Hi all,

Moving one small implementation point out of the FATT thread, since it
seems orthogonal to the main discussion there.

I think a useful near-term artifact would be a short, non-blocking set of
negative cases for hybrid key exchange implementations. The goal would not
be to add a new proof obligation or delay any draft. It would just give
implementers a few concrete cases that exercise the “both components are
bound to the negotiated hybrid group and transcript” property.

A first cut might include:

   - the negotiated group is hybrid, but one component is missing,
   malformed, or associated with a different group;
   - ECDHE and KEM values are individually valid, but assembled from
   different handshakes or transcript contexts;
   - a peer attempts to continue as standalone ECDHE or standalone ML-KEM
   after a hybrid group was negotiated;
   - traffic secrets are derived before both components have been validated
   and accepted under the negotiated hybrid group;
   - logs, exports, or API surfaces make a hybrid exchange appear as if
   only one component was used.

If this seems useful, I can collect feedback for about a week and then turn
the cleaned-up version into a GitHub issue or a small PR against the
implementation-guidance text.

Best,
Songbo Bu
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