How to Decode Obfuscated JS Video URLs with Python: A Step‑by‑Step Guide

Introduction

Hello, I am a Python advanced learner.

JS Encryption Function

The target website uses a JavaScript encryption function to hide the video URL. The encrypted function is shown below.

The core of the encryption is the decodeMp4.decode() function, which relies on several helper functions.

define("tool", function(a, b, c) {
    var d = a("jquery"),
        e = a("support"),
        f = a("constants"),
        g = a("base64"),
        h = "substring",
        i = "split",
        j = "replace",
        k = "substr";
    b.decodeMp4 = {
        getHex: function(a) {
            return { str: a[h](4), hex: a[h](0, 4)[i]("").reverse().join("") };
        },
        getDec: function(a) {
            var b = parseInt(a, 16).toString();
            return { pre: b[h](0, 2)[i](""), tail: b[h](2)[i]("") };
        },
        substr: function(a, b) {
            var c = a[h](0, b[0]), d = a[k](b[0], b[1]);
            return c + a[h](b[0])[j](d, "");
        },
        getPos: function(a, b) {
            return b[0] = a.length - b[0] - b[1], b;
        },
        decode: function(a) {
            var b = this.getHex(a), c = this.getDec(b.hex), d = this[k](b.str, c.pre);
            return g.atob(this[k](d, this.getPos(d, c.tail)));
        }
    };
});

Conversion Process

Each helper function is re‑implemented in Python to reproduce the same behavior.

1. getHex

def getHex(a):
    return {
        "str": a[4:],  # JS substring(4)
        "hex": "".join(list(a[0:4])[::-1])  # reverse first 4 chars
    }

2. getDec

def getDec(a):
    b = str(int(a, 16))
    return {
        "pre": list(b[:2]),
        "tail": list(b[2:])
    }

3. substr

def substr(a, b):
    c = a[0:int(b[0])]
    d = a[int(b[0]):int(b[0]) + int(b[1])]
    return c + a[int(b[0]):].replace(d, '')

4. getPos

def getPos(a, b):
    b[0] = len(a) - int(b[0]) - int(b[1])
    return b

5. decode

def decode(a):
    b = getHex(a)
    c = getDec(b['hex'])
    d = substr(b['str'], c['pre'])
    return base64.b64decode(substr(d, getPos(d, c['tail'])))

Result Demonstration

Running the Python implementation on the captured string variable a yields the same encrypted video URL as the original site.

The obtained URL can be opened in a browser and the video plays normally; downloading the video is then straightforward.

Conclusion

This tutorial shows how to tackle a typical Python web‑scraping obstacle where JavaScript dynamically generates encrypted video URLs. By reverse‑engineering the JS logic and translating it into Python, the hidden URL can be recovered and accessed.