Read Minified Javascript Jan 24th 2021 Words: 7.1k
This post is created a year ago, the content may be outdated.

Background

I am trying to make a plugin that uses a 3rd party service. Lucky for me there is an web app so I can “guess” the API one by one by using Chrome Development Tools. The service is using RESTful and it seems all the arguments and and return fields are intuitive. I Thought I could finish the documentation in one night ;)

However, not all goes on well. There are two APIs that returns the data in the “encrypted” form (just obfuscated encoding actually), as in the following example:

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{"data":"MANBAYFBUEWDOQFP62SDOXFNJFNVOYP6AJAYFBU2SAYL7PKFXLJD2SFXFBVM7MWXFORDKDOABSOPZDFMWJLQ7SWMWPKQ7AJQF7PK2XPWVM7VOY2X2SDOQXYFYEYFSFUOYYFOY6CXFDOQFVR72UMW2UASDOMW2UQF2XQF2UASYFSFSWPZYFOYASDKDOOY6CR7VDKAVDKDOABSOPZDFMWJLQ7SWMWPKQ7SWAB...truncated...OMWUQ7DFMWXRDK6BDKUYEAY6I7OYDOFBUJLABAB25="}

It looks like base32, so let’s try to decode it… and FAILED.

Nahh, it can’t be that simple, right? OK, Challenge acceppted, let’s do it the hard way.

By searching the name of the API endpoint, it is easy to locate the function used to decode the message, in this case window.bays4.d, which lies in a script tag in the header of html.

