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Security Transport Layer Security TLS features TLS 1.2 is in widespread use and can be configured such that it provides good security properties. TLS 1.3 is also in widespread use and fixes some known deficiencies with TLS 1.2, such as removing error-prone cryptographic primitives and encrypting more of the traffic so that it is not readable by outsiders. Both versions have several extension points, so items like new cryptographic algorithms, new supported groups (formerly "named curves"), etc., can be added without defining a new protocol. This document specifies that TLS 1.2 is frozen: no new algorithms or extensions will be approved. Further, TLS 1.3 use is widespread, and new protocols should require and assume its existence. About This Document Status information for this document may be found at . Discussion of this document takes place on the Transport Layer Security Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction TLS 1.2 is in widespread use and can be configured such that it provides good security properties. However, this protocol version suffers from several deficiencies:

1. While application layer traffic is always encrypted, most of the handshake messages are not encrypted. Therefore, the privacy provided is suboptimal.
2. The list of cryptographic primitives specified for the protocol, both in-use primitives and deprecated ones, includes several primitives that were a source for vulnerabilities throughout the years, such as RSA key exchange, CBC cipher suites, and problematic finite-field Diffie-Hellman group negotiation. See for elaboration.
3. The original protocol, as-is, does not provide security (cite Renegotiation attack). Rather, some extensions are required to provide security.

In contrast, TLS 1.3 is also in widespread use and fixes most known deficiencies with TLS 1.2, such as encrypting more of the traffic so that it is not readable by outsiders and removing most cryptographic primitives considered dangerous. Importantly, TLS 1.3 enjoys robust security proofs and provides excellent security as-is. Both versions have several extension points, so items like new cryptographic algorithms, new supported groups (formerly "named curves"), etc., can be added without defining a new protocol. This document specifies that TLS 1.2 is frozen: no new algorithms or extensions will be approved. Further, TLS 1.3 use is widespread, and new protocols should require and assume its existence.

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 Quantum computers, once available, will have a huge impact on TLS. In 2016, the US National Institute of Standards and Technology started a multi-year effort to standardize algorithms that will be "safe" once quantum computers are feasible . First IETF discussions happened around the same time . While the industry is waiting for NIST to finish standardization, the IETF has several efforts underway. A working group was formed in early 2013 to work on use of PQC in IETF protocols, . Several other working groups, including TLS , are working on drafts to support hybrid algorithms and identifiers, for use during a transition from classic to a post-quantum world. For TLS it is important to note that the focus of these efforts is TLS 1.3 or later. TLS 1.2 is WILL NOT be supported (see ).

TLS Use by Other Protocols 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.

Security Considerations TLS 1.2 was specified with several cryptographic primitives and design choices that have historically hindered its security. 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 extension points, is vulnerable to the renegotiation attack and the Triple Handshake attack. 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. Refer to , , for elaboration. 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 present. 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. As before, to securely deploy the protocol, these key exchange methods must be disabled. Refer to draft-obsolete-kex for elaboration (TODO I guess we will anyway wait for WGLC for draft-obsolete-kex, so no sense to temporarily refer to the draft.) 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. And lastly, historically the protocol was affected by several other attacks that TLS 1.3 is immune to: BEAST , Logjam , FREAK , and SLOTH .

IANA Considerations IANA will stop accepting registrations for any TLS parameters except for the following:

• TLS Exporter Labels
• TLS Application-Layer Protocol Negotiation (ALPN) Protocol IDs

Entries in any other TLS protocol registry should have an indication like "For TLS 1.3 or later" in their entry.

References Normative References 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] 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 updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. This document describes a number of changes to TLS and DTLS IANA registries that range from adding notes to the registry all the way to changing the registration policy. These changes were mostly motivated by WG review of the TLS- and DTLS-related registries undertaken as part of the TLS 1.3 development process. This document updates the following RFCs: 3749, 5077, 4680, 5246, 5705, 5878, 6520, and 7301. This document describes how Transport Layer Security (TLS) is used to secure QUIC. 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. 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. 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. Informative References n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

Acknowledgments None yet.