united-states open-source-software
Is it theoretically possible to create a free software WinRAR compressor?
<p>Please forgive me if this has already been answered somewhere; I searched high and low and found quite a few answers dealing with the RAR file format but none that answered this specific question.</p>

<p>Is it <em>theoretically</em> possible to create a fully Free Software (i.e. GPL or similar) program that <em>compresses</em> RAR?</p>

<p>I am already aware of the hairy licensing issues surrounding the unrar code and the end user license agreement that supposedly prevents reverse-engineering of the binaries and usage of the unrar source to create a compression routine.</p>

<p>But suppose one has only an existing RAR archive in their possession. Having <em>never seen the RAR binaries or unrar source</em> (i.e., clean-room reverse engineering), <strong>would one be legally able to create a completely free decompression AND compression routine for RAR</strong>?</p>

<p>There already exists a "free software" unarchiver for RAR archives. The Mac unarchiving software "The Unarchiver" (the latest version is proprietary but older versions until 2017 were LGPL), as well as its cross-platform command-line version "unar", implemented extraction of RARv3 files in free software starting <a href="https://web.archive.org/web/20100722095811/http://wakaba.c3.cx/s/apps/unarchiver.html" rel="nofollow noreferrer">in 2010</a>. See the Free Software Foundation announcement <a href="https://www.fsf.org/blogs/licensing/free-rarv3-extraction" rel="nofollow noreferrer">here</a>. The last free software version of The Unarchiver <a href="https://web.archive.org/web/20170706222632/http://unarchiver.c3.cx/changes" rel="nofollow noreferrer">supported</a> extraction of RARv5 archives.</p>

<p>I am not exactly sure how they managed to write a free software implementation of RAR extraction. Perhaps it was some kind of "clean room" reimplementation? But presumably the same technique can be used to implement RAR archiving.</p>
