How do I protect Python code from being read by users?


I am developing a piece of software in Python that will be distributed to my employer’s customers. My employer wants to limit the usage of the software with a time-restricted license file.

If we distribute the .py files or even .pyc files it will be easy to (decompile and) remove the code that checks the license file.

Another aspect is that my employer does not want the code to be read by our customers, fearing that the code may be stolen or at least the "novel ideas".

Is there a good way to handle this problem?

Asked By: Jordfräs



Python, being a byte-code-compiled interpreted language, is very difficult to lock down. Even if you use a exe-packager like py2exe, the layout of the executable is well-known, and the Python byte-codes are well understood.

Usually in cases like this, you have to make a tradeoff. How important is it really to protect the code? Are there real secrets in there (such as a key for symmetric encryption of bank transfers), or are you just being paranoid? Choose the language that lets you develop the best product quickest, and be realistic about how valuable your novel ideas are.

If you decide you really need to enforce the license check securely, write it as a small C extension so that the license check code can be extra-hard (but not impossible!) to reverse engineer, and leave the bulk of your code in Python.

Answered By: Ned Batchelder

You should take a look at how the guys at do it for their client software, including Linux. It’s quite tricky to crack and requires some quite creative disassembly to get past the protection mechanisms.

Answered By: fwzgekg

Is your employer aware that he can “steal” back any ideas that other people get from your code? I mean, if they can read your work, so can you theirs. Maybe looking at how you can benefit from the situation would yield a better return of your investment than fearing how much you could lose.

[EDIT] Answer to Nick’s comment:

Nothing gained and nothing lost. The customer has what he wants (and paid for it since he did the change himself). Since he doesn’t release the change, it’s as if it didn’t happen for everyone else.

Now if the customer sells the software, they have to change the copyright notice (which is illegal, so you can sue and will win -> simple case).

If they don’t change the copyright notice, the 2nd level customers will notice that the software comes from you original and wonder what is going on. Chances are that they will contact you and so you will learn about the reselling of your work.

Again we have two cases: The original customer sold only a few copies. That means they didn’t make much money anyway, so why bother. Or they sold in volume. That means better chances for you to learn about what they do and do something about it.

But in the end, most companies try to comply to the law (once their reputation is ruined, it’s much harder to do business). So they will not steal your work but work with you to improve it. So if you include the source (with a license that protects you from simple reselling), chances are that they will simply push back changes they made since that will make sure the change is in the next version and they don’t have to maintain it. That’s win-win: You get changes and they can make the change themselves if they really, desperately need it even if you’re unwilling to include it in the official release.

Answered By: Aaron Digulla

I have looked at software protection in general for my own projects and the general philosophy is that complete protection is impossible. The only thing that you can hope to achieve is to add protection to a level that would cost your customer more to bypass than it would to purchase another license.

With that said I was just checking google for python obsfucation and not turning up a lot of anything. In a .Net solution, obsfucation would be a first approach to your problem on a windows platform, but I am not sure if anyone has solutions on Linux that work with Mono.

The next thing would be to write your code in a compiled language, or if you really want to go all the way, then in assembler. A stripped out executable would be a lot harder to decompile than an interpreted language.

It all comes down to tradeoffs. On one end you have ease of software development in python, in which it is also very hard to hide secrets. On the other end you have software written in assembler which is much harder to write, but is much easier to hide secrets.

Your boss has to choose a point somewhere along that continuum that supports his requirements. And then he has to give you the tools and time so you can build what he wants. However my bet is that he will object to real development costs versus potential monetary losses.

Answered By: Peter M

"Is there a good way to handle this problem?" No. Nothing can be protected against reverse engineering. Even the firmware on DVD machines has been reverse engineered and the AACS Encryption key exposed. And that’s in spite of the DMCA making that a criminal offense.

Since no technical method can stop your customers from reading your code, you have to apply ordinary commercial methods.

  1. Licenses. Contracts. Terms and Conditions. This still works even when people can read the code. Note that some of your Python-based components may require that you pay fees before you sell software using those components. Also, some open-source licenses prohibit you from concealing the source or origins of that component.

