properly handle the case where the (flag-derived) key is invalid (e.g., namely, unpopulated field)

this practically rewrites the JavaScript code in the index.html file

now the tool works without manual intervention

flag_encoder/index.html

@@ -21,46 +21,43 @@
 		var inpFlag = document.querySelector("#inp-flag");
 		var outpObscuredFlag = document.querySelector("#outp-obscured-flag");
 		var outpSignature = document.querySelector("#outp-signature");
+		var secp256k1n = new BN('115792089237316195423570985008687907852837564279074904382605163141518161494337', 10);
 
-		function displayObscuredFlag() {
+		function getPrivateKey() {
-			var k = inpFlag.value;
+			var s = inpFlag.value;
-			var t = "0x";
+			var k = new BN(0);
-			for (i in k) {
+			var h = "0x";
-				t += k.charCodeAt(i).toString(16).padStart(2, "0");
+			for (i in s) {
+				var c = s.charCodeAt(i);
+				k = k.muln(16).addn(c);
+				h += c.toString(16).padStart(2, "0");
 			}
-			try {
+			if (k.mod(secp256k1n).eqn(0)) {
-				outpObscuredFlag.value = addressFromKey(t);
+				return null;
-			} catch {
+			}
+			return h;
+		}
+
+		function updateOutputValues(ev) {
+			var k = getPrivateKey();
+			if (k === null) {
 				outpObscuredFlag.value = "";
-			}
-		}
-		function displaySignature() {
-			var m = inpContestantAddress.value;
-			var k = inpFlag.value;
-			var t = "0x";
-			for (i in k) {
-				t += k.charCodeAt(i).toString(16).padStart(2, "0");
-			}
-			try {
-				outpSignature.value = signMessage(m, t);
-			} catch {
 				outpSignature.value = "";
+			} else {
+				outpObscuredFlag.value = addressFromKey(k);
+				var m = inpContestantAddress.value;
+				if (m.slice(0, 2) !== "0x") {
+					outpSignature.value = "";
+				} else {
+					outpSignature.value = signMessage(m, k);
+				}
 			}
 		}
 
-		function onContestantAddressChange(ev) {
+		inpContestantAddress.oninput = updateOutputValues;
-			displaySignature();
+		inpFlag.oninput = updateOutputValues;
-		}
-		function onFlagChange(ev) {
-			displayObscuredFlag();
-			displaySignature();
-		}
 
-		inpContestantAddress.oninput = onContestantAddressChange;
+		updateOutputValues();
-		inpFlag.oninput = onFlagChange;
-
-		displayObscuredFlag();
-		displaySignature();
 	</script>
 
 	</body>