I notice that knot 3.1 does not support EdDSA (22519 and 448) when using softhsm as a  PKCS #11 backend. Since this is supported by knot when using the default cryptographic provider, and also by gnutls 3.6.0 (at least for the 25519 version) for release 3.6.0 and later, my guess is that this a limitation in softhsm itself. Could anybody in this forum with the necessary savvy please confirm (or not) this?