that some TLS implementations do
 not validate host names.  If the TLS implementation they are using
 does not validate host names, authors might need to write their own
 validation code or consider using a different TLS implementation.
 It is noted that the requirements regarding host name validation
 (and, in general, binding between the TLS layer and the protocol that
 runs above it) vary between different protocols.  For HTTPS, these
 requirements are defined by Section 3 of [RFC2818].
 Readers are referred to [RFC6125] for further details regarding
 generic host name validation in the TLS context.  In addition, that
 RFC contains a long list of example protocols, some of which
 implement a policy very different from HTTPS.
 If the host name is discovered indirectly and in an insecure manner
 (e.g., by an insecure DNS query for an MX or SRV record), it SHOULD
 NOT be used as a reference identifier [RFC6125] even when it matches
 the presented certificate.  This proviso does not apply if the host
 name is discovered securely (for further discussion, see [DANE-SRV]
 and [DANE-SMTP]).
 Host name validation typically applies only to the leaf "end entity"
 certificate.  Naturally, in order to ensure proper authentication in
 the context of the PKI, application clients need to verify the entire
 certification path in accordance with [RFC5280] (see also [RFC6125]).

Sheffer, et al. Best Current Practice [Page 17] RFC 7525 TLS Recommendations May 2015

6.2. AES-GCM

 Section 4.2 above recommends the use of the AES-GCM authenticated
 encryption algorithm.  Please refer to Section 11 of [RFC5246] for
 general security considerations when using TLS 1.2, and to Section 6
 of [RFC5288] for security considerations that apply specifically to
 AES-GCM when used with TLS.

6.3. Forward Secrecy

 Forward secrecy (also called "perfect forward secrecy" or "PFS" and
 defined in [RFC4949]) is a defense against an attacker who records
 encrypted conversations where the session keys are only encrypted
 with the communicating parties' long-term keys.  Should the attacker
 be able to obtain these long-term keys at some point later in time,
 the session keys and thus the entire conversation could be decrypted.
 In the context of TLS and DTLS, such compromise of long-term keys is
 not entirely implausible.  It can happen, for example, due to:
 o  A client or server being attacked by some other attack vector, and
    the private key retrieved.
 o  A long-term key retrieved from a device that has been sold or
    otherwise decommissioned without prior wiping.
 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
    retrieved from it either by extortion or compromise
    [Soghoian2011].
 o  A cryptographic break-through, or the use of asymmetric keys with
    insufficient length [Kleinjung2010].
 o  Social engineering attacks against system administrators.
 o  Collection of private keys from inadequately protected backups.
 Forward secrecy ensures in such cases that it is not feasible for an
 attacker to determine the session keys even if the attacker has
 obtained the long-term keys some time after the conversation.  It
 also protects against an attacker who is in possession of the long-
 term keys but remains passive during the conversation.
 Forward secrecy is generally achieved by using the Diffie-Hellman
 scheme to derive session keys.  The Diffie-Hellman scheme has both
 parties maintain private secrets and send parameters over the network
 as modular powers over certain cyclic groups.  The properties of the

Sheffer, et al. Best Current Practice [Page 18] RFC 7525 TLS Recommendations May 2015

 so-called Discrete Logarithm Problem (DLP) allow the parties to
 derive the session keys without an eavesdropper being able to do so.
 There is currently no known attack against DLP if sufficiently large
 parameters are chosen.  A variant of the Diffie-Hellman scheme uses
 Elliptic Curves instead of the originally proposed modular
 arithmetics.
 Unfortunately, many TLS/DTLS cipher suites were defined that do not
 feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256.  This
 document therefore advocates strict use of forward-secrecy-only
 ciphers.