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!function(t,e){"object"==typeof exports&&"object"==typeof module?module.exports=e():"function"==typeof define&&define.amd?define([],e):"object"==typeof exports?exports.bays4=e():t.bays4=e()}(this,function(){return function(r){var n={};function o(t){if(n[t])return n[t].exports;var e=n[t]={i:t,l:!1,exports:{}};return r[t].call(e.exports,e,e.exports,o),e.l=!0,e.exports}return o.m=r,o.c=n,o.i=function(t){return t},o.d=function(t,e,r){o.o(t,e)||Object.defineProperty(t,e,{configurable:!1,enumerable:!0,get:r})},o.n=function(t){var e=t&&t.__esModule?function(){return t.default}:function(){return t};return o.d(e,"a",e),e},o.o=function(t,e){return Object.prototype.hasOwnProperty.call(t,e)},o.p="",o(o.s=3)}([function(t,e,r){"use strict";Object.defineProperty(e,"__esModule",{value:!0});var n=function(){function n(t,e){for(var r=0;r<e.length;r++){var n=e[r];n.enumerable=n.enumerable||!1,n.configurable=!0,"value"in n&&(n.writable=!0),Object.defineProperty(t,n.key,n)}}return function(t,e,r){return e&&n(t.prototype,e),r&&n(t,r),t}}(),o=function(){function r(){!function(t,e){if(!(t instanceof r))throw new TypeError("Cannot call a class as a function")}(this)}return n(r,null,[{key:"loop",value:function(t,r){"v".repeat(t).split("").map(function(t,e){return r(e)})}}]),r}();e.default=o},function(t,e,r){"use strict";Object.defineProperty(e,"__esModule",{value:!0});var n=function(){function n(t,e){for(var r=0;r<e.length;r++){var n=e[r];n.enumerable=n.enumerable||!1,n.configurable=!0,"value"in n&&(n.writable=!0),Object.defineProperty(t,n.key,n)}}return function(t,e,r){return e&&n(t.prototype,e),r&&n(t,r),t}}(),o=a(r(5)),u=a(r(0));function a(t){return t&&t.__esModule?t:{default:t}}function i(t,e){if(!(t instanceof e))throw new TypeError("Cannot call a class as a function")}var f=function(){function t(){i(this,t),this._char=".",this._children={}}return n(t,[{key:"getChar",value:function(){return this._char}},{key:"getChildren",value:function(){return this._children}},{key:"setChar",value:function(t){this._char=t}},{key:"setChildren",value:function(t,e){this._children[t]=e}}]),t}(),s="ABCDEFGHIJKLMNOPQRSTUVWXYZ234567",c=[1,2,2,2,2,2],l=function(){function e(t){i(this,e),this._random=new o.default,this._sign="",this._inter={},this._head=new f}return n(e,[{key:"init",value:function(t){var e=this;this._random.seed(t),this._sign=t,u.default.loop(64,function(t){e._addSymbol("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"[t],c[parseInt((t+1)/11)])}),this._inter["="]="="}},{key:"_addSymbol",value:function(t,e){var r=this,n=this._head,o="";return u.default.loop(e,function(t){for(var e=s[r._random.generate(32)];e in n.getChildren()&&"."!==n.getChildren()[e].getChar();)e=s[r._random.generate(32)];o+=e,e in n.getChildren()||n.setChildren(e,new f),n=n.getChildren()[e]}),n.setChar(t),this._inter[t]=o}},{key:"decode",value:function(t){for(var e="",r=4;r<t.length;)if("="!==t[r]){for(var n=this._head;t[r]in n.getChildren();)n=n.getChildren()[t[r]],r++;e+=n.getChar()}else e+="=",r++;return e}}]),e}();e.default=l},function(module,exports,__webpack_require__){var __WEBPACK_AMD_DEFINE_ARRAY__,__WEBPACK_AMD_DEFINE_RESULT__,ya,za;ya="undefined"!=typeof self?self:"undefined"!=typeof window?window:"undefined"!=typeof global?global:this,za=function(global){"use strict";global=global||{};var _Base64=global.Base64,version="2.5.1",buffer;if(void 0!==module&&module.exports)try{buffer=eval("require('buffer').Buffer")}catch(t){buffer=void 0}var b64chars="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/",b64tab=function(t){for(var e={},r=0,n=t.length;r<n;r++)e[t.charAt(r)]=r;return e}(b64chars),fromCharCode=String.fromCharCode,cb_utob=function(t){if(t.length<2)return(e=t.charCodeAt(0))<128?t:e<2048?fromCharCode(192|e>>>6)+fromCharCode(128|63&e):fromCharCode(224|e>>>12&15)+fromCharCode(128|e>>>6&63)+fromCharCode(128|63&e);var e=65536+1024*(t.charCodeAt(0)-55296)+(t.charCodeAt(1)-56320);return fromCharCode(240|e>>>18&7)+fromCharCode(128|e>>>12&63)+fromCharCode(128|e>>>6&63)+fromCharCode(128|63&e)},re_utob=/[\uD800-\uDBFF][\uDC00-\uDFFFF]|[^\x00-\x7F]/g,utob=function(t){return t.replace(re_utob,cb_utob)},cb_encode=function(t){var e=[0,2,1][t.length%3],r=t.charCodeAt(0)<<16|(1<t.length?t.charCodeAt(1):0)<<8|(2<t.length?t.charCodeAt(2):0);return[b64chars.charAt(r>>>18),b64chars.charAt(r>>>12&63),2<=e?"=":b64chars.charAt(r>>>6&63),1<=e?"=":b64chars.charAt(63&r)].join("")},btoa=global.btoa?function(t){return global.btoa(t)}:function(t){return t.replace(/[\s\S]{1,3}/g,cb_encode)},_encode=buffer?buffer.from&&Uint8Array&&buffer.from!==Uint8Array.from?function(t){return(t.constructor===buffer.constructor?t:buffer.from(t)).toString("base64")}:function(t){return(t.constructor===buffer.constructor?t:new buffer(t)).toString("base64")}:function(t){return btoa(utob(t))},encode=function(t,e){return e?