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Security Transport Layer Security This document defines a mechanism for servers to communicate supported key share algorithms 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 supported groups in the supported_groups extension, but also generates key shares for a subset in the key_share extension. Named groups in this subset can be used in one round trip, while named groups outside the subset require a HelloRetryRequest and hence 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 supported named groups 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 supported TLS named groups, 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 configuration is 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's list against its configuration to predict which named group will be chosen. When evaluating the server's list, the client MUST ignore any codepoints that it does not support or recognize. If the client predicts a named group, the client SHOULD send a key_share extension containing just that named group in the initial ClientHello. The client MAY continue to send other key shares to reduce mispredictions (see ), though this comes at additional network and computational cost. The client MAY also ignore the prediction, e.g., it chooses not to apply this process to some groups (see ). If there were no named groups in common, the client SHOULD proceed as if the tls-supported-groups parameter was not present and predict some default set of key shares. The HTTPS or SVCB record may have been stale, so it is possible the server still has a named group in common. This process does not modify the supported_groups extension. To avoid downgrade attacks, the client MUST continue to send all its supported groups, in preference order, in supported_groups. 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 configuration
• The server is in the process of deploying a change to named group configuration, and different server instances temporarily evaluate different configuration
• 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. An attacker may be able to forge an HTTPS or SVCB record and influence 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. Provided the client's supported_groups list always reflects the unmodified client preference list, this is safe. The scope of attacker influence depends on how the server selects a group. Servers are expected to evaluate the combination of key_share and supported_groups according to their selection goals and the definitions in . When deciding between multiple common groups, a server might consider:

• The server's local preferences, picking one it considers best.
• The client's preference order in supported_groups, picking one the client considers best.
• Which groups appear in key_share, picking one that avoids a HelloRetryRequest.

The last case, presence in key_share, is under attacker influence in this mechanism. However, already permits the client to omit its most preferred groups in key_share. Servers are thus expected to only select by key_share when they opt to consider neither the client's preference nor their own. That is, it is only appropriate in cases where the two groups have comparable preference, such that round-trip costs dominate. Servers SHOULD NOT use key_share to select a classical named group over a post-quantum named group. 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 might prefer post-quantum groups, but support ECDH groups with older servers. It MAY then ignore DNS-based ECDH predictions, limiting tls-supported-groups to post-quantum options. In this case, transitions between post-quantum groups, where the bandwidth concerns are more pronounced, remain optimized, but ECDH-only servers cannot take advantage of tls-supported-groups.

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, Marc Penninga, Sophie Schmieg, Martin Thomson, and Bas Westerbaan for discussions and review of this document.