  2. Offer significant value. If your stuff is so good — at a price that is hard to refuse — there’s no incentive to waste time and money reverse engineering anything. Reverse engineering is expensive. Make your product slightly less expensive.

  3. Offer upgrades and enhancements that make any reverse engineering a bad idea. When the next release breaks their reverse engineering, there’s no point. This can be carried to absurd extremes, but you should offer new features that make the next release more valuable than reverse engineering.

  4. Offer customization at rates so attractive that they’d rather pay you to build and support the enhancements.

  5. Use a license key which expires. This is cruel, and will give you a bad reputation, but it certainly makes your software stop working.

  6. Offer it as a web service. SaaS involves no downloads to customers.

Answered By: S.Lott

In some circumstances, it may be possible to move (all, or at least a key part) of the software into a web service that your organization hosts.

That way, the license checks can be performed in the safety of your own server room.

Answered By: Oddthinking

Depending in who the client is, a simple protection mechanism, combined with a sensible license agreement will be far more effective than any complex licensing/encryption/obfuscation system.

The best solution would be selling the code as a service, say by hosting the service, or offering support – although that isn’t always practical.

Shipping the code as .pyc files will prevent your protection being foiled by a few #s, but it’s hardly effective anti-piracy protection (as if there is such a technology), and at the end of the day, it shouldn’t achieve anything that a decent license agreement with the company will.

Concentrate on making your code as nice to use as possible – having happy customers will make your company far more money than preventing some theoretical piracy..

Answered By: dbr

What about signing your code with standard encryption schemes by hashing and signing important files and checking it with public key methods?

In this way you can issue license file with a public key for each customer.

Additional you can use an python obfuscator like this one (just googled it).

Answered By: Peter Parker

Python is not the tool you need

You must use the right tool to do the right thing, and Python was not designed to be obfuscated. It’s the contrary; everything is open or easy to reveal or modify in Python because that’s the language’s philosophy.

If you want something you can’t see through, look for another tool. This is not a bad thing, it is important that several different tools exist for different usages.

Obfuscation is really hard

Even compiled programs can be reverse-engineered so don’t think that you can fully protect any code. You can analyze obfuscated PHP, break the flash encryption key, etc. Newer versions of Windows are cracked every time.

Having a legal requirement is a good way to go

You cannot prevent somebody from misusing your code, but you can easily discover if someone does. Therefore, it’s just a casual legal issue.

Code protection is overrated

Nowadays, business models tend to go for selling services instead of products. You cannot copy a service, pirate nor steal it. Maybe it’s time to consider to go with the flow…

Answered By: e-satis

The best you can do with Python is to obscure things.

  • Strip out all docstrings
  • Distribute only the .pyc compiled files.
  • freeze it
  • Obscure your constants inside a class/module so that help(config) doesn’t show everything

You may be able to add some additional obscurity by encrypting part of it and decrypting it on the fly and passing it to eval(). But no matter what you do someone can break it.

None of this will stop a determined attacker from disassembling the bytecode or digging through your api with help, dir, etc.

Answered By: Brian C. Lane

Do not rely on obfuscation. As You have correctly concluded, it offers very limited protection.
UPDATE: Here is a link to paper which reverse engineered obfuscated python code in Dropbox. The approach – opcode remapping is a good barrier, but clearly it can be defeated.

Instead, as many posters have mentioned make it:

  • Not worth reverse engineering time (Your software is so good, it makes sense to pay)
  • Make them sign a contract and do a license audit if feasible.

Alternatively, as the kick-ass Python IDE WingIDE does: Give away the code. That’s right, give the code away and have people come back for upgrades and support.

Answered By: Konrads

The reliable only way to protect code is to run it on a server you control and provide your clients with a client which interfaces with that server.

Answered By: Alex Coventry

Though there’s no perfect solution, the following can be done:

  1. Move some critical piece of startup code into a native library.
  2. Enforce the license check in the native library.

If the call to the native code were to be removed, the program wouldn’t start anyway. If it’s not removed then the license will be enforced.

