How to Reverse Engineer JS Video Encryption with Python: A Step‑by‑Step Guide

Preface

The author, a Python enthusiast, noticed several community members asking for JavaScript reverse‑engineering tutorials and decided to share a practical example that converts a JS video‑encryption routine into Python.

JS Code

The target JavaScript code, extracted from a small video‑hosting site, defines a decodeMp4.decode() function that relies on helper functions such as getHex, getDec, substr, and getPos. The original script is shown below:

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)));
        }
    };
});

An accompanying screenshot of the function on the original page is included below:

Conversion Process

The variable a represents a long encrypted string obtained via breakpoint debugging. An example value is shown:

a = "c0b1Ly9tdnPflQ3cQpPZpZGVvMTAubWVpdHVkYXRhLmNvbS82MWM0NDNlOGI1MmFmMTYzMi5tcDkBOyQ"

1. getHex(a) – JavaScript → Python

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

2. getDec(a) – JavaScript → Python

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

3. substr(a, b) – JavaScript → Python

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(a, b) – JavaScript → Python

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

5. decode(a) – JavaScript → Python

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"])) )

Effect Demonstration

Running the translated Python code reproduces the same encrypted video URL that the original webpage generates. The resulting URL can be opened in a browser to stream the video, and a direct download request retrieves the video file. Screenshots of the successful decryption are shown below:

Summary

The article demonstrates a complete workflow for handling JavaScript‑based video encryption in Python web‑scraping scenarios: extracting the obfuscated string, translating the JS helper functions into Python, and finally reconstructing the real video URL for playback or download.