UTA Working Group                                             Y. Sheffer
Internet-Draft                                                    Intuit
Obsoletes: 7525 (if approved)                                    R. Holz
Updates: 5288, 6066 (if approved)                   University of Twente
Intended status: Best Current Practice                    P. Saint-Andre
Expires: 1 March 2022                                            Mozilla
                                                              T. Fossati
                                                                     arm
                                                          28 August 2021


  Recommendations for Secure Use of Transport Layer Security (TLS) and
                Datagram Transport Layer Security (DTLS)
                      draft-ietf-uta-rfc7525bis-02

Abstract

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

   This document was published as RFC 7525 when the industry was in the
   midst of its transition to TLS 1.2.  Years later this transition is
   largely complete and TLS 1.3 is widely available.  Given the new
   environment, we believe new guidance is needed.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 1 March 2022.




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Copyright Notice

   Copyright (c) 2021 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 Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  General Recommendations . . . . . . . . . . . . . . . . . . .   5
     3.1.  Protocol Versions . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  SSL/TLS Protocol Versions . . . . . . . . . . . . . .   5
       3.1.2.  DTLS Protocol Versions  . . . . . . . . . . . . . . .   6
       3.1.3.  Fallback to Lower Versions  . . . . . . . . . . . . .   7
     3.2.  Strict TLS  . . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Compression . . . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  TLS Session Resumption  . . . . . . . . . . . . . . . . .   8
     3.5.  TLS Renegotiation . . . . . . . . . . . . . . . . . . . .   9
     3.6.  Post-Handshake Authentication . . . . . . . . . . . . . .  10
     3.7.  Server Name Indication  . . . . . . . . . . . . . . . . .  10
     3.8.  Application-Layer Protocol Negotiation  . . . . . . . . .  10
     3.9.  Zero Round Trip Time (0-RTT) Data in TLS 1.3  . . . . . .  11
   4.  Recommendations: Cipher Suites  . . . . . . . . . . . . . . .  11
     4.1.  General Guidelines  . . . . . . . . . . . . . . . . . . .  12
     4.2.  Recommended Cipher Suites . . . . . . . . . . . . . . . .  13
       4.2.1.  Implementation Details  . . . . . . . . . . . . . . .  14
     4.3.  Cipher Suites for TLS 1.3 . . . . . . . . . . . . . . . .  14
     4.4.  Limits on Key Usage . . . . . . . . . . . . . . . . . . .  15
     4.5.  Public Key Length . . . . . . . . . . . . . . . . . . . .  15
     4.6.  Truncated HMAC  . . . . . . . . . . . . . . . . . . . . .  16
   5.  Applicability Statement . . . . . . . . . . . . . . . . . . .  16
     5.1.  Security Services . . . . . . . . . . . . . . . . . . . .  17
     5.2.  Opportunistic Security  . . . . . . . . . . . . . . . . .  18
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
     6.1.  Host Name Validation  . . . . . . . . . . . . . . . . . .  18
     6.2.  AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . .  19
       6.2.1.   Nonce Reuse in TLS 1.2 . . . . . . . . . . . . . . .  19
     6.3.  Forward Secrecy . . . . . . . . . . . . . . . . . . . . .  20
     6.4.  Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . .  21



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     6.5.  Certificate Revocation  . . . . . . . . . . . . . . . . .  22
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  23
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  23
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  26
   Appendix A.  Differences from RFC 7525  . . . . . . . . . . . . .  32
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  33
     B.1.  draft-ietf-uta-rfc7525bis-02  . . . . . . . . . . . . . .  33
     B.2.  draft-ietf-uta-rfc7525bis-01  . . . . . . . . . . . . . .  33
     B.3.  draft-ietf-uta-rfc7525bis-00  . . . . . . . . . . . . . .  34
     B.4.  draft-sheffer-uta-rfc7525bis-00 . . . . . . . . . . . . .  34
     B.5.  draft-sheffer-uta-bcp195bis-00  . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

1.  Introduction

   Transport Layer Security (TLS) [RFC5246] and Datagram Transport
   Security Layer (DTLS) [RFC6347] are widely used to protect data
   exchanged over application protocols such as HTTP, SMTP, IMAP, POP,
   SIP, and XMPP.  Over the years leading to 2015, several serious
   attacks on TLS have emerged, including attacks on its most commonly
   used cipher suites and their modes of operation.  For instance, both
   the AES-CBC [RFC3602] and RC4 [RFC7465] encryption algorithms, which
   together have been the most widely deployed ciphers, have been
   attacked in the context of TLS.  A companion document [RFC7457]
   provides detailed information about these attacks and will help the
   reader understand the rationale behind the recommendations provided
   here.

