Turn a simple socket into an SSL socket

Question

I wrote simple C programs  which are using sockets   client  and  server      UNIX Linux usage   The server side simply creates a socket   sockfd   socket AF INET  SOCK STREAM  0     And then binds it to sockaddr   bind sockfd   struct sockaddr     amp serv addr  sizeof serv addr      And listens  and accepts and reads    listen sockfd 5   newsockfd   accept sockfd   struct sockaddr     amp cli addr   amp clilen   read newsockfd buffer 255     The client creates the socket  and then writes to it   Now  I want to convert this simple connection into an SSL connection  in the plainest  most idyllic  neatest and quickest way     I ve tried to add OpenSSL to my project  but I can t find an easy way to implement what I want

User · Answer

There are several steps when using OpenSSL. You must have an SSL certificate made which can contain the certificate with the private key be sure to specify the exact location of the certificate (this example has it in the root). There are a lot of good tutorials out there.

Some includes:

#include <openssl/applink.c>
#include <openssl/bio.h>
#include <openssl/ssl.h>
#include <openssl/err.h>

You will need to initialize OpenSSL:

void InitializeSSL()
{
    SSL_load_error_strings();
    SSL_library_init();
    OpenSSL_add_all_algorithms();
}

void DestroySSL()
{
    ERR_free_strings();
    EVP_cleanup();
}

void ShutdownSSL()
{
    SSL_shutdown(cSSL);
    SSL_free(cSSL);
}

Now for the bulk of the functionality. You may want to add a while loop on connections.

int sockfd, newsockfd;
SSL_CTX *sslctx;
SSL *cSSL;

InitializeSSL();
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd< 0)
{
    //Log and Error
    return;
}
struct sockaddr_in saiServerAddress;
bzero((char *) &saiServerAddress, sizeof(saiServerAddress));
saiServerAddress.sin_family = AF_INET;
saiServerAddress.sin_addr.s_addr = serv_addr;
saiServerAddress.sin_port = htons(aPortNumber);

bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr));

listen(sockfd,5);
newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);

sslctx = SSL_CTX_new( SSLv23_server_method());
SSL_CTX_set_options(sslctx, SSL_OP_SINGLE_DH_USE);
int use_cert = SSL_CTX_use_certificate_file(sslctx, "/serverCertificate.pem" , SSL_FILETYPE_PEM);

int use_prv = SSL_CTX_use_PrivateKey_file(sslctx, "/serverCertificate.pem", SSL_FILETYPE_PEM);

cSSL = SSL_new(sslctx);
SSL_set_fd(cSSL, newsockfd );
//Here is the SSL Accept portion.  Now all reads and writes must use SSL
ssl_err = SSL_accept(cSSL);
if(ssl_err <= 0)
{
    //Error occurred, log and close down ssl
    ShutdownSSL();
}

You are then able read or write using:

SSL_read(cSSL, (char *)charBuffer, nBytesToRead);
SSL_write(cSSL, "Hi :3\n", 6);

Update The SSL_CTX_new should be called with the TLS method that best fits your needs in order to support the newer versions of security, instead of SSLv23_server_method(). See: OpenSSL SSL_CTX_new description

TLS_method(), TLS_server_method(), TLS_client_method(). These are the general-purpose version-flexible SSL/TLS methods. The actual protocol version used will be negotiated to the highest version mutually supported by the client and the server. The supported protocols are SSLv3, TLSv1, TLSv1.1, TLSv1.2 and TLSv1.3.

User · Answer

OpenSSL is quite difficult  It s easy to accidentally throw away all your security by not doing negotiation exactly right   Heck  I ve been personally bitten by a bug where curl wasn t reading the OpenSSL alerts exactly right  and couldn t talk to some sites    If you really want quick and simple  put stud in front of your program an call it a day   Having SSL in a different process won t slow you down  http   vincent bernat im en blog 2011-ssl-benchmark html

User · Answer

For others like me   There was once an example in the SSL source in the directory demos ssl  with example code in C    Now it s available only via the history  https   github com openssl openssl tree 691064c47fd6a7d11189df00a0d1b94d8051cbe0 demos ssl  You probably will have to find a working version  I originally posted this answer at Nov 6 2015  And I had to edit the source -- not much   Certificates   pem in demos certs apps   https   github com openssl openssl tree master demos certs apps

User · Answer

Here my example ssl socket server threads  multiple connection  https   github com breakermind CppLinux blob master QtSslServerThreads breakermindsslserver cpp    include  lt pthread h gt   include  lt stdio h gt   include  lt stdlib h gt   include  lt string gt   include  lt unistd h gt   include  lt iostream gt    include  lt breakermindsslserver h gt   using namespace std   int main int argc  char  argv          BreakermindSslServer boom      boom Start 123   home user c   qt BreakermindServer certificate crt     home user c   qt BreakermindServer private key        return 0

[c] Turn a simple socket into an SSL socket

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