View difference between Paste ID: 3kLdRnCa and KnMb0ZWM

 Number.prototype.mod = function(n) {
     return ((this%n)+n)%n;
 }
 
-function rotXMod(str, startIdx, addEachRot) {
+function rotplus(str, startIdx, addEachRot) {
     var result = '';
 	for (var i = 0; i < str.length; ++i) {
 		var rot = (startIdx + i * addEachRot);
 		var c = str.charCodeAt(i) - 33 + rot;
 		result += String.fromCharCode(33 + (c.mod(90)));
     }
 	return result;
 }
 
 function encode(str, b64Count, rotStart, addEachRot) {
     result = str;
     for (var i = 0; i < b64Count; ++i) {
         result = btoa(result);
     }
 
-    return rotXMod(result, rotStart, addEachRot);
+    return rotplus(result, rotStart, addEachRot);
 }
 
 function decode(str, b64Count, rotStart, addEachRot) {
-    result = undoRotXMod(str, -rotStart, -addEachRot);
+    result = rotplus(str, -rotStart, -addEachRot); // to undo rotplus, just pass parameters as negative
     for (var i = 0; i < b64Count; ++i) {
         result = atob(result);
     }
     return result;
 }
 
 // The following code is a way you can brute-force any parameters.
 // The use of base64 as the final decoding step means _most_ parameters are
 // obviously invalid as they don't result in a valid base64 string.
 
 // additionally, due to the use of modulus there is a set amount of values
 // that can be used for rotStart and addEachRot so brute-forcing is a
 // pretty reasonable thing to do
 
 for (var b64Count = 0; b64Count < 30; ++b64Count) {
 	for (var rs = 0; rs < 90; ++rs) {
 		for (var aer = 0; aer < 90; ++aer) {
 			try {
 				console.log(decode(str, b64Count, rs, aer) + " (used params " + b64Count + "," + rs + "," + aer + ")"); // replace str with whatever you're trying to decode
 			}
 			catch (e) {
 				// if an error is thrown (from atob(), which is the base64 decoder) then
 				// we know this solution is not valid.
 			}
 		}
 	}
 }
 
 // after running the above, you should get a bunch of text in your console. It should
 // be fairly obvious which bit of text is the correct answer