client_ssl.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <openssl/bio.h> /* BasicInput/Output streams */
#include <openssl/err.h> /* errors */
#include <openssl/ssl.h> /* core library */

#define BUFF_SIZE 1024

void report_and_exit(const char* msg) { 
  perror(msg); 
  ERR_print_errors_fp(stderr); 
  exit(-1); 
}

void init_ssl() { 
  SSL_load_error_strings(); 
  SSL_library_init(); 
}

void cleanup(SSL_CTX* ctx, BIO* bio) { 
  SSL_CTX_free(ctx); 
  BIO_free_all(bio); 
}

void secure_connect(const char* hostname) { 
  char name[BUFF_SIZE]; 
  char request[BUFF_SIZE]; 
  char response[BUFF_SIZE];

  const SSL_METHOD* method = TLSv1_2_client_method(); 
  if (NULL == method) report_and_exit("TLSv1_2_client_method...");

  SSL_CTX* ctx = SSL_CTX_new(method); 
  if (NULL == ctx) report_and_exit("SSL_CTX_new...");

  BIO* bio = BIO_new_ssl_connect(ctx); 
  if (NULL == bio) report_and_exit("BIO_new_ssl_connect...");

  SSL* ssl = NULL;

  /* link bio channel, SSL session, and server endpoint */ 

  sprintf(name, "%s:%s", hostname, "https"); 
  BIO_get_ssl(bio, &ssl); /* session */ 
  SSL_set_mode(ssl, SSL_MODE_AUTO_RETRY); /* robustness */ 
  BIO_set_conn_hostname(bio, name); /* prepare to connect */

  /* try to connect */ 
  if (BIO_do_connect(bio) <= 0) { 
    cleanup(ctx, bio); 
    report_and_exit("BIO_do_connect..."); 
  }

  /* verify truststore, check cert */ 
  if (!SSL_CTX_load_verify_locations(ctx, 
                                      "/etc/ssl/certs/ca-certificates.crt", /* truststore */ 
                                      "/etc/ssl/certs/")) /* more truststore */
    report_and_exit("SSL_CTX_load_verify_locations...");

  long verify_flag = SSL_get_verify_result(ssl); 
  if (verify_flag != X509_V_OK) 
    fprintf(stderr, 
            "##### Certificate verification error (%i) but continuing...\n", 
            (int) verify_flag);

  /* now fetch the homepage as sample data */ 
  sprintf(request, 
          "GET / HTTP/1.1\x0D\x0AHost: %s\x0D\x0A\x43onnection: Close\x0D\x0A\x0D\x0A", 
          hostname); 
  BIO_puts(bio, request);

  /* read HTTP response from server and print to stdout */ 
  while (1) { 
    memset(response, '\0', sizeof(response)); 
    int n = BIO_read(bio, response, BUFF_SIZE); 
    if (n <= 0) break; /* 0 is end-of-stream, < 0 is an error */ 
  puts(response); 
  }

  cleanup(ctx, bio); 
}

int main() { 
  init_ssl();

  const char* hostname = "www.google.com:443"; 
  fprintf(stderr, "Trying an HTTPS connection to %s...\n", hostname); 
  secure_connect(hostname);

return 0; 
}