int main( int argc, char *argv[] )
{
    FILE *f;

    int ret;
    size_t n, buflen;
    int listen_fd = -1;
    int client_fd = -1;

    unsigned char buf[2048];
    unsigned char hash[20];
    unsigned char buf2[2];
    const char *pers = "dh_server";

    entropy_context entropy;
    ctr_drbg_context ctr_drbg;
    rsa_context rsa;
    dhm_context dhm;
    aes_context aes;

    ((void) argc);
    ((void) argv);

    memset( &rsa, 0, sizeof( rsa ) );
    memset( &dhm, 0, sizeof( dhm ) );

    /*
     * 1. Setup the RNG
     */
    printf( "\n  . Seeding the random number generator" );
    fflush( stdout );

    entropy_init( &entropy );
    if( ( ret = ctr_drbg_init( &ctr_drbg, entropy_func, &entropy,
                               (const unsigned char *) pers,
                               strlen( pers ) ) ) != 0 )
    {
        printf( " failed\n  ! ctr_drbg_init returned %d\n", ret );
        goto exit;
    }

    /*
     * 2a. Read the server's private RSA key
     */
    printf( "\n  . Reading private key from rsa_priv.txt" );
    fflush( stdout );

    if( ( f = fopen( "rsa_priv.txt", "rb" ) ) == NULL )
    {
        ret = 1;
        printf( " failed\n  ! Could not open rsa_priv.txt\n" \
                "  ! Please run rsa_genkey first\n\n" );
        goto exit;
    }

    rsa_init( &rsa, RSA_PKCS_V15, 0 );

    if( ( ret = mpi_read_file( &rsa.N , 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.E , 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.D , 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.P , 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.Q , 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.DP, 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.DQ, 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.QP, 16, f ) ) != 0 )
    {
        printf( " failed\n  ! mpi_read_file returned %d\n\n", ret );
        goto exit;
    }

    rsa.len = ( mpi_msb( &rsa.N ) + 7 ) >> 3;
    
    fclose( f );

    /*
     * 2b. Get the DHM modulus and generator
     */
    printf( "\n  . Reading DH parameters from dh_prime.txt" );
    fflush( stdout );

    if( ( f = fopen( "dh_prime.txt", "rb" ) ) == NULL )
    {
        ret = 1;
        printf( " failed\n  ! Could not open dh_prime.txt\n" \
                "  ! Please run dh_genprime first\n\n" );
        goto exit;
    }

    if( mpi_read_file( &dhm.P, 16, f ) != 0 ||
        mpi_read_file( &dhm.G, 16, f ) != 0 )
    {
        printf( " failed\n  ! Invalid DH parameter file\n\n" );
        goto exit;
    }

    fclose( f );

    /*
     * 3. Wait for a client to connect
     */
    printf( "\n  . Waiting for a remote connection" );
    fflush( stdout );

    if( ( ret = net_bind( &listen_fd, NULL, SERVER_PORT ) ) != 0 )
    {
        printf( " failed\n  ! net_bind returned %d\n\n", ret );
        goto exit;
    }

    if( ( ret = net_accept( listen_fd, &client_fd, NULL ) ) != 0 )
    {
        printf( " failed\n  ! net_accept returned %d\n\n", ret );
        goto exit;
    }

    /*
     * 4. Setup the DH parameters (P,G,Ys)
     */
    printf( "\n  . Sending the server's DH parameters" );
    fflush( stdout );

    memset( buf, 0, sizeof( buf ) );

    if( ( ret = dhm_make_params( &dhm, mpi_size( &dhm.P ), buf, &n,
                                 ctr_drbg_random, &ctr_drbg ) ) != 0 )
    {
        printf( " failed\n  ! dhm_make_params returned %d\n\n", ret );
        goto exit;
    }

    /*
     * 5. Sign the parameters and send them
     */
    sha1( buf, n, hash );

    buf[n    ] = (unsigned char)( rsa.len >> 8 );
    buf[n + 1] = (unsigned char)( rsa.len      );

    if( ( ret = rsa_pkcs1_sign( &rsa, NULL, NULL, RSA_PRIVATE, SIG_RSA_SHA1,
                                0, hash, buf + n + 2 ) ) != 0 )
    {
        printf( " failed\n  ! rsa_pkcs1_sign returned %d\n\n", ret );
        goto exit;
    }

    buflen = n + 2 + rsa.len;
    buf2[0] = (unsigned char)( buflen >> 8 );
    buf2[1] = (unsigned char)( buflen      );

    if( ( ret = net_send( &client_fd, buf2, 2 ) ) != 2 ||
        ( ret = net_send( &client_fd, buf, buflen ) ) != (int) buflen )
    {
        printf( " failed\n  ! net_send returned %d\n\n", ret );
        goto exit;
    }

