ext/ipp/sources/ippcp/gsmod_montfactor.c

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/* 
// 
//  Purpose:
//     Cryptography Primitive. Modular Arithmetic Engine. General Functionality
// 
//  Contents:
//        gsMontFactor()
//
*/

#include "owndefs.h"
#include "owncp.h"
#include "pcpbnumisc.h"
#include "pcpbnuarith.h"
#include "gsmodstuff.h"
#include "pcptool.h"

/*
// montfomery factor k0 = -((modulus^-1 mod B) %B)
*/
BNU_CHUNK_T gsMontFactor(BNU_CHUNK_T m0)
{
   BNU_CHUNK_T y = 1;
   BNU_CHUNK_T x = 2;
   BNU_CHUNK_T mask = 2*x-1;

   int i;
   for(i=2; i<=BNU_CHUNK_BITS; i++, x<<=1) {
      BNU_CHUNK_T rH, rL;
      MUL_AB(rH, rL, m0, y);
      if( x < (rL & mask) ) /* x < ((m0*y) mod (2*x)) */
         y+=x;
      mask += mask + 1;
   }
   return 0-y;
}