Telegram has been in the news in recent times as having strong security that's even got the Russians worried. Telegram Messenger is the official client for all Windows phones and it's still getting decent updates, even though WP8.1 itself is now not officially supported. Which is quite impressive, perhaps speaking to the fact that an awful lot of older Windows phones are still in use around the world and that many of them don't like governments (or even Google or Apple) tracking what they want to say...(!)