File Formats, Reverse Engineering, and the Rocksmith vs. Slopsmith Debate

This is a biased article — I have already made up my mind that open-source software released under licenses like GPLv3 should not be stopped from being distributed because a company doesn’t want their software’s inputs and outputs to be usable in other software. This is the heart of Ubisoft’s recent DMCA takedown of the open‑source project Slopsmith: telling people how they can use what they have bought after they have bought it under different license terms and seeking to prevent interoperability for remixing or better technological uses. For me, this is like saying don’t use MP3s, CDs or vinyl because we invented the format — we should control how it is used. I can’t agree with that.

This doesn’t pertain to piracy because Slopsmith, as a program that plays a format, is not pirating the format; it is just utilizing files that people have already accessed, such as through legal purchase. Any piracy that goes on is not done through the software itself, so this is not the issue. Anyone who says it is is distorting the truth and maligning the facts. Open‑source software with a license to remake, redistribute, borrow, or copy is a cat that is out of the bag once released, and attempts to put it back in without actually violating people’s rights is a wrongdoing and an infringement on people’s freedoms.

As someone who spends time making content, I can understand both sides of the coin. On one side is the law; on the other side is free will. Like, I get both sides as someone who spends time making content — in the event of something I do having value, simply to have 30 years of development turned to no tangible benefits is still a bit not good. However, on the flip side, it’s pretty obvious that people will come in and steal what you have done if it is free to recreate and distribute. This has always been the case with digital information. It is just expected. However, that is where the legal stuff comes into play: there are laws to ensure people who have authority over stuff get to say how it is used, and with that there are licensing processes. If something is released under open‑source licensing, then you have access to see and usually freely distribute that thing; when it is not, you do not.

Digital goods differ fundamentally from physical ones in how easy and free they are to create and distribute. In the U.S., copyright protects expression, not ideas, methods, or functional systems. As the Supreme Court affirmed in Feist Publications, Inc. v. Rural Telephone Service Co. (1991): Copyright protection extends only to expression, not to ideas.

File formats — such as Rocksmith’s .psarc archives or internal note data structures — are generally treated as functional systems rather than expressive works. As such, they often fall outside the core of copyright protection, especially when what is being replicated is structural behavior rather than creative expression. This distinction is especially important in music software and tools designed for compatibility and learning. So we can set that aside in consideration.

Can You Really Own a File Format?

A central question in the Rocksmith/Slopsmith discussion is whether a company can legally control a file format in a way that prevents compatible tools.

Generally speaking:

• Copyright does not protect functional file formats as such.
• Patents may protect specific technical innovations (e.g. novel compression, encryption, or parsing methods), but not abstract data structures or general file layout concepts.

In other words, while specific implementation techniques inside a format may be protected, the general idea of a format is often not.

This distinction has been reinforced in several interoperability cases. In Sega Enterprises Ltd. v. Accolade, Inc. (1992), the Ninth Circuit held that reverse engineering for the purpose of interoperability can be a lawful use when it is necessary to access unprotected functional elements of a system.

Similarly, in Sony Computer Entertainment, Inc. v. Connectix Corp. (2000), the court upheld reverse engineering of the PlayStation BIOS to build an emulator, emphasizing that intermediate copying can be lawful when it serves the purpose of creating a new, compatible platform rather than substituting for the original.

Interoperability: The Right to Build Compatible Tools

Building software that works with existing file formats is not automatically infringement. A useful analogy is a word processor opening a .docx file or an audio player supporting multiple codecs. In such cases, developers are typically working with publicly observable structure, not copying proprietary implementation code.

The principle was also reinforced in Lotus Development Corp. v. Borland International (1995), where the First Circuit held that a menu command hierarchy was a method of operation and therefore not protectable expression, even if it was central to user interaction.

More recently, in Google LLC v. Oracle America, Inc. (2021), the U.S. Supreme Court found that Google's reuse of certain Java API structures was fair use in the context of building a new platform, placing significant weight on interoperability and transformative use.

In the context of Rocksmith and Slopsmith, the legal questions typically come down to specifics rather than the concept of compatibility itself:

• Was any Ubisoft source code copied?
• Was any DRM or encryption bypassed in violation of the DMCA?
• Or was the file format independently analyzed and reimplemented?

The DMCA §1201 prohibits circumvention of technological protection measures used to control access to copyrighted works. This appears central to Ubisoft's claims regarding proprietary encryption in Rocksmith's .psarc and .sng files. However, in practice, there are exemptions, limitations, and fact‑specific defenses that can apply, particularly where interoperability and legitimate reverse engineering are involved.

