UTA Working Group Y. Sheffer Internet-Draft Intuit Obsoletes: 7525 (if approved) P. Saint-Andre Updates: 5288, 6066 (if approved) Mozilla Intended status: Best Current Practice T. Fossati Expires: 7 August 2022 arm 3 February 2022 Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) draft-ietf-uta-rfc7525bis-05 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 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 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, updated 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 7 August 2022. Sheffer, et al. Expires 7 August 2022 [Page 1] Internet-Draft TLS Recommendations February 2022 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. General Recommendations . . . . . . . . . . . . . . . . . . . 5 3.1. Protocol Versions . . . . . . . . . . . . . . . . . . . . 5 3.1.1. SSL/TLS Protocol Versions . . . . . . . . . . . . . . 5 3.1.2. DTLS Protocol Versions . . . . . . . . . . . . . . . 6 3.1.3. Fallback to Lower Versions . . . . . . . . . . . . . 7 3.2. Strict TLS . . . . . . . . . . . . . . . . . . . . . . . 7 3.3. Compression . . . . . . . . . . . . . . . . . . . . . . . 8 3.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 8 3.5. Renegotiation in TLS 1.2 . . . . . . . . . . . . . . . . 9 3.6. Post-Handshake Authentication . . . . . . . . . . . . . . 10 3.7. Server Name Indication . . . . . . . . . . . . . . . . . 10 3.8. Application-Layer Protocol Negotiation . . . . . . . . . 11 3.9. Zero Round Trip Time (0-RTT) Data in TLS 1.3 . . . . . . 11 4. Recommendations: Cipher Suites . . . . . . . . . . . . . . . 12 4.1. General Guidelines . . . . . . . . . . . . . . . . . . . 12 4.2. Cipher Suites for TLS 1.2 . . . . . . . . . . . . . . . . 13 4.2.1. Implementation Details . . . . . . . . . . . . . . . 14 4.3. Cipher Suites for TLS 1.3 . . . . . . . . . . . . . . . . 15 4.4. Limits on Key Usage . . . . . . . . . . . . . . . . . . . 15 4.5. Public Key Length . . . . . . . . . . . . . . . . . . . . 16 4.6. Truncated HMAC . . . . . . . . . . . . . . . . . . . . . 17 5. Applicability Statement . . . . . . . . . . . . . . . . . . . 17 5.1. Security Services . . . . . . . . . . . . . . . . . . . . 18 5.2. Opportunistic Security . . . . . . . . . . . . . . . . . 19 6. Security Considerations . . . . . . . . . . . . . . . . . . . 19 6.1. Host Name Validation . . . . . . . . . . . . . . . . . . 19 6.2. AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.2.1. Nonce Reuse in TLS 1.2 . . . . . . . . . . . . . . . 20 6.3. Forward Secrecy . . . . . . . . . . . . . . . . . . . . . 21 6.4. Diffie-Hellman Exponent Reuse . . . . . . . . . . . . . . 22 Sheffer, et al. Expires 7 August 2022 [Page 2] Internet-Draft TLS Recommendations February 2022 6.5. Certificate Revocation . . . . . . . . . . . . . . . . . 23 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 25 8.1. Normative References . . . . . . . . . . . . . . . . . . 25 8.2. Informative References . . . . . . . . . . . . . . . . . 27 Appendix A. Differences from RFC 7525 . . . . . . . . . . . . . 34 Appendix B. Document History . . . . . . . . . . . . . . . . . . 35 B.1. draft-ietf-uta-rfc7525bis-05 . . . . . . . . . . . . . . 36 B.2. draft-ietf-uta-rfc7525bis-04 . . . . . . . . . . . . . . 36 B.3. draft-ietf-uta-rfc7525bis-03 . . . . . . . . . . . . . . 36 B.4. draft-ietf-uta-rfc7525bis-02 . . . . . . . . . . . . . . 36 B.5. draft-ietf-uta-rfc7525bis-01 . . . . . . . . . . . . . . 36 B.6. draft-ietf-uta-rfc7525bis-00 . . . . . . . . . . . . . . 37 B.7. draft-sheffer-uta-rfc7525bis-00 . . . . . . . . . . . . . 37 B.8. draft-sheffer-uta-bcp195bis-00 . . . . . . . . . . . . . 37 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37 1. Introduction Transport Layer Security (TLS) and Datagram Transport Security Layer (DTLS) 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, the industry has 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, 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. 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 these attacks 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 and version 1.3 of DTLS was finalized [I-D.ietf-tls-dtls13]; these versions largely mitigate or resolve the described attacks. Sheffer, et al. Expires 7 August 2022 [Page 3] Internet-Draft TLS Recommendations February 2022 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. 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. Sheffer, et al. Expires 7 August 2022 [Page 4] Internet-Draft TLS Recommendations February 2022 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. 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]. Sheffer, et al. Expires 7 August 2022 [Page 5] Internet-Draft TLS Recommendations February 2022 * 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. * 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). Sheffer, et al. Expires 7 August 2022 [Page 6] Internet-Draft TLS Recommendations February 2022 * Implementations MUST support DTLS 1.2 [RFC6347] and MUST prefer to negotiate DTLS version 1.2 over earlier versions of DTLS. 