/* LibTomCrypt, modular cryptographic library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
#include "tomcrypt_private.h"
/**
@file ecc_shared_secret.c
ECC Crypto, Tom St Denis
*/
#ifdef LTC_MECC
/**
Create an ECC shared secret between two keys
@param private_key The private ECC key
@param public_key The public key
@param out [out] Destination of the shared secret (Conforms to EC-DH from ANSI X9.63)
@param outlen [in/out] The max size and resulting size of the shared secret
@return CRYPT_OK if successful
*/
int ecc_shared_secret(const ecc_key *private_key, const ecc_key *public_key,
unsigned char *out, unsigned long *outlen)
{
unsigned long x;
ecc_point *result;
void *prime, *a;
int err;
LTC_ARGCHK(private_key != NULL);
LTC_ARGCHK(public_key != NULL);
LTC_ARGCHK(out != NULL);
LTC_ARGCHK(outlen != NULL);
/* type valid? */
if (private_key->type != PK_PRIVATE) {
return CRYPT_PK_NOT_PRIVATE;
}
/* make new point */
result = ltc_ecc_new_point();
if (result == NULL) {
return CRYPT_MEM;
}
prime = private_key->dp.prime;
a = private_key->dp.A;
if ((err = ltc_mp.ecc_ptmul(private_key->k, &public_key->pubkey, result, a, prime, 1)) != CRYPT_OK) { goto done; }
x = (unsigned long)mp_unsigned_bin_size(prime);
if (*outlen < x) {
*outlen = x;
err = CRYPT_BUFFER_OVERFLOW;
goto done;
}
zeromem(out, x);
if ((err = mp_to_unsigned_bin(result->x, out + (x - mp_unsigned_bin_size(result->x)))) != CRYPT_OK) { goto done; }
err = CRYPT_OK;
*outlen = x;
done:
ltc_ecc_del_point(result);
return err;
}
#endif