// generate a fresh key each session
pk, err := ecdsa.GenerateKey(ed448.Curve448(), rand.Reader)

// ...

// derive a shared secret for signed hashing
// and a shared key for aes-256
func deriveShared(pk *ecdsa.PrivateKey, pub ecdsa.PublicKey) (secret [64]byte, key [32]byte) {
    s := func() [64]byte {
        x, y := ed448.Curve448().ScalarMult(pub.X, pub.Y, pk.D.Bytes())
        return sha3.Sum512(append(x.Bytes(), y.Bytes()...))
    }()
    k := func() [32]byte {
        return sha3.Sum256(s[:])
    }()
    return s, k
}