



Internet Engineering Task Force (IETF)                        Y. Sheffer
Request for Comments: 9325                                        Intuit
BCP: 195                                                  P. Saint-Andre
Obsoletes: 7525                                              Independent
Updates: 5288, 6066                                           T. Fossati
Category: Best Current Practice                              ARM Limited
ISSN: 2070-1721                                            November 2022


  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 used to protect data exchanged over a wide range of
   application protocols and can also form the basis for secure
   transport protocols.  Over the years, the industry has witnessed
   several serious attacks on TLS and DTLS, including attacks on the
   most commonly used cipher suites and their modes of operation.  This
   document provides the latest recommendations for ensuring the
   security of deployed services that use TLS and DTLS.  These
   recommendations are applicable to the majority of use cases.

   RFC 7525, an earlier version of the TLS recommendations, was
   published when the industry was transitioning to TLS 1.2.  Years
   later, this transition is largely complete, and TLS 1.3 is widely
   available.  This document updates the guidance given the new
   environment and obsoletes RFC 7525.  In addition, this document
   updates RFCs 5288 and 6066 in view of recent attacks.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9325.

Copyright Notice

   Copyright (c) 2022 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
   (https://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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  General Recommendations
     3.1.  Protocol Versions
       3.1.1.  SSL/TLS Protocol Versions
       3.1.2.  DTLS Protocol Versions
       3.1.3.  Fallback to Lower Versions
     3.2.  Strict TLS
     3.3.  Compression
       3.3.1.  Certificate Compression
     3.4.  TLS Session Resumption
     3.5.  Renegotiation in TLS 1.2
     3.6.  Post-Handshake Authentication
     3.7.  Server Name Indication (SNI)
     3.8.  Application-Layer Protocol Negotiation (ALPN)
     3.9.  Multi-Server Deployment
     3.10. Zero Round-Trip Time (0-RTT) Data in TLS 1.3
   4.  Recommendations: Cipher Suites
     4.1.  General Guidelines
     4.2.  Cipher Suites for TLS 1.2
       4.2.1.  Implementation Details
     4.3.  Cipher Suites for TLS 1.3
     4.4.  Limits on Key Usage
     4.5.  Public Key Length
     4.6.  Truncated HMAC
   5.  Applicability Statement
     5.1.  Security Services
     5.2.  Opportunistic Security
   6.  IANA Considerations
   7.  Security Considerations
     7.1.  Host Name Validation
     7.2.  AES-GCM
       7.2.1.   Nonce Reuse in TLS 1.2
     7.3.  Forward Secrecy
     7.4.  Diffie-Hellman Exponent Reuse
     7.5.  Certificate Revocation
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  Differences from RFC 7525
   Acknowledgments
   Authors' Addresses

1.  Introduction

   Transport Layer Security (TLS) and Datagram Transport Layer Security
   (DTLS) are used to protect data exchanged over a wide variety of
   application protocols, including HTTP [RFC9112] [RFC9113], IMAP
   [RFC9051], Post Office Protocol (POP) [STD53], SIP [RFC3261], SMTP
   [RFC5321], and the Extensible Messaging and Presence Protocol (XMPP)
   [RFC6120].  Such protocols use both the TLS or DTLS handshake
   protocol and the TLS or DTLS record layer.  Although the TLS
   handshake protocol can also be used with different record layers to
   define secure transport protocols (the most prominent example is QUIC
   [RFC9000]), such transport protocols are not directly in scope for
   this document; nevertheless, many of the recommendations here might
   apply insofar as such protocols use the TLS handshake protocol.

   Over the years leading to 2015, the industry had witnessed serious
   attacks on TLS and DTLS, including attacks on the most commonly used
   cipher suites and their modes of operation.  For instance, both the
   AES-CBC [RFC3602] and RC4 [RFC7465] encryption algorithms, which
   together were once the most widely deployed ciphers, were attacked in
   the context of TLS.  Detailed information about the attacks known
   prior to 2015 is provided in a companion document [RFC7457] to the
   previous version of the TLS recommendations [RFC7525], which will
   help the reader understand the rationale behind the recommendations
   provided here.  That document has not been updated in concert with
   this one; instead, newer attacks are described in this document, as
   are mitigations for those attacks.

   The TLS community reacted to the attacks described in [RFC7457] in
   several ways:

   *  Detailed guidance was published on the use of TLS 1.2 [RFC5246]
      and DTLS 1.2 [RFC6347] along with earlier protocol versions.  This
      guidance is included in the original [RFC7525] and mostly retained
      in this revised version; note that this guidance was mostly
      adopted by the industry since the publication of RFC 7525 in 2015.

   *  Versions of TLS earlier than 1.2 were deprecated [RFC8996].

   *  Version 1.3 of TLS [RFC8446] was released, followed by version 1.3
      of DTLS [RFC9147]; these versions largely mitigate or resolve the
      described attacks.

   Those who implement and deploy TLS and TLS-based protocols need
   guidance on how they can be used securely.  This document provides
   guidance for deployed services as well as for software
   implementations, assuming the implementer expects their code to be
   deployed in the environments defined in Section 5.  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.

   This document attempts to minimize new guidance to TLS 1.2
   implementations, and the overall approach is to encourage systems to
   move to TLS 1.3.  However, this is not always practical.  Newly
   discovered attacks, as well as ecosystem changes, necessitated some
   new requirements that apply to TLS 1.2 environments.  Those are
   summarized in Appendix A.

   Naturally, future attacks are likely, and this document cannot
   address them.  Those who implement and deploy TLS/DTLS and protocols
   based on TLS/DTLS are strongly advised to pay attention to future
   developments.  In particular, although it is known that the creation
   of quantum computers will have a significant impact on the security
   of cryptographic primitives and the technologies that use them,
   currently post-quantum cryptography is a work in progress and it is
   too early to make recommendations; once the relevant specifications
   are standardized in the IETF or elsewhere, this document should be
   updated to reflect best practices at that time.

   As noted, the TLS 1.3 specification resolves 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.
   Therefore, this document replaces [RFC7525], with an explicit goal to
   encourage migration of most uses of TLS 1.2 to TLS 1.3.

   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 specific
   application protocol); when that is the case, implementers are
   advised to adhere to those stricter requirements.  Furthermore, this
   document provides a floor, not a ceiling: where feasible,
   administrators of services are encouraged to go beyond the minimum
   support available in implementations to provide the strongest
   security possible.  For example, based on knowledge about the
   deployed base for an existing application protocol and a cost-benefit
   analysis regarding security strength vs. interoperability, a given
   service provider might decide to disable TLS 1.2 entirely and offer
   only TLS 1.3.

   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.

   This document updates [RFC5288] in view of the [Boeck2016] attack.
   See Section 7.2.1 for the details.

   This document updates [RFC6066] in view of the [ALPACA] attack.  See
   Section 3.7 for the details.

2.  Terminology

   A number of security-related terms in this document are used in the
   sense defined in [RFC4949], including "attack", "authentication",
   "certificate", "cipher", "compromise", "confidentiality",
   "credential", "data integrity", "encryption", "forward secrecy",
   "key", "key length", "self-signed certificate", "strength", and
   "strong".

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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:

   *  Implementations MUST NOT negotiate SSL version 2.

      Rationale: Today, SSLv2 is considered insecure [RFC6176].

   *  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 Padding Oracle On
      Downgraded Legacy Encryption (POODLE) attack [POODLE], SSLv3 is
      now widely recognized as fundamentally insecure.  See [RFC7568]
      for further details.

   *  Implementations MUST NOT negotiate TLS version 1.0 [RFC2246].

      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 cipher suites based on
      cipher block chaining (CBC) and does not warn against common
      padding errors.  This and other recommendations in this section
      are in line with [RFC8996].

   *  Implementations MUST NOT negotiate TLS version 1.1 [RFC4346].

      Rationale: TLS 1.1 (published in 2006) is a security improvement
      over TLS 1.0 but still does not support certain stronger cipher
      suites that were introduced with the standardization of TLS 1.2 in
      2008, including the cipher suites recommended for TLS 1.2 by this
      document (see Section 4.2 below).

