Solving Dorateatcheese's message (ROT13)

I hate number 13.

Story

2 days ago I just smashing my keyboard, but suddenly my friend Dorateatcheese, sending weird message.

No one know what he saying.

Until Jan 18th 2023, Dorateatcheese said in Zalo: You will never decode my message, this challenge me.

I tried CyberChef, but... doesn't detect anything.

How I decoded it

To find it, we need to know the magic. I'll take Uryyb thlf jung'f hc? as example.

First, find the magic. I found the word jung'f look familiar to that's or what's
Second, see the range of each word in ASCII Table. I take the range of j with t and w

| j | 106 + 10 = 116  | t |
| j | 106 + 13 = 119  | w |

Next, take the range of u with h

| u | 117 - 13 = 104  | h |

And continue, we have:

| j | 106 + 13 = 119  | w |
| u | 117 - 13 = 104  | h |
| n | 110 - 13 =  97  | a |
| g | 103 + 13 = 110  | t |
| ' | N O T   U S E D | ' | if this is space or not a char, don't.
| f | 102 + 13 = 115  | s |

This have a lot of number 13 (i hate that number), so the offset is +13 and -13

Third, I try with other character, however... If we take z for example, the ASCII code is 122, if we using +13 this will go to other one.

So, we have to use -13 instead.

Note: If UPPERCASE CHAR, the range must be 65 (A) to 90 (Z). if lowercase char, the range must be 97 (a) to 122 (z). Or else it won't be working. I fully solved in Notepad (i wanted to do it on Excel, but lazy).

Later I coded C++ code to solve the Dorateatcheese puzzle. You can check here.

Process making C++ code

Since i'm too lazy to do it by hand, I've to do it in C++

// offset is int.
if(s[i] >= 'a' && s[i] <='z' || s[i] >= 'A' && s[i] <='Z'){
    if(s[i] + offset >= 'a' && s[i] + offset <='z' || s[i] + offset >= 'A' && s[i] + offset <='Z')
        cout<<char(s[i] + offset);
    else if (s[i] - offset >= 'a' && s[i] - offset <='z' || s[i] - offset >= 'A' && s[i] - offset <='Z')
        cout<<char(s[i] - offset);
}

However, when I run it, this will fail on second rule:

If UPPERCASE CHAR, the range must be 65 (A) to 90 (Z). if lowercase char, the range must be 97 (a) to 122 (z). Or else it won't be working. So, I recode it.

for(int i=0; i<=s.size(); i++){
        if(s[i] >= 'a' && s[i] <='z'){
            if(s[i] + offset >= 'a' && s[i] + offset <='z')
                cout<<char(s[i] + offset);
            else if (s[i] - offset >= 'a' && s[i] - offset <='z')
                cout<<char(s[i] - offset);
        }
        else if (s[i] >= 'A' && s[i] <='Z'){
            if(s[i] + offset >= 'A' && s[i] + offset <='Z')
                cout<<char(s[i] + offset);
            else if (s[i] - offset >= 'A' && s[i] - offset <='Z')
                cout<<char(s[i] - offset);
        }
        else if (s[i] == ' ') cout<<" ";
        else cout<<s[i];
    }

What this thing do?

if(s[i] >= 'a' && s[i] <='z'){

}
else if (s[i] >= 'A' && s[i] <='Z'){

}

If the current i is valid lowercase char or UPPERCASE CHAR, we do something.

if(s[i] + offset >= 'a' && s[i] + offset <='z')
        cout<<char(s[i] + offset);
else if (s[i] - offset >= 'a' && s[i] - offset <='z')
        cout<<char(s[i] - offset);

This is for lowercase char
If we take current character plus the number (offset) we shift is valid char, we print out.
Else if we take current character minus the number (offset) we shift is valid char, we print out.

if(s[i] + offset >= 'a' && s[i] + offset <='z')
        cout<<char(s[i] + offset);
else if (s[i] - offset >= 'a' && s[i] - offset <='z')
        cout<<char(s[i] - offset);

Same thing as lowercase char, but this is for UPPERCASE CHAR

else if (s[i] == ' ') cout<<" ";
else cout<<s[i];

If it have space , we print
Else we print something else (like @)

The end...?

After done a while, I already have nightmare on number 13.
When I played CyberChef a while, I found out ROT13. I was thinking: oh man not number 13..., I drag it and...