Finish challenge 20 statistical CTR.

src/set3.rs

@@ -268,5 +268,38 @@ pub fn challenge19() {
 }
 
 pub fn challenge20() {
-    println!("[xxxx] Challenge 20: TBD");
+    fn read(path: &str) -> Vec<Bytes> {
+        let file = std::fs::File::open(path).unwrap();
+        let br = BufReader::new(file);
+        br.lines()
+            .map(|line| BytesBase64::from_base64(&line.unwrap()).to_bytes())
+            .collect()
+    }
+
+    fn attack(ciphers: Vec<Bytes>) -> Vec<Bytes> {
+        let min_cipher_len = ciphers.iter().map(|c| c.len()).min().unwrap_or(0);
+        let mut key: Vec<u8> = vec![];
+        for byte_index in 0..min_cipher_len {
+            let bytes = Bytes(ciphers.iter().map(|c| c.0[byte_index]).collect());
+            let key_char = Bytes::guess_key(&bytes);
+            key.push(key_char);
+        }
+        let key = Bytes(key);
+        ciphers
+            .iter()
+            .map(|cipher| Bytes::xor(&key, cipher))
+            .collect()
+    }
+
+    let plaintexts = read("data/20.txt");
+    let key = Bytes::from_utf8("YELLOW SUBMARINE");
+    let encrypt = |plaintext: &Bytes| -> Bytes { ctr::encrypt(&key, 0, plaintext) };
+    let ciphers: Vec<Bytes> = plaintexts.iter().map(|ct| encrypt(&ct)).collect();
+    let plaintexts = attack(ciphers);
+
+    println!("[okay] Challenge 20: {}", plaintexts[0].to_utf8());
+}
+
+pub fn challenge21() {
+    println!("[xxxx] Challenge 21: TBD");
 }