   *  Implementations MUST support TLS 1.2 [RFC5246].

      Rationale: TLS 1.2 is implemented and deployed more widely than
      TLS 1.3 at this time, and when the recommendations in this
      document are followed to mitigate known attacks, the use of TLS
      1.2 is as safe as the use of TLS 1.3.  In most application
      protocols that reuse TLS and DTLS, there is no immediate need to
      migrate solely to TLS 1.3.  Indeed, because many application
      clients are dependent on TLS libraries or operating systems that
      do not yet support TLS 1.3, proactively deprecating TLS 1.2 would
      introduce significant interoperability issues, thus harming
      security more than helping it.  Nevertheless, it is expected that
      a future version of this BCP will deprecate the use of TLS 1.2
      when it is appropriate to do so.

   *  Implementations SHOULD support TLS 1.3 [RFC8446] and, if
      implemented, MUST prefer to negotiate TLS 1.3 over earlier
      versions of TLS.

      Rationale: TLS 1.3 is a major overhaul to the protocol and
      resolves many of the security issues with TLS 1.2.  To the extent
      that an implementation supports TLS 1.2 (even if it defaults to
      TLS 1.3), it MUST follow the recommendations regarding TLS 1.2
      specified in this document.

   *  New transport protocols that integrate the TLS/DTLS handshake
      protocol and/or record layer MUST use only TLS/DTLS 1.3 (for
      instance, QUIC [RFC9001] took this approach).  New application
      protocols that employ TLS/DTLS for channel or session encryption
      MUST integrate with both TLS/DTLS versions 1.2 and 1.3;
      nevertheless, in rare cases where broad interoperability is not a
      concern, application protocol designers MAY choose to forego TLS
      1.2.

      Rationale: Secure deployment of TLS 1.3 is significantly easier
      and less error prone than secure deployment of TLS 1.2.  When
      designing a new secure transport protocol such as QUIC, there is
      no reason to support TLS 1.2.  By contrast, new application
      protocols that reuse TLS need to support both TLS 1.3 and TLS 1.2
      in order to take advantage of underlying library or operating
      system support for both versions.

   This BCP applies to TLS 1.3, TLS 1.2, and 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:

   *  Implementations MUST NOT negotiate DTLS version 1.0 [RFC4347].

      Version 1.0 of DTLS correlates to version 1.1 of TLS (see above).

   *  Implementations MUST support DTLS 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.)

   *  Implementations SHOULD support DTLS 1.3 [RFC9147] and, if
      implemented, MUST prefer to negotiate DTLS version 1.3 over
      earlier versions of DTLS.

      Version 1.3 of DTLS correlates to version 1.3 of TLS (see above).

3.1.3.  Fallback to Lower Versions

   TLS/DTLS 1.2 clients MUST NOT fall back to earlier TLS versions,
   since those versions have been deprecated [RFC8996].  As a result,
   the downgrade-protection Signaling Cipher Suite Value (SCSV)
   mechanism [RFC7507] is no longer needed for clients.  In addition,
   TLS 1.3 implements a new version-negotiation mechanism.

3.2.  Strict TLS

   The following recommendations are provided to help prevent "SSL
   Stripping" and STARTTLS command injection (attacks that are
   summarized in [RFC7457]):

   *  Many existing application protocols were designed before the use
      of TLS became common.  These protocols typically support TLS in
      one of two ways: either via a separate port for TLS-only
      communication (e.g., port 443 for HTTPS) or via a method for
      dynamically upgrading a channel from unencrypted to TLS protected
      (e.g., STARTTLS, which is used in protocols such as IMAP and
      XMPP).  Regardless of the mechanism for protecting the
      communication channel (TLS-only port or dynamic upgrade), what
      matters is the end state of the channel.  When a protocol defines
      both a dynamic upgrade method and a separate TLS-only method, then
      the separate TLS-only method MUST be supported by implementations
      and MUST be configured by administrators to be used in preference
      to the dynamic upgrade method.  When a protocol supports only a
      dynamic upgrade method, implementations MUST provide a way for
      administrators to set a strict local policy that forbids use of
      plaintext in the absence of a negotiated TLS channel, and
      administrators MUST use this policy.

   *  HTTP client and server implementations intended for use in the
      World Wide Web (see Section 5) MUST support the HTTP Strict
      Transport Security (HSTS) header field [RFC6797] so that web
      servers can advertise that they are willing to accept TLS-only
      clients.  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]).  Similar
      technologies exist for non-HTTP application protocols, such as
      Mail Transfer Agent Strict Transport Security (MTA-STS) for mail
      transfer agents [RFC8461] and methods based on DNS-Based
      Authentication of Named Entities (DANE) [RFC6698] for SMTP
      [RFC7672] and XMPP [RFC7712].

   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]) when using TLS 1.2, implementations and
   deployments SHOULD NOT support TLS-level compression (Section 6.2.2
   of [RFC5246]); the only exception is when the application protocol in
   question has been proven not to be open to such attacks.  However,
   even in this case, extreme caution is warranted because of the
   potential for future attacks related to TLS compression.  More
   specifically, the HTTP protocol is known to be vulnerable to
   compression-related attacks.  (This recommendation applies to TLS 1.2
   only, because compression has been removed from TLS 1.3.)

   Rationale: TLS compression has been subject to security attacks such
   as the Compression Ratio Info-leak Made Easy (CRIME) attack.

   Implementers should note that compression at higher protocol levels
   can allow an active attacker to extract cleartext information from
   the connection.  The Browser Reconnaissance and Exfiltration via
   Adaptive Compression of Hypertext (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.

3.3.1.  Certificate Compression

   Certificate chains often take up most of the bytes transmitted during
   the handshake.  In order to manage their size, some or all of the
   following methods can be employed (see also Section 4 of [RFC9191]
   for further suggestions):

   *  Limit the number of names or extensions.

   *  Use keys with small public key representations, like the Elliptic
      Curve Digital Signature Algorithm (ECDSA).

   *  Use certificate compression.

   To achieve the latter, TLS 1.3 defines the compress_certificate
   extension in [RFC8879].  See also Section 5 of [RFC8879] for security
   and privacy considerations associated with its use.  For the
   avoidance of doubt, CRIME-style attacks on TLS compression do not
   apply to certificate compression.

   Due to the strong likelihood of middlebox interference, compression
   in the style of [RFC8879] has not been made available in TLS 1.2.  In
   theory, the cached_info extension defined in [RFC7924] could be used,
   but it is not supported widely enough to be considered a practical
   alternative.

3.4.  TLS Session Resumption

   Session resumption drastically reduces the number of full TLS
   handshakes and thus is an essential performance feature for most
   deployments.

   Stateless session resumption with session tickets is a popular
   strategy.  For TLS 1.2, it is specified in [RFC5077].  For TLS 1.3, a
   more secure mechanism based on the use of a pre-shared key (PSK) is
   described in Section 4.6.1 of [RFC8446].  See [Springall16] for a
   quantitative study of the risks induced by TLS cryptographic
   "shortcuts", including session resumption.

   When it is used, the resumption information MUST be authenticated and
   encrypted to prevent modification or eavesdropping by an attacker.
   Further recommendations apply to session tickets:

   *  A strong cipher MUST be used when encrypting the ticket (at least
      as strong as the main TLS cipher suite).

   *  Ticket-encryption keys MUST be changed regularly, e.g., once every
      week, so as not to negate the benefits of forward secrecy (see
      Section 7.3 for details on forward secrecy).  Old ticket-
      encryption keys MUST be destroyed at the end of the validity
      period.

   *  For similar reasons, session ticket validity MUST be limited to a
      reasonable duration (e.g., half as long as ticket-encryption key
      validity).

   *  TLS 1.2 does not roll the session key forward within a single
      session.  Thus, to prevent an attack where the server's ticket-
      encryption key is stolen and used to decrypt the entire content of
      a session (negating the concept of forward secrecy), a TLS 1.2
      server SHOULD NOT resume sessions that are too old, e.g., sessions
      that have been open longer than two ticket-encryption key rotation
      periods.

