In a “C” program how can I store a hexadecimal value in a string variable?

问题

I have a program which takes the input data as a plaintext and then decrypts the message using 3DES method in CBC mode. But the values are hardcoded in the program I want to provide the encrypted value myself which should be decryted. How can I do this in the following program?

int main(void)
{
    unsigned char in[BUFSIZE], out[BUFSIZE], back[BUFSIZE];
    unsigned char *e = out;
    int len;

    DES_cblock key;
    DES_cblock seed = {0xFE, 0xDC, 0xBA, 0x98, 0x76, 0x54, 0x32, 0x10};
    DES_cblock ivsetup = {0xE1, 0xE2, 0xE3, 0xD4, 0xD5, 0xC6, 0xC7, 0xA8};
    DES_key_schedule keysched;
    DES_cblock ivec;

    memset(in, 0, sizeof(in));
    memset(out, 0, sizeof(out));
    memset(back, 0, sizeof(back));

    RAND_seed(seed, sizeof(DES_cblock));

    DES_random_key(&key);
    DES_set_odd_parity(&key);
    if (DES_set_key_checked((C_Block *)key, &keysched))
    {
        fprintf(stderr, "ERROR: Unable to set key schedule\n");
        exit(1);
    }

    /* 64 bytes of plaintext */

    /* From here, encryption starts for the plaintext below. */

    strcpy(in, "Now is the time for all men to stand up and be counted");

    printf("Plaintext: [%s]\n", in);

    len = strlen(in);
    memcpy(ivec, ivsetup, sizeof(ivsetup));
    DES_ncbc_encrypt(in, out, len, &keysched, &ivec, DES_ENCRYPT);

    printf("Ciphertext:");
    while (*e) printf(" [%02x]", *e++);
    printf("\n");

    /* Till here, encryption is over. After this we have to decrypt
     * the value which has been encoded, but I want to remove all this
     * part and to provide my own encrypted message, and get the
     * proper output.
     */

    memcpy(ivec, ivsetup, sizeof(ivsetup));

    /* The problem I am facing is how to provide the value properly
     * to the parameter "out" and "keysched", which should be of my
     * choice. For "out" I want to provide THIS value:
     * "2DC39619B4450A8C27A3976C50DE5799".
     */

    DES_ncbc_encrypt(out, back, len, &keysched, &ivec, DES_DECRYPT);

    printf("Decrypted Text: [%s]\n", back);

    exit(0);
}

Read more: http://blog.fpmurphy.com/2010/04/openssl-des-api.html#ixzz1uqOp1Yhv

回答1:

Read C FAQ 20.10. Hexadecimal is a representation. All numbers are stored in binary internally. Your DES_cblock is probably a typedef for an (unsigned, perhaps!) integral type. So, what you have in effect is an array of integers. You can put numbers in decimal, hexadecimal or binary -- but they will all work. Hexadecimal is typically used in cryptography because it has some notational advantages.

回答2:

I got it done. I did it in a childish way for the time being but it is working now. I did it like this.

out[0]=0xA0; out[1]=0x69; out[2]=0x57; out[3]=0x3B;
out[4]=0x70; out[5]=0x26; out[6]=0x1C; out[7]=0xE8;
out[8]=0xEF; out[9]=0xF2; out[10]=0x9F;out[11]=0x60;
out[12]=0x80;out[13]=0x60;out[14]=0xB2;out[15]=0xE5;

Later I will do this thing in a for loop.

回答3:

Create a dump function like this:

hexdump(char *buff, int len) { 
  int i,tmp;
  for(i=0; i < len; i++) {
    tmp = buff[i] & 0xff; /** to avoid sign extension */
    printf("%02x",tmp);
  }
}

And use it.

hexdump(back,len);

If you have to write it in memory, you can use sprintf, but you may have to write your own binary to hex function.

回答4:

Using the nonstandard itoa function that stores the value it as a string, you can do the following:

char* hexstr = itoa(back,16); 

// print out a string
printf("Decrypted Text: [%X]\n", back);

来源：https://stackoverflow.com/questions/10583023/in-a-c-program-how-can-i-store-a-hexadecimal-value-in-a-string-variable

标签

encryption

des