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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_j_to_a_itch (const struct ecc_curve *ecc)
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/* Needs 2*ecc->size + scratch for ecc_modq_inv */
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return ECC_J_TO_A_ITCH (ecc->size);
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ecc_j_to_a (const struct ecc_curve *ecc,
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mp_limb_t *r, const mp_limb_t *p,
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#define up (scratch + ecc->size)
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#define iz2p (scratch + ecc->size)
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#define iz3p (scratch + 2*ecc->size)
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/* Set v = (r_z / B^2)^-1,
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r_x = p_x v^2 / B^3 = ((v/B * v)/B * p_x)/B
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r_y = p_y v^3 / B^4 = (((v/B * v)/B * v)/B * p_x)/B
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Skip the first redc, if we want to stay in Montgomery
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mpn_copyi (up, p + 2*ecc->size, ecc->size);
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mpn_zero (up + ecc->size, ecc->size);
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mpn_zero (up + ecc->size, ecc->size);
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ecc_modp_inv (ecc, izp, up, up + ecc->size);
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/* Divide this common factor by B */
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mpn_copyi (iz3p, izp, ecc->size);
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mpn_zero (iz3p + ecc->size, ecc->size);
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ecc->redc (ecc, iz3p);
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ecc_modp_mul (ecc, iz2p, izp, iz3p);
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ecc_modp_sqr (ecc, iz2p, izp);
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/* Set s = p_z^{-1}, r_x = p_x s^2, r_y = p_y s^3 */
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mpn_copyi (up, p+2*ecc->size, ecc->size); /* p_z */
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ecc_modp_inv (ecc, izp, up, up + ecc->size);
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ecc_modp_sqr (ecc, iz2p, izp);
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ecc_modp_mul (ecc, iz3p, iz2p, p);
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/* ecc_modp (and ecc_modp_mul) may return a value up to 2p - 1, so
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do a conditional subtraction. */
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cy = mpn_sub_n (r, iz3p, ecc->p, ecc->size);
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cnd_copy (cy, r, iz3p, ecc->size);
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/* Skip y coordinate */
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ecc_modp_mul (ecc, iz3p, iz2p, izp);
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ecc_modp_mul (ecc, tp, iz3p, p + ecc->size);
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/* And a similar subtraction. */
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cy = mpn_sub_n (r + ecc->size, tp, ecc->p, ecc->size);
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cnd_copy (cy, r + ecc->size, tp, ecc->size);
added
removed
ecc-j-to-a.c
