Akamai Technologies

rsalz@akamai.com

nimrod.aviram@gmail.com

Security Using TLS in Applications TLS TLS Transition TLS Support Interoperability features TLS 1.3 is widely used, has had comprehensive security proofs, and improves both security and privacy deficiencies in TLS 1.2. Therefore, new protocols that use TLS must require TLS 1.3. As DTLS 1.3 is not widely available or deployed, this prescription does not pertain to DTLS (in any DTLS version); it pertains to TLS only. This document updates RFC 9325. It discusses post-quantum cryptography and the security and privacy improvements in TLS 1.3 as the rationale for the update.

Status of This Memo This memo documents an Internet Best Current Practice. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

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Table of Contents

• .  Introduction
• .  Conventions
• .  Implications for Post-Quantum Cryptography (PQC)
• .  TLS Use by Other Protocols and Applications
• .  Changes to RFC 9325
• .  Security Considerations
• .  IANA Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Authors' Addresses

Introduction This document specifies that new protocols that use TLS must assume that TLS 1.3 is available and require its use. As DTLS 1.3 is not widely available or deployed, this prescription does not pertain to DTLS (in any DTLS version); it pertains to TLS only. TLS 1.3 is in widespread use and fixes most known deficiencies with TLS 1.2. Examples of this include encrypting more of the traffic so that it is not readable by outsiders and removing most cryptographic primitives now considered weak. Importantly, the protocol has had comprehensive security proofs and should provide excellent security without any additional configuration. TLS 1.2 is in use and can be configured such that it provides good security properties. However, TLS 1.2 suffers from several deficiencies, as described in . Addressing them usually requires bespoke configuration. This document updates . It discusses post-quantum cryptography and the security and privacy improvements in TLS 1.3 as the rationale for the update. See .

Conventions 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 when, and only when, they appear in all capitals, as shown here.

Implications for Post-Quantum Cryptography (PQC) Cryptographically Relevant Quantum Computers (CRQCs), once available, will have a huge impact on TLS traffic (see, e.g., ). To mitigate this, TLS applications will need to migrate to Post-Quantum Cryptography (PQC) . Detailed considerations of when an application requires PQC or when a CRQC is a threat that an application needs to protect against are beyond the scope of this document. It is important to note that the TLS Working Group is focusing its efforts on TLS 1.3 or later; TLS 1.2 will not be supported (see ). This is one more reason for new protocols to require TLS to default to TLS 1.3, where PQC is actively being standardized, as this gives new applications the option to use PQC.

TLS Use by Other Protocols and Applications Any new protocol that uses TLS MUST specify TLS 1.3 as its default. For example, QUIC requires TLS 1.3 and specifies that endpoints MUST terminate the connection if an older version is used. If deployment considerations are a concern, the protocol MAY specify TLS 1.2 as an additional, non-default option. As a counter example, the Usage Profile for DNS over TLS specifies TLS 1.2 as the default, while also allowing TLS 1.3. For newer specifications that choose to support TLS 1.2, those preferences are to be reversed. The initial TLS handshake allows a client to specify which versions of TLS it supports, and the server is intended to pick the highest version that it also supports. This is known as "TLS version negotiation"; protocol and negotiation details are discussed in and . Many TLS libraries provide a way for applications to specify the range of versions they want, including an open interval where only the lowest or highest version is specified. If the application is using a TLS implementation that supports TLS version negotiation and if it knows that the TLS implementation will use the highest version supported, then clients SHOULD specify just the minimum version they want. This MUST be TLS 1.3 or TLS 1.2, depending on the circumstances described in the above paragraphs.

Changes to RFC 9325 provides recommendations for ensuring the security of deployed services that use TLS and, unlike this document, DTLS as well. describes TLS 1.3 as "widely available", and the transition to TLS 1.3 has further increased since publication of that document. This document thus makes two changes to the recommendations in :

• That section says that TLS 1.3 SHOULD be supported; this document mandates that TLS 1.3 MUST be supported for new protocols using TLS.
• That section says that TLS 1.2 MUST be supported; this document says that TLS 1.2 MAY be supported as described above.

Again, these changes only apply to TLS, and not DTLS.

Security Considerations TLS 1.2 was specified with several cryptographic primitives and design choices that have, over time, become significantly weaker. The purpose of this section is to briefly survey several such prominent problems that have affected the protocol. It should be noted, however, that TLS 1.2 can be configured securely; it is merely much more difficult to configure it securely as opposed to using its modern successor, TLS 1.3. See for a more thorough guide on the secure deployment of TLS 1.2. First, without any extensions, TLS 1.2 is vulnerable to renegotiation attacks (see and ) and the Triple Handshake attack (see ). Broadly, these attacks exploit the protocol's support for renegotiation in order to inject a prefix chosen by the attacker into the plaintext stream. This is usually a devastating threat in practice (e.g., it allows an attacker to obtain secret cookies in a web setting). In light of the above problems, specifies an extension that prevents this category of attacks. To securely deploy TLS 1.2, either renegotiation must be disabled entirely, or this extension must be used. Additionally, clients must not allow servers to renegotiate the certificate during a connection. Second, the original key exchange methods specified for TLS 1.2, namely RSA key exchange and finite field Diffie-Hellman, suffer from several weaknesses. To securely deploy the protocol, most of these key exchange methods must be disabled. See for details. Third, symmetric ciphers that are widely used in TLS 1.2, namely RC4 and Cipher Block Chaining (CBC) cipher suites, suffer from several weaknesses. RC4 suffers from exploitable biases in its key stream; see . CBC cipher suites have been a source of vulnerabilities throughout the years. A straightforward implementation of these cipher suites inherently suffers from the Lucky13 timing attack . The first attempt to implement the cipher suites in constant time introduced an even more severe vulnerability . Refer to for another example of a vulnerability with CBC cipher suites and a survey of similar works. In addition, TLS 1.2 was affected by several other attacks that TLS 1.3 is immune to: BEAST , Logjam , FREAK , and SLOTH . Finally, while application-layer traffic in TLS 1.2 is always encrypted, most of the content of the handshake messages is not. Therefore, the privacy provided is suboptimal. This is a protocol issue that cannot be addressed by configuration.

