Signal

Signal is widely regarded as the most accessible, secure messaging service in existence today. Available for Android, iOS, macOS, and Windows, the free app -- developed by Open Whisper Systems -- implements end-to-end encryption and no data is stored by the company's servers, which means that none of your conversations can be seized or read by law enforcement or hackers.

In order to use the service, you will need to tie a phone number to the app. You can also use Signal to replace traditional SMS messaging, but the same encryption and protections do not apply unless both recipients are using the app.