code: def to_curve25519_public_key(self):
          
          raw_pk = nacl.bindings.crypto_sign_ed25519_pk_to_curve25519(self._key)
          return _Curve25519_PublicKey(raw_pk)
docs: Converts a :class:`~nacl.signing.VerifyKey` to a
          :class:`~nacl.public.PublicKey`
  
          :rtype: :class:`~nacl.public.PublicKey`