   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.

   TLS 1.3 provides the powerful option of forward secrecy even within a
   long-lived connection that is periodically resumed.  Section 2.2 of
   [RFC8446] recommends that clients SHOULD send a "key_share" when
   initiating session resumption.  In order to gain forward secrecy,
   this document recommends that server implementations SHOULD select
   the "psk_dhe_ke" PSK key exchange mode and respond with a "key_share"
   to complete an Ephemeral Elliptic Curve Diffie-Hellman (ECDHE)
   exchange on each session resumption.  As a more performant
   alternative, server implementations MAY refrain from responding with
   a "key_share" until a certain amount of time (e.g., measured in
   hours) has passed since the last ECDHE exchange; this implies that
   the "key_share" operation would not occur for the presumed majority
   of session resumption requests (which would occur within a few hours)
   while still ensuring forward secrecy for longer-lived sessions.

   TLS session resumption introduces potential privacy issues where the
   server is able to track the client, in some cases indefinitely.  See
   [Sy2018] for more details.

3.5.  Renegotiation in TLS 1.2

   The recommendations in this section apply to TLS 1.2 only, because
   renegotiation has been removed from TLS 1.3.

   Renegotiation in TLS 1.2 is a handshake that establishes new
   cryptographic parameters for an existing session.  The mechanism
   existed in TLS 1.2 and in earlier protocol versions and was improved
   following several major attacks including a plaintext injection
   attack, CVE-2009-3555 [CVE].

   TLS 1.2 clients and servers MUST implement the renegotiation_info
   extension, as defined in [RFC5746].

   TLS 1.2 clients MUST send renegotiation_info in the Client Hello.  If
   the server does not acknowledge the extension, the client MUST
   generate a fatal handshake_failure alert prior to terminating the
   connection.

   Rationale: It is not safe for a client to connect to a TLS 1.2 server
   that does not support renegotiation_info regardless of whether either
   endpoint actually implements renegotiation.  See also Section 4.1 of
   [RFC5746].

   A related attack resulting from TLS session parameters not being
   properly authenticated is a Triple Handshake [Triple-Handshake].  To
   address this attack, TLS 1.2 implementations MUST support the
   extended_master_secret extension defined in [RFC7627].

3.6.  Post-Handshake Authentication

   Renegotiation in TLS 1.2 was (partially) replaced in TLS 1.3 by
   separate post-handshake authentication and key update mechanisms.  In
   the context of protocols that multiplex requests over a single
   connection (such as HTTP/2 [RFC9113]), post-handshake authentication
   has the same problems as TLS 1.2 renegotiation.  Multiplexed
   protocols SHOULD follow the advice provided for HTTP/2 in
   Section 9.2.3 of [RFC9113].

3.7.  Server Name Indication (SNI)

   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.  At the time of writing, a technology for encrypting the SNI
   (called Encrypted Client Hello) is being worked on in the TLS Working
   Group [TLS-ECH].  Once that method has been standardized and widely
   implemented, it will likely be appropriate to recommend its usage in
   a future version of this BCP.

   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.  However, SNI also leaks the target domain for a
   given connection; this information leak will be closed by use of TLS
   Encrypted Client Hello once that method has been standardized.

   In order to prevent the attacks described in [ALPACA], a server that
   does not recognize the presented server name SHOULD NOT continue the
   handshake and instead SHOULD fail with a fatal-level
   unrecognized_name(112) alert.  Note that this recommendation updates
   Section 3 of [RFC6066], which stated:

   |  If the server understood the ClientHello extension but does not
   |  recognize the server name, the server SHOULD take one of two
   |  actions: either abort the handshake by sending a fatal-level
   |  unrecognized_name(112) alert or continue the handshake.

   Clients SHOULD abort the handshake if the server acknowledges the SNI
   extension but presents a certificate with a different hostname than
   the one sent by the client.

3.8.  Application-Layer Protocol Negotiation (ALPN)

   TLS implementations (both client- and server-side) MUST support the
   Application-Layer Protocol Negotiation (ALPN) extension [RFC7301].

   In order to prevent "cross-protocol" attacks resulting from failure
   to ensure that a message intended for use in one protocol cannot be
   mistaken for a message for use in another protocol, servers are
   advised to strictly enforce the behavior prescribed in Section 3.2 of
   [RFC7301]:

   |  In the event that the server supports no protocols that the client
   |  advertises, then the server SHALL respond with a fatal
   |  'no_application_protocol' alert.

   Clients SHOULD abort the handshake if the server acknowledges the
   ALPN extension but does not select a protocol from the client list.
   Failure to do so can result in attacks such those described in
   [ALPACA].

   Protocol developers are strongly encouraged to register an ALPN
   identifier for their protocols.  This applies both to new protocols
   and to well-established protocols; however, because the latter might
   have a large deployed base, strict enforcement of ALPN usage may not
   be feasible when an ALPN identifier is registered for a well-
   established protocol.

3.9.  Multi-Server Deployment

   Deployments that involve multiple servers or services can increase
   the size of the attack surface for TLS.  Two scenarios are of
   interest:

   1.  Deployments in which multiple services handle the same domain
       name via different protocols (e.g., HTTP and IMAP).  In this
       case, an attacker might be able to direct a connecting endpoint
       to the service offering a different protocol and mount a cross-
       protocol attack.  In a cross-protocol attack, the client and
       server believe they are using different protocols, which the
       attacker might exploit if messages sent in one protocol are
       interpreted as messages in the other protocol with undesirable
       effects (see [ALPACA] for more detailed information about this
       class of attacks).  To mitigate this threat, service providers
       SHOULD deploy ALPN (see Section 3.8).  In addition, to the extent
       possible, they SHOULD ensure that multiple services handling the
       same domain name provide equivalent levels of security that are
       consistent with the recommendations in this document; such
       measures SHOULD include the handling of configurations across
       multiple TLS servers and protections against compromise of
       credentials held by those servers.

   2.  Deployments in which multiple servers providing the same service
       have different TLS configurations.  In this case, an attacker
       might be able to direct a connecting endpoint to a server with a
       TLS configuration that is more easily exploitable (see [DROWN]
       for more detailed information about this class of attacks).  To
       mitigate this threat, service providers SHOULD ensure that all
       servers providing the same service provide equivalent levels of
       security that are consistent with the recommendations in this
       document.

3.10.  Zero Round-Trip Time (0-RTT) Data in TLS 1.3

   The 0-RTT early data feature is new in TLS 1.3.  It provides reduced
   latency when TLS connections are resumed, at the potential cost of
   certain security properties.  As a result, it requires special
   attention from implementers on both the server and the client side.
   Typically, this extends to the TLS library as well as protocol layers
   above it.

   For HTTP over TLS, refer to [RFC8470] for guidance.

   For QUIC on TLS, refer to Section 9.2 of [RFC9001].

   For other protocols, generic guidance is given in Section 8 and
   Appendix E.5 of [RFC8446].  To paraphrase Appendix E.5, applications
   MUST avoid this feature unless an explicit specification exists for
   the application protocol in question to clarify when 0-RTT is
   appropriate and secure.  This can take the form of an IETF RFC, a
   non-IETF standard, or documentation associated with a non-standard
   protocol.

4.  Recommendations: Cipher Suites

   TLS 1.2 provided considerable flexibility in the selection of cipher
   suites.  Unfortunately, the security of some of these cipher suites
   has degraded over time to the point where some are known to be
   insecure (this is one reason why TLS 1.3 restricted such
   flexibility).  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.
   Consequently, cipher suites using weak algorithms need to be phased
   out 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 well over 20 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.

   *  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.

   *  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.

   *  Implementations MUST NOT negotiate cipher suites offering less
      than 112 bits of security, including so-called "export-level"
      encryption (which provides 40 or 56 bits of security).