   The TLS community reacted to these attacks in two ways:

   *  Detailed guidance was published on the use of TLS 1.2 and earlier
      protocol versions.  This guidance is included in the original
      [RFC7525] and mostly retained in this revised version.

   *  A new protocol version was released, TLS 1.3 [RFC8446], which
      largely mitigates or resolves these attacks.

   Those who implement and deploy TLS and DTLS, in particular versions
   1.2 or earlier of these protocols, need guidance on how TLS can be
   used securely.  This document provides guidance for deployed services
   as well as for software implementations, assuming the implementer
   expects his or her code to be deployed in environments defined in
   Section 5.  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.




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   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.

   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.  This
   document is being republished with this in mind, and with an explicit
   goal to migrate most uses of TLS 1.2 into 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 particular
   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, so stronger options are
   always allowed (e.g., depending on differing evaluations of the
   importance of cryptographic strength vs. computational load).

   Community knowledge about the strength of various algorithms and
   feasible attacks can change quickly, and experience shows that a Best
   Current Practice (BCP) document about security is a point-in-time
   statement.  Readers are advised to seek out any errata or updates
   that apply to this document.

2.  Terminology

   A number of security-related terms in this document are used in the
   sense defined in [RFC4949].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "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.



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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 POODLE attack [POODLE],
      SSLv3 is now widely recognized as fundamentally insecure.  See
      [DEP-SSLv3] 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 CBC-based cipher suites 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.

      NOTE: This recommendation has been changed from SHOULD NOT to MUST
      NOT on the assumption that [I-D.ietf-tls-oldversions-deprecate]
      will be published as an RFC before this document.





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   *  Implementations MUST support TLS 1.2 [RFC5246] and MUST prefer to
      negotiate TLS version 1.2 over earlier versions of TLS.

      Rationale: Several stronger cipher suites are available only with
      TLS 1.2 (published in 2008).  In fact, the cipher suites
      recommended by this document for TLS 1.2 (Section 4.2 below) are
      only available in this version.

   *  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.  We note that
      as long as TLS 1.2 is still allowed by a particular
      implementation, even if it defaults to TLS 1.3, implementers MUST
      still follow all the recommendations in this document.

   *  Implementations of "greenfield" protocols or deployments, where
      there is no need to support legacy endpoints, SHOULD support TLS
      1.3, with no negotiation of earlier versions.  Similarly, we
      RECOMMEND that new protocol designs that embed the TLS mechanisms
      (such as QUIC has done [RFC9001]) include TLS 1.3.

      Rationale: secure deployment of TLS 1.3 is significantly easier
      and less error prone than the secure deployment of TLS 1.2.

   This BCP applies to TLS 1.2, 1.3 and to earlier versions.  It is not
   safe for readers to assume that the recommendations in this BCP apply
   to any future version of TLS.

3.1.2.  DTLS Protocol Versions

   DTLS, an adaptation of TLS for UDP datagrams, was introduced when TLS
   1.1 was published.  The following are the recommendations with
   respect to DTLS:

   *  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 and (unless a higher version is
      available) MUST prefer to negotiate DTLS version 1.2 [RFC6347]

      Version 1.2 of DTLS correlates to version 1.2 of TLS (see above).
      (There is no version 1.1 of DTLS.)





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   *  Implementations SHOULD support and, if available, MUST prefer to
      negotiate DTLS version 1.3 as specified in [I-D.ietf-tls-dtls13].

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

3.1.3.  Fallback to Lower Versions

   Clients that "fall back" to lower versions of the protocol after the
   server rejects higher versions of the protocol MUST NOT fall back to
   SSLv3 or earlier.  Implementations of TLS/DTLS 1.2 or earlier MUST
   implement the Fallback SCSV mechanism [RFC7507] to prevent such
   fallback being forced by an attacker.

