“In general, the idea behind the app is to make privacy and communication as simple as possible,” said Moxie Marlinspike, the founder of Open Whisper Systems, the organization that developed Signal. WhatsApp, the popular chat tool, uses Signal’s software to encrypt its messaging. And in Facebook Messenger and Google’s texting app Allo, you can turn on an option that encrypts your messages. Here’s how to do that on Facebook. Here’s how to do it on Allo.