Start with challenge 18.

src/set3.rs

@@ -1,6 +1,7 @@
 use crate::bytes::Bytes;
 use crate::bytes_base64::BytesBase64;
 use crate::cbc;
+use crate::ctr;
 use rand::Rng;
 use std::io::{BufRead, BufReader};
 
@@ -95,5 +96,11 @@ pub fn challenge17() {
 }
 
 pub fn challenge18() {
-    println!("[xxxx] Challenge 18: ");
+    let key = Bytes::from_utf8("YELLOW SUBMARINE");
+    let cipher = BytesBase64::from_base64(
+        "L77na/nrFsKvynd6HzOoG7GHTLXsTVu9qvY/2syLXzhPweyyMTJULu/6/kXX0KSvoOLSFQ==",
+    )
+    .to_bytes();
+    let cleartext = ctr::decrypt(&key, &cipher).to_utf8();
+    println!("[xxxx] Challenge 18: {cleartext}");
 }