      Rationale: Based on [RFC3766], at least 112 bits of security is
      needed.  40-bit and 56-bit security (found in so-called "export
      ciphers") are considered insecure today.

   *  Implementations SHOULD NOT negotiate cipher suites that use
      algorithms offering less than 128 bits of security.

      Rationale: Cipher suites that offer 112 or more bits but less than
      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 Triple DES (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.

   *  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.

   *  Implementations SHOULD NOT negotiate cipher suites based on non-
      ephemeral (static) finite-field Diffie-Hellman (DH) key agreement.
      Similarly, implementations SHOULD NOT negotiate non-ephemeral
      Elliptic Curve DH key agreement.

      Rationale: The former cipher suites, which have assigned values
      prefixed by "TLS_DH_*", have several drawbacks, especially the
      fact that they do not support forward secrecy.  The latter
      ("TLS_ECDH_*") also lack forward secrecy and are subject to
      invalid curve attacks [Jager2015].

   *  Implementations MUST support and prefer to negotiate cipher suites
      offering forward secrecy.  However, TLS 1.2 implementations SHOULD
      NOT negotiate cipher suites based on ephemeral finite-field
      Diffie-Hellman key agreement (i.e., "TLS_DHE_*" suites).  This is
      justified by the known fragility of the construction (see
      [RACCOON]) and the limitation around negotiation, including using
      [RFC7919], which has seen very limited uptake.

      Rationale: Forward secrecy (sometimes called "perfect forward
      secrecy") prevents the recovery of information that was encrypted
      with older session keys, thus limiting how far back in time data
      can be decrypted when an attack is successful.  See Sections 7.3
      and 7.4 for a detailed discussion.

4.2.  Cipher Suites for TLS 1.2

   Given the foregoing considerations, implementation and deployment of
   the following cipher suites is RECOMMENDED:

   *  TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256

   *  TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384

   *  TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256

   *  TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384

   As these are Authenticated Encryption with Associated Data (AEAD)
   algorithms [RFC5116], these cipher suites are supported only in TLS
   1.2 and not in earlier protocol versions.

   Typically, to prefer these suites, the order of suites needs to be
   explicitly configured in server software.  It would be ideal if
   server software implementations were to prefer these suites by
   default.

   Some devices have hardware support for AES Counter Mode with CBC-MAC
   (AES-CCM) but not AES Galois/Counter Mode (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 these are out of scope entirely.

   A cipher suite that operates in CBC (cipher block chaining) mode
   (e.g., TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256) SHOULD NOT be used
   unless the encrypt_then_mac extension [RFC7366] is also successfully
   negotiated.  This requirement applies to both client and server
   implementations.

   When using ECDSA signatures for authentication of TLS peers, it is
   RECOMMENDED that implementations use the NIST curve P-256.  In
   addition, to avoid predictable or repeated nonces (which could reveal
   the long-term signing key), it is RECOMMENDED that implementations
   implement "deterministic ECDSA" as specified in [RFC6979] and in line
   with the recommendations in [RFC8446].

   Note that implementations of "deterministic ECDSA" may be vulnerable
   to certain side-channel and fault injection attacks precisely because
   of their determinism.  While most fault injection attacks described
   in the literature assume physical access to the device (and therefore
   are more relevant in Internet of Things (IoT) deployments with poor
   or non-existent physical security), some can be carried out remotely
   [Poddebniak2017], e.g., as Rowhammer [Kim2014] variants.  In
   deployments where side-channel attacks and fault injection attacks
   are a concern, implementation strategies combining both randomness
   and determinism (for example, as described in [CFRG-DET-SIGS]) can be
   used to avoid the risk of successful extraction of the signing key.

4.2.1.  Implementation Details

   Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the
   first proposal to any server.  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.

   The previous version of the TLS recommendations [RFC7525] implicitly
   allowed the old RFC 5246 mandatory-to-implement cipher suite,
   TLS_RSA_WITH_AES_128_CBC_SHA.  At the time of writing, this cipher
   suite does not provide additional interoperability, except with very
   old clients.  As with other cipher suites that do not provide forward
   secrecy, implementations SHOULD NOT support this cipher suite.  Other
   application protocols specify other cipher suites as mandatory to
   implement (MTI).

   [RFC8422] allows clients and servers to negotiate ECDH parameters
   (curves).  Both clients and servers SHOULD include the "Supported
   Elliptic Curves Extension" [RFC8422].  Clients and servers SHOULD
   support the NIST P-256 (secp256r1) [RFC8422] and X25519 (x25519)
   [RFC7748] curves.  Note that [RFC8422] deprecates all but the
   uncompressed point format.  Therefore, if the client sends an
   ec_point_formats extension, the ECPointFormatList MUST contain a
   single element, "uncompressed".

4.3.  Cipher Suites for TLS 1.3

   This document does not specify any cipher suites for TLS 1.3.
   Readers are referred to Section 9.1 of [RFC8446] for cipher suite
   recommendations.

4.4.  Limits on Key Usage

   All ciphers have an upper limit on the amount of traffic that can be
   securely protected with any given key.  In the case of AEAD cipher
   suites, two separate limits are maintained for each key:

   1.  Confidentiality limit (CL), i.e., the number of records that can
       be encrypted.

   2.  Integrity limit (IL), i.e., the number of records that are
       allowed to fail authentication.

   The latter applies to DTLS (and also to QUIC) but not to TLS itself,
   since TLS connections are torn down on the first decryption failure.

   When a sender is approaching CL, the implementation SHOULD initiate a
   new handshake (in TLS 1.3, this can be achieved by sending a
   KeyUpdate message on the established session) to rotate the session
   key.  When a receiver has reached IL, the implementation SHOULD close
   the connection.  Although these recommendations are a best practice,
   implementers need to be aware that it is not always easy to
   accomplish them in protocols that are built on top of TLS/DTLS
   without introducing coordination across layer boundaries.  See
   Section 6 of [RFC9001] for an example of the cooperation that was
   necessary in QUIC between the crypto and transport layers to support
   key updates.  Note that in general, application protocols might not
   be able to emulate that method given their more constrained
   interaction with TLS/DTLS.  As a result of these complexities, these
   recommendations are not mandatory.

   For all TLS 1.3 cipher suites, readers are referred to Section 5.5 of
   [RFC8446] for the values of CL and IL.  For all DTLS 1.3 cipher
   suites, readers are referred to Section 4.5.3 of [RFC9147].

   For all AES-GCM cipher suites recommended for TLS 1.2 and DTLS 1.2 in
   this document, CL can be derived by plugging the corresponding
   parameters into the inequalities in Section 6.1 of [AEAD-LIMITS] that
   apply to random, partially implicit nonces, i.e., the nonce
   construction used in TLS 1.2.  Although the obtained figures are
   slightly higher than those for TLS 1.3, it is RECOMMENDED that the
   same limit of 2^24.5 records is used for both versions.

   For all AES-GCM cipher suites recommended for DTLS 1.2, IL (obtained
   from the same inequalities referenced above) is 2^28.

4.5.  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 REQUIRED.

   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].  A DH key of 2048
   bits (equivalent to a 112-bit symmetric key) is the minimum allowed
   by the latest revision of [NIST.SP.800-56A] as of this writing (see
   in particular Appendix D of that document).

   As noted in [RFC3766], correcting for the emergence of The Weizmann
   Institute Relation Locator (TWIRL) machine [TWIRL] would imply that
   1024-bit DH keys yield about 61 bits of equivalent strength and that
   a 2048-bit DH key would yield about 92 bits of equivalent strength.
   The Logjam attack [Logjam] further demonstrates that 1024-bit Diffie-
   Hellman parameters should be avoided.

   With regard to ECDH keys, implementers are referred to the IANA "TLS
   Supported Groups" registry (formerly known as the "EC Named Curve
   Registry") within the "Transport Layer Security (TLS) Parameters"
   registry [IANA_TLS] and in particular to the "recommended" groups.
   Curves of less than 224 bits MUST NOT be used.  This recommendation
   is in line with the latest revision of [NIST.SP.800-56A].

