Encryption

• https://xmpp.org/extensions/xep-0384.html

Assumptions

• Client might be malicious;
• Client device is assumed to be secure;
• Communication channel is assumed to be secure;
• Server might be insecure;

Encryption modes

Unencrypted

• All data is stored in a plain text;
• Absolutely fine for guilds with public invite (anyone can access guild data anyways) and guild with a large number of participants (such guild is insecure in a practical sense);

Basic

• What is encrypted: guild name, channel name, channel topic, message text, role name, nickname, image data;
• How it is encrypted:
  • DM: DH/ECDH;
  • GDM/guild: pre-shared key;
• Model/api changes:
  • Guilds/DMs/GDMs now have “security mode” property:
    • server: implement this filed for guilds and channels;
    • server and client: reject plaintext-looking msgs in secure channels;
    • client: show guild/channel security mode (and explain what is it);
  • Users have asymmetric key pairs, possibly time-limited;
    • client: store private keys secure;
    • client: allow user to generate, view and manage keys;
    • client: notify user on their or others’ key expiration;
  • Users can share their public keys manually;
    • there should be message type / embed to show pubkey to the user;
    • client: allow user to easily add and auto-verify others’ pubkeys;
  • Server stores public keys and provides em w/ user profile;
    • server: store and send pubkeys;
    • client: show something like “emoji hash” to make it easier to manually validate peers pubkeys;
  • Text and image data is transmitted and stored encrypted:
    • cdn: do not attempt to transform encrypted images;
  • Users/channels/guilds may have multiple keys associated with it:
    • client: choose decryption key depending on message edit time;
    • client: if have sufficient rights, allow guild key re-issue and automatic key transmission to all/chosen members via encrypted DM;
• Notes:
  • server now has no access to message data, but meta-data is still public;
  • almost no forward secrecy and post-compromise security;

Fancy

• Double ratchet?