+/**
+ * @ingroup crypto
+ * Derive key material from a ECDH public key and a private ECDSA key.
+ * Dual to #GNUNET_CRRYPTO_ecdh_eddsa.
+ *
+ * @param priv private key from ECDSA to use for the ECDH (x)
+ * @param pub public key to use for the ECDH (yG)
+ * @param key_material where to write the key material H(h(x)yG)
+ * @return #GNUNET_SYSERR on error, #GNUNET_OK on success
+ */
+int
+GNUNET_CRYPTO_ecdsa_ecdh (const struct GNUNET_CRYPTO_EcdsaPrivateKey *priv,
+ const struct GNUNET_CRYPTO_EcdhePublicKey *pub,
+ struct GNUNET_HashCode *key_material)
+{
+ gcry_mpi_point_t result;
+ gcry_mpi_point_t q;
+ gcry_mpi_t d;
+ gcry_ctx_t ctx;
+ gcry_sexp_t pub_sexpr;
+ gcry_mpi_t result_x;
+ unsigned char xbuf[256 / 8];
+ size_t rsize;
+
+ /* first, extract the q = dP value from the public key */
+ if (0 != gcry_sexp_build (&pub_sexpr, NULL,
+ "(public-key(ecc(curve " CURVE ")(q %b)))",
+ (int)sizeof (pub->q_y), pub->q_y))
+ return GNUNET_SYSERR;
+ GNUNET_assert (0 == gcry_mpi_ec_new (&ctx, pub_sexpr, NULL));
+ gcry_sexp_release (pub_sexpr);
+ q = gcry_mpi_ec_get_point ("q", ctx, 0);
+
+ /* second, extract the d value from our private key */
+ GNUNET_CRYPTO_mpi_scan_unsigned (&d, priv->d, sizeof (priv->d));
+
+ /* then call the 'multiply' function, to compute the product */
+ result = gcry_mpi_point_new (0);
+ gcry_mpi_ec_mul (result, d, q, ctx);
+ gcry_mpi_point_release (q);
+ gcry_mpi_release (d);
+
+ /* finally, convert point to string for hashing */
+ result_x = gcry_mpi_new (256);
+ if (gcry_mpi_ec_get_affine (result_x, NULL, result, ctx))
+ {
+ LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "get_affine failed", 0);
+ gcry_mpi_point_release (result);
+ gcry_ctx_release (ctx);
+ return GNUNET_SYSERR;
+ }
+ gcry_mpi_point_release (result);
+ gcry_ctx_release (ctx);
+
+ rsize = sizeof (xbuf);
+ GNUNET_assert (! gcry_mpi_get_flag (result_x, GCRYMPI_FLAG_OPAQUE));
+ /* result_x can be negative here, so we do not use 'GNUNET_CRYPTO_mpi_print_unsigned'
+ as that does not include the sign bit; x should be a 255-bit
+ value, so with the sign it should fit snugly into the 256-bit
+ xbuf */
+ GNUNET_assert (0 ==
+ gcry_mpi_print (GCRYMPI_FMT_STD, xbuf, rsize, &rsize,
+ result_x));
+ GNUNET_CRYPTO_hash (xbuf,
+ rsize,
+ key_material);
+ gcry_mpi_release (result_x);
+ return GNUNET_OK;
+}
+
+