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* Copyright (c) 2015 Andrew Poelstra *
* Distributed under the MIT software license, see the accompanying *
* file COPYING or*
#include "ecmult_const_impl.h"
int secp256k1_ecdh(const secp256k1_context_t* ctx, unsigned char *result, const secp256k1_pubkey_t *point, const unsigned char *scalar) {
int ret = 0;
int overflow = 0;
secp256k1_gej_t res;
secp256k1_ge_t pt;
secp256k1_scalar_t s;
ARG_CHECK(result != NULL);
ARG_CHECK(point != NULL);
ARG_CHECK(scalar != NULL);
secp256k1_pubkey_load(ctx, &pt, point);
secp256k1_scalar_set_b32(&s, scalar, &overflow);
if (overflow || secp256k1_scalar_is_zero(&s)) {
ret = 0;
} else {
unsigned char x[32];
unsigned char y[1];
secp256k1_sha256_t sha;
secp256k1_ecmult_const(&res, &pt, &s);
secp256k1_ge_set_gej(&pt, &res);
/* Compute a hash of the point in compressed form
* Note we cannot use secp256k1_eckey_pubkey_serialize here since it does not
* expect its output to be secret and has a timing sidechannel. */
secp256k1_fe_get_b32(x, &pt.x);
y[0] = 0x02 | secp256k1_fe_is_odd(&pt.y);
secp256k1_sha256_write(&sha, y, sizeof(y));
secp256k1_sha256_write(&sha, x, sizeof(x));
secp256k1_sha256_finalize(&sha, result);
ret = 1;
return ret;