GcrSecretExchange is an API which models the method we use for transferring passwords across dbus and other exposed places. We've been doing this in gnome-keyring for some time now and this just exposes it for use elsewhere.

I want to use this to implement the Gnome Keyring DBus Prompting API. See bug #656954.

We use DH and AES to keep passwords safe going through exposed places. This is not safe against active attacks like MITM, but does solve problems with dbus calls, logging, use of pageable memory, and calls through VM type languages with unpredictable memory behavior.

The method uses a single request/response: 

 1. Caller sets up half of a DH key agreement context, and sends public parameter
    along with request to the Callee.
 2. Callee sets up its half of DH key agreement. Callee completes DH using public
    parameter from Caller. Callee encrypts password response, and sends the
    encrypted response, and its public parameter back to Caller.
 3. Caller complets DH using public parameter from Callee, and decrypts the
    password.

This is similar to what's used in the Secret Service API. However in that API we do things in a more persistent way, so that we're not constantly doing DH for each call.