   Rationale: Some client implementations revert to lower versions of
   TLS or even to SSLv3 if the server rejected higher versions of the
   protocol.  This fallback can be forced by a man-in-the-middle (MITM)
   attacker.  TLS 1.0 and SSLv3 are significantly less secure than TLS
   1.2 but at least TLS 1.0 is still allowed by many web servers.  As of
   this writing, the Fallback SCSV solution is widely deployed and
   proven as a robust solution to this problem.

3.2.  Strict TLS

   The following recommendations are provided to help prevent SSL
   Stripping (an attack that is summarized in Section 2.1 of [RFC7457]):

   *  In cases where an application protocol allows implementations or
      deployments a choice between strict TLS configuration and dynamic
      upgrade from unencrypted to TLS-protected traffic (such as
      STARTTLS), clients and servers SHOULD prefer strict TLS
      configuration.

   *  Application protocols typically provide a way for the server to
      offer TLS during an initial protocol exchange, and sometimes also
      provide a way for the server to advertise support for TLS (e.g.,
      through a flag indicating that TLS is required); unfortunately,
      these indications are sent before the communication channel is
      encrypted.  A client SHOULD attempt to negotiate TLS even if these
      indications are not communicated by the server.

   *  HTTP client and server implementations MUST support the HTTP
      Strict Transport Security (HSTS) header [RFC6797], in order to
      allow Web servers to advertise that they are willing to accept
      TLS-only clients.







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   *  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]).

   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 disable TLS-level compression (Section 6.2.2 of
   [RFC5246]), unless the application protocol in question has been
   shown not to be open to such attacks.  Note: 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 CRIME attack.

   Implementers should note that compression at higher protocol levels
   can allow an active attacker to extract cleartext information from
   the connection.  The BREACH attack is one such case.  These issues
   can only be mitigated outside of TLS and are thus outside the scope
   of this document.  See Section 2.6 of [RFC7457] for further details.

3.4.  TLS Session Resumption

   Session resumption drastically reduces the number of 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,
   an equivalent PSK-based mechanism is described in Section 4.6.1 of
   [RFC8446].  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 suite MUST be used when encrypting the ticket (as
      least as strong as the main TLS cipher suite).






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   *  Ticket keys MUST be changed regularly, e.g., once every week, so
      as not to negate the benefits of forward secrecy (see Section 6.3
      for details on forward secrecy).

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

   Rationale: session resumption is another kind of TLS handshake, and
   therefore must be as secure as the initial handshake.  This document
   (Section 4) recommends the use of cipher suites that provide forward
   secrecy, i.e. that prevent an attacker who gains momentary access to
   the TLS endpoint (either client or server) and its secrets from
   reading either past or future communication.  The tickets must be
   managed so as not to negate this security property.

   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 respond
   with a "key_share", to complete an ECDHE exchange on each session
   resumption.

   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.  TLS Renegotiation

   Where handshake renegotiation is implemented, both clients and
   servers MUST implement the renegotiation_info extension, as defined
   in [RFC5746].  Note: this recommendation applies to TLS 1.2 only,
   because renegotiation has been removed from TLS 1.3.

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






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3.6.  Post-Handshake Authentication

   Renegotiation in TLS 1.2 was 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), post-handshake authentication has the same problems as
   TLS 1.2 renegotiation.  Multiplexed protocols SHOULD follow the
   advice provided for HTTP/2 in [RFC8740].

3.7.  Server Name Indication

   TLS implementations MUST support the Server Name Indication (SNI)
   extension defined in Section 3 of [RFC6066] for those higher-level
   protocols that would benefit from it, including HTTPS.  However, the
   actual use of SNI in particular circumstances is a matter of local
   policy.  Implementers are strongly encouraged to support TLS
   Encrypted Client Hello (formerly called Encrypted SNI) once
   [I-D.ietf-tls-esni] has been standardized.

   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 plugged by use of TLS
   Encrypted Client Hello.

   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 fail with a fatal-level
   "unrecognized_name(112)" alert.  Note that this recommendation
   updates Section 3 of [RFC6066]: "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."  It is also RECOMMENDED that clients abort the handshake
   if the server acknowledges the SNI hostname with a different hostname
   than the one sent by the client.

