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Security Transport Layer Security This document defines a mechanism for servers to communicate key share preferences in DNS. Clients may use this information to reduce TLS handshake round-trips. About This Document The latest revision of this draft can be found at . 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 Named groups in TLS 1.3 are negotiated with two lists in the ClientHello: The client sends its full preferences in the supported_groups extension, but also generates key shares for a subset in the key_share extension. Named groups in this subset may be used in one, while named groups outside the subset requires a HelloRetryRequest and two round trips. The additional round trip is undesirable for performance, but unused key shares consume network and computational resources, so clients often do not generate key shares for all groups. Post-quantum key encapsulation methods (KEMs) have large keys and ciphertexts, so network costs are particularly pronounced. As a TLS ecosystem transitions from one post-quantum KEM to another, it is challenging to pick key shares without prior knowledge of the server's policies:

1. Predicting both post-quantum KEMs consumes excessive bandwidth on the unused option.
2. Predicting the old post-quantum KEM adds a round-trip cost to newer servers. Servers will be unlikely to transition as a result.
3. Predicting the new post-quantum KEM adds a round-trip cost to older servers. Particularly early in the transition, when most servers do not implement the new KEM, this may significantly regress performance.

This document defines a method for servers to declare their named group preferences in DNS, using SVCB or HTTPS resource records . This allows the client to predict key shares more accurately.

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.

DNS Service Parameter This document defines the tls-supported-groups SvcParamKey , which specifies the endpoint's TLS supported group preferences, as a non-empty sequence of TLS NamedGroup codepoints in order of decreasing preference, with no duplicates. This allows clients connecting to the endpoint to reduce the likelihood of needing a HelloRetryRequest.

Format The presentation value of the SvcParamValue is a non-empty comma-separated list () of decimal integers between 0 and 65535 (inclusive) in ASCII, with no duplicate integers. Any other value is a syntax error. To enable simpler parsing, this SvcParam MUST NOT contain escape sequences. The wire format of the SvcParamValue is a sequence of 2-octet numeric values in network byte order. An empty list of values is invalid, as is a list containing duplicates. For example, a TLS server which prefers x25519 (29) and also supports secp256r1 (23) would add a tls-supported-groups SvcParamValue containing 29 and 23. The presentation value would be "29,23". The wire format of the SvcParamValue would be four octets, represented in hexadecimal as 001d0017. The following is an example of the value appearing in a complete DNS record in the presentation syntax:

Configuring Services Services SHOULD include supported TLS named groups, in order of decreasing preference in the tls-supported-groups parameter of their HTTPS or SVCB endpoints. As TLS preferences are updated, services SHOULD update the DNS record to match. Services MAY include GREASE values in this list.

Client Behavior When connecting to a service endpoint whose HTTPS or SVCB record contains the tls-supported-groups parameter, the client evaluates the server preferences against its own to predict which named group will be chosen. When evaluating the server preferences, the client MUST ignore any codepoints that it does not support or recognize. If there is a named group in common, the client MAY send a key_share extension containing just that named group in the initial ClientHello. To avoid downgrade attacks, the client MUST continue to send its full preferences in the supported_groups extension. See for additional discussion on downgrades.

Misprediction Although this service parameter is intended to reduce key share mispredictions, mispredictions may still occur in some scenarios. For example:

• The client has fetched a stale HTTPS or SVCB record that no longer reflects the server preferences
• The server is in the process of deploying a change to named group preferences, and different server instances temporary evaluate different preferences
• The client was unable to fetch the HTTPS or SVCB record
• The client and server implement incompatible selection algorithms, such that client's evaluation of the service parameter did not match the server's final selection

Clients and servers MUST correctly handle mispredictions by responding to and sending HelloRetryRequest, respectively.

Security Considerations This document introduces a mechanism for clients to vary the key_share extension based on DNS. DNS responses are unauthenticated in many deployments, so this can permit attacker influence over the client's predicted named groups. That, in turn, can influence the named group selected by the TLS server, as TLS's downgrade protections only extend to the ClientHello itself. However, the client continues to send its full preferences in supported_groups, so this influence is limited by the server's named group selection policy: Servers which select purely based on preference orders will first select a named group on supported_groups, and then consider key_share only to send HelloRetryRequest or ServerHello. When connecting to such servers, attackers cannot influence the selection with this mechanism. However, some servers prioritize round-trip times over preference orders. That is, when choosing between a named group in key_share and a more preferable (e.g. more secure) named group not in key_share, these servers will select the less preferable one in key_share. In this case, an attacker may be able to influence the selection by forging an HTTPS or SVCB record. Per , the client's key_share extension does not reflect its full preference list in supported_groups. Thus, this server behavior is only appropriate when the two options are of comparable preference, such that round trip concerns dominate. In particular, it is NOT RECOMMENDED when choosing between post-quantum and classical named groups. As these semantics were already prescribed in , it is safe for clients to admit attacker control over the set of named groups preferred in key_share, provided supported_groups always reflects the true client preference. Servers are expected to evaluate the combination of key_share and supported_groups according to the defined semantics and their selection goals. To reduce the risk of downgrade attacks with incorrectly deployed servers, clients MAY choose to ignore tls-supported-groups when the result would predict a less preferred group. For example, a client that implements a combination of post-quantum groups and ECDH groups MAY limit its influence to predicting post-quantum groups. This optimizes transitions between post-quantum groups, where the bandwidth concerns are more pronounced, but means ECDH-only servers cannot take advantage of the mechanism.

IANA Considerations This document updates the Service Parameter Keys registry with the following entry:

Number	Name	Meaning	Format Reference	Change Controller
9	tls-supported-groups	Supported groups in TLS	(this document)	IETF

Normative References 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 specifies the "SVCB" ("Service Binding") and "HTTPS" DNS resource record (RR) types to facilitate the lookup of information needed to make connections to network services, such as for HTTP origins. SVCB records allow a service to be provided from multiple alternative endpoints, each with associated parameters (such as transport protocol configuration), and are extensible to support future uses (such as keys for encrypting the TLS ClientHello). They also enable aliasing of apex domains, which is not possible with CNAME. The HTTPS RR is a variation of SVCB for use with HTTP (see RFC 9110, "HTTP Semantics"). By providing more information to the client before it attempts to establish a connection, these records offer potential benefits to both performance and privacy. 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. This document describes GREASE (Generate Random Extensions And Sustain Extensibility), a mechanism to prevent extensibility failures in the TLS ecosystem. It reserves a set of TLS protocol values that may be advertised to ensure peers correctly handle unknown values.

Acknowledgments The author would like to thank David Adrian, Bob Beck, Sophie Schmieg, Martin Thomson, and Bas Westerbaan for discussions and review of this document.