People such as Rossman have been championing the right to repair, including the right to disable systems designed to prevent people fixing or using things they have paid for under one set of terms, and the company later changes and does things like lock off product functionality unless you pay subscription fees — which defeats the spirit the product was purchased under. When someone buys a file or any format such as a disc or record, they view that as their physical property to use as they see fit. They see the music contained in it as their right to put into any sound system they own, and any computer they own, not just the one the company wants them to use it with. Those are at worst monopolistic practices, at best anticompetitive practices.

Patents, Market Control, and Antitrust Limits

Some commentary around legacy software ecosystems raises the idea of patent blocking, where technologies are allegedly withheld from licensing or development. Refusing to license a patent is generally legal. Patent holders are granted exclusionary rights, including the right not to license, and non‑use alone is not typically unlawful. However, antitrust law can become relevant where patents are used as part of broader exclusionary conduct that harms competition. The Sherman Antitrust Act prohibits monopolization and certain restraints of trade, but enforcement requires demonstrating anti‑competitive behavior beyond mere ownership or non‑licensing of intellectual property. Cases such as United States v. Singer Manufacturing Co. (1963) illustrate that coordinated use of patents to suppress competition can violate antitrust law — but the legal issue was the broader anti‑competitive scheme, not patent ownership itself. These serve as examples of how companies should not try to use practices that stop others from developing their own products, and to force use of products simply by blocking off the means for alternative products to be usable.

In the EU there are provisions for interoperability through decompilation under strict conditions, and some countries maintain compulsory licensing frameworks in specific industries. This is all so the consumer is put first — not a single business to control, say, how file formats or a given tangible item is used in the scope of a larger set of use cases. You sell a solution in the form of a product you provide, but you do not sell the right to use the thing and maintain the right to limit how it is used, especially where those limits are not part of the original sale.

Open Source and the Irrevocability Reality

Once software is distributed under open‑source licenses such as GPLv3, recipients receive perpetual rights to use, modify, and redistribute it under the same license conditions. While licensors can stop distributing future versions, they generally cannot revoke rights already granted to existing recipients. Once it is open source, it is always open source. Once it is free to remake or redistribute under that license, that license carries forward.

In the Rocksmith/Slopsmith context, the legal and practical questions usually reduce to a few key issues:

• Did the tool copy protected source code or assets?
• Did it bypass technological protection measures in a way that violates the DMCA?
• Or does it independently implement compatibility with a publicly observable file structure?

From a technical standpoint, independently implementing support for a file format is often closer to learning a language than copying expressive artwork. Courts have historically been cautious about restricting interoperability too broadly, particularly in software ecosystems where compatibility drives innovation and user value.

This is not an unimportant legal dispute; it directly involves the right of software to stay open source and under the terms of its original license once it has started being spread. This could mirror into open‑source AI models, training data, or other useful precursor formats. This is like a company saying their open‑source AI model that was released on the terms that you can do what you'd like forever can turn around and say it can now only be used on computers it approves. Once software exists in the world, it becomes part of a shared technical environment. These types of legal decisions have real‑world impacts that go beyond the point they are viewed at in terms of music software; they reflect the entire digital format space.

The ability to have interoperability and technical progress are important values to support, not things that should only be supported by one company or person. On the other side, there is an unknown issue of restricting circumvention of security mechanisms and whether that should be protected if it is done in a way to prevent people from using what they purchased under the terms they were originally licensed or purchased it to the extent it was lawful at the time of the purchase.

Ubisoft has legitimate interests in protecting its intellectual property and maintaining control over its ecosystem, but it really has no right to tell other people they can't distribute information in a given format, because they don't control the format. The use of legal pressure to force someone to change an open‑source distribution is fairly reprehensible conduct. I don't have all the information, but the use of an injunction to remove other people's content or the distribution of other people's content is overreach, and an injunction to remove software because it gave access to use a format is also unwarranted. Now, I don't have all the information; I don't know the judge's reasoning specifically, but at face value this seems a deeply disturbing precedent if, in the U.S., file formats are no longer able to be used freely. For AI open models, this also matters because now there is a precedent saying you can't load this format into software you make — only software we are OK with — and for AI this really matters because there are different ways of loading LLM models. There is spillover, so this is significant for the future freedom of use of digital formats, and that should raise some eyebrows. However, if this case doesn't get furthered, it's unlikely to be a very strong case, I would think, as the ruling seems faulty. Due to the developer not having a legal war chest, it will likely die where it is, but people have a right to be concerned if formats are no longer free to be used as the owners and licensees of those files under the original license and purchase terms — this retroactive revision on terms of use is troubling in general.

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(I was only a legal studies minor undergrad; none of this is legal advice — just lay opinion.)