Tags: cipher hill java
Rating:
The flag is encrypted using [Hill cipher](https://en.wikipedia.org/wiki/Hill_cipher), in which every block of 3 is multiplied by a 3x3 matrix.
The official way to solve it is by solving a system of equations (using [Gaussian elimination](https://en.wikipedia.org/wiki/Gaussian_elimination) or similar methods).
Or it can be solved by bruteforcing all trigram conbinations, which is easier to implement :P
Modification of theclimb.java to allow us to reuse the code (you need to rename it to Main.java or remove the public access modifier for it to compile):
```
public String res(int len)
{
String res = "";
for (int i = 0; i < len; i++)
{
res += (char) (rmatrix[i] + 97);
}
//System.out.print(res);
return res;
}
```
And here's the solver code:
```
public class ClimbSolver {
static String encrypted = "lrzlhhombgichae";
static String key = "gybnqkurp";
public static void brute(int startPos) {
int size = (int) Math.sqrt(key.length());
String encChunk = encrypted.substring(startPos, startPos + size);
Main obj = new Main();
obj.keyconv(key, size);
for (char a = 'a'; a <= 'z'; a++)
for (char b = 'a'; b <= 'z'; b++)
for (char c = 'a'; c <= 'z'; c++) {
String text = "" + a + b + c;
obj.textconv(text);
obj.multiply(text.length());
String res = obj.res(text.length());
if (res.equals(encChunk)) {
System.out.print(text);
}
}
}
public static void main(String[] args) {
for (int i = 0; i < encrypted.length(); i += 3) {
brute(i);
}
System.out.println();
}
}
```
Output:
```
hillshaveeyesxx
```
Flag: `csictf{hillshaveeyes}`
