If I reverse engineer a file format used by proprietary software and write (a) a specification and (b) a third-party program for reading and writing the format, and I publish both (a) and (b) online under open source licenses, is that legal? I am not looking at the software's code, only its interface and the file.

  • I'm pretty sure that the answer is no, but I'll leave it to someone more knowledgable to get the details right.
    – ohwilleke
    Aug 24, 2022 at 18:31
  • @ohwilleke no it's not copyright infringement or no it's not legal? Sorry I put opposite questions in the title and body; I fixed it.
    – Someone
    Aug 24, 2022 at 18:34
  • 1
    Probably not IP infringement.
    – ohwilleke
    Aug 24, 2022 at 18:37

1 Answer 1


Yes, it is legal to do that

More exactly, it is not copyright infringement.

Reverse engineering has been found to be a fair use under US copyright law in:

  • Sega Enterprises v. Accolade 977 F.2d 1510 (9th Cir. 1992);
  • Sony Computer Entertainment v. Connectix 203 F.3d 596 (9th Cir. 2000).; and
  • Atari Games Corp. v. Nintendo of America, Inc. 975 F.2d 832 (Fed. Cir. 1992).

In general pure reverse engineering is fair use when the reuser has not agreed to a contract limiting reverse engineering and has not obtained a copy through deception.

But a file format is considered to be an idea or a method of operation, and so is not protected by copyright at all, and nothing that is done with it could ever be copyright infringement. See https://social.msdn.microsoft.com/Forums/windows/en-US/3269d4f3-8b39-4a2c-8205-1a55e0c6774d/are-file-types-copyrighted?forum=Vsexpressvcs and "Does copyright protect data file formats?" from Lexology, the latter citing EU law and the case of SAS Institute Inc. v World Programming Ltd in the Court of Justice of the European Union (CJEU).

Thus there is no copyright infringement in reverse engineering a file format, or in then writing and distributing code to read, write, or, modify files in such a format.

If a valid patent applies, that may prevent creating or using such software without a license from the patent holder. But my understanding is that in most cases a file format will not be subject to a patent.

  • What about patents? They may cover some file formats. I suppose in that case, the reverse engineering work itself might be OK and probably part (a) of the Question, but probably not publishing software for it as in part (b) without a license.
    – Brandin
    Aug 25, 2022 at 6:54
  • @Brandin I have edited the answer to address patents. A patent could protect a file format, but I think that would be unusual. Aug 25, 2022 at 12:46
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    Regarding patents protecting file format: There are quite a lot of those regarding audio and video compression formats. Some of them valid, some of them expired, some of them of questionable enforceability depending on jurisdiction. They really turned that area into a legal minefield. For just a glimpse of how deep that rabbit hole goes, check out the section on licensing, ownership and implementation of the Wikipedia article on the MP3 format.
    – Philipp
    Aug 25, 2022 at 13:35
  • The GIF format was patented. But actually, a compression algorithm turning images into smaller images in GIF format was patented. Someone got around this by converting images to GIF format without any compression; the result was actually about 40% bigger than the original, but any GIF reader could read it jand display it ust fine. Used in situations where you don't care if an image is 1,000 or 2,000 bytes as long as you get around the patent. Obviously you didn't get the benefits of the patent either.
    – gnasher729
    Aug 25, 2022 at 13:42
  • Some open source packages at the time it was still patented, did actually deliver the compression code in source form only, but it was disabled by default (i.e. hidden to the compiler), unless you specifically enabled it with an extra build option. I always wondered if that was actually a legal way to get around it, but since most people don't consume software in source code form only and building it themselves, maybe it was not worth any kind of lawsuit from the patent holder.
    – Brandin
    Aug 25, 2022 at 13:55

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