



Transport Layer Security                                B. E. Westerbaan
Internet-Draft                                                Cloudflare
Intended status: Standards Track                        23 February 2026
Expires: 27 August 2026


               Updated recommendations for TLS keyshares
            draft-westerbaan-tls-keyshare-recommendations-00

Abstract

   This document updates the recommendations for key shares algorithms
   (TLS supported groups; previously EC Named Curve Registry) in the
   light of the future arrival of cryptographically relevant quantum
   computers.

   [[ NOTE I use key share in the title and here as it's more accurate
   than "group" and perhaps more well known in the context TLS than key
   agreement or key exchange. ]]

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://bwesterb.github.io/draft-westerbaan-tls-keyshare-
   recommendations/draft-westerbaan-tls-keyshare-recommendations.html.
   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-westerbaan-tls-keyshare-
   recommendations/.

   Discussion of this document takes place on the Transport Layer
   Security Working Group mailing list (mailto:tls@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/tls/.  Subscribe
   at https://www.ietf.org/mailman/listinfo/tls/.

   Source for this draft and an issue tracker can be found at
   https://github.com/bwesterb/draft-westerbaan-tls-keyshare-
   recommendations.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.







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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Recommend . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Discourage  . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   A future cryptographically relevant quantum computer can decrypt TLS
   handshakes recorded today that do not post-quantum algorithms for
   their key shares: algorithms designed to be resistant against quantum
   attack.  This threat is known as store-now/decrypt-later (SNDL).






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   RFC9847 defines the permitted value of the "Recommended" column of
   the TLS Supported Groups registry (https://www.iana.org/assignments/
   tls-parameters/tls-parameters.xhtml#tls-parameters-8) as:

   Y:  Indicates that the IETF has consensus that the item is
      RECOMMENDED.  This only means that the associated mechanism is fit
      for the purpose for which it was defined.  Careful reading of the
      documentation for the mechanism is necessary to understand the
      applicability of that mechanism.  The IETF could recommend
      mechanisms that have limited applicability, but will provide
      applicability statements that describe any limitations of the
      mechanism or necessary constraints on its use.

   N:  Indicates that the item has not been evaluated by the IETF and
      that the IETF has made no statement about the suitability of the
      associated mechanism.  This does not necessarily mean that the
      mechanism is flawed, only that no consensus exists.  The IETF
      might have consensus to leave an items marked as "N" on the basis
      of its having limited applicability or usage constraints.

   D:  Indicates that the item is discouraged.  This marking could be
      used to identify mechanisms that might result in problems if they
      are used, such as a weak cryptographic algorithm or a mechanism
      that might cause interoperability problems in deployment.  When
      marking a registry entry as “D”, either the References or the
      Comments Column MUST include sufficient information to determine
      why the marking has been applied.  Implementers and users SHOULD
      consult the linked references associated with the item to
      determine the conditions under which the item SHOULD NOT or MUST
      NOT be used.

   Given the SNDL threat, the IETF cannot recommend key shares for
   general use that do not offer post-quantum resistance, and this
   document updates the TLS Supported Groups registry
   (https://www.iana.org/assignments/tls-parameters/tls-
   parameters.xhtml#tls-parameters-8) accordingly.

   Among the currently registered post-quantum key share algorithms,
   IETF recommends X25519MLKEM768 for its widespread support.

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 as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.




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3.  Security Considerations

   Before the arrival of a cryptographically relevant quantum computer
   (CRQC), a TLS connection that negotiated a non-post quantum key share
   can be recorded decrypted in the future.

   After the arrival of a CRQC, allowing a non-post quantum key share to
   be negotiated allows for an active quantum attack that achieves MITM,
   even if the server certificate is post quantum.

4.  IANA Considerations

   This document updates the TLS Supported Groups registry
   (https://www.iana.org/assignments/tls-parameters/tls-
   parameters.xhtml#tls-parameters-8), according to the procedures in
   Section 6 of [RFC9847] as follows.

4.1.  Recommend

                 +=======+================+=============+
                 | Value | Description    | Recommended |
                 +=======+================+=============+
                 | 4588  | X25519MLKEM768 | Y           |
                 +-------+----------------+-------------+

                                 Table 1

4.2.  Discourage

   +=======+======================+=============+======================+
   | Value | Description          | Recommended | Comment              |
   +=======+======================+=============+======================+
   | 9     | sect283k1            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 10    | sect283r1            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 11    | sect409k1            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |



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   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 12    | sect409r1            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 13    | sect571k1            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 14    | sect571r1            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 22    | secp256k1            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 23    | secp256r1            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 24    | secp384r1            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 25    | secp521r1            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 26    | brainpoolP256r1      | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |



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   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 27    | brainpoolP384r1      | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 28    | brainpoolP512r1      | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 29    | x25519               | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 30    | x448                 | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 31    | brainpoolP256r1tls13 | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 32    | brainpoolP384r1tls13 | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 33    | brainpoolP512r1tls13 | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 34    | GC256A               | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |



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   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 35    | GC256B               | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 36    | GC256C               | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 37    | GC256D               | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 38    | GC512A               | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 39    | GC512B               | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 40    | GC512C               | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 41    | curveSM2             | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 256   | ffdhe2048            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |



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   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 257   | ffdhe3072            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 258   | ffdhe4096            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 259   | ffdhe6144            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+
   | 260   | ffdhe8192            | D           | Vulnerable to        |
   |       |                      |             | store-now/decrypt-   |
   |       |                      |             | later quantum        |
   |       |                      |             | attack, see          |
   |       |                      |             | TBA:this-document    |
   +-------+----------------------+-------------+----------------------+

                                  Table 2

5.  References

5.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

5.2.  Informative References

   [RFC9847]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 9847, DOI 10.17487/RFC9847, December 2025,
              <https://www.rfc-editor.org/rfc/rfc9847>.




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Author's Address

   Bas Westerbaan
   Cloudflare
   Email: bas@cloudflare.com














































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