SPRUJB6B November 2024 – May 2025 AM2612
Arguments: address (length=1).
Treat one word of memory pointed to by the address as a divisor. Then, compute mand/divisor and write the result back to mand.
Invalidate the scratchpad.
The division must be exact, and the divisor must be odd.
This command will cause an error if:
This command is so restrictive because it is present only to support a particular DPA-resistance technique for RSA signing. The signing operation uses an n-word odd modulus p and a 1-word odd blinding value b. It performs arithmetic mod b*p. At the end of the computation, it calculates the (n+1)-word value b*x, where the n-word value x is the correct answer. The INT_DIV_EXACT_SMALL command is used to recover x from b*x.
The INT_DIV_SMALL_EXACT command erases the pbar register, so if a general modulus is set, the next Montgomery multiplication command will take slightly longer.