UnRAR Source Code Reverse Engineering Restrictions

The UnRAR utility source code, provided by Alexander Roshal and RARLAB, is distributed under a restrictive source-available license rather than an open-source license. While the code is freely available to read and integrate into third-party software for extraction purposes, its licensing terms strictly forbid developers from using it to reverse-engineer, recreate, or emulate the proprietary RAR compression algorithm. This article details the specific legal and technical boundaries governing the use and study of the UnRAR source code.

The Core Compression Prohibition

The primary legal restriction found in the UnRAR license explicitly bans the use of the source code to develop a RAR-compatible compressor. The license dictates that while the UnRAR sources may be used in any software to decompress RAR archives without fee, they cannot be used to:

  • Recreate or reverse-engineer the RAR compression algorithm.
  • Develop a RAR-compatible compression engine.
  • Assist in writing software that generates RAR archives without licensing the official algorithm.

While RAR decompression specifications are effectively laid bare through the UnRAR source, the compression algorithm itself remains proprietary intellectual property.

"Tainted" Status and Clean-Room Engineering

Because the UnRAR license specifically prohibits using the decompression code to discover or reimplement the compression mechanisms, developers who study the UnRAR source code are legally considered "tainted" regarding the creation of RAR compression tools.

If a developer reads the UnRAR source code to infer how compression tables, dictionary sizes, or encoding schemes function on the writing end, any resulting compression software could be subject to copyright and license violation claims. For developers attempting to build independent RAR-compatible tools, a strict clean-room reverse-engineering design is legally required—meaning the engineer analyzing binary formats or creating the compression software must not have reviewed the proprietary UnRAR source code.

Modifications and Distribution Limits

Beyond reverse engineering the algorithm, the UnRAR license limits how the source code itself can be redistributed:

  • No Standalone Redistribution of Modified Code: Developers may not distribute modified versions of the UnRAR source code as a separate product. Modifications may only be distributed as part of a compiled software product or in the form of patches/diffs relative to the original source.
  • Attribution Requirement: Any derivative application using the UnRAR decompression code must clearly acknowledge the original copyright held by Alexander Roshal in its documentation and source files.
  • No Sub-Licensing as Open Source: The UnRAR code cannot be re-licensed under copyleft or permissive open-source licenses such as the GNU General Public License (GPL) or MIT license, as the anti-compression restriction conflicts with the open-source definition of software freedom.

Permitted Uses vs. Prohibited Actions

To remain compliant with the UnRAR source terms:

  • Permitted: Compiling the source directly into an application to read and extract .rar archives.
  • Permitted: Modifying the decompression routines to optimize extraction performance or integrate with specific operating system APIs.
  • Prohibited: Analyzing the decompression logic to deductively construct an encoder that outputs .rar files.
  • Prohibited: Copying decoding structures to construct an independent, closed or open-source compression library.