Though this is not a cross-platform or a pure-Python solution, it will work.

Answered By: I K

I understand that you want your customers to use the power of python but do not want expose the source code.

Here are my suggestions:

(a) Write the critical pieces of the code as C or C++ libraries and then use SIP or swig to expose the C/C++ APIs to Python namespace.

(b) Use cython instead of Python

(c) In both (a) and (b), it should be possible to distribute the libraries as licensed binary with a Python interface.

Answered By: bhadra

Shipping .pyc files has its problems – they are not compatible with any other python version than the python version they were created with, which means you must know which python version is running on the systems the product will run on. That’s a very limiting factor.

Answered By: Erik Forsberg

Another attempt to make your code harder to steal is to use jython and then use java obfuscator.

This should work pretty well as jythonc translate python code to java and then java is compiled to bytecode. So ounce you obfuscate the classes it will be really hard to understand what is going on after decompilation, not to mention recovering the actual code.

The only problem with jython is that you can’t use python modules written in c.

Answered By: Piotr Czapla

Idea of having time restricted license and check for it in locally installed program will not work. Even with perfect obfuscation, license check can be removed. However if you check license on remote system and run significant part of the program on your closed remote system, you will be able to protect your IP.

Preventing competitors from using the source code as their own or write their inspired version of the same code, one way to protect is to add signatures to your program logic (some secrets to be able to prove that code was stolen from you) and obfuscate the python source code so, it’s hard to read and utilize.

Good obfuscation adds basically the same protection to your code, that compiling it to executable (and stripping binary) does. Figuring out how obfuscated complex code works might be even harder than actually writing your own implementation.

This will not help preventing hacking of your program. Even with obfuscation code license stuff will be cracked and program may be modified to have slightly different behaviour (in the same way that compiling code to binary does not help protection of native programs).

In addition to symbol obfuscation might be good idea to unrefactor the code, which makes everything even more confusing if e.g. call graphs points to many different places even if actually those different places does eventually the same thing.

Logical signature inside obfuscated code (e.g. you may create table of values which are used by program logic, but also used as signature), which can be used to determine that code is originated from you. If someone decides to use your obfuscated code module as part of their own product (even after reobfuscating it to make it seem different) you can show, that code is stolen with your secret signature.

Answered By: Mikael Lepistö

Compile python and distribute binaries!

Sensible idea:

Use Cython, Nuitka, Shed Skin or something similar to compile python to C code, then distribute your app as python binary libraries (pyd) instead.

That way, no Python (byte) code is left and you’ve done any reasonable amount of obscurification anyone (i.e. your employer) could expect from regular Code, I think. (.NET or Java less safe than this case, as that bytecode is not obfuscated and can relatively easily be decompiled into reasonable source.)

Cython is getting more and more compatible with CPython, so I think it should work. (I’m actually considering this for our product.. We’re already building some thirdparty libs as pyd/dlls, so shipping our own python code as binaries is not a overly big step for us.)

See This Blog Post (not by me) for a tutorial on how to do it. (thx @hithwen)

Crazy idea:

You could probably get Cython to store the C-files separately for each module, then just concatenate them all and build them with heavy inlining. That way, your Python module is pretty monolithic and difficult to chip at with common tools.

Beyond crazy:

You might be able to build a single executable if you can link to (and optimize with) the python runtime and all libraries (dlls) statically. That way, it’d sure be difficult to intercept calls to/from python and whatever framework libraries you use. This cannot be done if you’re using LGPL code though.

Answered By: Macke

using cxfreeze ( py2exe for linux ) will do the job.

it is available in ubuntu repositories

Answered By: Ali Alnoaimi

I think there is one more method to protect your Python code; part of the Obfuscation method. I believe there was a game like Mount and Blade or something that changed and recompiled their own python interpreter (the original interpreter which i believe is open source) and just changed the OP codes in the OP code table to be different then the standard python OP codes.

So the python source is unmodified but the file extensions of the *.pyc files are different and the op codes don’t match to the public python.exe interpreter. If you checked the games data files all the data was in Python source format.

