Generate RSA keys locally, and ONLY store locally.

RSA Asymmetric Encryption: Different from Symmetric Encryption, has a pair of Public Key and Private Key.
RSA Public Key: Can be published to everyone. Always used to encrypt data, as well as verify signature from Private Key.
RSA Private Key: Must keep it safe. Can decrypt data which encrypt with Public Key, and sign the data.

You'll exchange public key with your friends, and then encrypt message with their public key. And the message can only decrypt by the private key.

We won't upload any of your personal information, including RSA Keys and messages.