OpenSSL Client/Server

Client

#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <openssl/ssl.h>
#include <openssl/err.h>
#include <openssl/x509v3.h>

int create_socket(int port) {
  /* returns a valid socket fd */
}

int main(int argc, char **argv) {
  int sock;
  SSL *ssl;
  SSL_CTX *ctx;
  const SSL_METHOD *method;
  X509_VERIFY_PARAM *param;

  /* init */
  SSL_load_error_strings();
  OpenSSL_add_ssl_algorithms();

  /* create context */
  method = TLSv1_2_client_method();

  if (!(ctx = SSL_CTX_new(method))) {
    exit(1);
  }

  /* configure context */
  SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER, NULL);
  SSL_CTX_set_verify_depth(ctx, 4);
  SSL_CTX_load_verify_locations(ctx, "/path/to/cert.pem", NULL);
  SSL_CTX_set_options(ctx, SSL_OP_NO_SSLv2 | SSL_OP_NO_SSLv3 | SSL_OP_NO_TLSv1 | SSL_OP_NO_TLSv1_1 | SSL_OP_NO_COMPRESSION);

  /* open a socket */
  sock = create_socket(1337);

  /* create ssl instance from context */
  ssl = SSL_new(ctx);

  param = SSL_get0_param(ssl);

  X509_VERIFY_PARAM_set_hostflags(param, X509_CHECK_FLAG_NO_PARTIAL_WILDCARDS);
  X509_VERIFY_PARAM_set1_host(param, "localhost", 0);

  /* assign socket to ssl intance */
  SSL_set_fd(ssl, sock);

  /* perform ssl handshake & connection */
  SSL_connect(ssl);

  /* perform ssl reads / writes */

  // SSL_read(ssl, buff, 255);

  /* cleanup */
  close(sock);
  SSL_free(ssl);
  SSL_CTX_free(ctx);
  EVP_cleanup();
}

Server

#include <stdio.h>                                                                                                                       
#include <unistd.h>                                                                                                                      
#include <sys/socket.h>                                                                                                                  
#include <arpa/inet.h>                                                                                                                   
#include <openssl/ssl.h>                                                                                                                 
#include <openssl/err.h>   
#include <openssl/x509v3.h>                                                                                                              

int create_socket(int port) {                                                                                                            
  /* returns a valid socket fd */                                                                                                        
}                                                                                                                                        

int main(int argc, char **argv) {                                                                                                        
  int sock;                                                                                                                              
  SSL_CTX *ctx;                                                                                                                          

  /* init */                                                                                                                             
  SSL_load_error_strings();                                                                                                              
  OpenSSL_add_ssl_algorithms();                                                                                                          

  /* create context */                                                                                                                   
  method = TLSv1_2_server_method();                                                                                                       

  if (!(ctx = SSL_CTX_new(method))) {                                                                                                    
    exit(1);                                                                                                                             
  }

  /* configure context */                                                                                                                
  SSL_CTX_set_ecdh_auto(ctx, 1);

  /* Set the key and cert */                                                                                                             
  if (SSL_CTX_use_certificate_file(ctx, "cert.pem", SSL_FILETYPE_PEM) <= 0) {                                                            
    exit(1);
  }

  if (SSL_CTX_use_PrivateKey_file(ctx, "key.pem", SSL_FILETYPE_PEM) <= 0) {                                                              
    exit(1);
  }                                                                                                                                      

  /* open a socket */
  sock = create_socket(5555);                                                                                                            

  /* Handle connections */                                                                                                               
  while(1) {                                                                                                                             
    int nsock;
    SSL *ssl;

    /* accept client connections
       if there are any */                                                                                                               
    nsock = accept_connection(sock);                                                                                                     

    /* create ssl instance from context */                                                                                               
    ssl = SSL_new(ctx);

    /* assign socket to ssl intance */                                                                                                   
    SSL_set_fd(ssl, client);                                                                                                             

    /* perform ssl handshake & connection */                                                                                             
    SSL_accept(ssl);                                                                                                                     

    /* perform ssl reads / writes */
    SSL_write(ssl, buff, sizeof(buff));

    /* free ssl instance */
    SSL_free(ssl);

    /* close client connection */
    close(client);
  }
}