Add test for bug in padding validation and get ready for challenge 18.

src/main.rs

@@ -25,4 +25,5 @@ fn main() {
     set2::challenge15();
     set2::challenge16();
     set3::challenge17();
+    set3::challenge18();
 }

src/set2.rs

@@ -404,6 +404,14 @@ pub fn challenge15() {
         Bytes::from_utf8("ICE ICE BABY\u{3}\u{3}\u{4}\u{4}").has_valid_pkcs7(16),
         false
     );
+    assert_eq!(
+        Bytes::from_utf8("ICE ICE BABY!!!\u{0}").has_valid_pkcs7(16),
+        false
+    );
+    assert_eq!(
+        Bytes::from_utf8("ICE ICE BABY!!!\u{1}").has_valid_pkcs7(16),
+        true
+    );
     let mut bytes = Bytes::from_utf8("ICE ICE BABY\u{3}\u{3}\u{4}\u{4}");
     bytes.pad_pkcs7(16);
     assert_eq!(bytes.has_valid_pkcs7(16), true);

src/set3.rs

@@ -93,3 +93,7 @@ pub fn challenge17() {
     assert_eq!(roundtrip, cleartext);
     println!("[okay] Challenge 17: {}", roundtrip.to_utf8());
 }
+
+pub fn challenge18() {
+    println!("[xxxx] Challenge 18: ");
+}