Rating:
# FooBar NET_DOT Write Up
## Details:
Jeopardy style CTF
Category: Reverse Engineering
## Write up:
This was a .net Dll so I used dnSpy to decompile:
```c#
using System;
namespace win
{
// Token: 0x02000002 RID: 2
internal class Program
{
// Token: 0x06000001 RID: 1 RVA: 0x00002050 File Offset: 0x00000250
private static int sum_all(string password)
{
int num = 0;
foreach (char c in password)
{
num += (int)c;
}
return num;
}
// Token: 0x06000002 RID: 2 RVA: 0x0000208C File Offset: 0x0000028C
private static int check(int[] values)
{
int[] array = new int[]
{
2410,
2404,
2430,
2408,
2391,
2381,
2333,
2396,
2369,
2332,
2398,
2422,
2332,
2397,
2416,
2370,
2393,
2304,
2393,
2333,
2416,
2376,
2371,
2305,
2377,
2391
};
int result = 0;
for (int i = 0; i < array.Length; i++)
{
bool flag = array[i] == values[i];
if (!flag)
{
result = 0;
break;
}
result = 1;
}
return result;
}
// Token: 0x06000003 RID: 3 RVA: 0x000020E4 File Offset: 0x000002E4
private static void Main(string[] args)
{
Console.WriteLine("Hello there mate \nJust enter the flag to check : ");
string text = Console.ReadLine();
int[] array = new int[26];
bool flag = text.Length != 26;
if (flag)
{
Console.WriteLine("Input length error");
Console.ReadLine();
}
else
{
for (int i = 0; i < text.Length; i++)
{
array[i] = (int)text[i];
}
int[] array2 = new int[26];
for (int j = 0; j < 26; j++)
{
array2[j] = (array[j] - (j % 2 * 2 + j % 3) ^ Program.sum_all(text));
}
int num = Program.check(array2);
bool flag2 = num == 1;
if (flag2)
{
Console.WriteLine("Your flag : " + text);
Console.ReadLine();
}
else
{
Console.WriteLine("try harder");
Console.ReadLine();
}
}
}
}
}
```
For this challenge I assumed that the flag started with the correct flag text and used that to find the sum of the total text which was 2349. I then wrote a python script for reverse the key:
```python
# key in code
key = [2410, 2404, 2430, 2408, 2391, 2381, 2333, 2396, 2369, 2332, 2398, 2422, 2332, 2397, 2416, 2370, 2393, 2304, 2393, 2333, 2416, 2376, 2371, 2305, 2377, 2391]
# string to store result
s = ""
# loop through the 26 characters specified
for i in range(0, 26):
# do the needed math
s += chr((key[i]^2349)+((i%2)*2 + (i%3)))
# print
print(s)
```
This gave me the following:
```
GLUG{d0tn3t_1s_qu1t3_go0d}
```
