How to load Client certificate during ssl connection for mutual authentication?

Question

I am not able toload client certificate on ssl connection, so that server can authenticate it. What may be the issue ? LoadCertificates() function is working fine for loading server certificate. I am able to get server certificate on client end. But not able to get client certificate on server end.

Also I want to vertify that the certificates are signed by correct CA. I have CA certificate on both ends. Can I do that ?

 //SSL-Client.c
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <malloc.h>
#include <string.h>
#include <sys/socket.h>
#include <resolv.h>
#include <netdb.h>
#include <openssl/ssl.h>
#include <openssl/err.h>

#define FAIL    -1

int OpenConnection(const char *hostname, int port)
{   int sd;
    struct hostent *host;
    struct sockaddr_in addr;

    if ( (host = gethostbyname(hostname)) == NULL )
    {
        perror(hostname);
        abort();
    }
    sd = socket(PF_INET, SOCK_STREAM, 0);
    bzero(&addr, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(port);
    addr.sin_addr.s_addr = *(long*)(host->h_addr);
    if ( connect(sd, (struct sockaddr*)&addr, sizeof(addr)) != 0 )
    {
        close(sd);
        perror(hostname);
        abort();
    }
    return sd;
}

SSL_CTX* InitCTX(void)
{   SSL_METHOD *method;
    SSL_CTX *ctx;

    OpenSSL_add_all_algorithms();  /* Load cryptos, et.al. */
    SSL_load_error_strings();   /* Bring in and register error messages */
    method = TLSv1_2_client_method();  /* Create new client-method instance */
    ctx = SSL_CTX_new(method);   /* Create new context */
    if ( ctx == NULL )
    {
        ERR_print_errors_fp(stderr);
        abort();
    }
    return ctx;
}

void LoadCertificates(SSL_CTX* ctx, char* CertFile, char* KeyFile)
{
    /* set the local certificate from CertFile */
    if ( SSL_CTX_use_certificate_file(ctx, CertFile, SSL_FILETYPE_PEM) <= 0 )
    {
        ERR_print_errors_fp(stderr);
        abort();
    }
    /* set the private key from KeyFile (may be the same as CertFile) */
    if ( SSL_CTX_use_PrivateKey_file(ctx, KeyFile, SSL_FILETYPE_PEM) <= 0 )
    {
        ERR_print_errors_fp(stderr);
        abort();
    }
    /* verify private key */
    if ( !SSL_CTX_check_private_key(ctx) )
    {
        fprintf(stderr, "Private key does not match the public certificate\n");
        abort();
    }
}

void ShowCerts(SSL* ssl)
{   X509 *cert;
    char *line;

    cert = SSL_get_peer_certificate(ssl); /* get the server's certificate */
    if ( cert != NULL )
    {
        printf("Server certificates:\n");
        line = X509_NAME_oneline(X509_get_subject_name(cert), 0, 0);
        printf("Subject: %s\n", line);
        free(line);       /* free the malloc'ed string */
        line = X509_NAME_oneline(X509_get_issuer_name(cert), 0, 0);
        printf("Issuer: %s\n", line);
        free(line);       /* free the malloc'ed string */
        X509_free(cert);     /* free the malloc'ed certificate copy */
    }
    else
        printf("Info: No client certificates configured.\n");
}

int main(int count, char *strings[])
{   SSL_CTX *ctx;
    int server;
    SSL *ssl;
    char buf[1024];
    int bytes;
    char *hostname, *portnum;

    if ( count != 3 )
    {
        printf("usage: %s <hostname> <portnum>\n", strings[0]);
        exit(0);
    }
    SSL_library_init();
    hostname=strings[1];
    portnum=strings[2];

    ctx = InitCTX();
    LoadCertificates(ctx, "cert.pem", "key.pem");
    server = OpenConnection(hostname, atoi(portnum));
    ssl = SSL_new(ctx);      /* create new SSL connection state */
    SSL_set_fd(ssl, server);    /* attach the socket descriptor */
    if ( SSL_connect(ssl) == FAIL )   /* perform the connection */
        ERR_print_errors_fp(stderr);
    else
    {   char *msg = "Hello???";

        printf("Connected with %s encryption\n", SSL_get_cipher(ssl));
        ShowCerts(ssl);        /* get any certs */
        SSL_write(ssl, msg, strlen(msg));   /* encrypt & send message */
        bytes = SSL_read(ssl, buf, sizeof(buf)); /* get reply & decrypt */
        buf[bytes] = 0;
        printf("Received: \"%s\"\n", buf);

        SSL_free(ssl);        /* release connection state */
    }
    close(server);         /* close socket */
    SSL_CTX_free(ctx);        /* release context */
    return 0;
}

Server.c

//SSL-Server.c
#include <errno.h>
#include <unistd.h>
#include <malloc.h>
#include <string.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <resolv.h>
#include "openssl/ssl.h"
#include "openssl/err.h"

