diff --git a/flag_encoder/index.html b/flag_encoder/index.html
index a15c5df..6f2a92f 100644
--- a/flag_encoder/index.html
+++ b/flag_encoder/index.html
@@ -27,6 +27,8 @@
 			var s = inpFlag.value;
 			var k = new BN(0);
 			var h = "0x";
+			if (s.slice(0, 5) !== "CCTF{" || s.slice(-1) !== "}")
+				return null;
 			for (i in s) {
 				var c = s.charCodeAt(i);
 				k = k.muln(16).addn(c);