### view libtommath/bn_mp_montgomery_calc_normalization.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_CALC_NORMALIZATION_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
*/

/*
* shifts with subtractions when the result is greater than b.
*
* The method is slightly modified to shift B unconditionally upto just under
* the leading bit of b.  This saves alot of multiple precision shifting.
*/
int mp_montgomery_calc_normalization (mp_int * a, mp_int * b)
{
int     x, bits, res;

/* how many bits of last digit does b use */
bits = mp_count_bits (b) % DIGIT_BIT;

if (b->used > 1) {
if ((res = mp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1)) != MP_OKAY) {
return res;
}
} else {
mp_set(a, 1);
bits = 1;
}

/* now compute C = A * B mod b */
for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
if ((res = mp_mul_2 (a, a)) != MP_OKAY) {
return res;
}
if (mp_cmp_mag (a, b) != MP_LT) {
if ((res = s_mp_sub (a, b, a)) != MP_OKAY) {
return res;
}
}
}

return MP_OKAY;
}
#endif

/* \$Source: /cvs/libtom/libtommath/bn_mp_montgomery_calc_normalization.c,v \$ */
/* \$Revision: 1.3 \$ */
/* \$Date: 2006/03/31 14:18:44 \$ */
```