Finally I found a messenger that I really like : Conversations, it uses the XMMP (formerly known as Jabber) protocol. It has not only encryption but it respects my privacy, I don’t need a phone number or e-mail adress to register an account at a XMPP server*. Encryption is done by OMEO, OTR (Off The Record) or OpenPGP.
Conversations is an Android app, you can download it in the PlayStore, but then you have to pay. If you install the F-droid app on your Android device you can download it legally for free
You can find more clients here
* Signal and WhatsApp have good encryption, but you need your phone number to register AND you have to give your number to people otherwise they can’t call or send you messages. An other problem of WhatsApp is the spying by Facebook.
Telegram needs a phonenumber to register, but you can use a nickname so your number stays private.
Wire don’t need a phonenumber, it uses only a nickname.