### view libtommath/bn_mp_mul_2d.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_MUL_2D_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
*/

/* shift left by a certain bit count */
int mp_mul_2d (mp_int * a, int b, mp_int * c)
{
mp_digit d;
int      res;

/* copy */
if (a != c) {
if ((res = mp_copy (a, c)) != MP_OKAY) {
return res;
}
}

if (c->alloc < (int)(c->used + b/DIGIT_BIT + 1)) {
if ((res = mp_grow (c, c->used + b / DIGIT_BIT + 1)) != MP_OKAY) {
return res;
}
}

/* shift by as many digits in the bit count */
if (b >= (int)DIGIT_BIT) {
if ((res = mp_lshd (c, b / DIGIT_BIT)) != MP_OKAY) {
return res;
}
}

/* shift any bit count < DIGIT_BIT */
d = (mp_digit) (b % DIGIT_BIT);
if (d != 0) {
register mp_digit *tmpc, shift, mask, r, rr;
register int x;

/* bitmask for carries */
mask = (((mp_digit)1) << d) - 1;

/* shift for msbs */
shift = DIGIT_BIT - d;

/* alias */
tmpc = c->dp;

/* carry */
r    = 0;
for (x = 0; x < c->used; x++) {
/* get the higher bits of the current word */
rr = (*tmpc >> shift) & mask;

/* shift the current word and OR in the carry */
*tmpc = ((*tmpc << d) | r) & MP_MASK;
++tmpc;

/* set the carry to the carry bits of the current word */
r = rr;
}

/* set final carry */
if (r != 0) {
c->dp[(c->used)++] = r;
}
}
mp_clamp (c);
return MP_OKAY;
}
#endif

/* \$Source: /cvs/libtom/libtommath/bn_mp_mul_2d.c,v \$ */
/* \$Revision: 1.3 \$ */
/* \$Date: 2006/03/31 14:18:44 \$ */
```