code: def _cert_callback(callback, der_cert, reason):
      
  
      if not callback:
          return
      callback(x509.Certificate.load(der_cert), reason)
docs: Constructs an asn1crypto.x509.Certificate object and calls the export
      callback
  
      :param callback:
          The callback to call
  
      :param der_cert:
          A byte string of the DER-encoded certificate
  
      :param reason:
          None if cert is being exported, or a unicode string of the reason it
          is not being exported