3.8.  Application-Layer Protocol Negotiation

   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 should
   strictly enforce the behavior prescribed in Section 3.2 of [RFC7301]:
   "In the event that the server supports no protocols that the client



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   advertises, then the server SHALL respond with a fatal
   "no_application_protocol" alert."  It is also RECOMMENDED that
   clients 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 to new protocols, as
   well as well-established protocols such as SMTP.

3.9.  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 improved
   latency when TLS connections are resumed, at the potential cost of
   security.  As a result, it requires special attention from
   implementers on both the server and the client side.  Typically this
   extends to both the TLS library as well as protocol layers above it.

   For use in HTTP-over-TLS, readers are referred to [RFC8470] for
   guidance.

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

   For other protocols, generic guidance is given in Sec. 8 and
   Appendix E.5 of [RFC8446].  Given the complexity, we RECOMMEND to
   avoid this feature altogether 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 even documentation associated with a non-
   standard protocol.

4.  Recommendations: Cipher Suites

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










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4.1.  General Guidelines

   Cryptographic algorithms weaken over time as cryptanalysis improves:
   algorithms that were once considered strong become weak.  Such
   algorithms need to be phased out over time and replaced with more
   secure cipher suites.  This helps to ensure that the desired security
   properties still hold.  SSL/TLS has been in existence for almost 20
   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 provide 40 or 56 bits of security).

      Rationale: Based on [RFC3766], at least 112 bits of security is
      needed.  40-bit and 56-bit security are considered insecure today.
      TLS 1.1 and 1.2 never negotiate 40-bit or 56-bit export ciphers.

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

      Rationale: Cipher suites that offer between 112-bits and 128-bits
      of security are not considered weak at this time; however, it is
      expected that their useful lifespan is short enough to justify
      supporting stronger cipher suites at this time.  128-bit ciphers
      are expected to remain secure for at least several years, and
      256-bit ciphers until the next fundamental technology
      breakthrough.  Note that, because of so-called "meet-in-the-
      middle" attacks [Multiple-Encryption], some legacy cipher suites



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      (e.g., 168-bit 3DES) have an effective key length that is smaller
      than their nominal key length (112 bits in the case of 3DES).
      Such cipher suites should be evaluated according to their
      effective key length.

   *  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 MUST support and prefer to negotiate cipher suites
      offering forward secrecy, such as those in the Ephemeral Diffie-
      Hellman and Elliptic Curve Ephemeral Diffie-Hellman ("DHE" and
      "ECDHE") families.

      Rationale: Forward secrecy (sometimes called "perfect forward
      secrecy") prevents the recovery of information that was encrypted
      with older session keys, thus limiting the amount of time during
      which attacks can be successful.  See Section 6.3 for a detailed
      discussion.

4.2.  Recommended Cipher Suites

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

   *  TLS_DHE_RSA_WITH_AES_128_GCM_SHA256

   *  TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256

   *  TLS_DHE_RSA_WITH_AES_256_GCM_SHA384

   *  TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384

   These cipher suites are supported only in TLS 1.2 and not in earlier
   protocol versions, because they are authenticated encryption (AEAD)
   algorithms [RFC5116].

   Typically, in order to prefer these suites, the order of suites needs
   to be explicitly configured in server software.  (See [BETTERCRYPTO]
   for helpful deployment guidelines, but note that its recommendations
   differ from the current document in some details.)  It would be ideal
   if server software implementations were to prefer these suites by
   default.





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   Some devices have hardware support for AES-CCM but not AES-GCM, so
   they are unable to follow the foregoing recommendations regarding
   cipher suites.  There are even devices that do not support public key
   cryptography at all, but they are out of scope entirely.

4.2.1.  Implementation Details

   Clients SHOULD include TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 as the
   first proposal to any server, unless they have prior knowledge that
   the server cannot respond to a TLS 1.2 client_hello message.

   Servers MUST prefer this cipher suite over weaker cipher suites
   whenever it is proposed, even if it is not the first proposal.

   Clients are of course free to offer stronger cipher suites, e.g.,
   using AES-256; when they do, the server SHOULD prefer the stronger
   cipher suite unless there are compelling reasons (e.g., seriously
   degraded performance) to choose otherwise.

