### view libtommath/bn_mp_montgomery_setup.c @ 457:e430a26064eeDROPBEAR_0.50

Make dropbearkey only generate 1024 bit keys
author Matt Johnston Wed, 08 Aug 2007 15:57:50 +0000 5ff8218bcee9 60fc6476e044
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```#include <tommath.h>
#ifdef BN_MP_MONTGOMERY_SETUP_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
* LibTomMath is a library that provides multiple-precision
* integer arithmetic as well as number theoretic functionality.
*
* The library was designed directly after the MPI library by
* Michael Fromberger but has been written from scratch with
* additional optimizations in place.
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, [email protected], http://math.libtomcrypt.com
*/

/* setups the montgomery reduction stuff */
int
mp_montgomery_setup (mp_int * n, mp_digit * rho)
{
mp_digit x, b;

/* fast inversion mod 2**k
*
* Based on the fact that
*
* XA = 1 (mod 2**n)  =>  (X(2-XA)) A = 1 (mod 2**2n)
*                    =>  2*X*A - X*X*A*A = 1
*                    =>  2*(1) - (1)     = 1
*/
b = n->dp;

if ((b & 1) == 0) {
return MP_VAL;
}

x = (((b + 2) & 4) << 1) + b; /* here x*a==1 mod 2**4 */
x *= 2 - b * x;               /* here x*a==1 mod 2**8 */
#if !defined(MP_8BIT)
x *= 2 - b * x;               /* here x*a==1 mod 2**16 */
#endif
#if defined(MP_64BIT) || !(defined(MP_8BIT) || defined(MP_16BIT))
x *= 2 - b * x;               /* here x*a==1 mod 2**32 */
#endif
#ifdef MP_64BIT
x *= 2 - b * x;               /* here x*a==1 mod 2**64 */
#endif

/* rho = -1/m mod b */
*rho = (unsigned long)(((mp_word)1 << ((mp_word) DIGIT_BIT)) - x) & MP_MASK;

return MP_OKAY;
}
#endif

/* \$Source: /cvs/libtom/libtommath/bn_mp_montgomery_setup.c,v \$ */
/* \$Revision: 1.4 \$ */
/* \$Date: 2006/12/04 21:34:03 \$ */
```