tommath/bn_mp_expt_d_ex.c

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#include <tommath_private.h>
#ifdef BN_MP_EXPT_D_EX_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, tstdenis82@gmail.com, http://libtom.org
*/
/* calculate c = a**b using a square-multiply algorithm */
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int mp_expt_d_ex(const mp_int *a, mp_digit b, mp_int *c, int fast)
{
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int res;
unsigned int x;
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mp_int g;
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if ((res = mp_init_copy(&g, a)) != MP_OKAY) {
return res;
}
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/* set initial result */
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mp_set(c, 1uL);
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if (fast != 0) {
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while (b > 0u) {
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/* if the bit is set multiply */
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if ((b & 1u) != 0u) {
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if ((res = mp_mul(c, &g, c)) != MP_OKAY) {
mp_clear(&g);
return res;
}
}
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/* square */
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if (b > 1u) {
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if ((res = mp_sqr(&g, &g)) != MP_OKAY) {
mp_clear(&g);
return res;
}
}
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/* shift to next bit */
b >>= 1;
}
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} else {
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for (x = 0; x < (unsigned)DIGIT_BIT; x++) {
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/* square */
if ((res = mp_sqr(c, c)) != MP_OKAY) {
mp_clear(&g);
return res;
}
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/* if the bit is set multiply */
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if ((b & (1uL << (DIGIT_BIT - 1))) != 0u) {
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if ((res = mp_mul(c, &g, c)) != MP_OKAY) {
mp_clear(&g);
return res;
}
}
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/* shift to next bit */
b <<= 1;
}
} /* if ... else */
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mp_clear(&g);
return MP_OKAY;
}
#endif
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/* ref: $Format:%D$ */
/* git commit: $Format:%H$ */
/* commit time: $Format:%ai$ */