   When using RSA, servers MUST 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 and SHA-1 or MD5 MUST NOT
   be used [RFC9155] (for more details, see also [CAB-Baseline], for
   which the current version at the time of writing is 1.8.4).  Clients
   MUST indicate to servers that they request SHA-256 by using the
   "Signature Algorithms" extension defined in TLS 1.2.  For TLS 1.3,
   the same requirement is already specified by [RFC8446].


4.6.  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].

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:

   *  Web software and services that wish to protect HTTP traffic with
      TLS.

   *  Email software and services that wish to protect IMAP, Post Office
      Protocol version 3 (POP3), or SMTP traffic with TLS.

   *  Instant-messaging software and services that wish to protect
      Extensible Messaging and Presence Protocol (XMPP) or Internet
      Relay Chat (IRC) traffic with TLS.

   *  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 [RFC7590],
   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).

   Although many of the recommendations provided here might also apply
   to QUIC insofar that it uses the TLS 1.3 handshake protocol, QUIC and
   other such secure transport protocols are out of scope of this
   document.  For QUIC specifically, readers are referred to Section 9.2
   of [RFC9001].

   This document does not address the use of TLS in constrained-node
   networks [RFC7228].  For recommendations regarding the profiling of
   TLS and DTLS for small devices with severe constraints on power,
   memory, and processing resources, the reader is referred to [RFC7925]
   and [IOT-PROFILE].

5.1.  Security Services

   This document provides recommendations for an audience that wishes to
   secure their communication with TLS to achieve the following:

   Confidentiality:  all application-layer communication is encrypted
      with the goal that no party should be able to decrypt it except
      the intended receiver.

   Data integrity:  any changes made to the communication in transit are
      detectable by the receiver.

   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 requires 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 clients.

   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.  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.

   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.

6.  IANA Considerations

   This document has no IANA actions.

7.  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.  The reader is referred to the Security
   Considerations sections of TLS 1.3 [RFC8446], DTLS 1.3 [RFC9147], TLS
   1.2 [RFC5246], and DTLS 1.2 [RFC6347] for further context.

7.1.  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 Sections 4.3.3, 4.3.4, and 4.3.5 of
   [RFC9110].

   Host name validation is security-critical for all common TLS use
   cases.  Without it, TLS ensures that the certificate is valid and
   guarantees possession of the private key but does not ensure that the
   connection terminates at the desired endpoint.  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 application protocols, some of which implement a policy very
   different from HTTPS.

   If the host name is discovered indirectly and insecurely (e.g., by a
   cleartext DNS query for an SRV or Mail Exchange (MX) 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
   [RFC7673] and [RFC7672]).

   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].

7.2.  AES-GCM

   Section 4.2 recommends the use of the AES-GCM authenticated
   encryption algorithm.  Please refer to Section 6 of [RFC5288] for
   security considerations that apply specifically to AES-GCM when used
   with TLS.

7.2.1.   Nonce Reuse in TLS 1.2

   The existence of deployed TLS stacks that mistakenly reuse the AES-
   GCM nonce is documented in [Boeck2016], showing there is an actual
   risk of AES-GCM getting implemented insecurely and thus making TLS
   sessions that use an AES-GCM cipher suite vulnerable to attacks such
   as [Joux2006].  (See [CVE] records: CVE-2016-0270, CVE-2016-10213,
   CVE-2016-10212, and CVE-2017-5933.)

   While this problem has been fixed in TLS 1.3, which enforces a
   deterministic method to generate nonces from record sequence numbers
   and shared secrets for all its AEAD cipher suites (including AES-
   GCM), TLS 1.2 implementations could still choose their own
   (potentially insecure) nonce generation methods.

   It is therefore RECOMMENDED that TLS 1.2 implementations use the
   64-bit sequence number to populate the nonce_explicit part of the GCM
   nonce, as described in the first two paragraphs of Section 5.3 of
   [RFC8446].  This stronger recommendation updates Section 3 of
   [RFC5288], which specifies that the use of 64-bit sequence numbers to
   populate the nonce_explicit field is optional.

   We note that at the time of writing, there are no cipher suites
   defined for nonce-reuse-resistant algorithms such as AES-GCM-SIV
   [RFC8452].

7.3.  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:

   *  A client or server being attacked by some other attack vector, and
      the private key retrieved.

   *  A long-term key retrieved from a device that has been sold or
      otherwise decommissioned without prior wiping.

   *  A long-term key used on a device as a default key [Heninger2012].

   *  A key generated by a trusted third party like a CA and later
      retrieved from it by either extortion or compromise
      [Soghoian2011].

   *  A cryptographic breakthrough or the use of asymmetric keys with
      insufficient length [Kleinjung2010].

   *  Social engineering attacks against system administrators.

   *  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
   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
   arithmetic.  Given the current state of the art, Elliptic Curve
   Diffie-Hellman appears to be more efficient, permits shorter key
   lengths, and allows less freedom for implementation errors than
   finite-field Diffie-Hellman.

   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.

7.4.  Diffie-Hellman Exponent Reuse

   For performance reasons, it is not uncommon for TLS implementations
   to reuse Diffie-Hellman and Elliptic Curve Diffie-Hellman exponents
   across multiple connections.  Such reuse can result in major security
   issues:

   *  If exponents are reused for too long (in some cases, even as
      little as 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.

   *  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, although general guidance in this area is
      provided by [NIST.SP.800-56A] and available in many protocol
      implementations.

   *  Under certain conditions, the use of static finite-field DH keys,
      or of ephemeral finite-field DH keys that are reused across
      multiple connections, can lead to timing attacks (such as those
      described in [RACCOON]) on the shared secrets used in Diffie-
      Hellman key exchange.

   *  An "invalid curve" attack can be mounted against Elliptic Curve DH
      if the victim does not verify that the received point lies on the
      correct curve.  If the victim is reusing the DH secrets, the
      attacker can repeat the probe varying the points to recover the
      full secret (see [Antipa2003] and [Jager2015]).

   To address these concerns:

   *  TLS implementations SHOULD NOT use static finite-field DH keys and
      SHOULD NOT reuse ephemeral finite-field DH keys across multiple
      connections.

   *  Server implementations that want to reuse Elliptic Curve DH keys
      SHOULD either use a "safe curve" [SAFECURVES] (e.g., X25519) or
      perform the checks described in [NIST.SP.800-56A] on the received
      points.

7.5.  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]:

   *  Certificate revocation is an important tool when recovering from
      attacks on the TLS implementation as well as cases of misissued
      certificates.  TLS implementations MUST implement a strategy to
      distrust revoked certificates.

   *  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.  The more
      modern [CRLite] and the follow-on Let's Revoke [LetsRevoke] build
      on the availability of Certificate Transparency [RFC9162] logs and
      aggressive compression to allow practical use of the CRL
      infrastructure, but at the time of writing, neither solution is
      deployed for client-side revocation processing at scale.

   *  Proprietary mechanisms that embed revocation lists in the web
      browser's configuration database cannot scale beyond the few most
      heavily used web servers.

   *  The Online Certification Status Protocol (OCSP) [RFC6960] in its
      basic form 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.)  For a recent survey of the
      status of OCSP deployment in the web PKI, see [Chung18].

   *  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 an active on-path attacker because the attacker
      can simply ignore the client's request for a stapled OCSP
      response.

   *  [RFC7633] defines a certificate extension that indicates that
      clients must expect stapled OCSP responses for the certificate and
      must abort the handshake ("hard-fail") if such a response is not
      available.

   *  OCSP stapling as used in TLS 1.2 does not extend to intermediate
      certificates within a certificate chain.  The Multiple Certificate
      Status extension [RFC6961] addresses this shortcoming, but it has
      seen little deployment and had been deprecated by [RFC8446].  As a
      result, although this extension was recommended for TLS 1.2 in
      [RFC7525], it is no longer recommended by this document.