6.4. Diffie-Hellman Exponent Reuse

 For performance reasons, many TLS implementations reuse Diffie-
 Hellman and Elliptic Curve Diffie-Hellman exponents across multiple
 connections.  Such reuse can result in major security issues:
 o  If exponents are reused for too long (e.g., even more than a few
    hours), an attacker who gains access to the host can decrypt
    previous connections.  In other words, exponent reuse negates the
    effects of forward secrecy.
 o  TLS implementations that reuse exponents should test the DH public
    key they receive for group membership, in order to avoid some
    known attacks.  These tests are not standardized in TLS at the
    time of writing.  See [RFC6989] for recipient tests required of
    IKEv2 implementations that reuse DH exponents.

6.5. Certificate Revocation

 The following considerations and recommendations represent the
 current state of the art regarding certificate revocation, even
 though no complete and efficient solution exists for the problem of
 checking the revocation status of common public key certificates
 [RFC5280]:
 o  Although Certificate Revocation Lists (CRLs) are the most widely
    supported mechanism for distributing revocation information, they
    have known scaling challenges that limit their usefulness (despite
    workarounds such as partitioned CRLs and delta CRLs).
 o  Proprietary mechanisms that embed revocation lists in the Web
    browser's configuration database cannot scale beyond a small
    number of the most heavily used Web servers.

Sheffer, et al. Best Current Practice [Page 19] RFC 7525 TLS Recommendations May 2015

 o  The On-Line Certification Status Protocol (OCSP) [RFC6960]
    presents both scaling and privacy issues.  In addition, clients
    typically "soft-fail", meaning that they do not abort the TLS
    connection if the OCSP server does not respond.  (However, this
    might be a workaround to avoid denial-of-service attacks if an
    OCSP responder is taken offline.)
 o  The TLS Certificate Status Request extension (Section 8 of
    [RFC6066]), commonly called "OCSP stapling", resolves the
    operational issues with OCSP.  However, it is still ineffective in
    the presence of a MITM attacker because the attacker can simply
    ignore the client's request for a stapled OCSP response.
 o  OCSP stapling as defined in [RFC6066] does not extend to
    intermediate certificates used in a certificate chain.  Although
    the Multiple Certificate Status extension [RFC6961] addresses this
    shortcoming, it is a recent addition without much deployment.
 o  Both CRLs and OCSP depend on relatively reliable connectivity to
    the Internet, which might not be available to certain kinds of
    nodes (such as newly provisioned devices that need to establish a
    secure connection in order to boot up for the first time).
 With regard to common public key certificates, servers SHOULD support
 the following as a best practice given the current state of the art
 and as a foundation for a possible future solution:
 1.  OCSP [RFC6960]
 2.  Both the status_request extension defined in [RFC6066] and the
     status_request_v2 extension defined in [RFC6961] (This might
     enable interoperability with the widest range of clients.)
 3.  The OCSP stapling extension defined in [RFC6961]
 The considerations in this section do not apply to scenarios where
 the DANE-TLSA resource record [RFC6698] is used to signal to a client
 which certificate a server considers valid and good to use for TLS
 connections.

Sheffer, et al. Best Current Practice [Page 20] RFC 7525 TLS Recommendations May 2015

7. References

7.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000,
            <http://www.rfc-editor.org/info/rfc2818>.
 [RFC3766]  Orman, H. and P. Hoffman, "Determining Strengths For
            Public Keys Used For Exchanging Symmetric Keys", BCP 86,
            RFC 3766, April 2004,
            <http://www.rfc-editor.org/info/rfc3766>.
 [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
            Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
            for Transport Layer Security (TLS)", RFC 4492, May 2006,
            <http://www.rfc-editor.org/info/rfc4492>.
 [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", FYI
            36, RFC 4949, August 2007,
            <http://www.rfc-editor.org/info/rfc4949>.
 [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.2", RFC 5246, August 2008,
            <http://www.rfc-editor.org/info/rfc5246>.
 [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
            Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
            August 2008, <http://www.rfc-editor.org/info/rfc5288>.
 [RFC5289]  Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
            256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
            August 2008, <http://www.rfc-editor.org/info/rfc5289>.
 [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
            "Transport Layer Security (TLS) Renegotiation Indication
            Extension", RFC 5746, February 2010,
            <http://www.rfc-editor.org/info/rfc5746>.
 [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
            Extensions: Extension Definitions", RFC 6066, January
            2011, <http://www.rfc-editor.org/info/rfc6066>.

