Decode Obfuscated JS Video URLs Using Python: A Step‑by‑Step Guide

Preface

Recently, members of a Python community asked for JavaScript reverse‑engineering videos and code. The author shares a case study that records the process of decoding a JS‑based video URL encryption.

JS Code

The encrypted function originates from a small video site and is shown below.

The core of the encryption is the decodeMp4.decode() function.

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

Assume the variable a contains a long string obtained by breakpoint debugging: a = "c0b1Ly9md..."; The following diagram lists the functions to be translated.

1. getHex(a) function

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

2. getDec(a) function

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

3. substr(a, b) function

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) function

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

5. decode(a) function

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 script yields the same encrypted URL that the web page uses, allowing the video to be played or downloaded.

Placing the obtained address in a browser plays the video and enables a direct download.

Summary

The article walks through a practical example of reverse‑engineering a JavaScript video‑URL encryption routine, translating each JavaScript helper into Python, and demonstrates how to retrieve the hidden video link using a web‑scraping approach.