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 [I-D.ietf-tls-dtls13] 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]. We note that as a result of that, the 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 (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. Sheffer, et al. Expires 7 August 2022 [Page 7] Internet-Draft TLS Recommendations February 2022 * 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, a more secure PSK-based mechanism is described in Section 4.6.1 of [RFC8446]. See this post (https://blog.filippo.io/we-need-to-talk- about-session-tickets/) by Filippo Valsorda for a comparison of TLS 1.2 and 1.3 session resumption, and [Springall16] for a quantitative study of 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: Sheffer, et al. Expires 7 August 2022 [Page 8] Internet-Draft TLS Recommendations February 2022 * A strong cipher suite MUST be used when encrypting the ticket (as least as strong as the main TLS cipher suite). * Ticket keys MUST be changed regularly, e.g., once every week, so as not to negate the benefits of forward secrecy (see Section 6.3 for details on forward secrecy). Old ticket keys MUST be destroyed shortly after a new key version is made available. * For similar reasons, session ticket validity SHOULD be limited to a reasonable duration (e.g., half as long as ticket key validity). * TLS 1.2 does not roll the session key forward within a single session. Thus, to prevent an attack where a stolen ticket key is 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 key rotation periods. Note that this implies that some server implementations might need to abort sessions after a certain duration. 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. 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. TLS 1.2 clients and servers MUST implement the renegotiation_info extension, as defined in [RFC5746]. Sheffer, et al. Expires 7 August 2022 [Page 9] Internet-Draft TLS Recommendations February 2022 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 properly authenticated is Triple Handshake [triple-handshake]. To address this attack, TLS 1.2 implementations SHOULD support the extended_master_secret extension defined in [RFC7627]. 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 SHOULD 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 Sheffer, et al. Expires 7 August 2022 [Page 10] Internet-Draft TLS Recommendations February 2022 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 extension, but presents a certificate 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 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]. 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 even documentation associated with a non- standard protocol. Sheffer, et al. Expires 7 August 2022 [Page 11] Internet-Draft TLS Recommendations February 2022 4. Recommendations: Cipher Suites TLS and its implementations provide 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. Incorrectly configuring a server leads to no or reduced security. This section includes recommendations on the selection and negotiation of cipher suites. 4.1. General Guidelines Cryptographic algorithms weaken over time as cryptanalysis improves: algorithms that were once considered strong become weak. Such algorithms need to be phased out over time and replaced with more secure cipher suites. This helps to ensure that the desired security properties still hold. SSL/TLS has been in existence for almost 20 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. Sheffer, et al. Expires 7 August 2022 [Page 12] Internet-Draft TLS Recommendations February 2022 Rationale: Cipher suites that offer between 112-bits and 128-bits of security are not considered weak at this time; however, it is expected that their useful lifespan is short enough to justify supporting stronger cipher suites at this time. 128-bit ciphers are expected to remain secure for at least several years, and 256-bit ciphers until the next fundamental technology breakthrough. Note that, because of so-called "meet-in-the- middle" attacks [Multiple-Encryption], some legacy cipher suites (e.g., 168-bit 3DES) have an effective key length that is smaller than their nominal key length (112 bits in the case of 3DES). Such cipher suites should be evaluated according to their effective key length. * 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 key agreement. Rationale: These cipher suites, which have assigned values prefixed by "TLS_DH_*", 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. 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 the amount of time during which attacks can be successful. See Section 6.3 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 Sheffer, et al. Expires 7 August 2022 [Page 13] Internet-Draft TLS Recommendations February 2022 * TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 * TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 * TLS_ECDHE_ECDSA_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. 