Sheffer, et al. Best Current Practice [Page 21] RFC 7525 TLS Recommendations May 2015

 [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
            Verification of Domain-Based Application Service Identity
            within Internet Public Key Infrastructure Using X.509
            (PKIX) Certificates in the Context of Transport Layer
            Security (TLS)", RFC 6125, March 2011,
            <http://www.rfc-editor.org/info/rfc6125>.
 [RFC6176]  Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
            (SSL) Version 2.0", RFC 6176, March 2011,
            <http://www.rfc-editor.org/info/rfc6176>.
 [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
            Security Version 1.2", RFC 6347, January 2012,
            <http://www.rfc-editor.org/info/rfc6347>.
 [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
            February 2015, <http://www.rfc-editor.org/info/rfc7465>.

7.2. Informative References

 [BETTERCRYPTO]
            bettercrypto.org, "Applied Crypto Hardening", April 2015,
            <https://bettercrypto.org/static/
            applied-crypto-hardening.pdf>.
 [CAB-Baseline]
            CA/Browser Forum, "Baseline Requirements for the Issuance
            and Management of Publicly-Trusted Certificates Version
            1.1.6", 2013, <https://www.cabforum.org/documents.html>.
 [DANE-SMTP]
            Dukhovni, V. and W. Hardaker, "SMTP security via
            opportunistic DANE TLS", Work in Progress, draft-ietf-
            dane-smtp-with-dane-16, April 2015.
 [DANE-SRV] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
            Based Authentication of Named Entities (DANE) TLSA Records
            with SRV Records", Work in Progress,
            draft-ietf-dane-srv-14, April 2015.
 [DEP-SSLv3]
            Barnes, R., Thomson, M., Pironti, A., and A. Langley,
            "Deprecating Secure Sockets Layer Version 3.0", Work in
            Progress, draft-ietf-tls-sslv3-diediedie-03, April 2015.

Sheffer, et al. Best Current Practice [Page 22] RFC 7525 TLS Recommendations May 2015

 [DegabrieleP07]
            Degabriele, J. and K. Paterson, "Attacking the IPsec
            Standards in Encryption-only Configurations", IEEE
            Symposium on Security and Privacy (SP '07), 2007,
            <http://dx.doi.org/10.1109/SP.2007.8>.
 [ECRYPT-II]
            Smart, N., "ECRYPT II Yearly Report on Algorithms and
            Keysizes (2011-2012)", 2012,
            <http://www.ecrypt.eu.org/ecrypt2/>.
 [Heninger2012]
            Heninger, N., Durumeric, Z., Wustrow, E., and J.
            Halderman, "Mining Your Ps and Qs: Detection of Widespread
            Weak Keys in Network Devices", Usenix Security Symposium
            2012, 2012.
 [IANA-TLS] IANA, "Transport Layer Security (TLS) Parameters",
            <http://www.iana.org/assignments/tls-parameters>.
 [Kleinjung2010]
            Kleinjung, T., "Factorization of a 768-Bit RSA modulus",
            CRYPTO 10, 2010, <https://eprint.iacr.org/2010/006.pdf>.
 [Krawczyk2001]
            Krawczyk, H., "The Order of Encryption and Authentication
            for Protecting Communications (Or: How Secure is SSL?)",
            CRYPTO 01, 2001,
            <https://www.iacr.org/archive/crypto2001/21390309.pdf>.
 [Multiple-Encryption]
            Merkle, R. and M. Hellman, "On the security of multiple
            encryption", Communications of the ACM, Vol. 24, 1981,
            <http://dl.acm.org/citation.cfm?id=358718>.
 [NIST.SP.800-56A]
            Barker, E., Chen, L., Roginsky, A., and M. Smid,
            "Recommendation for Pair-Wise Key Establishment Schemes
            Using Discrete Logarithm Cryptography", NIST Special
            Publication 800-56A, 2013,
            <http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
            NIST.SP.800-56Ar2.pdf>.
 [POODLE]   US-CERT, "SSL 3.0 Protocol Vulnerability and POODLE
            Attack", Alert TA14-290A, October 2014,
            <https://www.us-cert.gov/ncas/alerts/TA14-290A>.