_encode(String(t)).replace(/[+\/]/g,function(t){return"+"==t?"-":"_"}).replace(/=/g,""):_encode(String(t))},encodeURI=function(t){return encode(t,!0)},re_btou=new RegExp(["[À-ß][€-¿]","[à-ï][€-¿]{2}","[ð-÷][€-¿]{3}"].join("|"),"g"),cb_btou=function(t){switch(t.length){case 4:var e=((7&t.charCodeAt(0))<<18|(63&t.charCodeAt(1))<<12|(63&t.charCodeAt(2))<<6|63&t.charCodeAt(3))-65536;return fromCharCode(55296+(e>>>10))+fromCharCode(56320+(1023&e));case 3:return fromCharCode((15&t.charCodeAt(0))<<12|(63&t.charCodeAt(1))<<6|63&t.charCodeAt(2));default:return fromCharCode((31&t.charCodeAt(0))<<6|63&t.charCodeAt(1))}},btou=function(t){return t.replace(re_btou,cb_btou)},cb_decode=function(t){var e=t.length,r=e%4,n=(0<e?b64tab[t.charAt(0)]<<18:0)|(1<e?b64tab[t.charAt(1)]<<12:0)|(2<e?b64tab[t.charAt(2)]<<6:0)|(3<e?b64tab[t.charAt(3)]:0),o=[fromCharCode(n>>>16),fromCharCode(n>>>8&255),fromCharCode(255&n)];return o.length-=[0,0,2,1][r],o.join("")},_atob=global.atob?function(t){return global.atob(t)}:function(t){return t.replace(/\S{1,4}/g,cb_decode)},atob=function(t){return _atob(String(t).replace(/[^A-Za-z0-9\+\/]/g,""))},_decode=buffer?buffer.from&&Uint8Array&&buffer.from!==Uint8Array.from?function(t){return(t.constructor===buffer.constructor?t:buffer.from(t,"base64")).toString()}:function(t){return(t.constructor===buffer.constructor?t:new buffer(t,"base64")).toString()}:function(t){return btou(_atob(t))},decode=function(t){return _decode(String(t).replace(/[-_]/g,function(t){return"-"==t?"+":"/"}).replace(/[^A-Za-z0-9\+\/]/g,""))},noConflict=function(){var t=global.Base64;return global.Base64=_Base64,t};if(global.Base64={VERSION:version,atob:atob,btoa:btoa,fromBase64:decode,toBase64:encode,utob:utob,encode:encode,encodeURI:encodeURI,btou:btou,decode:decode,noConflict:noConflict,__buffer__:buffer},"function"==typeof Object.defineProperty){var 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r(){!function(t,e){if(!(t instanceof r))throw new TypeError("Cannot call a class as a function")}(this)}return n(r,null,[{key:"get",value:function(t){return t>>>0}},{key:"xor",value:function(t,e){return this.get(this.get(t)^this.get(e))}},{key:"and",value:function(t,e){return this.get(this.get(t)&this.get(e))}},{key:"mul",value:function(t,e){var r=((4294901760&t)>>>0)*e,n=(65535&t)*e;return this.get((r>>>0)+(n>>>0))}},{key:"or",value:function(t,e){return this.get(this.get(t)|this.get(e))}},{key:"not",value:function(t){return this.get(~this.get(t))}},{key:"shiftLeft",value:function(t,e){return this.get(this.get(t)<<e)}},{key:"shiftRight",value:function(t,e){return this.get(t)>>>e}},{key:"mod",value:function(t,e){return this.get(this.get(t)%e)}}]),r}();e.default=o},function(t,e,r){"use strict";Object.defineProperty(e,"__esModule",{value:!0});var n=function(){function n(t,e){for(var r=0;r<e.length;r++){var n=e[r];n.enumerable=n.enumerable||!1,n.configurable=!0,"value"in n&&(n.writable=!0),Object.defineProperty(t,n.key,n)}}return function(t,e,r){return e&&n(t.prototype,e),r&&n(t,r),t}}(),o=a(r(0)),u=a(r(4));function a(t){return t&&t.__esModule?t:{default:t}}var i=function(){function r(){!function(t,e){if(!(t instanceof r))throw new TypeError("Cannot call a class as a function")}(this),this._status=[],this._mat1=0,this._mat2=0,this._tmat=0}return n(r,[{key:"seed",value:function(e){var r=this;o.default.loop(4,function(t){e.length>t?r._status[t]=u.default.get(e.charAt(t).charCodeAt()):r._status[t]=u.default.get(110)}),this._mat1=this._status[1],this._mat2=this._status[2],this._tmat=this._status[3],this._init()}},{key:"_init",value:function(){var e=this;o.default.loop(7,function(t){e._status[t+1&3]=u.default.xor(e._status[t+1&3],t+1+u.default.mul(1812433253,u.default.xor(e._status[3&t],u.default.shiftRight(e._status[3&t],30))))}),0==(2147483647&this._status[0])&&0===this._status[1]&&0===this._status[2]&&0===this._status[3]&&(this._status[0]=66,this._status[1]=65,this._status[2]=89,this._status[3]=83),o.default.loop(8,function(){return e._next_state()})}},{key:"_next_state",value:function(){var t=void 0,e=void 0;e=this._status[3],t=u.default.xor(u.default.and(this._status[0],2147483647),u.default.xor(this._status[1],this._status[2])),t=u.default.xor(t,u.default.shiftLeft(t,1)),e=u.default.xor(e,u.default.xor(u.default.shiftRight(e,1),t)),this._status[0]=this._status[1],this._status[1]=this._status[2],this._status[2]=u.default.xor(t,u.default.shiftLeft(e,10)),this._status[3]=e,this._status[1]=u.default.xor(this._status[1],u.default.and(-u.default.and(e,1),this._mat1)),this._status[2]=u.default.xor(this._status[2],u.default.and(-u.default.and(e,1),this._mat2))}},{key:"generate",value:function(t){this._next_state();var e,r=void 0;return r=this._status[3],e=u.default.xor(this._status[0],u.default.shiftRight(this._status[2],8)),r=u.default.xor(r,e),(r=u.default.xor(u.default.and(-u.default.and(e,1),this._tmat),r))%t}}]),r}();e.default=i}])})

