8-bit modular exponentiation code?

[Bill Frantz]

At 10:08 AM -0700 5/30/03, Major Variola (ret) wrote:
>At 07:30 AM 5/24/03 +0100, Adam Back wrote:
>>Colin Plumb's crypto library bnlib supports multiple word size I
>>believe.
>>On Fri, May 23, 2003 at 11:36:58AM -0700, Major Variola (ret.) wrote:
>>> Anyone know of any open-source modexp code for 8-bit cpus?
>
>Thank you for your response, however (for the record) that code requires
>
>at least a 16bit CPU.  From bnlib.doc (an amusing read, BTW):
>
>It is written in C, and should compile on any platform with an ANSI C
>compiler and 16 and 32-bit unsigned data types
>
>"Small" is defined as less than 65536, the minimum 16-bit word size
>supported by the library.

I think your best bet for an 8 bit CPU will be an assembly language
routine.  These processors normally include the carry/borrow out of the
high order bit in a status register.  An assembly routine can use that bit
to efficiently implement multiple precision arithmetic, which can, in turn,
be used to implement modexp.

Cheers - Bill


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