   This document does not change the mandatory-to-implement TLS cipher
   suite(s) prescribed by TLS.  To maximize interoperability, RFC 5246
   mandates implementation of the TLS_RSA_WITH_AES_128_CBC_SHA cipher
   suite, which is significantly weaker than the cipher suites
   recommended here.  (The GCM mode does not suffer from the same
   weakness, caused by the order of MAC-then-Encrypt in TLS
   [Krawczyk2001], since it uses an AEAD mode of operation.)
   Implementers should consider the interoperability gain against the
   loss in security when deploying the TLS_RSA_WITH_AES_128_CBC_SHA
   cipher suite.  Other application protocols specify other cipher
   suites as mandatory to implement (MTI).

   Note that some profiles of TLS 1.2 use different cipher suites.  For
   example, [RFC6460] defines a profile that uses the
   TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
   TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suites.

   [RFC4492] allows clients and servers to negotiate ECDH parameters
   (curves).  Both clients and servers SHOULD include the "Supported
   Elliptic Curves" extension [RFC4492].  For interoperability, clients
   and servers SHOULD support the NIST P-256 (secp256r1) curve
   [RFC4492].  In addition, clients SHOULD send an ec_point_formats
   extension with a single element, "uncompressed".

4.3.  Cipher Suites for TLS 1.3

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



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4.4.  Limits on Key Usage

   All ciphers have an upper limit on the amount of traffic that can be
   securely encrypted with any given key.  In the case of AEAD cipher
   suites, the limit is typically determined by the cipher's integrity
   guarantees.  When the amount of traffic for a particular connection
   has reached the limit, an implementation SHOULD perform a new
   handshake (or in TLS 1.3, a Key Update) to rotate the session key.

   For all AES-GCM cipher suites recommended for TLS 1.2 in this
   document, the limit for one connection is 2^24.5 full-size records
   (about 24 million).  This is the same number as for TLS 1.3 with the
   equivalent cipher suites.


   // TODO: refer to {{I-D.irtf-cfrg-aead-limits}} once it has added the
   // derivation for TLS 1.2, which is different from TLS 1.3.
   // Different derivation, same numbers.

   For all TLS 1.3 cipher suites, readers are referred to Section 5.5 of
   [RFC8446].

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).








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   As noted in [RFC3766], correcting for the emergence of a TWIRL
   machine would imply that 1024-bit DH keys yield about 65 bits of
   equivalent strength and that a 2048-bit DH key would yield about 92
   bits of equivalent strength.  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
   "Supported Groups Registry" (former "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 SHOULD authenticate using certificates with
   at least a 2048-bit modulus for the public key.  In addition, the use
   of the SHA-256 hash algorithm is RECOMMENDED and SHA-1 or MD5 MUST
   NOT be used (see [CAB-Baseline] for more details).  Clients MUST
   indicate to servers that they request SHA-256, by using the
   "Signature Algorithms" extension defined in TLS 1.2.

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, 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.





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   *  Realtime media software and services that wish to protect Secure
      Realtime Transport Protocol (SRTP) traffic with DTLS.

   This document does not modify the implementation and deployment
   recommendations (e.g., mandatory-to-implement cipher suites)
   prescribed by existing application protocols that employ TLS or DTLS.
   If the community that uses such an application protocol wishes to
   modernize its usage of TLS or DTLS to be consistent with the best
   practices recommended here, it needs to explicitly update the
   existing application protocol definition (one example is [TLS-XMPP],
   which updates [RFC6120]).

   Designers of new application protocols developed through the Internet
   Standards Process [RFC2026] are expected at minimum to conform to the
   best practices recommended here, unless they provide documentation of
   compelling reasons that would prevent such conformance (e.g.,
   widespread deployment on constrained devices that lack support for
   the necessary algorithms).

5.1.  Security Services

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

   *  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 recommends algorithms and configuration options that
   enforce secrecy of the data in transit.




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   This document also assumes that data integrity protection is always
   one of the goals of a deployment.  In cases where integrity is not
   required, it does not make sense to employ TLS in the first place.
   There are attacks against confidentiality-only protection that
   utilize the lack of integrity to also break confidentiality (see, for
   instance, [DegabrieleP07] in the context of IPsec).

   This document addresses itself to application protocols that are most
   commonly used on the Internet with TLS and DTLS.  Typically, all
   communication between TLS clients and TLS servers requires all three
   of the above security services.  This is particularly true where TLS
   clients are user agents like Web browsers or email software.