After change the input blob several times, I found the decryption script is generic. There is no dynamic server generated keys, that’s a good news.

Of course, many libraries in Python or Node are able to evaluate the js script directly, so you don’t need to dig into the minified shit, but I want to do it just for fun.

Workspace setup

First, fire up a new project with my old friend Webstorm.

Now create an index file which loads the messy blob and invokes the decryption method once. 

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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>TEST</title>
    <script src="./decode.js"></script>
    <script>
        console.log(window.bays4.d("3GL7DI5MHU7CX3RD426CX4SFIWYXWRD4DGDI5MHWRDGP25DUEQQUKQ5EDXWRJ...truncated...X2Q26GTE3UKBAWRX2Q5D4QWPNHP2DGP2BAECCX2QPB4PCXUGHQ5UGP2263JXWRJXJQDIUGD44VCXP24GJQRCPB5DW5"))
    </script>
</head>
<body>

</body>
</html>

Now the code can be debugged with fancy tools like breakpoints and variable tracker.

Beautify the file

Replace !0 to true and !1 to false

Ctrl + R

Manually add curly bracket

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// short
for (var i = 0; i < 5; i++) this_is_a_very_long_expression()

// long
for (var i = 0; i < 5; i++) {
    this_is_a_very_long_expression()
}
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// short
a == 1 && b === 2 && c !== 3 && this_is_a_very_long_expression();

// long
if (a == 1 &&
    b === 2 &&
    c !== 3) {
        this_is_a_very_long_expression();
    }

Substitute comma expression

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// short
var a,b,c = x

// long
var a = x;
var b = x;
var c = x;
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// short
return do_a(), b = x, do_c(y)

// long
do_a();
b = x;
return do_c(y);

Search for keyword of library

Final JS output

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!function (t, e) {
    "object" == typeof exports && "object" == typeof module ? module.exports = e() : "function" == typeof define && define.amd ? define([], e) : "object" == typeof exports ? exports.bays4 = e() : t.bays4 = e()
}
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                    for (var r = 0; r < e.length; r++) {
                        var n = e[r];
                        n.enumerable = n.enumerable || false, n.configurable = true, "value" in n && (n.writable = true), Object.defineProperty(t, n.key, n)
                    }
                }

                return function (t, e, r) {
                    return e && n(t.prototype, e), r && n(t, r), t
                }
            }();
            const Looper = function () {
                function constructor() {
                    !function (t, e) {
                        if (!(t instanceof constructor)) throw new TypeError("Cannot call a class as a function")
                    }(this)
                }

                setClassFunc(constructor, null, [{
                    // just alias of a basic for loop with count
                    key: "loop", value: function (count, func) {
                        "v".repeat(count).split("").map(function (currVal, index) {
                            return func(index)
                        })
                    }
                }]);
                return constructor;
            }();
            exports.default = Looper;
        },
        /** Decryption class */
        function (module, exports, __webpack_require__) {
            "use strict";
            Object.defineProperty(exports, "__esModule", {value: true});
            const setClassFunc = function () {
                function n(t, e) {
                    for (var r = 0; r < e.length; r++) {
                        var n = e[r];
                        n.enumerable = n.enumerable || false, n.configurable = true, "value" in n && (n.writable = true), Object.defineProperty(t, n.key, n)
                    }
                }

                return function (t, e, r) {
                    return e && n(t.prototype, e), r && n(t, r), t
                }
            }();
            const Random = a(__webpack_require__(5));
            const Looper = a(__webpack_require__(0));

            function a(t) {
                return t && t.__esModule ? t : {default: t}
            }

            function checkIsCallable(t, e) {
                if (!(t instanceof e)) throw new TypeError("Cannot call a class as a function")
            }

            const Head = function () {
                function constructor() {
                    checkIsCallable(this, constructor);
                    this._char = ".";
                    this._children = {};
                }

                setClassFunc(constructor, [
                    {
                        key: "getChar", value: function () {
                            return this._char;
                        }
                    },
                    {
                        key: "getChildren", value: function () {
                            return this._children;
                        }
                    },
                    {
                        key: "setChar", value: function (char) {
                            this._char = char;
                        }
                    },
                    {
                        key: "setChildren", value: function (key, value) {
                            this._children[key] = value;
                        }
                    }]);
                return constructor;
            }();
            const cipherSet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";
            const Decryption = function () {
                function constructor(t) {
                    checkIsCallable(this, constructor);
                    this._random = new Random.default;
                    this._inter = {};
                    this._head = new Head;
                }

