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/* ecc-generic-redc.c */
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/* nettle, low-level cryptographics library
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* Copyright (C) 2013 Niels Möller
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* The nettle library is free software; you can redistribute it and/or modify
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* it under the terms of the GNU Lesser General Public License as published by
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* the Free Software Foundation; either version 2.1 of the License, or (at your
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* option) any later version.
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* The nettle library is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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* or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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* License for more details.
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* You should have received a copy of the GNU Lesser General Public License
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* along with the nettle library; see the file COPYING.LIB. If not, write to
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* the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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/* Development of Nettle's ECC support was funded by the .SE Internet Fund. */
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#include "ecc-internal.h"
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ecc_generic_redc (const struct ecc_curve *ecc, mp_limb_t *rp)
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unsigned shift = ecc->size * GMP_NUMB_BITS - ecc->bit_size;
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mp_size_t k = ecc->redc_size;
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/* Use that 1 = p + 1, and that at least one low limb of p + 1 is zero. */
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for (i = 0; i < ecc->size; i++)
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rp[i] = mpn_addmul_1 (rp + i + k,
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ecc->redc_ppm1, ecc->size - k, rp[i]);
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hi = mpn_add_n (rp, rp, rp + ecc->size, ecc->size);
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hi = (hi << shift) | (rp[ecc->size - 1] >> (GMP_NUMB_BITS - shift));
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rp[ecc->size - 1] = (rp[ecc->size - 1]
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& (((mp_limb_t) 1 << (GMP_NUMB_BITS - shift)) - 1))
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+ mpn_addmul_1 (rp, ecc->Bmodp_shifted, ecc->size-1, hi);
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cy = cnd_sub_n (hi, rp, ecc->p, ecc->size);
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/* Use that 1 = - (p - 1), and that at least one low limb of p -
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for (i = 0; i < ecc->size; i++)
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rp[i] = mpn_submul_1 (rp + i + k,
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ecc->redc_ppm1, ecc->size - k, rp[i]);
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hi = mpn_sub_n (rp, rp + ecc->size, rp, ecc->size);
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cy = cnd_add_n (hi, rp, ecc->p, ecc->size);
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/* Result is always < 2p, provided that
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2^shift * Bmodp_shifted <= p */
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hi = (rp[ecc->size - 1] >> (GMP_NUMB_BITS - shift));
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rp[ecc->size - 1] = (rp[ecc->size - 1]
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& (((mp_limb_t) 1 << (GMP_NUMB_BITS - shift)) - 1))
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+ mpn_addmul_1 (rp, ecc->Bmodp_shifted, ecc->size-1, hi);