Code change:
It seems like we have a few separable issues here: Whether we require a single signature scheme for the whole group Whether signature schemes are included in the ciphersuite Whether we have an MTI ciphersuite / signature algorithm What new ciphersuites should be defined, and what algorithms should go in them Let's discuss this at the interim and maybe break this PR into a few bits
Note that specification required registries imply that there is a designated expert pool. I will add some text to create one. The text is very similar to the TLS DE pool text from with some tweaks that, hopefully, address the misunderstandings uncovered during non-standards track registry requests. Note the process is: (assuming that the IESG eventually approves this draft) When the IESG approves this draft, the responsible AD assigns some DEs (like 2-3 suggested by the WG chairs). These DEs are in charge of all registrations regardless of whether the draft comes from the WG or elsewhere as all requests go to the MLS DEs mailing list.
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This seems to accurately reflect the consensus we had at the last interim, and there's nothing I can't live with otherwise. I will probably file a follow-up to convert the Derive-Key-Pair operations to use HKDF directly, as discussed, avoiding the need for intermediate values or truncation of SHA-512.

Related discussion:
text: curves, they SHOULD perform the additional checks specified in Section 7 of <del> 6.1.2. This ciphersuite uses the following primitives: </del> <ins> 6.1.1.2. </ins> <del> Hash function: SHA-256 AEAD: AES-128-GCM When HPKE is used with this ciphersuite, it uses the following algorithms: KEM: 0x0001 = DHKEM(P-256) </del> <ins> Given an octet string X, the private key produced by the Derive-Key- Pair operation is SHA-512(X) truncated to 448 bits (Recall that any 56-octet string is a valid X448 private key).  The corresponding public key is X448(SHA-512(X), 5). </ins> <del> KDF: 0x0001 = HKDF-SHA256 </del> <ins> Implementations SHOULD use the approach specified in RFC7748 to calculate the Diffie-Hellman shared secret.  Implementations MUST check whether the computed Diffie-Hellman shared secret is the all- zero value and abort if so, as described in Section 6 of RFC7748.  If implementers use an alternative implementation of these elliptic curves, they SHOULD perform the additional checks specified in Section 7 of </ins> <del> AEAD: 0x0001 = AES-GCM-128 </del> <ins> 6.1.1.3. </ins> Given an octet string X, the private key produced by the Derive-Key- Pair operation is SHA-256(X), interpreted as a big-endian integer.