EPUB3 OCF unobfuscate test code (not sure it is correct or not)

unobfuscate.rs

// unobfuscate test code in OCF https://www.w3.org/publishing/epub3/epub-ocf.html#obfus-algorithm
// [dependencies]
// sha-1 = "0.9.8"

use sha1::{Digest, Sha1};
use std::fs::File;
use std::io::prelude::*;
use std::path::Path;

const KEY: &[u8] = b"key string";
const SRC: &str = "resource_to_decrypt.bin";
const DEST: &str = "resource_decrypted.bin";

fn main() {
    let mut hasher = Sha1::new();
    hasher.update(KEY);
    let r = hasher.finalize();
    println!("{:?}", r);

    let path = Path::new(SRC);
    let opath = Path::new(DEST);
    let mut file = File::open(&path).unwrap();
    let mut buffer = Vec::new();
    file.read_to_end(&mut buffer).unwrap();
    for i in 0..52 {
        for (index, v) in r.iter().enumerate() {
            buffer[i * r.len() + index] = buffer[i * r.len() + index] ^ v;
        }
    }
    let mut outfile = File::create(opath).unwrap();
    outfile.write_all(&buffer).unwrap();
}