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/* eddsa-compress.c
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Copyright (C) 2014 Niels Möller
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This file is part of GNU Nettle.
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GNU Nettle is free software: you can redistribute it and/or
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modify it under the terms of either:
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* the GNU Lesser General Public License as published by the Free
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Software Foundation; either version 3 of the License, or (at your
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option) any later version.
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or
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* the GNU General Public License as published by the Free
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Software Foundation; either version 2 of the License, or (at your
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option) any later version.
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or both in parallel, as here.
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GNU Nettle is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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General Public License for more details.
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You should have received copies of the GNU General Public License and
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the GNU Lesser General Public License along with this program. If
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not, see http://www.gnu.org/licenses/.
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*/
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#if HAVE_CONFIG_H
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# include "config.h"
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#endif
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#include "eddsa.h"
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#include "eddsa-internal.h"
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#include "ecc-internal.h"
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#include "gmp-glue.h"
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mp_size_t
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_eddsa_compress_itch (const struct ecc_curve *ecc)
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{
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return 2*ecc->p.size + ecc->h_to_a_itch;
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}
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void
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_eddsa_compress (const struct ecc_curve *ecc, uint8_t *r, mp_limb_t *p,
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mp_limb_t *scratch)
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{
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#define xp scratch
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#define yp (scratch + ecc->p.size)
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#define scratch_out (scratch + 2*ecc->p.size)
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size_t nbytes = 1 + ecc->p.bit_size / 8;
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ecc->h_to_a (ecc, 0, xp, p, scratch_out);
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/* Encoding is the y coordinate and an appended "sign" bit, which is
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the low bit of x. The sign bit is stored as the most significant
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bit of the last byte. */
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mpn_get_base256_le (r, nbytes, yp, ecc->p.size);
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r[nbytes - 1] += (xp[0] & 1) << 7;
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}
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