[Cryptech Tech] arm

[Peter Gutmann]

Fredrik Thulin <fredrik at thulin.net> writes:

>The conversation in Stockholm as I interpreted it sort of pointed towards
>possibly moving the external interface (USB / Ethernet / other) to a separate
>processor outside the tamper boundary but inside a fictional HSM "box" in the
>future.

The problem is that no matter what you do you're at some point going to be
dealing with a device that's actually a general-purpose CPU emulating a dumb
wire.  So one possibility is to use something like a USB-to-SPI bridge
(there's lots of these, FTDI, Microchip, Cypress, Silicon Labs, etc) so that
an attacker can target the USB device but that'll only get them to the SPI bus
rather than straight onto the host CPU.  In effect you're adding a USB
firewall between the main CPU and an attacker... OK, not really a firewall
since an attacker who fully controls the bridge can then try and attack the
host CPU over the SPI bus, but at least you're getting some level of isolation
from standard USB attacks.

Peter.