Implement challenge 15 pkcs7 validation.

src/set2.rs

@@ -390,3 +390,26 @@ pub fn challenge14() {
         roundtrip_text.to_utf8()[..17].to_string()
     );
 }
+
+pub fn challenge15() {
+    assert_eq!(
+        Bytes::from_utf8("ICE ICE BABY\u{4}\u{4}\u{4}").has_valid_pkcs7(16),
+        false
+    );
+    assert_eq!(
+        Bytes::from_utf8("ICE ICE BABY\u{4}\u{4}\u{4}\u{4}").has_valid_pkcs7(16),
+        true
+    );
+    assert_eq!(
+        Bytes::from_utf8("ICE ICE BABY\u{3}\u{3}\u{4}\u{4}").has_valid_pkcs7(16),
+        false
+    );
+    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);
+    println!("[okay] Challenge 15: PKCS7 works");
+}
+
+pub fn challenge16() {
+    println!("[xxxx] Challenge 16: TBD");
+}