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 /* 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, tomstdenis@iahu.ca, http://math.libtomcrypt.org */ #include /* reduces x mod m, assumes 0 < x < m**2, mu is * precomputed via mp_reduce_setup. * From HAC pp.604 Algorithm 14.42 */ int mp_reduce (mp_int * x, mp_int * m, mp_int * mu) { mp_int q; int res, um = m->used; /* q = x */ if ((res = mp_init_copy (&q, x)) != MP_OKAY) { return res; } /* q1 = x / b**(k-1) */ mp_rshd (&q, um - 1); /* according to HAC this optimization is ok */ if (((unsigned long) um) > (((mp_digit)1) << (DIGIT_BIT - 1))) { if ((res = mp_mul (&q, mu, &q)) != MP_OKAY) { goto CLEANUP; } } else { if ((res = s_mp_mul_high_digs (&q, mu, &q, um - 1)) != MP_OKAY) { goto CLEANUP; } } /* q3 = q2 / b**(k+1) */ mp_rshd (&q, um + 1); /* x = x mod b**(k+1), quick (no division) */ if ((res = mp_mod_2d (x, DIGIT_BIT * (um + 1), x)) != MP_OKAY) { goto CLEANUP; } /* q = q * m mod b**(k+1), quick (no division) */ if ((res = s_mp_mul_digs (&q, m, &q, um + 1)) != MP_OKAY) { goto CLEANUP; } /* x = x - q */ if ((res = mp_sub (x, &q, x)) != MP_OKAY) { goto CLEANUP; } /* If x < 0, add b**(k+1) to it */ if (mp_cmp_d (x, 0) == MP_LT) { mp_set (&q, 1); 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; } } CLEANUP: mp_clear (&q); return res; }
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