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. 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 (that would allow revealing 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]. 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. The previous version of this document 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 extremely old clients. As with other cipher suites that do not Sheffer, et al. Expires 7 August 2022 [Page 14] Internet-Draft TLS Recommendations February 2022 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 Sec. 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 only applies to DTLS since TLS connections are torn down on the first decryption failure. When a sender is approaching CL, the implementation SHOULD initiate a new handshake (or in TLS 1.3, a Key Update) to rotate the session key. When a receiver has reached IL, the implementation SHOULD close the connection. 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 [I-D.ietf-tls-dtls13]. Sheffer, et al. Expires 7 August 2022 [Page 15] Internet-Draft TLS Recommendations February 2022 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 [I-D.irtf-cfrg-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). 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]. Sheffer, et al. Expires 7 August 2022 [Page 16] Internet-Draft TLS Recommendations February 2022 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 ([RFC9155], and 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. * 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 Sheffer, et al. Expires 7 August 2022 [Page 17] Internet-Draft TLS Recommendations February 2022 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. 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. Sheffer, et al. Expires 7 August 2022 [Page 18] Internet-Draft TLS Recommendations February 2022 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. 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. Sheffer, et al. Expires 7 August 2022 [Page 19] Internet-Draft TLS Recommendations February 2022 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. 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]. Sheffer, et al. Expires 7 August 2022 [Page 20] Internet-Draft TLS Recommendations February 2022 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. 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 Sheffer, et al. Expires 7 August 2022 [Page 21] Internet-Draft TLS Recommendations February 2022 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. 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 (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. Sheffer, et al. Expires 7 August 2022 [Page 22] Internet-Draft TLS Recommendations February 2022 * 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. 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]: * 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 a small number of the most heavily used Web servers. * The On-Line 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 an up-to-date 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 a MITM attacker because the attacker can simply ignore the client's request for a stapled OCSP response. Sheffer, et al. Expires 7 August 2022 [Page 23] Internet-Draft TLS Recommendations February 2022 * [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, we no longer recommend this extension for TLS 1.2. * 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 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 to review the operational conditions before deciding on this approach. 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 Thanks to Alexey Melnikov, Andrei Popov, Christian Huitema, Daniel Kahn Gillmor, David Benjamin, Eric Rescorla, Hannes Tschofenig, Hubert Kario, Ilari Liusvaara, John Mattsson, John R Levine, Julien Élie, Martin Thomson, Mohit Sahni, Nick Sullivan, Nimrod Aviram, Rich Salz, Ryan Sleevi, Sean Turner, Valery Smyslov, Viktor Dukhovni for helpful comments and discussions that have shaped this document. Sheffer, et al. Expires 7 August 2022 [Page 24] Internet-Draft TLS Recommendations February 2022 The authors gratefully acknowledge the contribution of Ralph Holz, who was a coauthor of RFC 7525, the previous version of this document. See RFC 7525 for additional acknowledgments for the previous revision of this document. 