New Protocols Using TLS Must Require TLS 1.3

Introduction This document specifies that, since TLS 1.3 use is widespread, new protocols that use TLS must require and assume its existence. It updates as described in . 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 RFC9325 and discusses post-quantum cryptography and fixed weaknesses in TLS 1.2 as a rationale for that update.

Conventions and Definitions 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 (CRQC), 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 need to protect against, are beyond the scope of this document. For TLS it is important to note that the focus of these efforts within the TLS WG is TLS 1.3 or later, and that TLS 1.2 will not be supported (see ). This is one more reason for new protocols 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 as its default TLS 1.3. 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 the TLS protocol it supports and the server is intended to pick the highest version that it also supports. This is known as the "TLS version negotiation," and 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 this, 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. At the time it was published, it described availability of TLS 1.3 as "widely available." The transition and adoption mentioned in that document has grown, and this document now 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 TLS-using protocols.
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. Firstly, the TLS 1.2 protocol, without any extensions, 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, that allows e.g. obtaining 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. Secondly, the original key exchange methods specified for the protocol, namely RSA key exchange and finite field Diffie-Hellman, suffer from several weaknesses. Similarly, to securely deploy the protocol, these key exchange methods must be disabled. See for details. Thirdly, symmetric ciphers which were widely-used in the protocol, namely RC4 and 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 . There have been further similar vulnerabilities throughout the years exploiting CBC cipher suites; refer to, e.g., for an example 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 . And finally, while application layer traffic is always encrypted, most of the handshake messages are not. Therefore, the privacy provided is suboptimal. This is a protocol issue that cannot be addressed by configuration.

IANA Considerations This document makes no requests to IANA.