Most encrypted messaging services require both parties to use the same Internet-based app. The Android-only TextSecure, however, works with standard SMS and MMS messages, so it's an easy way to add some protection to your text messaging without having to change your friends' behavior.
1. Download TextSecure from the Google Play store.
2. Choose an encryption passphrase (optional). When you first open TextSecure, you'll be prompted to set a password for encrypting the TextSecure data stored on your phone. You can skip this step, or enter a password and tap "Create." For additional information, see our article on how to encrypt an Android device.
3. Choose whether to encrypt archived text messages. If you chose to encrypt local TextSecure data, you'll also be asked if you want to import your existing text messages into TextSecure's encrypted database. Choose yes, and your messages will be stored securely on your phone.
4. Verify your device phone number. TextSecure will send you a confirmation text message. After this point, you won't be able to take screenshots within TextSecure, giving your text messages another layer of protection.
4. Tap 'yes' to make TextSecure your default messaging app. If you had another messaging app set to default, such as Google Hangouts, TextSecure will now supercede it as the default.
5. Send and receive SMS and MMS messages just as before. TextSecure stores SMS and MMS messages in an encrypted form on your phone. However, if you access the contents of a MMS message from another app, such as a photo viewer, it becomes unencrypted. If you're texting with someone who also has TextSecure, your entire conversation will be encrypted in transit.
6. Set a passphrase timeout by tapping the app's menu button, then tapping Settings. By default, every time you close and reopen TextSecure, you'll need to re-enter your passphrase. In the app's settings, however, you can add a "timeout" function so that TextSecure will also require you to re-enter the passphrase after a specific interval of time.
7. Scroll down to Passphrase settings and check "Timeout Passphrase."
8. Tap "Timeout Interval" and choose how often you want TextSecure to require you to re-enter your passphrase.