### view libtommath/bn_mp_sub_d.c @ 763:f744321ac048ecc

ecdh works against OpenSSH
author Matt Johnston Mon, 08 Apr 2013 23:12:20 +0800 5ff8218bcee9 60fc6476e044
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```#include <tommath.h>
#ifdef BN_MP_SUB_D_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
*
* The library is free for all purposes without any express
* guarantee it works.
*
* Tom St Denis, [email protected], http://math.libtomcrypt.com
*/

/* single digit subtraction */
int
mp_sub_d (mp_int * a, mp_digit b, mp_int * c)
{
mp_digit *tmpa, *tmpc, mu;
int       res, ix, oldused;

/* grow c as required */
if (c->alloc < a->used + 1) {
if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) {
return res;
}
}

/* if a is negative just do an unsigned
*/
if (a->sign == MP_NEG) {
a->sign = MP_ZPOS;
a->sign = c->sign = MP_NEG;

/* clamp */
mp_clamp(c);

return res;
}

/* setup regs */
oldused = c->used;
tmpa    = a->dp;
tmpc    = c->dp;

/* if a <= b simply fix the single digit */
if ((a->used == 1 && a->dp[0] <= b) || a->used == 0) {
if (a->used == 1) {
*tmpc++ = b - *tmpa;
} else {
*tmpc++ = b;
}
ix      = 1;

/* negative/1digit */
c->sign = MP_NEG;
c->used = 1;
} else {
/* positive/size */
c->sign = MP_ZPOS;
c->used = a->used;

/* subtract first digit */
*tmpc    = *tmpa++ - b;
mu       = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);

/* handle rest of the digits */
for (ix = 1; ix < a->used; ix++) {
*tmpc    = *tmpa++ - mu;
mu       = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1);
}
}

/* zero excess digits */
while (ix++ < oldused) {
*tmpc++ = 0;
}
mp_clamp(c);
return MP_OKAY;
}

#endif

/* \$Source: /cvs/libtom/libtommath/bn_mp_sub_d.c,v \$ */
/* \$Revision: 1.5 \$ */
/* \$Date: 2006/03/31 14:18:44 \$ */
```