Challenge 12 decrypt first block.

src/bytes.rs

@@ -16,6 +16,10 @@ impl Bytes {
         String::from(std::str::from_utf8(&v).unwrap())
     }
 
+    pub fn first_block(&self, length: usize) -> Bytes {
+        Bytes(self.0[0..length].to_vec())
+    }
+
     pub fn random(length: usize) -> Bytes {
         Bytes(
             (0..length)