[Cryptech Tech] arm

[Fredrik Thulin]

On Tuesday, January 20, 2015 10:13:06 AM Jakob Schlyter wrote:
> On 20 jan 2015, at 10:07, Fredrik Thulin <fredrik at thulin.net> wrote:
> > One way of being careful is what I believe Peter G. is advocating here and
> > adding an external chip with a USB host controller and a SPI interface, in
> > the hopes that the SPI interface in our main MCU has less bugs than the
> > USB interface in the main MCU (likely true IMHO).
> 
> When the HSM is the USB client, the SPI interface makes a lot of sense, but
> I do not think it is possible when the HSM is the USB host. As a host, the
> main MCU must be able to communicate with different types of devices (mass
> storage, HID, ...).

Well it's up to us to define what types of devices we _must_ be able to 
communicate with. At least for simple things like HID and likely virtual com 
ports, this chip used in the Facedancer would achieve what we wanted I 
believe:

  http://www.maximintegrated.com/en/products/interface/controllers-expanders/MAX3421E.html

For more complicated protocols like USB mass storage then maybe this is more 
an electrical barrier - but maybe there are other USB host controllers that 
can handle more things... or we implement the USB host to SPI ourselves:

  FPGA etc. <--> MCU <-- SPI --> MCU <-- USB -->

If the protocol spoken in the "SPI" part is simple enough, then hopefully only 
the rightmost MCU would be pwned.

/Fredrik