Struggling to reverse decryption algorithm in C

Question

This is my first ever question on StackOverflow so if I do something wrong don't go too hard on me. Anyways, I have found a certain encryption in a C file I was looking in and I have the decryption code but I have no clue on how to reverse it so I can encrypt my own string into it.

    int r;
    char str[] = {"4:;145;14;81583"};
    for(r = 0; (r < 100 && str[r] != '\0'); r++)
    str[r] = str[r] - 3;

The encrypted string (4:;145;14;81583) is the IP 178.128.185.250 Like I said, I am wondering how to reverse the decryption method so that I can encrypt my own strings. Help?

This was the part that confused me. for(r = 0; (r < 100 && str[r] != '\0'); r++) — Studooo
– Studooo, Commented Aug 3, 2018 at 23:02
It just means: Continue the loop until end of string (i.e. str[r] != '\0') or at maximum 100 chars (i.e. r < 100) I have no idea why the max of 100 chars is added. Seems like pure nonsense — 4386427
– 4386427, Commented Aug 3, 2018 at 23:04

4386427 · Accepted Answer · 2018-10-03 11:32:39Z

2

It's straight forward: Decrypt is - 3 so encrypt is + 3

Decrypt

#include <stdio.h>

int main(void) {
    int r;
    char str[] = {"4:;145;14;81583"};
    for(r = 0; (r < 100 && str[r] != '\0'); r++)
        str[r] = str[r] - 3;
                //      ^ notice: minus
    printf("%s\n", str);
    return 0;
}

Output:

178.128.185.250

Encrypt

include <stdio.h>

int main(void) {
    int r;
    char str[] = {"178.128.185.250"};
    for(r = 0; (r < 100 && str[r] != '\0'); r++)
        str[r] = str[r] + 3;
                //      ^ notice: plus
    printf("%s\n", str);
    return 0;
}

Output:

4:;145;14;81583

Edit:

The line

for(r = 0; (r < 100 && str[r] != '\0'); r++)

is copied directly from OPs question. As far as I can see, it can be simplified to:

for(r = 0; str[r] != '\0'; r++)

without any problems.

edited Oct 3, 2018 at 11:32

answered Aug 3, 2018 at 22:54

4386427

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2 Comments

Yunnosch Over a year ago

I am puzzling over the r < 100. Could you explain?

4386427 Over a year ago

@Yunnosch Well, I simply copied it directly from the question and didn't wanted to change it. But you're correct that it seems strange so I added some extra text to the answer.

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