   This document does not address the rarer deployment scenarios where
   one of the above three properties is not desired, such as the use
   case described in Section 5.2 below.  As another scenario where
   confidentiality is not needed, consider a monitored network where the
   authorities in charge of the respective traffic domain require full
   access to unencrypted (plaintext) traffic, and where users
   collaborate and send their traffic in the clear.

5.2.  Opportunistic Security

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

   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.  Security Considerations

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

6.1.  Host Name Validation

   Application authors should take note that some TLS implementations do
   not validate host names.  If the TLS implementation they are using
   does not validate host names, authors might need to write their own
   validation code or consider using a different TLS implementation.



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   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
   [I-D.ietf-httpbis-semantics].

   Readers are referred to [RFC6125] for further details regarding
   generic host name validation in the TLS context.  In addition, that
   RFC contains a long list of example protocols, some of which
   implement a policy very different from HTTPS.

   If the host name is discovered indirectly and in an insecure manner
   (e.g., by an insecure DNS query for an MX or SRV record), it SHOULD
   NOT be used as a reference identifier [RFC6125] even when it matches
   the presented certificate.  This proviso does not apply if the host
   name is discovered securely (for further discussion, see [DANE-SRV]
   and [DANE-SMTP]).

   Host name validation typically applies only to the leaf "end entity"
   certificate.  Naturally, in order to ensure proper authentication in
   the context of the PKI, application clients need to verify the entire
   certification path in accordance with [RFC5280] (see also [RFC6125]).

6.2.  AES-GCM

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

6.2.1.   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 in an insecure way 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, 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 of its AEAD cipher suites (including AES-
   GCM), TLS 1.2 implementations could still choose their own
   (potentially insecure) nonce generation methods.






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   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].  Note that this recommendation updates Section 3 of
   [RFC5288]: "The nonce_explicit MAY be the 64-bit sequence number."

   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].

6.3.  Forward Secrecy

   Forward secrecy (also called "perfect forward secrecy" or "PFS" and
   defined in [RFC4949]) is a defense against an attacker who records
   encrypted conversations where the session keys are only encrypted
   with the communicating parties' long-term keys.

   Should the attacker be able to obtain these long-term keys at some
   point later in time, the session keys and thus the entire
   conversation could be decrypted.

   In the context of TLS and DTLS, such compromise of long-term keys is
   not entirely implausible.  It can happen, for example, due to:

   *  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 either by extortion or compromise
      [Soghoian2011].

   *  A cryptographic break-through, 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.



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   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.

   Unfortunately, many TLS/DTLS cipher suites were defined that do not
   feature forward secrecy, e.g., TLS_RSA_WITH_AES_256_CBC_SHA256.  This
   document therefore advocates strict use of forward-secrecy-only
   ciphers.

6.4.  Diffie-Hellman Exponent Reuse

   For performance reasons, many TLS implementations reuse Diffie-
   Hellman and Elliptic Curve Diffie-Hellman exponents across multiple
   connections.  Such reuse can result in major security issues:

   *  If exponents are reused for too long (e.g., even more than a few
      hours), an attacker who gains access to the host can decrypt
      previous connections.  In other words, exponent reuse negates the
      effects of forward secrecy.

   *  TLS implementations that reuse exponents should test the DH public
      key they receive for group membership, in order to avoid some
      known attacks.  These tests are not standardized in TLS at the
      time of writing.  See [RFC6989] for recipient tests required of
      IKEv2 implementations that reuse DH exponents.

   *  Under certain conditions, the use of static DH keys, or of
      ephemeral 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.

   To address these concerns, TLS implementations SHOULD NOT use static
   DH keys and SHOULD NOT reuse ephemeral DH keys across multiple
   connections.


   // TODO: revisit when draft-bartle-tls-deprecate-ffdhe becomes a TLS
   // WG item, since it specifies MUST NOT rather than SHOULD NOT.






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6.5.  Certificate Revocation

   The following considerations and recommendations represent the
   current state of the art regarding certificate revocation, even
   though no complete and efficient solution exists for the problem of
   checking the revocation status of common public key certificates
   [RFC5280]:

   *  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).

   *  Proprietary mechanisms that embed revocation lists in the Web
      browser's configuration database cannot scale beyond a small
      number of the most heavily used Web servers.

