Finish challenge 20 statistical CTR.

src/bytes.rs

@@ -93,6 +93,16 @@ impl Bytes {
         r
     }
 
+    pub fn guess_key(&self) -> u8 {
+        // Assuming all bytes have been xor-encrypted by the same u8 key, find that key
+        // by trying all u8 values and compute an ascii score for the resulting "plaintext".
+        // The u8 key for the "plaintext" with the highest score is returned as the guessed
+        // key.
+        let mut h: Vec<(Bytes, u8)> = (0..=255).map(|i| (Bytes::xor_byte(self, i), i)).collect();
+        h.sort_by(|a, b| a.0.ascii_score().partial_cmp(&b.0.ascii_score()).unwrap());
+        h.last().unwrap().1
+    }
+
     pub fn pad_pkcs7(&mut self, block_size: usize) -> () {
         let Bytes(v) = self;
         let padding_value = (block_size - v.len() % block_size) as u8;