This is a simple example that shows how to encrypt and decrypt data using the DES implementation provided by OpenSSL.

We will define two functions, Encrypt and Decrypt. They both use the function DES_cfb64_encrypt to encrypt and decrypt data. See comments inline for explanations.

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <openssl/des.h>
 
 
char *
Encrypt( char *Key, char *Msg, int size)
{
 
        static char*    Res;
        int             n=0;
        DES_cblock      Key2;
        DES_key_schedule schedule;
 
        Res = ( char * ) malloc( size );
 
        /* Prepare the key for use with DES_cfb64_encrypt */
        memcpy( Key2, Key,8);
        DES_set_odd_parity( &Key2 );
        DES_set_key_checked( &Key2, &schedule );
 
        /* Encryption occurs here */
        DES_cfb64_encrypt( ( unsigned char * ) Msg, ( unsigned char * ) Res,
                           size, &schedule, &Key2, &n, DES_ENCRYPT );
 
         return (Res);
}
 
 
char *
Decrypt( char *Key, char *Msg, int size)
{
 
        static char*    Res;
        int             n=0;
 
        DES_cblock      Key2;
        DES_key_schedule schedule;
 
        Res = ( char * ) malloc( size );
 
        /* Prepare the key for use with DES_cfb64_encrypt */
        memcpy( Key2, Key,8);
        DES_set_odd_parity( &Key2 );
        DES_set_key_checked( &Key2, &schedule );
 
        /* Decryption occurs here */
        DES_cfb64_encrypt( ( unsigned char * ) Msg, ( unsigned char * ) Res,
                           size, &schedule, &Key2, &n, DES_DECRYPT );
 
        return (Res);
 
}

Note that the functions above assume a key of 8 characters. If a longer key is provided, only the first 8 bytes will be used. To support longer keys, additional processing and concatenation work must be done to prepare key2.

For testing purpose, this is a main function that calls the Encrypt and Decrypt functions shown above.

int main() {
 
char key[]="password";
char clear[]="This is a secret message";
char *decrypted;
char *encrypted;
 
encrypted=malloc(sizeof(clear));
decrypted=malloc(sizeof(clear));
 
printf("Clear text\t : %s \n",clear);
memcpy(encrypted,Encrypt(key,clear,sizeof(clear)), sizeof(clear));
printf("Encrypted text\t : %s \n",encrypted);
memcpy(decrypted,Decrypt(key,encrypted,sizeof(clear)), sizeof(clear));
printf("Decrypted text\t : %s \n",decrypted);
 
 
 
return (0);
}

To compile this code, copy the three functions Encrypt, Decrypt and main in a file (e.g. test.c), then compile as follows

gcc test.c -o test -lssl

The -lssl flag allows the code to be linked against the OpenSSL library.

If you don't have the OpenSSL library, you can install it using apt

sudo apt-get install libssl-dev

Once the program compiled, you can test it

$./test
Clear text	 : This is a secret message
Enecrypted text	 : 8��0t�E�E� \@�Z9T�)��a�
Decrypted text	 : This is a secret message

Labels: coding, security, howto