   *  TLS 1.3 (Section 4.4.2.1 of [RFC8446]) allows the association of
      OCSP information with intermediate certificates by using an
      extension to the CertificateEntry structure.  However, using this
      facility remains impractical because many certification
      authorities (CAs) either do not publish OCSP for CA certificates
      or publish OCSP reports with a lifetime that is too long to be
      useful.

   *  Both CRLs and OCSP depend on relatively reliable connectivity to
      the Internet, which might not be available to certain kinds of
      nodes.  A common example is newly provisioned devices that need to
      establish a secure connection in order to boot up for the first
      time.

   For the common use cases of public key certificates in TLS, 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:
   OCSP [RFC6960] and OCSP stapling using the status_request extension
   defined in [RFC6066].  Note that the exact mechanism for embedding
   the status_request extension differs between TLS 1.2 and 1.3.  As a
   matter of local policy, server operators MAY request that CAs issue
   must-staple [RFC7633] certificates for the server and/or for client
   authentication, but we recommend reviewing the operational conditions
   before deciding on this approach.

   The considerations in this section do not apply to scenarios where
   the DNS-Based Authentication of Named Entities (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.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3766]  Orman, H. and P. Hoffman, "Determining Strengths For
              Public Keys Used For Exchanging Symmetric Keys", BCP 86,
              RFC 3766, DOI 10.17487/RFC3766, April 2004,
              <https://www.rfc-editor.org/info/rfc3766>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://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,
              DOI 10.17487/RFC5288, August 2008,
              <https://www.rfc-editor.org/info/rfc5288>.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
              <https://www.rfc-editor.org/info/rfc5746>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [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, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC6176]  Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer
              (SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176, March
              2011, <https://www.rfc-editor.org/info/rfc6176>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <https://www.rfc-editor.org/info/rfc6979>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/info/rfc7301>.

   [RFC7366]  Gutmann, P., "Encrypt-then-MAC for Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
              <https://www.rfc-editor.org/info/rfc7366>.

   [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
              DOI 10.17487/RFC7465, February 2015,
              <https://www.rfc-editor.org/info/rfc7465>.

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,
              <https://www.rfc-editor.org/info/rfc7627>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8422]  Nir, Y., Josefsson, S., and M. Pegourie-Gonnard, "Elliptic
              Curve Cryptography (ECC) Cipher Suites for Transport Layer
              Security (TLS) Versions 1.2 and Earlier", RFC 8422,
              DOI 10.17487/RFC8422, August 2018,
              <https://www.rfc-editor.org/info/rfc8422>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8996]  Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
              1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021,
              <https://www.rfc-editor.org/info/rfc8996>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.

   [RFC9155]  Velvindron, L., Moriarty, K., and A. Ghedini, "Deprecating
              MD5 and SHA-1 Signature Hashes in TLS 1.2 and DTLS 1.2",
              RFC 9155, DOI 10.17487/RFC9155, December 2021,
              <https://www.rfc-editor.org/info/rfc9155>.

8.2.  Informative References

   [AEAD-LIMITS]
              Günther, F., Thomson, M., and C. A. Wood, "Usage Limits on
              AEAD Algorithms", Work in Progress, Internet-Draft, draft-
              irtf-cfrg-aead-limits-05, 11 July 2022,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              aead-limits-05>.

   [ALPACA]   Brinkmann, M., Dresen, C., Merget, R., Poddebniak, D.,
              Müller, J., Somorovsky, J., Schwenk, J., and S. Schinzel,
              "ALPACA: Application Layer Protocol Confusion - Analyzing
              and Mitigating Cracks in TLS Authentication", 30th USENIX
              Security Symposium (USENIX Security 21), August 2021,
              <https://www.usenix.org/conference/usenixsecurity21/
              presentation/brinkmann>.

   [Antipa2003]
              Antipa, A., Brown, D. R. L., Menezes, A., Struik, R., and
              S. Vanstone, "Validation of Elliptic Curve Public Keys",
              Public Key Cryptography - PKC 2003, December 2003,
              <https://doi.org/10.1007/3-540-36288-6_16>.

   [Boeck2016]
              Böck, H., Zauner, A., Devlin, S., Somorovsky, J., and P.
              Jovanovic, "Nonce-Disrespecting Adversaries: Practical
              Forgery Attacks on GCM in TLS", May 2016,
              <https://eprint.iacr.org/2016/475.pdf>.

   [CAB-Baseline]
              CA/Browser Forum, "Baseline Requirements for the Issuance
              and Management of Publicly-Trusted Certificates",
              Version 1.8.4, April 2022,
              <https://cabforum.org/documents/>.

   [CFRG-DET-SIGS]
              Preuß Mattsson, J., Thormarker, E., and S. Ruohomaa,
              "Deterministic ECDSA and EdDSA Signatures with Additional
              Randomness", Work in Progress, Internet-Draft, draft-irtf-
              cfrg-det-sigs-with-noise-00, 8 August 2022,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              det-sigs-with-noise-00>.

   [Chung18]  Chung, T., Lok, J., Chandrasekaran, B., Choffnes, D.,
              Levin, D., Maggs, B., Mislove, A., Rula, J., Sullivan, N.,
              and C. Wilson, "Is the Web Ready for OCSP Must-Staple?",
              Proceedings of the Internet Measurement Conference 2018,
              DOI 10.1145/3278532.3278543, October 2018,
              <https://doi.org/10.1145/3278532.3278543>.

   [CRLite]   Larisch, J., Choffnes, D., Levin, D., Maggs, B., Mislove,
              A., and C. Wilson, "CRLite: A Scalable System for Pushing
              All TLS Revocations to All Browsers", 2017 IEEE Symposium
              on Security and Privacy (SP), DOI 10.1109/sp.2017.17, May
              2017, <https://doi.org/10.1109/sp.2017.17>.

   [CVE]      MITRE, "Common Vulnerabilities and Exposures",
              <https://cve.mitre.org>.

   [DegabrieleP07]
              Degabriele, J. and K. Paterson, "Attacking the IPsec
              Standards in Encryption-only Configurations", 2007 IEEE
              Symposium on Security and Privacy (SP '07),
              DOI 10.1109/sp.2007.8, May 2007,
              <https://doi.org/10.1109/sp.2007.8>.

   [DROWN]    Aviram, N., Schinzel, S., Somorovsky, J., Heninger, N.,
              Dankel, M., Steube, J., Valenta, L., Adrian, D.,
              Halderman, J., Dukhovni, V., Käsper, E., Cohney, S.,
              Engels, S., Paar, C., and Y. Shavitt, "DROWN: Breaking TLS
              using SSLv2", 25th USENIX Security Symposium (USENIX
              Security 16), August 2016,
              <https://www.usenix.org/conference/usenixsecurity16/
              technical-sessions/presentation/aviram>.

   [Heninger2012]
              Heninger, N., Durumeric, Z., Wustrow, E., and J. A.
              Halderman, "Mining Your Ps and Qs: Detection of Widespread
              Weak Keys in Network Devices", 21st Usenix Security
              Symposium, August 2012.

   [IANA_TLS] IANA, "Transport Layer Security (TLS) Parameters",
              <https://www.iana.org/assignments/tls-parameters>.

   [IOT-PROFILE]
              Tschofenig, H. and T. Fossati, "TLS/DTLS 1.3 Profiles for
              the Internet of Things", Work in Progress, Internet-Draft,
              draft-ietf-uta-tls13-iot-profile-05, 6 July 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-uta-
              tls13-iot-profile-05>.

   [Jager2015]
              Jager, T., Schwenk, J., and J. Somorovsky, "Practical
              Invalid Curve Attacks on TLS-ECDH", Computer Security --
              ESORICS 2015, pp. 407-425,
              DOI 10.1007/978-3-319-24174-6_21, 2015,
              <https://doi.org/10.1007/978-3-319-24174-6_21>.

   [Joux2006] Joux, A., "Authentication Failures in NIST version of
              GCM", 2006, <https://csrc.nist.gov/csrc/media/projects/
              block-cipher-techniques/documents/bcm/comments/800-38-
              series-drafts/gcm/joux_comments.pdf>.