    /*
     * 6. Get the client's public value: Yc = G ^ Xc mod P
     */
    printf( "\n  . Receiving the client's public value" );
    fflush( stdout );

    memset( buf, 0, sizeof( buf ) );
    n = dhm.len;

    if( ( ret = net_recv( &client_fd, buf, n ) ) != (int) n )
    {
        printf( " failed\n  ! net_recv returned %d\n\n", ret );
        goto exit;
    }

    if( ( ret = dhm_read_public( &dhm, buf, dhm.len ) ) != 0 )
    {
        printf( " failed\n  ! dhm_read_public returned %d\n\n", ret );
        goto exit;
    }

    /*
     * 7. Derive the shared secret: K = Ys ^ Xc mod P
     */
    printf( "\n  . Shared secret: " );
    fflush( stdout );

    if( ( ret = dhm_calc_secret( &dhm, buf, &n ) ) != 0 )
    {
        printf( " failed\n  ! dhm_calc_secret returned %d\n\n", ret );
        goto exit;
    }

    for( n = 0; n < 16; n++ )
        printf( "%02x", buf[n] );

    /*
     * 8. Setup the AES-256 encryption key
     *
     * This is an overly simplified example; best practice is
     * to hash the shared secret with a random value to derive
     * the keying material for the encryption/decryption keys
     * and MACs.
     */
    printf( "...\n  . Encrypting and sending the ciphertext" );
    fflush( stdout );

    aes_setkey_enc( &aes, buf, 256 );
    memcpy( buf, PLAINTEXT, 16 );
    aes_crypt_ecb( &aes, AES_ENCRYPT, buf, buf );

    if( ( ret = net_send( &client_fd, buf, 16 ) ) != 16 )
    {
        printf( " failed\n  ! net_send returned %d\n\n", ret );
        goto exit;
    }

    printf( "\n\n" );

exit:

    net_close( client_fd );
    rsa_free( &rsa );
    dhm_free( &dhm );

#if defined(_WIN32)
    printf( "  + Press Enter to exit this program.\n" );
    fflush( stdout ); getchar();
#endif

    return( ret );
}
int main( int argc, char *argv[] )
{
    FILE *f;

    int ret;
    size_t n, buflen;
    int listen_fd = -1;
    int client_fd = -1;

    unsigned char buf[2048];
    unsigned char hash[20];
    unsigned char buf2[2];
    const char *pers = "dh_server";

    entropy_context entropy;
    ctr_drbg_context ctr_drbg;
    rsa_context rsa;
    dhm_context dhm;
    aes_context aes;

    ((void) argc);
    ((void) argv);

    memset( &rsa, 0, sizeof( rsa ) );
    memset( &dhm, 0, sizeof( dhm ) );

     
    printf( "\n  . Seeding the random number generator" );
    fflush( stdout );

    entropy_init( &entropy );
    if( ( ret = ctr_drbg_init( &ctr_drbg, entropy_func, &entropy,
                               (const unsigned char *) pers,
                               strlen( pers ) ) ) != 0 )
    {
        printf( " failed\n  ! ctr_drbg_init returned %d\n", ret );
        goto exit;
    }

     
    printf( "\n  . Reading private key from rsa_priv.txt" );
    fflush( stdout );

    if( ( f = fopen( "rsa_priv.txt", "rb" ) ) == NULL )
    {
        ret = 1;
        printf( " failed\n  ! Could not open rsa_priv.txt\n" \
                "  ! Please run rsa_genkey first\n\n" );
        goto exit;
    }

    rsa_init( &rsa, RSA_PKCS_V15, 0 );

    if( ( ret = mpi_read_file( &rsa.N , 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.E , 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.D , 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.P , 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.Q , 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.DP, 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.DQ, 16, f ) ) != 0 ||
        ( ret = mpi_read_file( &rsa.QP, 16, f ) ) != 0 )
    {
        printf( " failed\n  ! mpi_read_file returned %d\n\n", ret );
        goto exit;
    }

