Number.prototype.mod = function(n) {
    return ((this%n)+n)%n;
}

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 rotplus(result, rotStart, addEachRot);
}

function decode(str, b64Count, 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