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>And yet another technical remark. The use of external RAM could solve the
>problem of lacking memory, however it is unsafe. If this RAM were used
>for "challenge" encryption, monitoring of PIC<->RAM communication
>could lead to the private key recovery. Well, it should be done very fast,
>untill the device remains "armed", but it is not impossible.


Our terminals were approved by the banks, and the card industry, simply by
potting the processor and memory into a module, with the SRAM backup
battery outside the module.

Knowing what I know now, we should have had an algorithm to move the keys
around, or maybe Xor them periodically with an incrementing value.
Data that sits in an SRAM for a long time becomes quasi-permanent. In many
cases, if the data has been in for 6 months to a year without moving, you
can power down the ram for hours, with VCC shorted to ground, and on
powerup, retrieve the key. This cheap scrambling system adds no
cryptographic security, but when the lights go out, the keys would be GONE.

I like this end of the discussion. One of the major weaknesses I've seen in
access control is that it all ends up in something like "short these two
wires to grant access", so the obvious attack is to get to those two wires.
Tamper switches are ok, as far as they go, but obviously if you've bought
one and taken it apart, you know where the switches are, and how they work.

This is one reason I never put access pads outside a secured area. I want
you to open a door, trip a sensor, and be on a short fuse before you get to
muck around with a keypad.  I've been known to install weak key switches
(easily picked) or cheap keypads outside the protected area, that will trip
the alarm if operated.

Another fun one is a wire running along the external wall to the bell.
(where used)  The real wire rund directly back through the wall of course,
the accessable wire is the trap.


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