[Cryptech Tech] USB interface

[Rob Austein]

On Wed, 27 May 2020 22:22:20 -0400, Peter Stuge wrote:
...
> On rev03, PCs can only try to use a heuristic with the RPC protocol to
> guess if the board is running/available or not.
> 
> This proposed USB interface is self-powered, by VCCO_3V3, so when this
> interface is connected to a PC the USB device comes and goes on the PC
> as the 18V supply is connected and disconnected from the rev04 board.
> 
> Is that change desirable?

The only reason I can think of why the host side might care is if the
OS drivers are going to renumber the USB device every time it appears.
Otherwise, I don't see why the host would expect the HSM to do
anything in particular when not powered.

What I would like very much, though, if it's not a lot of trouble,
would be some way for the host side to identify these USB devices
easily and tell them apart from each other.

What we do right now is seriously disgusting: we look for USB devices
matching "VID:PID=0403:6014", then throw a particular magic byte
string at any such device; that byte string is guaranteed to parse as
(relatively) harmless garbage both in the RPC protocol and on the
management CLI, and we can guess which one we found by examining the
resulting error response.  Feh.  Please let's do better.