   *  The On-Line Certification Status Protocol (OCSP) [RFC6960]
      presents both scaling and privacy issues.  In addition, clients
      typically "soft-fail", meaning that they do not abort the TLS
      connection if the OCSP server does not respond.  (However, this
      might be a workaround to avoid denial-of-service attacks if an
      OCSP responder is taken offline.)

   *  The TLS Certificate Status Request extension (Section 8 of
      [RFC6066]), commonly called "OCSP stapling", resolves the
      operational issues with OCSP.  However, it is still ineffective in
      the presence of a MITM attacker because the attacker can simply
      ignore the client's request for a stapled OCSP response.

   *  OCSP stapling as defined in [RFC6066] does not extend to
      intermediate certificates used in a certificate chain.  Although
      the Multiple Certificate Status extension [RFC6961] addresses this
      shortcoming, it is a recent addition without much deployment.

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

   With regard to common public key certificates, servers SHOULD support
   the following as a best practice given the current state of the art
   and as a foundation for a possible future solution:

   1.  OCSP [RFC6960]






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   2.  Both the status_request extension defined in [RFC6066] and the
       status_request_v2 extension defined in [RFC6961] (This might
       enable interoperability with the widest range of clients.)

   3.  The OCSP stapling extension defined in [RFC6961]

   The considerations in this section do not apply to scenarios where
   the DANE-TLSA resource record [RFC6698] is used to signal to a client
   which certificate a server considers valid and good to use for TLS
   connections.

7.  Acknowledgments

   The following acknowledgments are inherited from [RFC7525].

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

   Robert Sparks and Dave Waltermire provided helpful reviews on behalf
   of the General Area Review Team and the Security Directorate,
   respectively.

   During IESG review, Richard Barnes, Alissa Cooper, Spencer Dawkins,
   Stephen Farrell, Barry Leiba, Kathleen Moriarty, and Pete Resnick
   provided comments that led to further improvements.

   Ralph Holz gratefully acknowledges the support by Technische
   Universitaet Muenchen.

   The authors gratefully acknowledge the assistance of Leif Johansson
   and Orit Levin as the working group chairs and Pete Resnick as the
   sponsoring Area Director.

8.  References

8.1.  Normative References






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   [I-D.ietf-httpbis-semantics]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-18, 18 August 2021,
              <https://www.ietf.org/archive/id/draft-ietf-httpbis-
              semantics-18.txt>.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-43, 30 April 2021,
              <https://www.ietf.org/archive/id/draft-ietf-tls-
              dtls13-43.txt>.

   [I-D.ietf-tls-oldversions-deprecate]
              Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS
              1.1", Work in Progress, Internet-Draft, draft-ietf-tls-
              oldversions-deprecate-12, 21 January 2021,
              <https://www.ietf.org/archive/id/draft-ietf-tls-
              oldversions-deprecate-12.txt>.

   [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>.

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492,
              DOI 10.17487/RFC4492, May 2006,
              <https://www.rfc-editor.org/info/rfc4492>.

   [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>.

   [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>.





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   [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>.

   [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>.

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

   [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>.

   [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>.




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   [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>.

   [RFC8740]  Benjamin, D., "Using TLS 1.3 with HTTP/2", RFC 8740,
              DOI 10.17487/RFC8740, February 2020,
              <https://www.rfc-editor.org/info/rfc8740>.

   [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>.

8.2.  Informative References

   [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) , 2021,
              <https://www.usenix.org/conference/usenixsecurity21/
              presentation/brinkmann>.

   [BETTERCRYPTO]
              bettercrypto.org, "Applied Crypto Hardening", April 2015,
              <https://bettercrypto.org/>.

   [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
              1.1.6", 2013, <https://www.cabforum.org/documents.html>.

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

   [DANE-SMTP]
              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>.





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   [DANE-SRV] 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>.

   [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>.

   [DEP-SSLv3]
              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>.

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

   [I-D.ietf-tls-esni]
              Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-13, 12 August 2021,
              <https://www.ietf.org/archive/id/draft-ietf-tls-esni-
              13.txt>.

   [I-D.irtf-cfrg-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-03, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-irtf-cfrg-aead-
              limits-03.txt>.

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

   [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>.






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   [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>.

   [Krawczyk2001]
              Krawczyk, H., "The Order of Encryption and Authentication
              for Protecting Communications (Or: How Secure is SSL?)",
              CRYPTO 01, 2001,
              <https://www.iacr.org/archive/crypto2001/21390309.pdf>.

