Built to know as little as possible.
The safest data is data that is never collected. Black Book delivers your messages without being able to read them, and your keys exist only on your phone.
How a message travels
On your phone
The message is locked before it leaves your phone, with a key only you and the recipient have.
On the way
The server sees who the message is for, but not what it says or who it is from.
With the recipient
The phone opens the message, checks that it really is from you, and stores it encrypted.
Afterwards
The message is deleted from the server as soon as it is delivered. With disappearing messages, it vanishes for both of you too.
The principles
Keys stay home
Keys are created on your phone and kept in its secure key store. They are never sent to the server and never included in backups.
Hidden sender
The server sees who a message is for, but not who it is from.
Your password never leaves your phone
Sign in is confirmed by your phone, not by your password. The password is never sent anywhere.
Encrypted storage
Conversations and photos are stored encrypted on your phone. Without the key, nothing can be read.
Photos without traces
Location, time and phone model are removed from photos before they are sent.
Protection from prying eyes
Lock the app with Face ID, fingerprint or password, hide its content in the app switcher, and block screenshots on Android.
Know who you are talking to.
Every conversation has a safety number only the two of you can compute. Compare it, or scan each other's QR codes, and you know nobody is in the middle. If someone's keys change, the app stops sending and tells you.
Nearby
When you use Nearby, your location is roughly rounded on the phone before it is sent, and it is never passed on. Others only see approximate distance. Moderators only see what is public and what has been reported, never private conversations, and never who reported.
More about NearbyWhen everything must go
Kill Switch deletes the key that locks everything on your phone. It takes a moment, and afterwards there is nothing left to recover.
How Kill Switch works