                setClassFunc(constructor, [
                    {
                        key: "init", value: function (seed) {
                            const self = this;
                            this._random.seed(seed);
                            Looper.default.loop(64, function (index) {
                                self._addSymbol(
                                    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"[index],
                                    [1, 2, 2, 2, 2, 2][parseInt((index + 1) / 11)])
                            });
                            this._inter["="] = "="
                        }
                    },
                    {
                        key: "_addSymbol", value: function (plainChar, count) {
                            const self = this;
                            let head = this._head;
                            let cipherStr = "";
                            Looper.default.loop(count, function () {
                                let char = cipherSet[self._random.generate(32)];
                                for (; char in head.getChildren() && "." !== head.getChildren()[char].getChar();) {
                                    char = cipherSet[self._random.generate(32)];
                                }
                                cipherStr += char;
                                char in head.getChildren() || head.setChildren(char, new Head);
                                head = head.getChildren()[char];    // cautious: `head` changes the reference
                            });
                            head.setChar(plainChar);
                            this._inter[plainChar] = cipherStr;
                        }
                    },
                    {
                        key: "decode", value: function (text) {
                            let result = ""
                            for (let index = 4; index < text.length;) {
                                if (text[index] !== "=") {
                                    let head = this._head;
                                    while (text[index] in head.getChildren()) {
                                        head = head.getChildren()[text[index]];
                                        index++;
                                    }
                                    result += head.getChar();
                                } else {
                                    result += "=";
                                    index++;
                                }
                            }
                            return result;
                        }
                    }]);
                return constructor;
            }();
            exports.default = Decryption;
        },
        /** Base64 library */
        function (module, exports, __webpack_require__) {
            var __WEBPACK_AMD_DEFINE_ARRAY__, __WEBPACK_AMD_DEFINE_RESULT__, ya, za;
            ya = "undefined" != typeof self ? self : "undefined" != typeof window ? window : "undefined" != typeof global ? global : this, za = function (global) {
                "use strict";
                global = global || {};
                var _Base64 = global.Base64, version = "2.5.1", buffer;
                if (void 0 !== module && module.exports) try {
                    buffer = eval("require('buffer').Buffer")
                } catch (t) {
                    buffer = void 0
                }
                var b64chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/",
                    b64tab = function (t) {
                        for (var e = {}, r = 0, n = t.length; r < n; r++) e[t.charAt(r)] = r;
                        return e
                    }(b64chars), fromCharCode = String.fromCharCode, cb_utob = function (t) {
                        if (t.length < 2) return (e = t.charCodeAt(0)) < 128 ? t : e < 2048 ? fromCharCode(192 | e >>> 6) + fromCharCode(128 | 63 & e) : fromCharCode(224 | e >>> 12 & 15) + fromCharCode(128 | e >>> 6 & 63) + fromCharCode(128 | 63 & e);
                        var e = 65536 + 1024 * (t.charCodeAt(0) - 55296) + (t.charCodeAt(1) - 56320);
                        return fromCharCode(240 | e >>> 18 & 7) + fromCharCode(128 | e >>> 12 & 63) + fromCharCode(128 | e >>> 6 & 63) + fromCharCode(128 | 63 & e)
                    }, re_utob = /[\uD800-\uDBFF][\uDC00-\uDFFFF]|[^\x00-\x7F]/g, utob = function (t) {
                        return t.replace(re_utob, cb_utob)
                    }, cb_encode = function (t) {
                        var e = [0, 2, 1][t.length % 3],
                            r = t.charCodeAt(0) << 16 | (1 < t.length ? t.charCodeAt(1) : 0) << 8 | (2 < t.length ? t.charCodeAt(2) : 0);
                        return [b64chars.charAt(r >>> 18), b64chars.charAt(r >>> 12 & 63), 2 <= e ? "=" : b64chars.charAt(r >>> 6 & 63), 1 <= e ? "=" : b64chars.charAt(63 & r)].join("")
                    }, btoa = global.btoa ? function (t) {
                        return global.btoa(t)
                    } : function (t) {
                        return t.replace(/[\s\S]{1,3}/g, cb_encode)
                    }, _encode = buffer ? buffer.from && Uint8Array && buffer.from !== Uint8Array.from ? function (t) {
                        return (t.constructor === buffer.constructor ? t : buffer.from(t)).toString("base64")
                    } : function (t) {
                        return (t.constructor === buffer.constructor ? t : new buffer(t)).toString("base64")
                    } : function (t) {
                        return btoa(utob(t))
                    }, encode = function (t, e) {
                        return e ? _encode(String(t)).replace(/[+\/]/g, function (t) {
                            return "+" == t ? "-" : "_"
                        }).replace(/=/g, "") : _encode(String(t))
                    }, encodeURI = function (t) {
                        return encode(t, true)
                    }, re_btou = new RegExp(["[À-ß][€-¿]", "[à-ï][€-¿]{2}", "[ð-÷][€-¿]{3}"].join("|"), "g"),
                    cb_btou = function (t) {
                        switch (t.length) {
                            case 4:
                                var e = ((7 & t.charCodeAt(0)) << 18 | (63 & t.charCodeAt(1)) << 12 | (63 & t.charCodeAt(2)) << 6 | 63 & t.charCodeAt(3)) - 65536;
                                return fromCharCode(55296 + (e >>> 10)) + fromCharCode(56320 + (1023 & e));
                            case 3:
                                return fromCharCode((15 & t.charCodeAt(0)) << 12 | (63 & t.charCodeAt(1)) << 6 | 63 & t.charCodeAt(2));
                            default:
                                return fromCharCode((31 & t.charCodeAt(0)) << 6 | 63 & t.charCodeAt(1))
                        }
                    }, btou = function (t) {
                        return t.replace(re_btou, cb_btou)
                    }, cb_decode = function (t) {
                        var e = t.length, r = e % 4,
                            n = (0 < e ? b64tab[t.charAt(0)] << 18 : 0) | (1 < e ? b64tab[t.charAt(1)] << 12 : 0) | (2 < e ? b64tab[t.charAt(2)] << 6 : 0) | (3 < e ? b64tab[t.charAt(3)] : 0),
                            o = [fromCharCode(n >>> 16), fromCharCode(n >>> 8 & 255), fromCharCode(255 & n)];
                        return o.length -= [0, 0, 2, 1][r], o.join("")
                    }, _atob = global.atob ? function (t) {
                        return global.atob(t)
                    } : function (t) {
                        return t.replace(/\S{1,4}/g, cb_decode)
                    }, atob = function (t) {
                        return _atob(String(t).replace(/[^A-Za-z0-9\+\/]/g, ""))
                    }, _decode = buffer ? buffer.from && Uint8Array && buffer.from !== Uint8Array.from ? function (t) {
                        return (t.constructor === buffer.constructor ? t : buffer.from(t, "base64")).toString()
                    } : function (t) {
                        return (t.constructor === buffer.constructor ? t : new buffer(t, "base64")).toString()
                    } : function (t) {
                        return btou(_atob(t))
                    }, decode = function (t) {
                        return _decode(String(t).replace(/[-_]/g, function (t) {
                            return "-" == t ? "+" : "/"
                        }).replace(/[^A-Za-z0-9\+\/]/g, ""))
                    }, noConflict = function () {
                        var t = global.Base64;
                        return global.Base64 = _Base64, t
                    };
                if (global.Base64 = {
                    VERSION: version,
                    atob: atob,
                    btoa: btoa,
                    fromBase64: decode,
                    toBase64: encode,
                    utob: utob,
                    encode: encode,
                    encodeURI: encodeURI,
                    btou: btou,
                    decode: decode,
                    noConflict: noConflict,
                    __buffer__: buffer
                }, "function" == typeof Object.defineProperty) {
                    var noEnum = function (t) {
                        return {value: t, enumerable: false, writable: true, configurable: true}
                    };
                    global.Base64.extendString = function () {
                        Object.defineProperty(String.prototype, "fromBase64", noEnum(function () {
                            return decode(this)
                        })), Object.defineProperty(String.prototype, "toBase64", noEnum(function (t) {
                            return encode(this, t)
                        })), Object.defineProperty(String.prototype, "toBase64URI", noEnum(function () {
                            return encode(this, true)
                        }))
                    }
                }
                return global.Meteor && (Base64 = global.Base64), void 0 !== module && module.exports ? module.exports.Base64 = global.Base64 : (__WEBPACK_AMD_DEFINE_ARRAY__ = [], __WEBPACK_AMD_DEFINE_RESULT__ = function () {
                    return global.Base64
                }.apply(exports, __WEBPACK_AMD_DEFINE_ARRAY__), void 0 === __WEBPACK_AMD_DEFINE_RESULT__ || (module.exports = __WEBPACK_AMD_DEFINE_RESULT__)), {Base64: global.Base64}
            }, module.exports = za(ya)
        },
        /** Decryption helper class */
        function (module, exports, __webpack_require__) {
            "use strict";
            Object.defineProperty(exports, "__esModule", {value: true});
            const setClassFunc = function () {
                function n(t, e) {
                    for (var r = 0; r < e.length; r++) {
                        var n = e[r];
                        n.enumerable = n.enumerable || false, n.configurable = true, "value" in n && (n.writable = true), Object.defineProperty(t, n.key, n)
                    }
                }

