docs: def load_verifying_key(verifying_key, crypto_backend=default_backend()):
      """ Optional: crypto backend object from the "cryptography" python library
      """
      if not isinstance(crypto_backend, (Backend, MultiBackend)):
          raise ValueError('backend must be a valid Backend object')
  
      if isinstance(verifying_key, EllipticCurvePublicKey):
          return verifying_key
      elif isinstance(verifying_key, (str, unicode)):
          try:
              return load_der_public_key(
                  verifying_key, backend=crypto_backend)
          except:
              try:
                  return load_pem_public_key(
                      verifying_key, backend=crypto_backend)
              except Exception:
                  raise ValueError('Invalid verifying key format')
      else:
          raise ValueError('Invalid verification key type')