   [Kim2014]  Kim, Y., Daly, R., Kim, J., Fallin, C., Lee, J. H., Lee,
              D., Wilkerson, C., Lai, K., and O. Mutlu, "Flipping Bits
              in Memory Without Accessing Them: An Experimental Study of
              DRAM Disturbance Errors", DOI 10.1109/ISCA.2014.6853210,
              July 2014, <https://users.ece.cmu.edu/~yoonguk/papers/kim-
              isca14.pdf>.

   [Kleinjung2010]
              Kleinjung, T., Aoki, K., Franke, J., Lenstra, A., Thomé,
              E., Bos, J., Gaudry, P., Kruppa, A., Montgomery, P.,
              Osvik, D., te Riele, H., Timofeev, A., and P. Zimmermann,
              "Factorization of a 768-Bit RSA Modulus", Advances in
              Cryptology - CRYPTO 2010, pp. 333-350,
              DOI 10.1007/978-3-642-14623-7_18, 2010,
              <https://doi.org/10.1007/978-3-642-14623-7_18>.

   [LetsRevoke]
              Smith, T., Dickinson, L., and K. Seamons, "Let's Revoke:
              Scalable Global Certificate Revocation", Proceedings 2020
              Network and Distributed System Security Symposium,
              DOI 10.14722/ndss.2020.24084, February 2020,
              <https://doi.org/10.14722/ndss.2020.24084>.

   [Logjam]   Adrian, D., Bhargavan, K., Durumeric, Z., Gaudry, P.,
              Green, M., Halderman, J., Heninger, N., Springall, D.,
              Thomé, E., Valenta, L., VanderSloot, B., Wustrow, E.,
              Zanella-Béguelin, S., and P. Zimmermann, "Imperfect
              Forward Secrecy: How Diffie-Hellman Fails in Practice",
              Proceedings of the 22nd ACM SIGSAC Conference on Computer
              and Communications Security, pp. 5-17,
              DOI 10.1145/2810103.2813707, October 2015,
              <https://doi.org/10.1145/2810103.2813707>.

   [Multiple-Encryption]
              Merkle, R. and M. Hellman, "On the security of multiple
              encryption", Communications of the ACM, Vol. 24, Issue 7,
              pp. 465-467, DOI 10.1145/358699.358718, July 1981,
              <https://doi.org/10.1145/358699.358718>.

   [NIST.SP.800-56A]
              National Institute of Standards and Technology,
              "Recommendation for Pair-Wise Key-Establishment Schemes
              Using Discrete Logarithm Cryptography", Revision 3, NIST
              Special Publication 800-56A,
              DOI 10.6028/NIST.SP.800-56Ar3, April 2018,
              <https://doi.org/10.6028/NIST.SP.800-56Ar3>.

   [PatersonRS11]
              Paterson, K., Ristenpart, T., and T. Shrimpton, "Tag Size
              Does Matter: Attacks and Proofs for the TLS Record
              Protocol", Proceedings of the 17th International
              conference on The Theory and Application of Cryptology and
              Information Security, pp. 372-389,
              DOI 10.1007/978-3-642-25385-0_20, December 2011,
              <https://doi.org/10.1007/978-3-642-25385-0_20>.

   [Poddebniak2017]
              Poddebniak, D., Somorovsky, J., Schinzel, S., Lochter, M.,
              and P. Rösler, "Attacking Deterministic Signature Schemes
              using Fault Attacks", Conference: 2018 IEEE European
              Symposium on Security and Privacy,
              DOI 10.1109/EuroSP.2018.00031, April 2018,
              <https://eprint.iacr.org/2017/1014.pdf>.

   [POODLE]   US-CERT, "SSL 3.0 Protocol Vulnerability and POODLE
              Attack", October 2014,
              <https://www.us-cert.gov/ncas/alerts/TA14-290A>.

   [RACCOON]  Merget, R., Brinkmann, M., Aviram, N., Somorovsky, J.,
              Mittmann, J., and J. Schwenk, "Raccoon Attack: Finding and
              Exploiting Most-Significant-Bit-Oracles in TLS-DH(E)",
              30th USENIX Security Symposium (USENIX Security 21), 2021,
              <https://www.usenix.org/conference/usenixsecurity21/
              presentation/merget>.

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996,
              <https://www.rfc-editor.org/info/rfc2026>.

   [RFC2246]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
              RFC 2246, DOI 10.17487/RFC2246, January 1999,
              <https://www.rfc-editor.org/info/rfc2246>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC3602]  Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC Cipher
              Algorithm and Its Use with IPsec", RFC 3602,
              DOI 10.17487/RFC3602, September 2003,
              <https://www.rfc-editor.org/info/rfc3602>.

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346,
              DOI 10.17487/RFC4346, April 2006,
              <https://www.rfc-editor.org/info/rfc4346>.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,
              <https://www.rfc-editor.org/info/rfc4347>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <https://www.rfc-editor.org/info/rfc5077>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://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, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/info/rfc5321>.

   [RFC6101]  Freier, A., Karlton, P., and P. Kocher, "The Secure
              Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
              DOI 10.17487/RFC6101, August 2011,
              <https://www.rfc-editor.org/info/rfc6101>.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <https://www.rfc-editor.org/info/rfc6120>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,
              <https://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, DOI 10.17487/RFC6960, June 2013,
              <https://www.rfc-editor.org/info/rfc6960>.

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              DOI 10.17487/RFC6961, June 2013,
              <https://www.rfc-editor.org/info/rfc6961>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <https://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, DOI 10.17487/RFC7457,
              February 2015, <https://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, DOI 10.17487/RFC7507, April 2015,
              <https://www.rfc-editor.org/info/rfc7507>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC7568]  Barnes, R., Thomson, M., Pironti, A., and A. Langley,
              "Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
              DOI 10.17487/RFC7568, June 2015,
              <https://www.rfc-editor.org/info/rfc7568>.

   [RFC7590]  Saint-Andre, P. and T. Alkemade, "Use of Transport Layer
              Security (TLS) in the Extensible Messaging and Presence
              Protocol (XMPP)", RFC 7590, DOI 10.17487/RFC7590, June
              2015, <https://www.rfc-editor.org/info/rfc7590>.

   [RFC7633]  Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS)
              Feature Extension", RFC 7633, DOI 10.17487/RFC7633,
              October 2015, <https://www.rfc-editor.org/info/rfc7633>.

   [RFC7672]  Dukhovni, V. and W. Hardaker, "SMTP Security via
              Opportunistic DNS-Based Authentication of Named Entities
              (DANE) Transport Layer Security (TLS)", RFC 7672,
              DOI 10.17487/RFC7672, October 2015,
              <https://www.rfc-editor.org/info/rfc7672>.

   [RFC7673]  Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
              Based Authentication of Named Entities (DANE) TLSA Records
              with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October
              2015, <https://www.rfc-editor.org/info/rfc7673>.

   [RFC7712]  Saint-Andre, P., Miller, M., and P. Hancke, "Domain Name
              Associations (DNA) in the Extensible Messaging and
              Presence Protocol (XMPP)", RFC 7712, DOI 10.17487/RFC7712,
              November 2015, <https://www.rfc-editor.org/info/rfc7712>.

   [RFC7919]  Gillmor, D., "Negotiated Finite Field Diffie-Hellman
              Ephemeral Parameters for Transport Layer Security (TLS)",
              RFC 7919, DOI 10.17487/RFC7919, August 2016,
              <https://www.rfc-editor.org/info/rfc7919>.

   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,
              <https://www.rfc-editor.org/info/rfc7924>.

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016,
              <https://www.rfc-editor.org/info/rfc7925>.

   [RFC8452]  Gueron, S., Langley, A., and Y. Lindell, "AES-GCM-SIV:
              Nonce Misuse-Resistant Authenticated Encryption",
              RFC 8452, DOI 10.17487/RFC8452, April 2019,
              <https://www.rfc-editor.org/info/rfc8452>.

   [RFC8461]  Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
              and J. Jones, "SMTP MTA Strict Transport Security (MTA-
              STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
              <https://www.rfc-editor.org/info/rfc8461>.

