code: def sign(self, data: bytes) -> bytes:
      signature = self._private_key.sign(
          data, _RSA_PADDING, _RSA_HASH_ALGORITHM
      )
      return base64.b64encode(signature)
  
imports: from __future__ import annotations
  import base64
  import threading
  from abc import ABC, abstractmethod
  from cryptography import exceptions
  from cryptography.hazmat.primitives import hashes, serialization
  from cryptography.hazmat.primitives.asymmetric import padding, rsa
label_desc: Use image node_id 3 for calling the RSAPrivateKey obj's underlying member method code with example usage: obj.sign(data) and returns: base64