The free Signal communications service from Open Whisper Systems lets you hold end-to-end encrypted text, voice, and video chats to keep your messages private and secure.
Secure communications: Signal uses end-to-end encryption to create a secure communication channel between Signal users. For encryption, the app uses Open Whisper Systems' own Signal Protocol, which is also used by WhatsApp, Facebook Messenger, and Google Allo to encrypt messages. The app is endorsed by the PrivacyTools.io site.
Tied to your phone number: The app uses your phone number to authorize your account, letting you use Signal without having to create a username or password. Through the app, connect with others through their numbers as well.
Chat or make voice or video calls: With the app, you can chat with other Android users as well as those using the Signal app on iOS, Windows, and Mac. You can also chat over Wi-Fi and your carrier's network.
One-to-one or group messaging: You can chat securely with an individual Signal user or with a group by first sorting those you want to communicate with into a group (by tapping the menu in the top-right corner and selecting New group) and then starting up starting up a chat.
Can also send unsecured SMS/MMS messages: If you long-press the blue Send icon, it turns gray, letting you send an insecure SMS message to a contact.
Your account is tied to one device, and the Android version doesn't work on tablets: Your account connects just to one device through your phone number, so you can't use Signal on multiple devices. For Android, the app does work on tablets, although the company says it's working on adding support for the larger devices.
A subset of your contacts will be available to chat: Unless you have a very privacy-focused circle of friends, you'll find using Signal gives you a limited group to chat with. Tap the blue Pencil icon to see which of your contacts are available to message with.
Group messages require creating a group first: You can't add friends to a group chat on the fly. You need to build a group list to have group chat.
The free Signal app for Android lets you keep your chats private and secure, using end-to-end encryption. The only downside is that you need the person you're communicating with to be on Signal, too, to ensure you have end-to-end encryption.
From Open Whisper Systems:
Privacy is possible, Signal makes it easy.
Using Signal, you can communicate instantly while avoiding SMS fees, create groups so that you can chat in real time with all your friends at once, and share media or attachments all with complete privacy. The server never has access to any of your communication and never stores any of your data.