Cryptography Consulting LLC TLS 1.3 runs its entire key schedule on HKDF over SHA-2. This document defines an extension that replaces that schedule with one built on an extendable-output function (XOF): the negotiated KDF governs every derivation, the Finished and binder MACs, and the transcript hash, so no SHA-2 remains in the key schedule. The cipher suites, AEAD algorithms, state machine, and record layer are unchanged, and a connection without the extension uses HKDF as today. Two KDFs are defined, SHAKE256 and the reduced-round TurboSHAKE256. This document updates RFC 9258.