Cryptography Messaging guide and how it works

First time operations
When you first open the 'cryptography' application, it will soon ask you to create messaging key pair. If you select "I'm not going to use messaging." application will set few variables so it wont ask you this again. It does not mean that it is disabled completely. If you select "Create key" a dialog will pop up. You can access this dialog any time at messaging --> tools --> Create keys section. With this you can always create new keys. Note that old messages will not be able to do decryption correctly because new key's override old ones. <-- this is good thing!
For sending message, you need:
  • Your own key pair, which is generated while connected to internet. Otherwise your contact won't be able to encrypt messages directed to you. Encryption happens with shared public key and decryption happens with users own private key. So public key will always be send to my server with your UUID where your opponent can get you public key via UUID to encrypt message. You get message back and decrypt it with your private key. Simple, isn't it?
  • Receivers UUID, which can be sent to other via Facebook, Whatsapp or any other method you prefer. Copy it at clipboard when needed. Your UUID is shown at Tools section. UUID will change if app is removed and re installed or application settings are removed.

How to actually send message:
  • Input receivers UUID as shown in the picture and press "Get key", if get key is successful, then you have your opponent's public key downloaded from my server and it is used to encrypt your message. You can't open your message yourself anymore after that. Only receivers private key can do that.
  • Press "Send", after that receiver can get your message by pressing "Get messages". Sending message back happens by same way but with different UUID. Try it out.
Received message looks something like this.

Message is not automatically shown as plain text. Press message to open it on new dialog window and it will be decrypted with your private key at that point.

On the second picture you can see my test message, which is sent from other phone. Marking it as read will change database bit so it's no more accessible after few seconds. All messages marked as read will be deleted COMPLETELY.
In this situation I have created new key pair while my opponents has just send me new message. I did not have my internet connection on so new public key was not sent at my server. Opponent got my old key and that why, there was wrong key used to encrypt message directed to me causing it to be unreadable mess.

If my server don't have your latest public key. You will not be able to see any messages directed to you.
At the database side, this is what I can see. Keys are stored as follows:
  • Users public key is always bind to the user UUID.
Messages are stored as follows:
  • Senders UUID, who has sent the message.
  • Receivers UUID, who's the message is directed to.
  • Actual message encrypted with receivers public key at senders phone.
  • Received bit, 0 = not received, 1 = received --> will be deleted.