                return function (t, e, r) {
                    return e && n(t.prototype, e), r && n(t, r), t
                }
            }();
            const Base64 = __webpack_require__(2);
            let Decryption = __webpack_require__(1);
            Decryption = Decryption.__esModule ? Decryption : {default: Decryption};
            const charToMagicInt = function (text) {
                var first_char_code = text.charCodeAt();
                return first_char_code >= 65 ? first_char_code - 65 : first_char_code - 65 + 41
            };
            const DecryptionHelper = function () {
                function constructor() {
                    !function (t, e) {
                        if (!(t instanceof constructor)) throw new TypeError("Cannot call a class as a function")
                    }(this)
                }

                setClassFunc(constructor, null, [
                    {
                        key: "_checkVersion", value: function (t) {
                            return ((32 * charToMagicInt(t[0]) + charToMagicInt(t[1])) * charToMagicInt(t[2]) + charToMagicInt(t[3])) % 32 <= 1
                        }
                    },
                    {
                        key: "d", value: function (encrypted_text) {
                            if (!this._checkVersion(encrypted_text)) return "";
                            var decryptor = new Decryption.default;
                            decryptor.init(encrypted_text.substr(0, 4));
                            var base64_text = decryptor.decode(encrypted_text);
                            return Base64.Base64.decode(base64_text)
                        }
                    }]);
                return constructor;
            }();
            exports.default = DecryptionHelper;
            module.exports = DecryptionHelper;
        },
        /** Bitwise operation class */
        function (module, exports, __webpack_require__) {
            "use strict";
            Object.defineProperty(exports, "__esModule", {value: true});
            const setClassFunc = function () {
                function n(t, e) {
                    for (var r = 0; r < e.length; r++) {
                        var n = e[r];
                        n.enumerable = n.enumerable || false, n.configurable = true, "value" in n && (n.writable = true), Object.defineProperty(t, n.key, n)
                    }
                }

                return function (t, e, r) {
                    return e && n(t.prototype, e), r && n(t, r), t
                }
            }();
            const BitOp = function () {
                function constructor() {
                    !function (t, e) {
                        if (!(t instanceof constructor)) throw new TypeError("Cannot call a class as a function")
                    }(this)
                }