8. References 8.1. Normative References [I-D.ietf-httpbis-semantics] Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP Semantics", Work in Progress, Internet-Draft, draft-ietf- httpbis-semantics-19, 12 September 2021, . [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, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [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, . [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, . [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, . Sheffer, et al. Expires 7 August 2022 [Page 25] Internet-Draft TLS Recommendations February 2022 [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, . [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, . [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, DOI 10.17487/RFC6066, January 2011, . [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, . [RFC6176] Turner, S. and T. Polk, "Prohibiting Secure Sockets Layer (SSL) Version 2.0", RFC 6176, DOI 10.17487/RFC6176, March 2011, . [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012, . [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, . [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, . [RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465, DOI 10.17487/RFC7465, February 2015, . Sheffer, et al. Expires 7 August 2022 [Page 26] Internet-Draft TLS Recommendations February 2022 [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, . [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves for Security", RFC 7748, DOI 10.17487/RFC7748, January 2016, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, . [RFC8740] Benjamin, D., "Using TLS 1.3 with HTTP/2", RFC 8740, DOI 10.17487/RFC8740, February 2020, . [RFC8996] Moriarty, K. and S. Farrell, "Deprecating TLS 1.0 and TLS 1.1", BCP 195, RFC 8996, DOI 10.17487/RFC8996, March 2021, . [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, . 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, . Sheffer, et al. Expires 7 August 2022 [Page 27] Internet-Draft TLS Recommendations February 2022 [Antipa2003] Antipa, A., Brown, D.R.L., Menezes, A., Struik, R., and S.A. Vanstone, "Validation of Elliptic Curve Public Keys", Public Key Cryptography - PKC 2003 , 2003. [BETTERCRYPTO] bettercrypto.org, "Applied Crypto Hardening", April 2015, . [Boeck2016] Böck, H., Zauner, A., Devlin, S., Somorovsky, J., and P. Jovanovic, "Nonce-Disrespecting Adversaries: Practical Forgery Attacks on GCM in TLS", May 2016, . [CAB-Baseline] CA/Browser Forum, "Baseline Requirements for the Issuance and Management of Publicly-Trusted Certificates Version 1.1.6", 2013, . [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, . [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, . [CVE] MITRE, "Common Vulnerabilities and Exposures", . [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, . [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, . Sheffer, et al. Expires 7 August 2022 [Page 28] Internet-Draft TLS Recommendations February 2022 [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, . [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, . [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, . [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, . [IANA_TLS] IANA, "Transport Layer Security (TLS) Parameters", . [Jager2015] Jager, T., Schwenk, J., and J. Somorovsky, "Practical Invalid Curve Attacks on TLS-ECDH", European Symposium on Research in Computer Security (ESORICS) 2015 , 2015. [Joux2006] Joux, A., "Authentication Failures in NIST version of GCM", 2006, . Sheffer, et al. Expires 7 August 2022 [Page 29] Internet-Draft TLS Recommendations February 2022 [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, . [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, 2020, . [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, . [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, . [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, . [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, . Sheffer, et al. Expires 7 August 2022 [Page 30] Internet-Draft TLS Recommendations February 2022 [POODLE] US-CERT, "SSL 3.0 Protocol Vulnerability and POODLE Attack", October 2014, . [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, . [RFC2026] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, DOI 10.17487/RFC2026, October 1996, . [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, DOI 10.17487/RFC2246, January 1999, . [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, . [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, DOI 10.17487/RFC4346, April 2006, . [RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security", RFC 4347, DOI 10.17487/RFC4347, April 2006, . [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, . [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008, . [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, . Sheffer, et al. Expires 7 August 2022 [Page 31] Internet-Draft TLS Recommendations February 2022 [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, . [RFC6120] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120, March 2011, . [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, . [RFC6797] Hodges, J., Jackson, C., and A. Barth, "HTTP Strict Transport Security (HSTS)", RFC 6797, DOI 10.17487/RFC6797, November 2012, . [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, . [RFC6961] Pettersen, Y., "The Transport Layer Security (TLS) Multiple Certificate Status Request Extension", RFC 6961, DOI 