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/* ecc-ecdh.c - Elliptic Curve Diffie-Hellman key agreement
* Copyright (C) 2019 g10 Code GmbH
*
* This file is part of Libgcrypt.
*
* Libgcrypt is free software; you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as
* published by the Free Software Foundation; either version 2.1 of
* the License, or (at your option) any later version.
*
* Libgcrypt is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* SPDX-License-Identifier: LGPL-2.1+
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include "g10lib.h"
#include "mpi.h"
#include "cipher.h"
#include "context.h"
#include "ec-context.h"
#include "ecc-common.h"
#define ECC_CURVE25519_BYTES 32
#define ECC_CURVE448_BYTES 56
static gpg_err_code_t
prepare_ec (mpi_ec_t *r_ec, const char *name)
{
int flags = 0;
if (!strcmp (name, "Curve25519"))
flags = PUBKEY_FLAG_DJB_TWEAK;
return _gcry_mpi_ec_internal_new (r_ec, &flags, "ecc_mul_point", NULL, name);
}
unsigned int
_gcry_ecc_get_algo_keylen (int curveid)
{
unsigned int len = 0;
if (curveid == GCRY_ECC_CURVE25519)
len = ECC_CURVE25519_BYTES;
else if (curveid == GCRY_ECC_CURVE448)
len = ECC_CURVE448_BYTES;
return len;
}
gpg_error_t
_gcry_ecc_mul_point (int curveid, unsigned char *result,
const unsigned char *scalar, const unsigned char *point)
{
unsigned int nbits;
unsigned int nbytes;
const char *curve;
gpg_err_code_t err;
gcry_mpi_t mpi_k;
mpi_ec_t ec;
mpi_point_struct Q;
gcry_mpi_t x;
unsigned int len;
unsigned char *buf;
if (curveid == GCRY_ECC_CURVE25519)
curve = "Curve25519";
else if (curveid == GCRY_ECC_CURVE448)
curve = "X448";
else
return gpg_error (GPG_ERR_UNKNOWN_CURVE);
err = prepare_ec (&ec, curve);
if (err)
return err;
nbits = ec->nbits;
nbytes = (nbits + 7)/8;
mpi_k = _gcry_mpi_set_opaque_copy (NULL, scalar, nbytes*8);
x = mpi_new (nbits);
point_init (&Q);
if (point)
{
gcry_mpi_t mpi_u = _gcry_mpi_set_opaque_copy (NULL, point, nbytes*8);
mpi_point_struct P;
point_init (&P);
err = _gcry_ecc_mont_decodepoint (mpi_u, ec, &P);
_gcry_mpi_release (mpi_u);
if (err)
goto leave;
_gcry_mpi_ec_mul_point (&Q, mpi_k, &P, ec);
point_free (&P);
}
else
_gcry_mpi_ec_mul_point (&Q, mpi_k, ec->G, ec);
_gcry_mpi_ec_get_affine (x, NULL, &Q, ec);
buf = _gcry_mpi_get_buffer (x, nbytes, &len, NULL);
if (!buf)
err = gpg_error_from_syserror ();
else
{
memcpy (result, buf, nbytes);
xfree (buf);
}
leave:
_gcry_mpi_release (x);
point_free (&Q);
_gcry_mpi_release (mpi_k);
_gcry_mpi_ec_free (ec);
return err;
}