New Protocols Must Require TLS 1.3 (2024)

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Table of Contents

1. Introduction

TLS 1.2 [TLS12] is in widespread use and can be configured such thatit provides goodsecurity properties. However, this protocol version suffers from severaldeficiencies:¶

1. While application layer traffic is always encrypted, most of the handshakemessages are not. Therefore, the privacy provided is suboptimal.This is a protocol issue that cannot be addressed by configuration.¶

2. The list of cryptographic primitives specified for the protocol, both in-useprimitives and deprecated ones, includes several primitives that havebeen a source forvulnerabilities throughout the years, such as RSA key exchange, CBC cipher suites,and problematic finite-field Diffie-Hellman group negotiation.These issues could be addressed through proper configuration; however,experience shows that configuration mistakes are common, especially whendeploying cryptography.See Section 6 for elaboration.¶

3. The base protocol does not provide security against sometypes of attacks (see Section 6);extensions are required to providesecurity.¶

TLS 1.3 [TLS13] is also inwidespread use and fixes most known deficiencies with TLS 1.2, such asencrypting more of the traffic so that it is not readable by outsiders andremoving most cryptographic primitives considered dangerous. Importantly, TLS1.3 enjoys robust security proofs and provides excellent security withoutany additional configuration.¶

This document specifies that, since TLS 1.3 use is widespread, new protocolsmust require and assume its existence.This prescription does not pertain to DTLS (in any DTLS version); it pertains toTLS only.¶

2. 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 asdescribed in BCP14 [RFC2119] [RFC8174] when, and only when, theyappear in all capitals, as shown here.¶

3. Implications for post-quantum cryptography

Cryptographically-relevantquantum computers, once available, will have a huge impact on TLS.In 2016, the US National Institute of Standards and Technology (NIST) started amulti-year effort to standardize algorithms that will be "safe"once quantum computers are feasible [PQC]. First IETF discussions happenedaround the same time [CFRGSLIDES].¶

While the industry is waiting for NIST to finish standardization, theIETF has several efforts underway.A working group was formed in early 2023 to work on use operational andtransitional uses of PQC in IETF protocols,[PQUIPWG].Several other working groups, notably LAMPS [LAMPSWG] and TLS [TLSWG],are working ondrafts to support hybrid algorithms and identifiers, for use during atransition from classic to a post-quantum world.¶

For TLS it is important to note that the focus of these efforts is TLS 1.3or later:TLS 1.2 WILL NOT be supported (see Section 7).This is one more reason for new protocols to default to TLS 1.3, wherepost-quantum cryptography is expected to be supported.¶

4. TLS Use by Other Protocols and Applications

Any new protocol that uses TLS MUST specify as its default TLS 1.3.For example, QUIC [QUICTLS] requires TLS 1.3 and specifies that endpointsMUST terminate the connection if an older version is used.¶

If deployment considerations are a concern, the protocol MAY specify TLS 1.2 asan additional, non-default option.As a counter example, the Usage Profile for DNS over TLS [DNSTLS] specifiesTLS 1.2 as the default, while also allowing TLS 1.3.For newer specifications that choose to support TLS 1.2, those preferences areto be reversed.¶

The initial TLS handshake allows a client to specify which versions ofthe TLS protocol it supports and the server is intended to pick the highestversion that it also supports.This is known as the "TLS version negotiation," andmany TLS libraries provide a way for applications to specify the rangeof versions.When the API allows it, clients SHOULD specify just the minimum version theywant.This SHOULD be TLS 1.3 or TLS 1.2, depending on the circ*mstances describedin the above paragraphs.¶

5. Changes to RFC 9325

This document makes two changes to the recommendations in[RFC9325], Section 3.1.1:¶

• That section says that TLS 1.3 SHOULD be supported; this document saysthat for new protocols it MUST be supported.¶

• That section says that TLS 1.2 MUST be supported; this document says thatit MAY be supported as described above.¶

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

6. Security Considerations

TLS 1.2 was specified with several cryptographic primitives and design choicesthat have, over time, weakened its security. The purpose of this section is tobriefly survey several such prominent problems that have affected the protocol.It should be noted, however, that TLS 1.2 can be configured securely; it ismerely much more difficult to configure it securely as opposed to using itsmodern successor, TLS 1.3. See [RFC9325] for a more thorough guide on thesecure deployment of TLS 1.2.¶

Firstly, the TLS 1.2 protocol, without any extension points, is vulnerable torenegotiation attacks (see [RENEG1] and [RENEG2]) and theTriple Handshake attack (see [TRIPLESHAKE]).Broadly, these attacksexploit the protocol's support for renegotiation in order to inject a prefixchosen by the attacker into the plaintext stream. This is usually a devastatingthreat in practice, that allows e.g. obtaining secret cookies in a web setting.In light ofthe above problems, [RFC5746] specifies an extension that prevents thiscategory of attacks. To securely deploy TLS 1.2, either renegotiation must bedisabled entirely, or this extension must be used. Additionally, clients mustnot allow servers to renegotiate the certificate during a connection.¶

Secondly, the original key exchange methods specified for the protocol, namelyRSA key exchange and finite field Diffie-Hellman, suffer from severalweaknesses. Similarly, to securely deploy the protocol, these key exchangemethods must be disabled.See [I-D.draft-ietf-tls-deprecate-obsolete-kex] for details.¶

Thirdly, symmetric ciphers which were widely-used in the protocol, namely RC4and CBC cipher suites, suffer from several weaknesses. RC4 suffers fromexploitable biases in its key stream; see [RFC7465]. CBC cipher suites havebeen a source of vulnerabilities throughout the years. A straightforwardimplementation of these cipher suites inherently suffers from the Lucky13 timingattack [LUCKY13]. The first attempt to implement the cipher suites inconstant time introduced an even more severe vulnerability [LUCKY13FIX].There have been further similar vulnerabilities throughout theyears exploiting CBC cipher suites; refer to e.g. [CBCSCANNING]for an example and a survey of similar works.¶

And lastly, historically the protocol was affected by several other attacks thatTLS 1.3 is immune to:BEAST [BEAST], Logjam [WEAKDH], FREAK [FREAK], and SLOTH [SLOTH].¶

7. IANA Considerations

This document makes no requests to IANA.¶

8. References

Authors' Addresses