Implement challenge 18 CTR cipher mode.

src/set3.rs

@@ -97,10 +97,20 @@ pub fn challenge17() {
 
 pub fn challenge18() {
     let key = Bytes::from_utf8("YELLOW SUBMARINE");
+
+    let cleartext = Bytes::from_utf8("Let's see if we can get the party started hard my friends.");
+    let cipher = ctr::encrypt(&key, 42351234, &cleartext);
+    let roundtrip = ctr::encrypt(&key, 42351234, &cipher);
+    assert_eq!(cleartext, roundtrip);
+
     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}");
+    let cleartext = ctr::decrypt(&key, 0, &cipher).to_utf8();
+    println!("[okay] Challenge 18: {cleartext}");
+}
+
+pub fn challenge19() {
+    println!("[xxxx] Challenge 19: TBD");
 }