Tags: crypto rust
Rating:
# SoulCrabber 2
## Description
Aliens realised that hard-coded values are bad, so added a little bit of entropy.
```rust
use rand::{Rng,SeedableRng};
use rand::rngs::StdRng;
use std::fs;
use std::io::Write;
use std::time::SystemTime;
fn get_rng() -> StdRng {
let seed = SystemTime::now()
.duration_since(SystemTime::UNIX_EPOCH)
.expect("Time is broken")
.as_secs();
return StdRng::seed_from_u64(seed);
}
fn rand_xor(input : String) -> String {
let mut rng = get_rng();
return input
.chars()
.into_iter()
.map(|c| format!("{:02x}", (c as u8 ^ rng.gen::<u8>())))
.collect::<Vec<String>>()
.join("");
}
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(())
}
```
`418a5175c38caf8c1cafa92cde06539d512871605d06b2d01bbc1696f4ff487e9d46ba0b5aaf659807`
## Solution
```rust
use rand::{Rng,SeedableRng};
use rand::rngs::StdRng;
use std::time::SystemTime;
fn get_rng() -> StdRng {
let seed = SystemTime::now()
.duration_since(SystemTime::UNIX_EPOCH)
.expect("Time is broken")
.as_secs();
return StdRng::seed_from_u64(seed);
}
fn is_sub<T: PartialEq>(mut haystack: &[T], needle: &[T]) -> bool {
if needle.len() == 0 { return true; }
while !haystack.is_empty() {
if haystack.starts_with(needle) { return true; }
haystack = &haystack[1..];
}
false
}
fn rand_xor(input : &Vec<u8>, mut rng: StdRng) -> Vec<u8> {
return input
.into_iter()
.map(|c| c ^ rng.gen::<u8>())
.collect::<Vec<u8>>();
}
fn main() -> std::io::Result<()> {
let out: Vec<u8> = vec![65, 138, 81, 117, 195, 140, 175, 140, 28, 175, 169, 44, 222, 6, 83, 157, 81, 40, 113, 96, 93, 6, 178, 208, 27, 188, 22, 150, 244, 255, 72, 126, 157, 70, 186, 11, 90, 175, 101, 152, 7];
let first_bytes: Vec<u8> = vec![67, 72, 84, 66, 123];
let mut seed = SystemTime::now()
.duration_since(SystemTime::UNIX_EPOCH)
.expect("Time is broken")
.as_secs();
loop {
let mut rng = StdRng::seed_from_u64(seed);
let r = rand_xor(&out, rng);
if is_sub(&r, &first_bytes) {
let s = r
.into_iter()
.map(|c| format!("{:02x}", c))
.collect::<Vec<String>>()
.join("");
println!("{}", s);
break;
}
seed = seed - 1;
}
Ok(())
}
```
and then:
```python
from binascii import unhexlify
unhexlify("434854427b636c34353531635f6368346c6c336e67335f72337772317474336e5f316e5f727535747d")
```
