docs: def data(self):
          """
          Returns signed data if present in the message
          """
          # Check if signatire is detached
          if self.detached:
              return None
          bio = Membio()
          if not libcrypto.CMS_verify(self.ptr, None, None, None, bio.bio,
                                      Flags.NO_VERIFY):
              raise CMSError("extract data")
          return str(bio)