All sorts of nasty tricks can be done to mess with immature hackers this way. Stopping a bunch of inexperienced hackers is easy. It’s the professional hackers that you will not likely beat. But most companies don’t keep pro hackers on staff long I imagine (likely because things get hacked). But immature hackers are all over the place (read as curious IT staff).

You could for example, in a modified interpreter, allow it to check for certain comments or doc strings in your source. You could have special OP codes for such lines of code. For example:

OP 234 is for source line “# Copyright I wrote this”
or compile that line into op codes that are equivalent to “if False:” if “# Copyright” is missing. Basically disabling a whole block of code for what appears to be some obscure reason.

One use case where recompiling a modified interpreter may be feasible is where you didn’t write the app, the app is big, but you are paid to protect it, such as when you’re a dedicated server admin for a financial app.

I find it a little contradictory to leave the source or opcodes open for eyeballs, but use SSL for network traffic. SSL is not 100% safe either. But it’s used to stop MOST eyes from reading it. A wee bit precaution is sensible.

Also, if enough people deem that Python source and opcodes are too visible, it’s likely someone will eventually develop at least a simple protection tool for it. So the more people asking “how to protect Python app” only promotes that development.

Answered By: DevPlayer

It is possible to have the py2exe byte-code in a crypted resource for a C launcher that loads and executes it in memory. Some ideas here and here.

Some have also thought of a self modifying program to make reverse engineering expensive.

You can also find tutorials for preventing debuggers, make the disassembler fail, set false debugger breakpoints and protect your code with checksums. Search for [“crypted code” execute “in memory”] for more links.

But as others already said, if your code is worth it, reverse engineers will succeed in the end.

Answered By: lalebarde

Have you had a look at pyminifier? It does Minify, obfuscate, and compress Python code. The example code looks pretty nasty for casual reverse engineering.

$ pyminifier --nonlatin --replacement-length=50 /tmp/
#!/usr/bin/env python3
ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲמּ=ImportError
ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ燱=print
ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ =False
ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ澨=object
 import demiurgic
except ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲמּ:
 ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ燱("Warning: You're not demiurgic. Actually, I think that's normal.")
 import mystificate
except ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲמּ:
 ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ燱("Warning: Dark voodoo may be unreliable.")
ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲﺬ=ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ 
class ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ (ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ澨):
 def __init__(self,*args,**kwargs):
 def ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ클(self,dactyl):
  ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ퐐=demiurgic.palpitation(dactyl)
  ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ =mystificate.dark_voodoo(ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ퐐)
  return ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ 
 def ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ (self,whatever):
  ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ燱(whatever)
if __name__=="__main__":
 ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ燱("Forming...")
 ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲﺃ=ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ ("epicaricacy","perseverate")
 ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲﺃ.ﺭ异 ﭞﰣﺁں 뻛 嬭ﱌ Ꝫﴹ뙫 퉊ﳦﲣפּܟﺶ ﶨࠔ ﶻ 䉊ﰸﭳᣲ ("Codswallop")
# Created by pyminifier (
Answered By: Lmwangi

Use Cython. It will compile your modules to high-performant C files, which can then be compiled to native binary libraries. This is basically un-reversable, compared to .pyc bytecode!

I’ve written a detailed article on how to set up Cython for a Python project, check it out:

Protecting Python Sources With Cython

Answered By: Vitaly Gordon

Use the same way to protect binary file of c/c++, that is, obfuscate each function body in executable or library binary file, insert an instruction “jump” at the begin of each function entry, jump to special function to restore obfuscated code. Byte-code is binary code of Python script, so

  • First compile python script to code object
  • Then iterate each code object, obfuscate co_code of each code object as the following
    0   JUMP_ABSOLUTE            n = 3 + len(bytecode)

    ... Here it's obfuscated bytecode

    n   LOAD_GLOBAL              ? (__pyarmor__)
    n+3 CALL_FUNCTION            0
    n+6 POP_TOP
    n+7 JUMP_ABSOLUTE            0
  • Save obfuscated code object as .pyc or .pyo file

Those obfuscated file (.pyc or .pyo) can be used by normal python interpreter, when those code object is called first time

