Currently xmpp relies on the SSL connection to the server for security. The server can see git repository data pushed through it. (Also, the SSL connection is not pinned or really checked well at all.)
Add an encryption layer that does not rely on trusting the XMPP server's security. There are a few options for how to generate the key for eg, AES encryption:
- Do a simple Diffie-Hellman shared key generation when starting each XMPP push session. Would not protect the users from active MITM by the XMPP server, but would prevent passive data gathering attacks from getting useful data. Since the key is ephemeral, would provide Forward Security.
- Do D-H key generation, but at pairing, not push time. Allows for an optional confirmation step, using eg, BubbleBabble to compare the keys out of band. ("I see xebeb-dibyb-gycub-kacyb-modib-pudub-sefab-vifuc-bygoc-daguc-gohec-kuxax .. do you too?")
- Prompt both users for a passphrase when XMPP pairing, and use SPEKE (or similar methods like J-PAKE) to generate a shared key. Avoids active MITM attacks. Makes pairing harder, especially pairing between one's own devices, since the passphrase has to be entered on all devices. Also problematic when pairing more than 2 devices, especially when adding a device to the set later, since there would then be multiple different keys in use.
- Rely on the user's gpg key, and do gpg key verification during XMPP pairing. Problematic because who wants to put their gpg key on their phone? Also, require the users be in the WOT and be gpg literate.