   [RFC8470]  Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
              Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
              2018, <https://www.rfc-editor.org/info/rfc8470>.

   [RFC8879]  Ghedini, A. and V. Vasiliev, "TLS Certificate
              Compression", RFC 8879, DOI 10.17487/RFC8879, December
              2020, <https://www.rfc-editor.org/info/rfc8879>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [RFC9001]  Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
              <https://www.rfc-editor.org/info/rfc9001>.

   [RFC9051]  Melnikov, A., Ed. and B. Leiba, Ed., "Internet Message
              Access Protocol (IMAP) - Version 4rev2", RFC 9051,
              DOI 10.17487/RFC9051, August 2021,
              <https://www.rfc-editor.org/info/rfc9051>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

   [RFC9112]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
              June 2022, <https://www.rfc-editor.org/info/rfc9112>.

   [RFC9113]  Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
              DOI 10.17487/RFC9113, June 2022,
              <https://www.rfc-editor.org/info/rfc9113>.

   [RFC9162]  Laurie, B., Messeri, E., and R. Stradling, "Certificate
              Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
              December 2021, <https://www.rfc-editor.org/info/rfc9162>.

   [RFC9191]  Sethi, M., Preuß Mattsson, J., and S. Turner, "Handling
              Large Certificates and Long Certificate Chains in TLS-
              Based EAP Methods", RFC 9191, DOI 10.17487/RFC9191,
              February 2022, <https://www.rfc-editor.org/info/rfc9191>.

   [SAFECURVES]
              Bernstein, D. J. and T. Lange, "SafeCurves: choosing safe
              curves for elliptic-curve cryptography", December 2014,
              <https://safecurves.cr.yp.to>.

   [Soghoian2011]
              Soghoian, C. and S. Stamm, "Certified Lies: Detecting and
              Defeating Government Interception Attacks Against SSL",
              SSRN Electronic Journal, DOI 10.2139/ssrn.1591033, April
              2010, <https://doi.org/10.2139/ssrn.1591033>.

   [Springall16]
              Springall, D., Durumeric, Z., and J. Halderman, "Measuring
              the Security Harm of TLS Crypto Shortcuts", Proceedings of
              the 2016 Internet Measurement Conference, pp. 33-47,
              DOI 10.1145/2987443.2987480, November 2016,
              <https://doi.org/10.1145/2987443.2987480>.

   [STD53]    Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

              <https://www.rfc-editor.org/info/std53>

   [Sy2018]   Sy, E., Burkert, C., Federrath, H., and M. Fischer,
              "Tracking Users across the Web via TLS Session
              Resumption", Proceedings of the 34th Annual Computer
              Security Applications Conference, pp. 289-299,
              DOI 10.1145/3274694.3274708, December 2018,
              <https://doi.org/10.1145/3274694.3274708>.

   [TLS-ECH]  Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-15, 3 October 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              esni-15>.

   [Triple-Handshake]
              Bhargavan, K., Lavaud, A., Fournet, C., Pironti, A., and
              P. Strub, "Triple Handshakes and Cookie Cutters: Breaking
              and Fixing Authentication over TLS", 2014 IEEE Symposium
              on Security and Privacy, DOI 10.1109/sp.2014.14, May 2014,
              <https://doi.org/10.1109/sp.2014.14>.

   [TWIRL]    Shamir, A. and E. Tromer, "Factoring Large Numbers with
              the TWIRL Device", 2014 IEEE Symposium on Security and
              Privacy, DOI 10.1007/978-3-540-45146-4_1, 2004,
              <https://cs.tau.ac.il/~tromer/papers/twirl.pdf>.

Appendix A.  Differences from RFC 7525

   This revision of the Best Current Practices contains numerous
   changes, and this section is focused on the normative changes.

   *  High-level differences:

      -  Described the expectations from new TLS-incorporating transport
         protocols and from new application protocols layered on TLS.

      -  Clarified items (e.g., renegotiation) that only apply to TLS
         1.2.

      -  Changed the status of TLS 1.0 and 1.1 from "SHOULD NOT" to
         "MUST NOT".

      -  Added TLS 1.3 at a "SHOULD" level.

      -  Made similar changes to DTLS.

      -  Included specific guidance for multiplexed protocols.

      -  MUST-level implementation requirement for ALPN and more
         specific SHOULD-level guidance for ALPN and SNI.

      -  Clarified discussion of strict TLS policies, including MUST-
         level recommendations.

      -  Limits on key usage.

      -  New attacks since [RFC7457]: ALPACA, Raccoon, Logjam, and
         "Nonce-Disrespecting Adversaries".

      -  RFC 6961 (OCSP status_request_v2) has been deprecated.

      -  MUST-level requirement for server-side RSA certificates to have
         a 2048-bit modulus at a minimum, replacing a "SHOULD".

   *  Differences specific to TLS 1.2:

      -  SHOULD-level guidance on AES-GCM nonce generation.

      -  SHOULD NOT use (static or ephemeral) finite-field DH key
         agreement.

      -  SHOULD NOT reuse ephemeral finite-field DH keys across multiple
         connections.

      -  SHOULD NOT use static Elliptic Curve DH key exchange.

      -  2048-bit DH is now a "MUST" and ECDH minimal curve size is 224
         (vs. 192 previously).

      -  Support for extended_master_secret is now a "MUST" (previously
         it was a soft recommendation, as the RFC had not been published
         at the time).  Also removed other, more complicated, related
         mitigations.

      -  MUST-level restriction on session ticket validity, replacing a
         "SHOULD".

      -  SHOULD-level restriction on the TLS session duration, depending
         on the rotation period of an [RFC5077] ticket key.

      -  Dropped TLS_DHE_RSA_WITH_AES from the recommended ciphers.

      -  Added TLS_ECDHE_ECDSA_WITH_AES to the recommended ciphers.

      -  SHOULD NOT use the old MTI cipher suite,
         TLS_RSA_WITH_AES_128_CBC_SHA.

      -  Recommended curve X25519 alongside NIST P-256.

   *  Differences specific to TLS 1.3:

      -  New TLS 1.3 capabilities: 0-RTT.

      -  Removed capabilities: renegotiation and compression.

      -  Added mention of TLS Encrypted Client Hello, but no
         recommendation for use until it is finalized.

      -  SHOULD-level requirement for forward secrecy in TLS 1.3 session
         resumption.

      -  Generic MUST-level guidance to avoid 0-RTT unless it is
         documented for the particular protocol.

Acknowledgments

   Thanks to Alexey Melnikov, Alvaro Retana, Andrei Popov, Ben Kaduk,
   Christian Huitema, Corey Bonnell, Cullen Jennings, Daniel Kahn
   Gillmor, David Benjamin, Eric Rescorla, Éric Vyncke, Francesca
   Palombini, Hannes Tschofenig, Hubert Kario, Ilari Liusvaara, John
   Preuß Mattsson, John R. Levine, Julien Élie, Lars Eggert, Leif
   Johansson, Magnus Westerlund, Martin Duke, Martin Thomson, Mohit
   Sahni, Nick Sullivan, Nimrod Aviram, Paul Wouters, Peter Gutmann,
   Rich Salz, Robert Sayre, Robert Wilton, Roman Danyliw, Ryan Sleevi,
   Sean Turner, Stephen Farrell, Tim Evans, Valery Smyslov, Viktor
   Dukhovni, and Warren Kumari for helpful comments and discussions that
   have shaped this document.

   The authors gratefully acknowledge the contribution of Ralph Holz,
   who was a coauthor of RFC 7525, the previous version of the TLS
   recommendations.

   See RFC 7525 for additional acknowledgments specific to the previous
   version of the TLS recommendations.

Authors' Addresses

   Yaron Sheffer
   Intuit
   Email: yaronf.ietf@gmail.com


   Peter Saint-Andre
   Independent
   Email: stpeter@stpeter.im


   Thomas Fossati
   ARM Limited
   Email: thomas.fossati@arm.com
