@LiquidBrain: Read USBNetwork's README.
In its default state, it'll *prompt* for a password, but will accept anything.
That said, that stops being the case once you enable SSH over WiFi.
You certainly don't need to build anything anywhere to setup shared key authentication (again, the peculiarities of the Kindle setup vs. standard OpenSSH locations are detailed in the doc), so I'm a bit confused as to what exactly you're trying to achieve
.