Hello @felix.herrmann !
You can check Steem Messenger !
It's been 8 months that we are working on it, security and privacy is our major concern !
Here's the latest release log :
https://steemit.com/utopian-io/@kingswisdom/steem-messenger-v0-0-5-safestorage-safesocket-last-update-before-release-and-many-more