  • First op is JUMP_ABSOLUTE, it will jump to offset n

  • At offset n, the instruction is to call a PyCFunction. This function will restore those obfuscated bytecode between offset 3 and n, and put the original byte-code at offset 0. The obfuscated code can be got by the following code

        char *obfucated_bytecode;
        Py_ssize_t len;
        PyFrameObject* frame = PyEval_GetFrame();
        PyCodeObject *f_code = frame->f_code;
        PyObject *co_code = f_code->co_code;      
        PyBytes_AsStringAndSize(co_code, &obfucated_bytecode, &len)
  • After this function returns, the last instruction is to jump to
    offset 0. The really byte-code now is executed.

There is a tool Pyarmor to obfuscate python scripts by this way.

Answered By: Jondy Zhao

I was surprised in not seeing pyconcrete in any answer. Maybe because it’s newer than the question?

It could be exactly what you need(ed).

Instead of obfuscating the code, it encrypts it and decrypts at load time.

From pypi page:

Protect python script work flow

  • import pyconcrete
  • pyconcrete will hook import module
  • when your script do import MODULE,
    pyconcrete import hook will try to find MODULE.pye first and then
    decrypt MODULE.pye via _pyconcrete.pyd and execute decrypted data (as
    .pyc content)
  • encrypt & decrypt secret key record in _pyconcrete.pyd
    (like DLL or SO) the secret key would be hide in binary code, can’t
    see it directly in HEX view
Answered By: mvallebr

If we focus on software licensing, I would recommend to take a look at another Stack Overflow answer I wrote here to get some inspiration of how a license key verification system can be constructed.

There is an open-source library on GitHub that can help you with the license verification bit.

You can install it by pip install licensing and then add the following code:

pubKey = "<RSAKeyValue><Modulus>sGbvxwdlDbqFXOMlVUnAF5ew0t0WpPW7rFpI5jHQOFkht/326dvh7t74RYeMpjy357NljouhpTLA3a6idnn4j6c3jmPWBkjZndGsPL4Bqm+fwE48nKpGPjkj4q/yzT4tHXBTyvaBjA8bVoCTnu+LiC4XEaLZRThGzIn5KQXKCigg6tQRy0GXE13XYFVz/x1mjFbT9/7dS8p85n8BuwlY5JvuBIQkKhuCNFfrUxBWyu87CFnXWjIupCD2VO/GbxaCvzrRjLZjAngLCMtZbYBALksqGPgTUN7ZM24XbPWyLtKPaXF2i4XRR9u6eTj5BfnLbKAU5PIVfjIS+vNYYogteQ==</Modulus><Exponent>AQAB</Exponent></RSAKeyValue>"

res = Key.activate(token="WyIyNTU1IiwiRjdZZTB4RmtuTVcrQlNqcSszbmFMMHB3aWFJTlBsWW1Mbm9raVFyRyJd",
                   product_id=3349, key="ICVLD-VVSZR-ZTICT-YKGXL", machine_code=Helpers.GetMachineCode())

if res[0] == None not Helpers.IsOnRightMachine(res[0]):
    print("An error occured: {0}".format(res[1]))

You can read more about the way the RSA public key, etc are configured here.

Answered By: Artem

There is a comprehensive answer on concealing the python source code, which can be find here.

Possible techniques discussed are:
– use compiled bytecode (python -m compileall)
– executable creators (or installers like PyInstaller)
– software as an service (the best solution to conceal your code in my opinion)
– python source code obfuscators

Answered By: Mike

Neiher Cython nor Nuitka were not the answer, because when running the solution that is compiled with Nuitka or Cython into .pyd or .exe files a cache directory is generated and all .pyc files are copied into the cache directory, so an attacker simply can decompile .pyc files and see your code or change it.

I documented how to obfuscate the python by converting it to .so file, and converting it to a python wheel file:

Answered By: Mohammad Arshi

First, here is a list of public Github repositories regarding obfuscation.

Among all choices out there I ended up using python-obfuscator followed by python-minifier to decrease also the script size. Be careful though to obfuscate first and then minify in order for the script to be still executable.

Answered By: vpap