    rsa.len = ( mpi_msb( &rsa.N ) + 7 ) >> 3;
    
    fclose( f );

     
    printf( "\n  . Reading DH parameters from dh_prime.txt" );
    fflush( stdout );

    if( ( f = fopen( "dh_prime.txt", "rb" ) ) == NULL )
    {
        ret = 1;
        printf( " failed\n  ! Could not open dh_prime.txt\n" \
                "  ! Please run dh_genprime first\n\n" );
        goto exit;
    }

    if( mpi_read_file( &dhm.P, 16, f ) != 0 ||
        mpi_read_file( &dhm.G, 16, f ) != 0 )
    {
        printf( " failed\n  ! Invalid DH parameter file\n\n" );
        goto exit;
    }

    fclose( f );

     
    printf( "\n  . Waiting for a remote connection" );
    fflush( stdout );

    if( ( ret = net_bind( &listen_fd, NULL, SERVER_PORT ) ) != 0 )
    {
        printf( " failed\n  ! net_bind returned %d\n\n", ret );
        goto exit;
    }

    if( ( ret = net_accept( listen_fd, &client_fd, NULL ) ) != 0 )
    {
        printf( " failed\n  ! net_accept returned %d\n\n", ret );
        goto exit;
    }

     
    printf( "\n  . Sending the server's DH parameters" );
    fflush( stdout );

    memset( buf, 0, sizeof( buf ) );

    if( ( ret = dhm_make_params( &dhm, mpi_size( &dhm.P ), buf, &n,
                                 ctr_drbg_random, &ctr_drbg ) ) != 0 )
    {
        printf( " failed\n  ! dhm_make_params returned %d\n\n", ret );
        goto exit;
    }

     
    sha1( buf, n, hash );

    buf[n    ] = (unsigned char)( rsa.len >> 8 );
    buf[n + 1] = (unsigned char)( rsa.len      );

    if( ( ret = rsa_pkcs1_sign( &rsa, NULL, NULL, RSA_PRIVATE, SIG_RSA_SHA1,
                                0, hash, buf + n + 2 ) ) != 0 )
    {
        printf( " failed\n  ! rsa_pkcs1_sign returned %d\n\n", ret );
        goto exit;
    }

    buflen = n + 2 + rsa.len;
    buf2[0] = (unsigned char)( buflen >> 8 );
    buf2[1] = (unsigned char)( buflen      );

    if( ( ret = net_send( &client_fd, buf2, 2 ) ) != 2 ||
        ( ret = net_send( &client_fd, buf, buflen ) ) != (int) buflen )
    {
        printf( " failed\n  ! net_send returned %d\n\n", ret );
        goto exit;
    }

     
    printf( "\n  . Receiving the client's public value" );
    fflush( stdout );

    memset( buf, 0, sizeof( buf ) );
    n = dhm.len;

    if( ( ret = net_recv( &client_fd, buf, n ) ) != (int) n )
    {
        printf( " failed\n  ! net_recv returned %d\n\n", ret );
        goto exit;
    }

    if( ( ret = dhm_read_public( &dhm, buf, dhm.len ) ) != 0 )
    {
        printf( " failed\n  ! dhm_read_public returned %d\n\n", ret );
        goto exit;
    }

     
    printf( "\n  . Shared secret: " );
    fflush( stdout );

    if( ( ret = dhm_calc_secret( &dhm, buf, &n ) ) != 0 )
    {
        printf( " failed\n  ! dhm_calc_secret returned %d\n\n", ret );
        goto exit;
    }

    for( n = 0; n < 16; n++ )
        printf( "%02x", buf[n] );

     
    printf( "...\n  . Encrypting and sending the ciphertext" );
    fflush( stdout );

    aes_setkey_enc( &aes, buf, 256 );
    memcpy( buf, PLAINTEXT, 16 );
    aes_crypt_ecb( &aes, AES_ENCRYPT, buf, buf );

    if( ( ret = net_send( &client_fd, buf, 16 ) ) != 16 )
    {
        printf( " failed\n  ! net_send returned %d\n\n", ret );
        goto exit;
    }

    printf( "\n\n" );

exit:

    net_close( client_fd );
    rsa_free( &rsa );
    dhm_free( &dhm );

#if defined(_WIN32)
    printf( "  + Press Enter to exit this program.\n" );
    fflush( stdout ); getchar();
#endif

    return( ret );
}