Briefly: I am wondering whether information gleaned about a proprietary binary format by perturbing the data using an unlicensed copy of the original software (no longer distributed by the copyright holder) could be legally distributed or used in new software, without express permission from the original publisher. This is assuming the data is not particularly obfuscated and the EULA doesn't expressly prohibit reverse engineering (though I don't know if the EULA would make a difference given the software is already not legally obtained). I am mostly interested in factors other than the EULA which would affect how I can use the results of reverse engineering, since the effect of the EULA has been covered in other answers, and because there would likely be no EULA in effect in this question's case. I would just assume for the purposes of this question that the EULA does not contain any terms that would prevent a licensed user from reverse engineering the software.

These are the main issues I am interested in:

  • It seems to me that the only immediate infringement is in unauthorized acquisition and use of the software, when doing the reversing. Is this true? This would distinguish my question from ones such as Reverse-Engineering an Application without EULA where unauthorized reproduction/derivation of the software is involved.

  • Would I own copyright on any artifacts (documentation, interop code) resulting from the reverse engineering, despite the infringement involved in producing it?

  • Such artifacts would not be direct output from the software itself. Does this make it any different from cases (such as unlicensed photoshop) where the purpose of the unauthorized use is to use the output of the program?

  • What could I be liable for in case I distributed the artifacts commercially? What if I make them available to other people without any personal commercial purpose, but allow commercial use (such as in an free software / open source license)?

I am aware that it's possible for software patents to apply, but a patent violation would not be affected by how I produced any code.

EDIT: Forgot to mention, I am in the US (Illinois), and the original publisher is based in Japan.

I do have a specific case in mind, so more details follow in case they make a difference.

The software


Without going into too much detail, the software is written for Windows and consists of an editor for creating interactive programs, which can package the data with an interpreter for distribution to end users. Due to its age, the program has compatibility issues with newer versions of Windows.


The software was discontinued by the original publisher. It is available from secondhand sellers (in the original packaging), but obtaining the newest version requires a patcher executable that used to be distributed on the publisher's website. The updater has apparently been archived in the Wayback Machine, but my gut tells me that this would not be a legitimate copy.

File Format

The format is a highly-structured, unobfuscated binary format. It is relatively clear how changes in the editor affect the data files, so I do not think any analysis would be considered circumvention of copy protection mechanisms.

So far I have only examined the editable data and do not plan to analyze or work with the packaged data if it will take significantly more work (for example if it is compressed or obfuscated).


I found an umbrella EULA applying to many different products on an archive of the publisher's website. I don't know whether a separate EULA with different terms was distributed with the software I am interested in, but the EULA I found does not expressly prohibit reverse engineering. If I can obtain the software legally, existing questions suggest it would at least not be copyright infringement to perform reverse engineering in the way I intend (Can software be legally rewritten if the original source code is not used?).


I plan to compare data files before and after changing them using the original software. I am not planning on disassembling the software or otherwise referencing the original algorithm used to serialize the data.

Intended purposes of reverse engineering


I wish to document the format and make this documentation freely available to other users.

Conversion tool

I wish to create and distribute a tool for converting a project in the original format into a common interchange format like JSON, for import into other projects.

Reimplement and extend interpreter

I wish to implement an interpreter that will run on the converted data files. This would be implemented from scratch, without trying to reverse engineer the original interpreter. The aim would be an accurate reproduction of the original behavior, but not necessarily perfect. My intent is to make the interpreter cross-platform and to include new features not originally present. Ideally, I can freely distribute this (for example under a copyleft license).

Implementing an editor is not one of my goals, as I think the original is adequate for existing projects, and there are better tools available for new projects.

  • Is the software discontinued, but the company is still in business?
    – Ron Beyer
    Apr 10 '20 at 16:43
  • The company is still in business and still holds the rights to the software, as far as I know.
    – Reimu
    Apr 10 '20 at 19:10

The EULA is Required to Provide a Complete Answer https://www.j-platpat.inpit.go.jp/p0100

Japan patent search

The Japanese copyright law is here: https://www.cric.or.jp/english/clj/doc/20151001_October,2015_Copyright_Law_of_Japan.pdf

If it wasn't patented, copyrighted or is open source, check GitHub, you can give them credit but make no profit from it as they declared it. If you repurpose it you may be able to seek intellectual property protection(s) under your name.

If you are concerned due to the way you obtained it that depends upon their current policy.

  • 1) As I mentioned, I believe that the EULA does not expressly prohibit or allow reverse engineering. Do you have an example of any other information which could be relevant? 2) If something "wasn't copyrighted" (I am guessing you mean is in the public domain) or is open source, then commercial use would be allowed. However, neither of these apply in this case (I think this is evident from my original question). 3) Does your answer reference any specific part of japanese copyright law, or did you just link it in case it would be useful to me?
    – Reimu
    Apr 10 '20 at 8:00
  • Also I believe that based on this resource (bradley.com/insights/publications/2012/03/…) that generally the copyright laws of my nation (the United States) would apply, even though the work is originally copyrighted in Japan. Is there something that causes Japanese law to apply in this case?
    – Reimu
    Apr 10 '20 at 8:10
  • The EULA may reference tos, toa, or other agreement, it may define how disputes are settled, etc. I wouldn't dare state anything as certain unless I can read included docs.
    – Leah
    Apr 10 '20 at 9:06
  • Well I can say that the EULA doesn't reference any other agreement, other than that products made with the tool should not violate the copyright of other rightsholders. And like I said, my question involves the case of an unlicensed copy, so no EULA would have been agreed to.
    – Reimu
    Apr 10 '20 at 9:12
  • I will just say that I am not going to post the EULA since I don't want to say what product I am talking about. I'm not interested in whether the EULA would cause it to be prohibited, because other answers on this site already say that it would be allowed under US law, as long as the EULA doesn't expressly prohibit it. When I think it's worth the investment to be absolutely sure then I will seek legal counsel, because any answer here about the EULA will either agree or contradict the other answers on this site. I will update my question to clarify this.
    – Reimu
    Apr 10 '20 at 9:17

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