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 problimatic 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. Problimatic because who wants to put their gpg key on their
    phone? Also, require the users be in the WOT and be gpg literate.
