code: def decrypt_string(self, token_str, expiration_in_seconds=None):
          
  
          # Add '=' padding if needed
          if len(token_str) % 4:
              token_str += '=' * (4 - len(token_str) % 4)
  
          # Convert string to bytes
          encrypted_bytes = token_str.encode()
  
          # Verify signature, verify expiration, and decrypt using ``cryptography.fernet.Fernet()``
          concatenated_bytes = self.fernet.decrypt(encrypted_bytes, expiration_in_seconds)
          concatenated_str = concatenated_bytes.decode('utf-8')
  
          return concatenated_str
docs: Verify signature, verify timestamp, and decrypt a token using ``cryptography.fernet.Fernet()``.