how I can enforce OpenSSL to use my system's root CA if I make my application for linux or for windows 10 or later?

Question

I am making the following piece of code:

#include <stdio.h>
#include <openssl/ssl.h>
#include <openssl/err.h>

#include <shlwapi.h>

#if defined(WIN32) || defined(_WIN32) || defined(__WIN32) && !defined(__CYGWIN__)

#define _WINSOCK_DEPRECATED_NO_WARNINGS
#define _CRT_SECURE_NO_WARNINGS


#include <winsock2.h>
#include <ws2tcpip.h>
#include <shlwapi.h>

// Need to link with Ws2_32.lib, Mswsock.lib, and Advapi32.lib
#pragma comment (lib, "Ws2_32.lib")
//#pragma comment (lib, "Mswsock.lib")
//#pragma comment (lib, "AdvApi32.lib")


#define WIN32_LEAN_AND_MEAN


int verifyCerts( SSL_CTX* ctx )
{
    // directory where executable is
    char path[MAX_PATH] = "";
    
    memset(path, 0, MAX_PATH);
    GetModuleFileName(0, path, MAX_PATH);
    PathRemoveFileSpec(path);

    sprintf(path,"%s\\%s",path,"certificates");
    printf("\nCert path %s\n",path);
    sprintf(path,"%s\\%s",path,"certificates");
    printf("\nCert path %s\n",path);
    int value = SSL_CTX_load_verify_locations(ctx,NULL,path);
}

#else // By default use system's CA root

int verifyCerts( SSL_CTX* ctx )
{
       
}

#endif

SSL_CTX* InitCTX(void)
{
    OpenSSL_add_all_algorithms(); 
    SSL_load_error_strings();
    const SSL_METHOD* method = SSLv23_method();
    SSL_CTX* ctx = SSL_CTX_new(method);
    SSL_CTX_set_options(ctx, SSL_OP_ALL | SSL_OP_NO_TLSv1_1 | SSL_OP_NO_TLSv1);
    
    if (ctx == NULL)
    {
        ERR_print_errors_fp(stderr);
        abort();
    }
   
    int value = verifyCerts( ctx );
    if(value == 0) {
        printf("Certificate error\n");
        exit(1);
    }

    SSL_CTX_set_verify(ctx,SSL_VERIFY_PEER,NULL);

    return ctx;
}

In my case I want to do the following:

• For windows XP (legacy application) to provide certificates alongside to my application so I can mame my application as sacure as possible.
• At any other case (for linux systems or windows 10 or above) I'll use the OS's default CA certs (no hussle to provide my own).

So how I can ensure that the latter case is applicable as well?

Answer 1

At #else section just place the following code:

int verifyCerts( SSL_CTX* ctx )
{    
    const char *path = getenv(X509_get_default_cert_dir_env());

    if (!path){
        path = X509_get_default_cert_dir();
    }

    return SSL_CTX_load_verify_locations(ctx,NULL,path);
}

That will allow for linux systems to verify using default certs path. So for wondows XP only we can use custom mingw flags.