Is it legal to develop a commercial player that a proprietary file format?
<p>I managed to reverse engineer a proprietary file format enough to be able to 'play' it. I am working on a project where I am building an app that reads these such a file and play it. Where do I stand legally if I want to sell this on steam or on a similar platform?</p>

<p>It depends on how you reverse-engineered the player, but the prospects are not good. In particular, it depends on whether you can do it without using the original program. For the most part, you can't buy software, you can only buy a license to use it, and the terms of use typically expressly forbid reverse engineering (even though copyright law would allow it, if you bought the software). The simple fact of the file format being proprietary does not mean that you can't create a conversion program, but you'd need some legal way to figure out what the format is, apart from using the software to determine what the effect of X is. If the EULA doesn't forbid it, then it is okay.</p>

intellectual-property commerce