Sheffer, et al. Best Current Practice [Page 23] RFC 7525 TLS Recommendations May 2015

 [PatersonRS11]
            Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag size
            does matter: attacks and proofs for the TLS record
            protocol", 2011,
            <http://dx.doi.org/10.1007/978-3-642-25385-0_20>.
 [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
            3", BCP 9, RFC 2026, October 1996,
            <http://www.rfc-editor.org/info/rfc2026>.
 [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
            RFC 2246, January 1999,
            <http://www.rfc-editor.org/info/rfc2246>.
 [RFC3602]  Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
            Algorithm and Its Use with IPsec", RFC 3602, September
            2003, <http://www.rfc-editor.org/info/rfc3602>.
 [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
            (TLS) Protocol Version 1.1", RFC 4346, April 2006,
            <http://www.rfc-editor.org/info/rfc4346>.
 [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
            Security", RFC 4347, April 2006,
            <http://www.rfc-editor.org/info/rfc4347>.
 [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
            "Transport Layer Security (TLS) Session Resumption without
            Server-Side State", RFC 5077, January 2008,
            <http://www.rfc-editor.org/info/rfc5077>.
 [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
            Encryption", RFC 5116, January 2008,
            <http://www.rfc-editor.org/info/rfc5116>.
 [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
            Housley, R., and W. Polk, "Internet X.509 Public Key
            Infrastructure Certificate and Certificate Revocation List
            (CRL) Profile", RFC 5280, May 2008,
            <http://www.rfc-editor.org/info/rfc5280>.
 [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
            Curve Cryptography Algorithms", RFC 6090, February 2011,
            <http://www.rfc-editor.org/info/rfc6090>.
 [RFC6101]  Freier, A., Karlton, P., and P. Kocher, "The Secure
            Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
            August 2011, <http://www.rfc-editor.org/info/rfc6101>.

Sheffer, et al. Best Current Practice [Page 24] RFC 7525 TLS Recommendations May 2015

 [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
            Protocol (XMPP): Core", RFC 6120, March 2011,
            <http://www.rfc-editor.org/info/rfc6120>.
 [RFC6460]  Salter, M. and R. Housley, "Suite B Profile for Transport
            Layer Security (TLS)", RFC 6460, January 2012,
            <http://www.rfc-editor.org/info/rfc6460>.
 [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
            of Named Entities (DANE) Transport Layer Security (TLS)
            Protocol: TLSA", RFC 6698, August 2012,
            <http://www.rfc-editor.org/info/rfc6698>.
 [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
            Transport Security (HSTS)", RFC 6797, November 2012,
            <http://www.rfc-editor.org/info/rfc6797>.
 [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
            Galperin, S., and C. Adams, "X.509 Internet Public Key
            Infrastructure Online Certificate Status Protocol - OCSP",
            RFC 6960, June 2013,
            <http://www.rfc-editor.org/info/rfc6960>.
 [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
            Multiple Certificate Status Request Extension", RFC 6961,
            June 2013, <http://www.rfc-editor.org/info/rfc6961>.
 [RFC6989]  Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman
            Tests for the Internet Key Exchange Protocol Version 2
            (IKEv2)", RFC 6989, July 2013,
            <http://www.rfc-editor.org/info/rfc6989>.
 [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
            Most of the Time", RFC 7435, December 2014,
            <http://www.rfc-editor.org/info/rfc7435>.
 [RFC7457]  Sheffer, Y., Holz, R., and P. Saint-Andre, "Summarizing
            Known Attacks on Transport Layer Security (TLS) and
            Datagram TLS (DTLS)", RFC 7457, February 2015,
            <http://www.rfc-editor.org/info/rfc7457>.
 [RFC7507]  Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
            Suite Value (SCSV) for Preventing Protocol Downgrade
            Attacks", RFC 7507, April 2015.

