Re: cryptophone ideas

>>They are capable of doing 2 data moves, a 16x16 multiply, a 40 bit
>>accumulate and a prefech of the next instruction all in 100ns.
>
>This is where a DSP really shines, since it's the fundamental
>operation in digital filtering; indeed it wouldn't be a DSP if it
>couldn't do a multiply/accumulate in a single clock cycle.
>
>But I wouldn't be too surprised if general purpose CPUs eventually get
>the same capability. And once they are, the distinction between a
>"DSP" and a "general purpose" CPU will pretty much vanish. DSPs are
>notoriously harder to program than general purpose CPUs, and being
>lower volume items they won't be able to compete in price or clock
>speed with general purpose CPUs made in the millions.

>
>Phil

How hard is it to reprogram the DSP that comes with a cellular
phone right now? I've never opened one up. Can you just unsolder
a rom, read it, insert your own code for DH key exchange, add
some encryption, burn a new ROM and have a secure phone?

>How hard is it to reprogram the DSP that comes with a cellular
>phone right now? I've never opened one up. Can you just unsolder
>a rom, read it, insert your own code for DH key exchange, add
>some encryption, burn a new ROM and have a secure phone? 

Actually, there'd be little reason to modify the code in the DSP, at
least the one in our CDMA phones. We use a DSP-16A to execute our
variable-rate variant of the CELP vocoder, but we also have a 80C186
general purpose CPU that does all of the other housekeeping functions
in the phone. This is where you'd probably want to add crypto code.

As an aside, a lot of people seem to think that DSP chips are
miniature Crays that can run any given program faster than any
"ordinary" computer. They're not. DSPs are special purpose CPUs
heavily optimized for the multiply-accumulate operation that is
fundamental to digital filtering.  They have no special gift for
general purpose computing.

This includes encryption.  Encryption is better done on a general
purpose CPU when you consider the much larger economies of scale for
general purpose CPUs as compared with DSPs, not to mention ease of
programming, higher clock speeds, wider availability, etc.

Phil

>How hard is it to reprogram the DSP that comes with a cellular
>phone right now? I've never opened one up. Can you just unsolder
>a rom, read it, insert your own code for DH key exchange, add
>some encryption, burn a new ROM and have a secure phone? 

Also, it is not sufficient to incorporate encryption merely into the
cellular phone itself. You need the cooperation of the base station,
at least if you want to interoperate with an ordinary telephone on the
land side of your connection.

Now it would be possible to provide your own encryption on an
end-to-end basis using a data (as opposed to voice) bearer service
from the carrier, but this would require the person you call to have
compatible equipment (vocoder, modem, encryption routines, keys, etc).

Phil

Peter Wayner says:
> How hard is it to reprogram the DSP that comes with a cellular
> phone right now? I've never opened one up. Can you just unsolder
> a rom, read it, insert your own code for DH key exchange, add
> some encryption, burn a new ROM and have a secure phone? 

You would also have to open up the base station for the cell, unsolder
its roms, read them, insert your own code for DH key exchange, add
some encryption, and put it back, and I suspect that the cellular
vendor would get mad at you.

Perry