TextSecure: Everything Right This Time
Now that we've established that going from WhatsApp to Telegram is a move that makes no sense at all: it's not safe, as discussed at UnhandledExpression, its "challenge" to hack it is no more than a publicity stunt for the ignorant masses, as Moxie sarcastically states, not to mention that if your reasoning is "I don't want Facebook to have all my data", switching to Telegram just means you can now worry about whether your contact info and that of all your friends went to the Russian big competitor VK, as your address book's numbers are saved onto their server, maybe it's time to shed some light on an alternative that does check all the right boxes, is as easy to use as WhatsApp, open source, and has people working on it that in fact know about encryption.
Enter TextSecure. First, I'm really charmed by the way WhisperSystems, co-founded by Moxie Marlinspike and Stuart Anderson, is approaching all this: have a look at their blog posts on WhisperSystems and Moxie's ThoughtCrime). What's more, they've already made name with Redphone (end-to-end encrypted calling over your data connection, used e.g. in the Egyptian revolution according to Wikipedia) and the integration of TextSecure into Cyanogenmod ROMs. Now, with the release of TextSecure v2 (more info in WhisperSystems' introduction and in this great comparison of potential WhatsApp alternatives) you can send messages, end-to-end encrypted, not only via SMS/MMS (as with v1 and Cyanogenmod's implementation for now), but also via your data connection or WiFi, reducing the costs to virtually nothing and thus coming dangerously close to being the best WhatsApp killer out there, and if anything, the only properly encrypted one. An iOS version is due soon.
For the cryptographers: Perfect Forward Secrecy, Axolotl Ratchet (an OTR derivate), Client-Server communication via SSL/TLS and deniability.
For the end-users: easy texting, integrate SMS/MMS messaging, securely send pictures, audio, video or contact information, encrypt all messages on your phone with a password.
Tip: if you want to make sure TextSecure never uses SMS to send messages, untick the SMS Fallback option. This could save you some unforeseen costs and make it work exactly like WhatsApp: only those using TextSecure will get your messages. Depending on the Android version, you might need to untick Use for all SMS/MMS as well to be able to do this. As far as I understood, a message with a blue background is sent by data, while a message with a green background is sent by SMS/MSS.