Sheffer, et al. Best Current Practice [Page 25] RFC 7525 TLS Recommendations May 2015

 [SESSION-HASH]
            Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
            Langley, A., and M. Ray, "Transport Layer Security (TLS)
            Session Hash and Extended Master Secret Extension", Work
            in Progress, draft-ietf-tls-session-hash-05, April 2015.
 [Smith2013]
            Smith, B., "Proposal to Change the Default TLS
            Ciphersuites Offered by Browsers.", 2013,
            <https://briansmith.org/browser-ciphersuites-01.html>.
 [Soghoian2011]
            Soghoian, C. and S. Stamm, "Certified lies: Detecting and
            defeating government interception attacks against SSL",
            Proc. 15th Int. Conf. Financial Cryptography and Data
            Security, 2011.
 [TLS-XMPP] Saint-Andre, P. and a. alkemade, "Use of Transport Layer
            Security (TLS) in the Extensible Messaging and Presence
            Protocol (XMPP)", Work in Progress,
            draft-ietf-uta-xmpp-07, April 2015.
 [triple-handshake]
            Delignat-Lavaud, A., Bhargavan, K., and A. Pironti,
            "Triple Handshakes Considered Harmful: Breaking and Fixing
            Authentication over TLS", 2014,
            <https://secure-resumption.com/>.

Acknowledgments

 Thanks to RJ Atkinson, Uri Blumenthal, Viktor Dukhovni, Stephen
 Farrell, Daniel Kahn Gillmor, Paul Hoffman, Simon Josefsson, Watson
 Ladd, Orit Levin, Ilari Liusvaara, Johannes Merkle, Bodo Moeller,
 Yoav Nir, Massimiliano Pala, Kenny Paterson, Patrick Pelletier, Tom
 Ritter, Joe St. Sauver, Joe Salowey, Rich Salz, Brian Smith, Sean
 Turner, and Aaron Zauner for their feedback and suggested
 improvements.  Thanks also to Brian Smith, who has provided a great
 resource in his "Proposal to Change the Default TLS Ciphersuites
 Offered by Browsers" [Smith2013].  Finally, thanks to all others who
 commented on the TLS, UTA, and other discussion lists but who are not
 mentioned here by name.
 Robert Sparks and Dave Waltermire provided helpful reviews on behalf
 of the General Area Review Team and the Security Directorate,
 respectively.

Sheffer, et al. Best Current Practice [Page 26] RFC 7525 TLS Recommendations May 2015

 During IESG review, Richard Barnes, Alissa Cooper, Spencer Dawkins,
 Stephen Farrell, Barry Leiba, Kathleen Moriarty, and Pete Resnick
 provided comments that led to further improvements.
 Ralph Holz gratefully acknowledges the support by Technische
 Universitaet Muenchen.  The authors gratefully acknowledge the
 assistance of Leif Johansson and Orit Levin as the working group
 chairs and Pete Resnick as the sponsoring Area Director.

Authors' Addresses

 Yaron Sheffer
 Intuit
 4 HaHarash St.
 Hod HaSharon  4524075
 Israel
 EMail: yaronf.ietf@gmail.com
 Ralph Holz
 NICTA
 13 Garden St.
 Eveleigh 2015 NSW
 Australia
 EMail: ralph.ietf@gmail.com
 Peter Saint-Andre
 &yet
 EMail: peter@andyet.com
 URI:   https://andyet.com/

Sheffer, et al. Best Current Practice [Page 27]

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