SoulCrabber 1 Writeup

Solved by : ava and thewhiteh4t

Solution :

• since the seed remains fixed and we know the seed the randomized values in each run will be same so we just have to inverse XOR the cipher text to get the flag
• hex decode cipher text
• convert each char to 8 bit int
• xor against random 8 bit integers
• convert integer to char

XOR Function :

    fn rand_xor(input : String) -> String {
        
        // get_rng function is called
        
        let mut rng = get_rng();
        return input
            .chars()        // converts string into array of chars
            .into_iter()    // converts vector into iterable
            // map applies a function to each element
            // {:02x} converts integers to 2 char hex format
            // u8 is 8 bit unsigned integer
            // ^ is bitwise XOR
            // XOR against random 8 bit integer
            .map(|c| format!("{:02x}", (c as u8 ^ rng.gen::<u8>())))
            .collect::<Vec<String>>()
            .join("");
    }

Get Numbers from hex string :

    fn rev_xor(input : String) {
        let mut rng = get_rng();
        let inp_arr = hex::decode(input);
        println!("{:?}", inp_arr);
    ...
    [27, 89, 20, 132, 219, 150, 47, 119, 130, 209, 65, 10, 250, 74, 56, 143, 121, 48, 6, 123, 206, 246, 223, 84, 106, 87, 217, 248, 115]

use those numbers into array and iterate it over :

    use rand::{Rng,SeedableRng};
    use rand::rngs::StdRng;
    use std::fs;
    use std::io::Write;
    
    fn get_rng() -> StdRng {
        let seed = 13371337;
        return StdRng::seed_from_u64(seed);
    }
    
    fn rand_xor(input : String) -> String {
        let mut rng = get_rng();
        //print!("{:?}",rng.gen::<u8>());
        let arr = [27, 89, 20, 132, 219, 150, 47, 119, 130, 209, 65, 10, 250, 74, 56, 143, 121, 48, 6, 123, 206, 246, 223, 84, 106, 87, 217, 248, 115];
        let space = "\t";
        for i in &arr
        {
            print!("{}", (i ^ rng.gen::<u8>()));
            print!("{}", space);
        }
        return input
            }
    
    fn main() -> std::io::Result<()> {
        let flag = fs::read_to_string("flag.txt")?;
        let xored = rand_xor(flag);
        //println!("{}", xored);
        let mut file = fs::File::create("out.txt")?;
        file.write(xored.as_bytes())?;
        Ok(())
    }