#define FAIL    -1

int OpenListener(int port)
{   int sd;
    struct sockaddr_in addr;

    sd = socket(PF_INET, SOCK_STREAM, 0);
    bzero(&addr, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(port);
    addr.sin_addr.s_addr = INADDR_ANY;
    if ( bind(sd, (struct sockaddr*)&addr, sizeof(addr)) != 0 )
    {
        perror("can't bind port");
        abort();
    }
    if ( listen(sd, 10) != 0 )
    {
        perror("Can't configure listening port");
        abort();
    }
    return sd;
}

int isRoot()
{
    if (getuid() != 0)
    {
        return 0;
    }
    else
    {
        return 1;
    }

}
SSL_CTX* InitServerCTX(void)
{   SSL_METHOD *method;
    SSL_CTX *ctx;

    OpenSSL_add_all_algorithms();  /* load & register all cryptos, etc. */
    SSL_load_error_strings();   /* load all error messages */
    method = TLSv1_2_server_method();  /* create new server-method instance */
    ctx = SSL_CTX_new(method);   /* create new context from method */
    if ( ctx == NULL )
    {
        ERR_print_errors_fp(stderr);
        abort();
    }
    return ctx;
}

void LoadCertificates(SSL_CTX* ctx, char* CertFile, char* KeyFile)
{
    /* set the local certificate from CertFile */
    if ( SSL_CTX_use_certificate_file(ctx, CertFile, SSL_FILETYPE_PEM) <= 0 )
    {
        ERR_print_errors_fp(stderr);
        abort();
    }
    /* set the private key from KeyFile (may be the same as CertFile) */
    if ( SSL_CTX_use_PrivateKey_file(ctx, KeyFile, SSL_FILETYPE_PEM) <= 0 )
    {
        ERR_print_errors_fp(stderr);
        abort();
    }
    /* verify private key */
    if ( !SSL_CTX_check_private_key(ctx) )
    {
        fprintf(stderr, "Private key does not match the public certificate\n");
        abort();
    }
}

void ShowCerts(SSL* ssl)
{   X509 *cert;
    char *line;

    cert = SSL_get_peer_certificate(ssl); /* Get certificates (if available) */
    if ( cert != NULL )
    {
        printf("Server certificates:\n");
        line = X509_NAME_oneline(X509_get_subject_name(cert), 0, 0);
        printf("Subject: %s\n", line);
        free(line);
        line = X509_NAME_oneline(X509_get_issuer_name(cert), 0, 0);
        printf("Issuer: %s\n", line);
        free(line);
        X509_free(cert);
    }
    else
        printf("No certificates.\n");
}

void Servlet(SSL* ssl) /* Serve the connection -- threadable */
{   char buf[1024];
    char reply[1024];
    int sd, bytes;
    const char* HTMLecho="<html><body><pre>%s</pre></body></html>\n\n";

    if ( SSL_accept(ssl) == FAIL )     /* do SSL-protocol accept */
        ERR_print_errors_fp(stderr);
    else
    {
        ShowCerts(ssl);        /* get any certificates */
        bytes = SSL_read(ssl, buf, sizeof(buf)); /* get request */
        if ( bytes > 0 )
        {
            buf[bytes] = 0;
            printf("Client msg: \"%s\"\n", buf);
            sprintf(reply, HTMLecho, buf);   /* construct reply */
            SSL_write(ssl, reply, strlen(reply)); /* send reply */
        }
        else
            ERR_print_errors_fp(stderr);
    }
    sd = SSL_get_fd(ssl);       /* get socket connection */
    SSL_free(ssl);         /* release SSL state */
    close(sd);          /* close connection */
}

int main(int count, char *strings[])
{   SSL_CTX *ctx;
    int server;
    char *portnum;

    if(!isRoot())
    {
        printf("This program must be run as root/sudo user!!");
        exit(0);
    }
    if ( count != 2 )
    {
        printf("Usage: %s <portnum>\n", strings[0]);
        exit(0);
    }
    SSL_library_init();

    portnum = strings[1];
    ctx = InitServerCTX();        /* initialize SSL */
    LoadCertificates(ctx, "mycert.pem", "mycert.pem"); /* load certs */
    server = OpenListener(atoi(portnum));    /* create server socket */
    while (1)
    {   struct sockaddr_in addr;
        socklen_t len = sizeof(addr);
        SSL *ssl;

        int client = accept(server, (struct sockaddr*)&addr, &len);  /* accept connection as usual */
        printf("Connection: %s:%d\n",inet_ntoa(addr.sin_addr), ntohs(addr.sin_port));
        ssl = SSL_new(ctx);              /* get new SSL state with context */
        SSL_set_fd(ssl, client);      /* set connection socket to SSL state */
        Servlet(ssl);         /* service connection */
    }
    close(server);          /* close server socket */
    SSL_CTX_free(ctx);         /* release context */
}

Answer 1

You should be calling the API SSL_CTX_set_verify and passing SSL_VERIFY_PEER as input to the second parameter mode.

In TLS, client requests a certificate from server depending on the cipher suites exchanged, whereas the server requests the certificate from client only when you explicitly tell it do so as client authentication is optional and not cipher suite driven. When you set the SSL_VERIFY_PEER mode, server will explicitly send a Certificate Request message to Client and request for a certificate and verify it.

You can also verify if a Certificate Request message is going from Server to Client using a Packet Capture.