code: def _validate_signing_cert(ca_certificate, certificate, crl=None):
      
      pyopenssl_cert = load_certificate(FILETYPE_PEM, certificate)
      pyopenssl_ca_cert = load_certificate(FILETYPE_PEM, ca_certificate)
  
      cert_store = X509Store()
      cert_store.add_cert(pyopenssl_ca_cert)
      if crl:
          pyopenssl_crl = load_crl(FILETYPE_PEM, crl)
          cert_store.add_crl(pyopenssl_crl)
          cert_store.set_flags(X509StoreFlags.CRL_CHECK | X509StoreFlags.CRL_CHECK_ALL)
  
      cert_store_context = X509StoreContext(cert_store, pyopenssl_cert)
      cert_store_context.verify_certificate()
docs: Validate an X509 certificate using pyOpenSSL.
  
      .. note::
          pyOpenSSL is a short-term solution to certificate validation. pyOpenSSL
          is basically in maintenance mode and there's a desire in upstream to move
          all the functionality into cryptography.
  
      Args:
          ca_certificate (str): A PEM-encoded Certificate Authority certificate to
              validate the ``certificate`` with.
          certificate (str): A PEM-encoded certificate that is in need of validation.
          crl (str): A PEM-encoded Certificate Revocation List which, if provided, will
              be taken into account when validating the certificate.
  
      Raises:
          X509StoreContextError: If the certificate failed validation. The
              exception contains the details of the error.