allow only printable ASCII characters in flag strings

flag_encoder/index.html

@@ -30,6 +30,8 @@
 				return null;
 			for (i in s) {
 				var c = s.charCodeAt(i);
+				if (c < 32 || c > 126)
+					return null;
 				h += c.toString(16).padStart(2, "0");
 			}
 			return "0x" + new BN(keccak256(h).slice(2), 16).mod(secp256k1n).toString(16).padStart(64, 0);