10.17487/RFC6961, June 2013, . [RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection Most of the Time", RFC 7435, DOI 10.17487/RFC7435, December 2014, . [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, . [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, . Sheffer, et al. Expires 7 August 2022 [Page 32] Internet-Draft TLS Recommendations February 2022 [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, . [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, . [RFC7633] Hallam-Baker, P., "X.509v3 Transport Layer Security (TLS) Feature Extension", RFC 7633, DOI 10.17487/RFC7633, October 2015, . [RFC7919] Gillmor, D., "Negotiated Finite Field Diffie-Hellman Ephemeral Parameters for Transport Layer Security (TLS)", RFC 7919, DOI 10.17487/RFC7919, August 2016, . [RFC8452] Gueron, S., Langley, A., and Y. Lindell, "AES-GCM-SIV: Nonce Misuse-Resistant Authenticated Encryption", RFC 8452, DOI 10.17487/RFC8452, April 2019, . [RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September 2018, . [RFC9001] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021, . [RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162, December 2021, . [SAFECURVES] Bernstein, D.J. and T. Lange, "SafeCurves: Choosing Safe Curves for Elliptic-Curve Cryptography", December 2014, . [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, . Sheffer, et al. Expires 7 August 2022 [Page 33] Internet-Draft TLS Recommendations February 2022 [Springall16] Springall, D., Durumeric, Z., and J. Halderman, "Measuring the Security Harm of TLS Crypto Shortcuts", Proceedings of the 2016 Internet Measurement Conference, DOI 10.1145/2987443.2987480, November 2016, . [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, . [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, . 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. - Limits on key usage. - New attacks since [RFC7457]: ALPACA, Raccoon, Logjam, "Nonce- Disrespecting Adversaries". - RFC 6961 (OCSP status_request_v2) has been deprecated. Sheffer, et al. Expires 7 August 2022 [Page 34] Internet-Draft TLS Recommendations February 2022 * 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. - 2048-bit DH now a MUST, ECDH minimal curve size is 224, vs. 192 previously. - Support for extended_master_secret is a SHOULD. Also removed other, more complicated, related mitigations. - SHOULD-level restriction on the TLS session duration, depending on the rotation period of an [RFC5077] ticket key. - Drop TLS_DHE_RSA_WITH_AES from the recommended ciphers - Add TLS_ECDHE_ECDSA_WITH_AES to the recommended ciphers - SHOULD NOT use the old MTI cipher suite, TLS_RSA_WITH_AES_128_CBC_SHA. - Recommend curve X25519 alongside NIST P-256 * Differences specific to TLS 1.3: - New TLS 1.3 capabilities: 0-RTT. - Removed capabilities: renegotiation, compression. - 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. Sheffer, et al. Expires 7 August 2022 [Page 35] Internet-Draft TLS Recommendations February 2022 B.1. draft-ietf-uta-rfc7525bis-05 * Addressed WG Last Call comments, specifically: - More clarity and guidance on session resumption. - Clarity on TLS 1.2 renegotiation. - Wording on the 0-RTT feature aligned with RFC 8446. - SHOULD NOT guidance on static and ephemeral finite field DH cipher suites. - Revamped the recommended TLS 1.2 cipher suites, removing DHE and adding ECDSA. The latter due to the wide adoption of ECDSA certificates and in line with RFC 8446. - Recommendation to use deterministic ECDSA. - Finally deprecated the old TLS 1.2 MTI cipher suite. - Deeper discussion of ECDH public key reuse issues, and as a result, recommended support of X25519. - Reworded the section on certificate revocation and OCSP following a long mailing list thread. B.2. draft-ietf-uta-rfc7525bis-04 * No version fallback from TLS 1.2 to earlier versions, therefore no SCSV. B.3. draft-ietf-uta-rfc7525bis-03 * Cipher integrity and confidentiality limits. * Require extended_master_secret. B.4. draft-ietf-uta-rfc7525bis-02 * Adjusted text about ALPN support in application protocols * Incorporated text from draft-ietf-tls-md5-sha1-deprecate B.5. draft-ietf-uta-rfc7525bis-01 * Many more changes, including: Sheffer, et al. Expires 7 August 2022 [Page 36] Internet-Draft TLS Recommendations February 2022 - 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. B.6. 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.7. draft-sheffer-uta-rfc7525bis-00 * Renamed, since the BCP number does not change. * Added an empty "Differences from RFC 7525" section. B.8. draft-sheffer-uta-bcp195bis-00 * Initial release, the RFC 7525 text as-is, with some minor editorial changes to the references. Authors' Addresses Sheffer, et al. Expires 7 August 2022 [Page 37] Internet-Draft TLS Recommendations February 2022 Yaron Sheffer Intuit Email: yaronf.ietf@gmail.com Peter Saint-Andre Mozilla Email: stpeter@mozilla.com Thomas Fossati arm Email: thomas.fossati@arm.com Sheffer, et al. Expires 7 August 2022 [Page 38]