Signal

Signal is an instant messaging app developed by Open Whisper Systems. This nonprofit software company develops open-source applications and has been supported at various times by Edward Snowden. The app aims to guarantee safe and completely private communication between its users.

As soon as you start Signal for the first time, you'll notice its security features. There's no independent login, no username or password – the app uses your phone number and your address book, so in theory, you can't pretend to be someone you're not.

The app uses end-to-end encryption, protecting every message you send to your contacts. Whether you're talking directly to an individual or a group, all your messages are always kept absolutely private. Not even Signal's servers have access to the information of your groups.

As you'd expect, Signal lets you send all kinds of files, including pictures, videos, and audio, always protecting them with the same protocol it uses for text messages. Everything is kept absolutely safe.

Signal is a secure and private instant messaging client that provides an additional layer of safety that other similar apps just don't have. WhatsApp, LINE and even Telegram might have more users, but they are not as safe as this app.