cryptophone ideas

[Phil Karn]

>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