Tamperproof devices and backdoors

Eric Murray ericm at lne.com
Fri May 25 18:18:04 EDT 2001

On Fri, May 25, 2001 at 09:48:19AM -0700, David Honig wrote:
> And the test structures (JTAG? JEDEC? lets you read out

(for software folks, JTAG is a 2-wire debgging interface
that's usually hooked up to all the registers.  It lets you
shift out the register contents via a seperate path
and often lets you shift in new values... very
useful for debugging a new chip that only half works).

> otherwise hidden internal state via a special test mode) 
> are almost always there ---even in crypto chips.
> A real Achilles heel one imagines.

Yes.  Hardware designers are very resistant to removing it though. :-)

I've been able to talk hardware designers into fusing the JTAG
lines with fusable links, or simply not bonding out the wires
in production.  But both of those solutions suck, since they're
relatively large features and would be subject to micromanipulation.

[Not to mention that JTAG interfaces are highly useful for people
trying to debug software problems. My company uses JTAG based
debugging equipment for this purpose... --Perry]


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