I am trying to implement ECDH in libgcrypt. I have found examples online for how to compute a shared secret and I know how to do symmetric encryption I just can’t figure out how to generate the keypairs.

I want the keys to be stored as unsigned char *’s . This is how I am currently trying to do it but it always segfaults on the line gcry_mpi_ec_mul(mpi_public_key, mpi_private_key, NULL, ctx);
Here is my code: `

void generate_curve25519_keypair(unsigned char *public_key, unsigned char *private_key) {
    gcry_error_t error;
    gcry_mpi_t mpi_private_key;
    gcry_mpi_point_t mpi_public_key;
    gcry_ctx_t ctx;

    // Generate a random private key
    gcry_randomize(private_key, CURVE25519_KEY_SIZE, GCRY_STRONG_RANDOM);

    // Clamp the private key for Curve25519
    private_key[0] &= 248;
    private_key[31] &= 127;
    private_key[31] |= 64;

    // Initialize the context for Curve25519
    error = gcry_mpi_ec_new(&ctx, NULL, "Curve25519");
    if (error) {
        fprintf(stderr, "Failed to create context: %sn", gpg_strerror(error));
        return;
    }

    // Convert the private key to an MPI
    error = gcry_mpi_scan(&mpi_private_key, GCRYMPI_FMT_USG, private_key, CURVE25519_KEY_SIZE, NULL);
    if (error) {
        fprintf(stderr, "Failed to convert private key to MPI: %sn", gpg_strerror(error));
        gcry_ctx_release(ctx);
        return;
    }

    // Allocate memory for the resulting public key point
    mpi_public_key = gcry_mpi_point_new(0);

    // Multiply the base point with the private key MPI to get the public key point
    gcry_mpi_ec_mul(mpi_public_key, mpi_private_key, NULL, ctx); // Use NULL for the default base point G
    if (error) {
        fprintf(stderr, "Failed to multiply with base point: %sn", gpg_strerror(error));
        gcry_mpi_release(mpi_private_key);
        gcry_mpi_point_release(mpi_public_key);
        gcry_ctx_release(ctx);
        return;
    }

    // Extract the x-coordinate of the public key point and convert it to a byte array
    gcry_mpi_t mpi_x = gcry_mpi_new(0);
    gcry_mpi_point_get(mpi_x, NULL, NULL, mpi_public_key);
    error = gcry_mpi_print(GCRYMPI_FMT_USG, public_key, CURVE25519_KEY_SIZE, NULL, mpi_x);
    if (error) {
        fprintf(stderr, "Failed to export public key: %sn", gpg_strerror(error));
        gcry_mpi_release(mpi_x);
        gcry_mpi_release(mpi_private_key);
        gcry_mpi_point_release(mpi_public_key);
        gcry_ctx_release(ctx);
        return;
    }

    // Release resources
    gcry_mpi_release(mpi_x);
    gcry_mpi_release(mpi_private_key);
    gcry_mpi_point_release(mpi_public_key);
    gcry_ctx_release(ctx);
}