IANA Considerations This document has no IANA actions.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are used to protect data exchanged over a wide range of application protocols and can also form the basis for secure transport protocols. Over the years, the industry has witnessed several serious attacks on TLS and DTLS, including attacks on the most commonly used cipher suites and their modes of operation. This document provides the latest recommendations for ensuring the security of deployed services that use TLS and DTLS. These recommendations are applicable to the majority of use cases. RFC 7525, an earlier version of the TLS recommendations, was published when the industry was transitioning to TLS 1.2. Years later, this transition is largely complete, and TLS 1.3 is widely available. This document updates the guidance given the new environment and obsoletes RFC 7525. In addition, this document updates RFCs 5288 and 6066 in view of recent attacks. This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] Use of TLS 1.3, which fixes some known deficiencies in TLS 1.2, is growing. This document specifies that no changes will be approved for TLS 1.2 outside of urgent security fixes (as determined by TLS Working Group consensus), new TLS Exporter Labels, and new Application-Layer Protocol Negotiation (ALPN) Protocol IDs. This applies to TLS only; it does not apply to DTLS (in any DTLS version). This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document obsoletes RFC 8446, which specified TLS 1.3. This document obsoletes RFC 5246 (specifying TLS 1.2) and RFCs 5077, 6961, 7627, and 8422, all of which pertain to TLS 1.2 or earlier, and updates RFCs 5705 and 6066. This document also specifies new requirements for TLS 1.2 implementations. Informative References 28th USENIX Security Symposium (USENIX Security 19) This document discusses usage profiles, based on one or more authentication mechanisms, which can be used for DNS over Transport Layer Security (TLS) or Datagram TLS (DTLS). These profiles can increase the privacy of DNS transactions compared to using only cleartext DNS. This document also specifies new authentication mechanisms -- it describes several ways that a DNS client can use an authentication domain name to authenticate a (D)TLS connection to a DNS server. Additionally, it defines (D)TLS protocol profiles for DNS clients and servers implementing DNS over (D)TLS. This document updates RFC 7858. IEEE Symposium on Security & Privacy 2015 CCS '16: Proceedings of the 2016 ACM SIGSAC Conference on Computer and Communications Security, pp. 1492-1504 NIST This document describes how Transport Layer Security (TLS) is used to secure QUIC. Wayback Machine archive Wayback Machine archive Secure Socket Layer (SSL) and Transport Layer Security (TLS) renegotiation are vulnerable to an attack in which the attacker forms a TLS connection with the target server, injects content of his choice, and then splices in a new TLS connection from a client. The server treats the client's initial TLS handshake as a renegotiation and thus believes that the initial data transmitted by the attacker is from the same entity as the subsequent client data. This specification defines a TLS extension to cryptographically tie renegotiations to the TLS connections they are being performed over, thus preventing this attack. [STANDARDS-TRACK] This document requires that Transport Layer Security (TLS) clients and servers never negotiate the use of RC4 cipher suites when they establish connections. This applies to all TLS versions. This document updates RFCs 5246, 4346, and 2246. The advent of a cryptographically relevant quantum computer (CRQC) would render state-of-the-art, traditional public key algorithms deployed today obsolete, as the mathematical assumptions underpinning their security would no longer hold. To address this, protocols and infrastructure must transition to post-quantum algorithms, which are designed to resist both traditional and quantum attacks. This document explains why engineers need to be aware of and understand post-quantum cryptography (PQC), and it details the impact of CRQCs on existing systems and the challenges involved in transitioning to post-quantum algorithms. Unlike previous cryptographic updates, this shift may require significant protocol redesign due to the unique properties of post-quantum algorithms. For (D)TLS 1.2, this document deprecates the use of two key exchanges, namely Diffie-Hellman (DH) over a finite field and RSA. It also discourages the use of static Elliptic Curve Diffie-Hellman (ECDH) cipher suites. These prescriptions apply only to (D)TLS 1.2, since (D)TLS 1.0 and TLS 1.1 are deprecated by RFC 8996 and (D)TLS 1.3 either does not use the affected algorithms or does not share the relevant configuration options. (There is no DTLS version 1.1.) This document updates RFCs 4162, 4279, 4346, 4785, 5246, 5288, 5289, 5469, 5487, 5932, 6209, 6347, 6367, 6655, 7905, 8422, and 9325 to either deprecate or discourage the use of cipher suites using the above key exchange methods in (D)TLS 1.2 connections. Network and Distributed System Security Symposium - NDSS 2016 Wayback Machine archive CCS '15: Proceedings of the 22nd ACM SIGSAC Conference on Computer and Communications Security, pp. 5-17

Authors' Addresses Akamai Technologies

rsalz@akamai.com

nimrod.aviram@gmail.com