tommath/bn_mp_reduce.c

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#include <tommath_private.h>
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#ifdef BN_MP_REDUCE_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
*
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* The library was designed directly after the MPI library by
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* 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
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*/
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/* reduces x mod m, assumes 0 < x < m**2, mu is
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* precomputed via mp_reduce_setup.
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* From HAC pp.604 Algorithm 14.42
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*/
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int mp_reduce(mp_int *x, const mp_int *m, const mp_int *mu)
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{
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mp_int q;
int res, um = m->used;
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/* q = x */
if ((res = mp_init_copy(&q, x)) != MP_OKAY) {
return res;
}
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/* q1 = x / b**(k-1) */
mp_rshd(&q, um - 1);
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/* according to HAC this optimization is ok */
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if ((mp_digit)um > (1uL << (DIGIT_BIT - 1))) {
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if ((res = mp_mul(&q, mu, &q)) != MP_OKAY) {
goto CLEANUP;
}
} else {
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#ifdef BN_S_MP_MUL_HIGH_DIGS_C
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if ((res = s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
goto CLEANUP;
}
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#elif defined(BN_FAST_S_MP_MUL_HIGH_DIGS_C)
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if ((res = fast_s_mp_mul_high_digs(&q, mu, &q, um)) != MP_OKAY) {
goto CLEANUP;
}
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#else
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{
res = MP_VAL;
goto CLEANUP;
}
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#endif
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}
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/* q3 = q2 / b**(k+1) */
mp_rshd(&q, um + 1);
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/* x = x mod b**(k+1), quick (no division) */
if ((res = mp_mod_2d(x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) {
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goto CLEANUP;
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}
/* q = q * m mod b**(k+1), quick (no division) */
if ((res = s_mp_mul_digs(&q, m, &q, um + 1)) != MP_OKAY) {
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goto CLEANUP;
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}
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/* x = x - q */
if ((res = mp_sub(x, &q, x)) != MP_OKAY) {
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goto CLEANUP;
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}
/* If x < 0, add b**(k+1) to it */
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if (mp_cmp_d(x, 0uL) == MP_LT) {
mp_set(&q, 1uL);
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if ((res = mp_lshd(&q, um + 1)) != MP_OKAY)
goto CLEANUP;
if ((res = mp_add(x, &q, x)) != MP_OKAY)
goto CLEANUP;
}
/* Back off if it's too big */
while (mp_cmp(x, m) != MP_LT) {
if ((res = s_mp_sub(x, m, x)) != MP_OKAY) {
goto CLEANUP;
}
}
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CLEANUP:
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mp_clear(&q);
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return res;
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}
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#endif
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/* ref: $Format:%D$ */
/* git commit: $Format:%H$ */
/* commit time: $Format:%ai$ */