   [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, 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, pp.
              465-467, DOI 10.1145/358699.358718, July 1981,
              <https://doi.org/10.1145/358699.358718>.

   [NIST.SP.800-56A]
              Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
              Davis, "Recommendation for pair-wise key-establishment
              schemes using discrete logarithm cryptography", National
              Institute of Standards and Technology report,
              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", Lecture Notes in Computer Science pp. 372-389,
              DOI 10.1007/978-3-642-25385-0_20, 2011,
              <https://doi.org/10.1007/978-3-642-25385-0_20>.

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



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   [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>.

   [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>.

   [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>.

   [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>.



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   [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>.

   [RFC6460]  Salter, M. and R. Housley, "Suite B Profile for Transport
              Layer Security (TLS)", RFC 6460, DOI 10.17487/RFC6460,
              January 2012, <https://www.rfc-editor.org/info/rfc6460>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, 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>.

   [RFC6989]  Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman
              Tests for the Internet Key Exchange Protocol Version 2
              (IKEv2)", RFC 6989, DOI 10.17487/RFC6989, July 2013,
              <https://www.rfc-editor.org/info/rfc6989>.

   [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>.

   [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>.



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   [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>.

   [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>.

   [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>.

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

   [Smith2013]
              Smith, B., "Proposal to Change the Default TLS
              Ciphersuites Offered by Browsers.", 2013,
              <https://briansmith.org/browser-ciphersuites-01.html>.

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

   [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,
              DOI 10.1145/3274694.3274708, December 2018,
              <https://doi.org/10.1145/3274694.3274708>.

   [TLS-XMPP] 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>.









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   [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>.

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:

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

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

      -  Added TLS 1.3 at a SHOULD level.

      -  Similar changes to DTLS, pending publication of DTLS 1.3.

      -  Specific guidance for multiplexed protocols.

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

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

   *  Differences specific to TLS 1.2:

      -  Fallback SCSV as a MUST for TLS 1.2.

      -  SHOULD-level guidance on AES-GCM nonce generation in TLS 1.2.

      -  SHOULD NOT use static DH keys or reuse ephemeral DH keys across
         multiple connections.

      -  2048-bit DH now a MUST, ECDH minimal curve size is 224, vs. 192
         previously.

   *  Differences specific to TLS 1.3:

      -  New TLS 1.3 capabilities: 0-RTT.

      -  Removed capabilities: renegotiation, compression.



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

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

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

Appendix B.  Document History


   // Note to RFC Editor: please remove before publication.

B.1.  draft-ietf-uta-rfc7525bis-02

   *  Adjusted text about ALPN support in application protocols

   *  Incorporated text from draft-ietf-tls-md5-sha1-deprecate

B.2.  draft-ietf-uta-rfc7525bis-01

   *  Many more changes, including:

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

      -  Removed TLS 1.2 capabilities: renegotiation, compression.

      -  Specific guidance for multiplexed protocols.

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

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

      -  SHOULD-level guidance on AES-GCM nonce generation in TLS 1.2.

      -  SHOULD NOT use static DH keys or reuse ephemeral DH keys across
         multiple connections.

      -  2048-bit DH now a MUST, ECDH minimal curve size is 224, up from
         192.







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B.3.  draft-ietf-uta-rfc7525bis-00

   *  Renamed: WG document.

   *  Started populating list of changes from RFC 7525.

   *  General rewording of abstract and intro for revised version.

   *  Protocol versions, fallback.

   *  Reference to ECHO.

B.4.  draft-sheffer-uta-rfc7525bis-00

   *  Renamed, since the BCP number does not change.

   *  Added an empty "Differences from RFC 7525" section.

B.5.  draft-sheffer-uta-bcp195bis-00

   *  Initial release, the RFC 7525 text as-is, with some minor
      editorial changes to the references.

Authors' Addresses

   Yaron Sheffer
   Intuit

   Email: yaronf.ietf@gmail.com


   Ralph Holz
   University of Twente

   Email: ralph.ietf@gmail.com


   Peter Saint-Andre
   Mozilla

   Email: stpeter@mozilla.com


   Thomas Fossati
   arm

   Email: thomas.fossati@arm.com




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