                setClassFunc(constructor, null, [
                    {
                        key: "get", value: function (t) {
                            return t >>> 0
                        }
                    },
                    {
                        key: "xor", value: function (t, e) {
                            return this.get(this.get(t) ^ this.get(e))
                        }
                    },
                    {
                        key: "and", value: function (t, e) {
                            return this.get(this.get(t) & this.get(e))
                        }
                    },
                    {
                        key: "mul", value: function (t, e) {
                            var r = ((4294901760 & t) >>> 0) * e, n = (65535 & t) * e;
                            return this.get((r >>> 0) + (n >>> 0))
                        }
                    },
                    {
                        key: "or", value: function (t, e) {
                            return this.get(this.get(t) | this.get(e))
                        }
                    },
                    {
                        key: "not", value: function (t) {
                            return this.get(~this.get(t))
                        }
                    },
                    {
                        key: "shiftLeft", value: function (t, e) {
                            return this.get(this.get(t) << e)
                        }
                    },
                    {
                        key: "shiftRight", value: function (t, e) {
                            return this.get(t) >>> e
                        }
                    },
                    {
                        key: "mod", value: function (t, e) {
                            return this.get(this.get(t) % e)
                        }
                    }]);
                return constructor;
            }();
            exports.default = BitOp;
        },
        /** Random generator class */
        function (module, exports, __webpack_require__) {
            "use strict";
            Object.defineProperty(exports, "__esModule", {value: true});
            const setClassFunc = function () {
                function n(t, e) {
                    for (var r = 0; r < e.length; r++) {
                        var n = e[r];
                        n.enumerable = n.enumerable || false, n.configurable = true, "value" in n && (n.writable = true), Object.defineProperty(t, n.key, n)
                    }
                }

                return function (t, e, r) {
                    return e && n(t.prototype, e), r && n(t, r), t
                }
            }();
            const Looper = a(__webpack_require__(0));
            const BitOp = a(__webpack_require__(4));

            function a(t) {
                return t && t.__esModule ? t : {default: t}
            }

            var Random = function () {
                function constructor() {
                    !function (t, e) {
                        if (!(t instanceof constructor)) throw new TypeError("Cannot call a class as a function")
                    }(this);
                    this._status = [];
                    this._mat1 = 0;
                    this._mat2 = 0;
                    this._tmat = 0;
                }

                setClassFunc(constructor, [
                    {
                        key: "seed", value: function (seed) {
                            const self = this;
                            Looper.default.loop(4, function (index) {
                                seed.length > index ?
                                    self._status[index] = BitOp.default.get(seed.charAt(index).charCodeAt()) :
                                    self._status[index] = BitOp.default.get(110);
                            });
                            this._mat1 = this._status[1];
                            this._mat2 = this._status[2];
                            this._tmat = this._status[3];
                            this._init();
                        }
                    },
                    {
                        key: "_init", value: function () {
                            const self = this;
                            Looper.default.loop(7, function (index) {
                                self._status[index + 1 & 3] = BitOp.default.xor(self._status[index + 1 & 3],
                                    index + 1 + BitOp.default.mul(1812433253,
                                    BitOp.default.xor(self._status[3 & index],
                                        BitOp.default.shiftRight(self._status[3 & index], 30))))
                            });
                            if (0 == (2147483647 & this._status[0]) &&
                                0 === this._status[1] &&
                                0 === this._status[2] &&
                                0 === this._status[3]) {
                                this._status[0] = 66;
                                this._status[1] = 65;
                                this._status[2] = 89;
                                this._status[3] = 83;
                            }
                            Looper.default.loop(8, function () {
                                return self._next_state();
                            });
                        }
                    },
                    {
                        key: "_next_state", value: function () {
                            let n1, n2;
                            n1 = BitOp.default.xor(BitOp.default.and(this._status[0], 2147483647),
                                BitOp.default.xor(this._status[1], this._status[2]));
                            n2 = this._status[3];
                            n1 = BitOp.default.xor(n1, BitOp.default.shiftLeft(n1, 1));
                            n2 = BitOp.default.xor(n2, BitOp.default.xor(BitOp.default.shiftRight(n2, 1), n1));
                            this._status[0] = this._status[1];
                            this._status[1] = this._status[2];
                            this._status[2] = BitOp.default.xor(n1, BitOp.default.shiftLeft(n2, 10));
                            this._status[3] = n2;
                            this._status[1] = BitOp.default.xor(this._status[1],
                                BitOp.default.and(-BitOp.default.and(n2, 1), this._mat1));
                            this._status[2] = BitOp.default.xor(this._status[2],
                                BitOp.default.and(-BitOp.default.and(n2, 1), this._mat2))
                        }
                    },
                    {
                        key: "generate", value: function (range) {
                            this._next_state();
                            let n1, n2;
                            n1 = BitOp.default.xor(this._status[0],
                                BitOp.default.shiftRight(this._status[2], 8));
                            n2 = this._status[3];
                            n2 = BitOp.default.xor(n2, n1);
                            return (BitOp.default.xor(BitOp.default.and(-BitOp.default.and(n1, 1), this._tmat), n2)) % range;
                        }
                    }]);
                return constructor
            }();
            exports.default = Random
        }
    ])
})

Python script for decryption

Caution:

  • Javascript bitwise operation uses int32, where in Python, there is no such bit limit, so it is better to wrap bitwise operation(especially shiftting) in the mask function. 
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import base64
import json


def int32(num: int):
    return int(0xFFFFFFFF & num)


class CustomMersenneTwister:
    _status: list
    _mat1: int
    _mat2: int
    _tmat: int

    def __init__(self, seed: str):
        self._status = list()
        self._mat1 = 0
        self._mat2 = 0
        self._tmat = 0
        for index in range(4):
            self._status.append(ord(seed[index]) if len(seed) > index else 110)
        self._mat1 = self._status[1]
        self._mat2 = self._status[2]
        self._tmat = self._status[3]

        for index in range(7):
            self._status[index + 1 & 3] ^= int32(index + 1 + int32(1812433253 * (
                    self._status[3 & index] ^ int32(self._status[3 & index] >> 30))))
        if 0 == (2147483647 & self._status[0]) and \
                0 == self._status[1] and \
                0 == self._status[2] and \
                0 == self._status[3]:
            self._status = [66, 65, 89, 83]
        for _ in range(8):
            self._twist()

    def _twist(self):
        n1 = (self._status[0] & 2147483647) ^ (self._status[1] ^ self._status[2])
        n2 = self._status[3]
        n1 ^= int32(n1 << 1)
        n2 ^= int32(n2 >> 1) ^ n1
        self._status[0] = self._status[1]
        self._status[1] = self._status[2]
        self._status[2] = n1 ^ int32(n2 << 10)
        self._status[3] = n2
        self._status[1] ^= -(n2 & 1) & self._mat1
        self._status[2] ^= -(n2 & 1) & self._mat2

    def generate(self, range: int):
        self._twist()
        n1 = self._status[0] ^ int32(self._status[2] >> 8)
        n2 = self._status[3]
        n2 ^= n1
        return ((-(n1 & 1) & self._tmat) ^ n2) % range


class Decryption:
    _head: dict
    _rng: CustomMersenneTwister
    _mapping: dict

    def __init__(self, seed: str):
        self._head = {
            "char": ".",
            "children": dict()
        }
        self._rng = CustomMersenneTwister(seed)
        self._init_mapping_dict()

    def _init_mapping_dict(self):
        self._mapping = dict()
        base64_charset = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
        cipher_charset = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
        for i in range(64):
            plain_char = base64_charset[i]
            cipher_str = ""
            head = self._head
            for _ in range([1, 2, 2, 2, 2, 2][int((i + 1) / 11)]):
                char = cipher_charset[self._rng.generate(32)]
                while char in head["children"].keys() and head["children"][char]["char"] != ".":
                    char = cipher_charset[self._rng.generate(32)]
                cipher_str += char
                if not char in head["children"].keys():
                    head["children"][char] = {
                        "char": ".",
                        "children": dict()
                    }
                head = head["children"][char]
            head["char"] = plain_char
            self._mapping[plain_char] = cipher_str
        self._mapping["="] = "="

    def decode_to_base64(self, encrypted_text: str):
        result = list()
        i = 4
        while i < len(encrypted_text):
            if encrypted_text[i] != "=":
                head = self._head
                while i < len(encrypted_text) and encrypted_text[i] in head["children"].keys():
                    head = head["children"][encrypted_text[i]]
                    i += 1
                result.append(head["char"])
            else:
                result.append("=")
                i += 1
        return ''.join(result)

    def decode_to_json(self, encrypt_text: str):
        return base64.b64decode(self.decode_to_base64(encrypt_text))


if __name__ == "__main__":
    # test
    rng = CustomMersenneTwister("3GL7")
    ans = [12, 18, 13, 10, 10, 11, 19, 21, 13, 31, 23]
    for i in range(11):
        assert rng.generate(32) == ans[i]
    decryption = Decryption("3GL7")

    ans = {
        "0": "ED",
        "1": "5D",
        "2": "P2",
        "3": "4S",
        "4": "ZL",
        "5": "Q5",
        "6": "GT",
        "7": "PJ",
        "8": "C2",
        "9": "W5",
        "A": "M",
        "B": "S",
        "C": "N",
        "D": "K",
        "E": "L",
        "F": "T",
        "G": "V",
        "H": "7",
        "I": "X",
        "J": "H",
        "K": "I3",
        "L": "WP",
        "M": "4G",
        "N": "BA",
        "O": "ZX",
        "P": "JA",
        "Q": "U2",
        "R": "FI",
        "S": "J7",
        "T": "J3",
        "U": "3B",
        "V": "PB",
        "W": "UK",
        "X": "UG",
        "Y": "CX",
        "Z": "QQ",
        "a": "ZA",
        "b": "DG",
        "c": "E3",
        "d": "4Q",
        "e": "DI",
        "f": "RC",
        "g": "P5",
        "h": "EC",
        "i": "JQ",
        "j": "WR",
        "k": "3J",
        "l": "26",
        "m": "3R",
        "n": "2Q",
        "o": "JX",
        "p": "D4",
        "q": "UV",
        "r": "UW",
        "s": "4P",
        "t": "UE",
        "u": "A4",
        "v": "U7",
        "w": "IX",
        "x": "4V",
        "y": "5M",
        "z": "WY",
        "+": "EX",
        "/": "A2",
        "=": "="
    }
    for k, v in ans.items():
        assert decryption._mapping[k] == v