Take stock of a tutorial on converting JS reverse code to Python code

Preface

A few days ago in the Python Xingyao and The Strongest King exchange group, several people were asking about JS reverse engineering videos and related codes. It seemed that they were all I’m learning advanced knowledge and I really can’t get enough of it. It just so happens that I have been reading some JS learning materials these days and saw a pretty good case. I will share it with you here and record it as well.

JS code

Regarding the search for JS code, it is quite difficult to write an article and explain it. It would be better to record a video explanation. Here, the ready-made JS code is directly arranged. It is quite difficult to find this JS encryption code at first. You need to constantly break points, find the encryption rules, and peel the onion layer by layer to find out. The JS encryption code used in this article comes from a small video website. The encryption function presented on the web page is as shown below:

The encryption method is not too difficult, among which decodeMp4. The core code of the decode() encryption function is as follows.

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();# 对应Python中的str(int(a, 16)) 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))) } };

You can see that the decode() function in decodeMp4 is called, and the decode() function calls getHex(a), getDec(b.hex), g.atob(), getPos( d, c.tail) and other functions, and what we have to do is to convert these functions into Python writing, then construct the corresponding encryption method, obtain the encrypted result, and then complete the reverse effect.

Conversion process

The variable a here is obtained by breaking points and is a long string. Here, the following variable is used as an example.

a = "c0b1Ly9tdnPflQ3cQpPZpZGVvMTAubWVpdHVkYXRhLmNvbS82MWM0NDNlOGI1MmFmMTYzMi5tcDkBOyQ"

Let’s briefly organize the functions that will be used later in advance, so that it will be easier for everyone to check later.

Let’s break down each function in turn, as follows:

1. getHex(a) function

var h = "substring"，i = "split"; getHex: function(a) { return { str: a[h](4), hex: a[h](0, 4)[i]("").reverse().join("") } },

The above Is the corresponding getHex() function JS code. You can see that a dictionary is directly returned. The keys of the dictionary are str and hex respectively. The corresponding value of str is a[h](4). The definition of h is substring. This function means that the string starts from the specified subscript until it reaches the end of the string. The translation here is a.substring(4), that is, the string a starts from the subscript 4 and ends at the end; a[h](0, 4)[i]("").reverse().join("") This is a bit more complicated to understand. First, the value of the string is taken, and the position is from 0 to 4. Then the function i, which is the split function, is called. Use spaces ("") as separation, call the reverse() function to sort in reverse order, and then call join("") to connect strings. After disassembly, it is much simpler. The next step is to construct the Python code. After writing the comparison, it will look like this:

def getHex(a): return { "str": a[4:],# JS中的substring(4)指的是从4开始取值到字符串末尾 "hex": "".join(list(a[0:4])[::-1])# [::-1]代表的是反向取值 }

Does it look familiar? It is exactly the same as the JS code above.

2. getDec(a) function

The JS code is as follows:

getDec: function(a) { var b = parseInt(a, 16).toString(); return { pre: b[h](0, 2)[i](""), tail: b[h](2)[i]("") } },

According to the corresponding relationship, the corresponding Python code can be written as follows:

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

3. substr(a, b) function

The JS code is as follows:

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, "") },

According to the corresponding relationship, the corresponding Python code can be written as follows:

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

4. getPos(a, b) function

The JS code is as follows:

getPos: function(a, b) { return b[0] = a.length - b[0] - b[1], b },

According to the corresponding relationship, the corresponding Python code can be written as follows:

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

5. decode(a, b) function

The JS code is as follows:

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))) }

According to the corresponding relationship, the corresponding Python code can be written as follows:

b = getHex(a) # print(b) c = getDec(b['hex']) print(c) # d = k(str(b), c.pre) d = substr(b['str'], c['pre']) # print(d) return base64.b64decode(substr(d, getPos(d, c['tail'])))

Effect display

Request directly through a web crawler. You cannot get the final encrypted address. No matter how you request, you cannot get it. You can only get the data-src, that is The string variable a mentioned above can only be reversed through the above analysis and run the code to get the same request address as on the web page, as shown in the figure below, the reverse is successful!

Put this address in the browser and it can be played. Then make a download request and the video can be downloaded.

Summary

Hello everyone, I am a Python advanced user. This article is mainly based on the JS reverse problem in Python web crawler and makes a case explanation. If the web page is loaded with JS, if you request it directly through a web crawler, you will not be able to get the final encrypted address. To address this reverse problem, we have made a simple reverse example implementation process.

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