code: def decryptPkmsg(self, ciphertext, textMsg=True):
          
          sessionRecord = self.sessionStore.loadSession(self.recipientId, self.deviceId)
          unsignedPreKeyId = self.sessionBuilder.process(sessionRecord, ciphertext)
          plaintext = self.decryptWithSessionRecord(sessionRecord, ciphertext.getWhisperMessage())
  
          # callback.handlePlaintext(plaintext)
          self.sessionStore.storeSession(self.recipientId, self.deviceId, sessionRecord)
  
          if unsignedPreKeyId is not None:
              self.preKeyStore.removePreKey(unsignedPreKeyId)
  
          return plaintext
docs: :type ciphertext: PreKeyWhisperMessage