This is the abridged version of an article I wrote last year about reverse engineering and fair use. You can read it here or here. The full article looks at Super Mario 64’s decompilation project and discusses where it sits legally. I think the context helps quite a bit.
Reverse engineering is awkward because there is very little law on it. There have been no supreme court cases on the matter. Accordingly, most law comes from the small number of cases that directly deal with reverse engineering, and extrapolation of more general copyright principles.
Under section 106 of the 1976 Copyright Act, copyright holders have the exclusive rights to do and to authorize a number of activities, most notably:
Exercising any of the rights afforded in section 106 without permission is copyright infringement. These rights, however, are not unlimited and are subject to several exceptions contained in sections 107 through 122 of the Copyright Act. Most relevant to reverse engineering are sections 107 and 117; with the latter providing that software owners may make copies necessary for utilization (i.e. loading a program) and for archival purposes.Id. § 117.
Section 107 serves as a codification of the fair use doctrine, which allows the use of copyrighted material to be non-infringing if used “for purposes such as criticism, comment, news reporting, teaching . . . , scholarship, or research.”Id. § 106. There are four statutory factors to consider when evaluating fair use. These factors are “illustrative and not limitative,” and should not “be treated in isolation . . . [a]ll are to be explored, and the results weighed together, in light of the purposes of copyright.”Campbell v. Acuff-Rose Music, Inc., 510 US 569, 577–578 (1994). The four factors are:
In 1998, the Digital Millennium Copyright Act (DMCA) amended US copyright law, complicating the status reverse engineering. Section 1201 of the Copyright ActId. § 1201. prohibits the “circumvent[ion of] technological measure that effectively controls access to a work.” The wording of this is vague, and it has been suggested that it may apply to more than just copy protection.Coders’ Rights Project Reverse Engineering FAQ, Electronic Frontier Foundation, https://www.eff.org/issues/coders/reverse-engineering-faq. Subsection F provides an exception for reverse engineering, wherein circumventing a technological measure is permissible to achieve interoperability (cross-compatibility of software). For the sake of simplicity, this article assumes this DMCA provision only applies to breaking copy protection.
Outside the remit of the DMCA, as a general rule, the reverse engineering of software in and of itself is a fair use when done to understand non-visible concepts. Most legal issues depend on what is done with the reverse engineered code.
Sega Enterprises Ltd. v. Accolade, Inc.Sega Enterprises Ltd. v. Accolade, Inc., 997 F.2d 1510 (9th Cir. 1992). held that the disassembly of object code is a fair use when used to understand elements “that are not visible to the user when operating—and then only when no alternative means of gaining an understanding of those ideas and functional concepts exists.”Id. 1520. The later case of Sony Computer Entertainment, Inc. v. Connectix Corp.Sony Computer Entertainment, Inc. v. Connectix Corp., 203 F.3d 596 (9th Cir. 2000). builds on this idea, holding that “the fair use doctrine preserves public access to the ideas and functional elements embedded in copyrighted computer software programs,”Id. 603. and that the number of times software is copied during the disassembly process is largely irrelevant.Id. 605.
The court in Atari Games Corp. v. Nintendo of America Inc.Atari Games Corp. v. Nintendo of America Inc., 975 F.3d 832 (Fed. Cir. 1992). was very careful to protect reverse engineering when finding against the party that engaged in it, noting that “[r]everse engineering, untainted by [a] purloined copy of the  program and necessary to understand [the program], is a fair use. An individual cannot . . . understand the object code on [a] chip without reverse engineering.”Id. 843. The court stressed that “[a]n author cannot acquire patent-like protection by putting an idea . . . in an unintelligible format and asserting copyright infringement against those who try to understand that idea.”Id. 842.
As mentioned earlier, most legal issues will arise after reverse engineering has taken place, where it is a matter of comparing the two works to determine if there is infringement. To determine this, both copyright ownership and evidence of copying must be proven. As direct evidence of copying is often unavailable, copying may be inferred where the two works are substantially similar.Atari, Inc. v. North American Philips Consumer Elecs. Corp., 672 F.2d 607, 614 (7th Cir. 1982).
Substantial similarity in reverse engineering cases is often established when more code than necessary is copied. For instance, in EF Johnson Co. v. Uniden Corp. of AmericaEF Johnson Co. v. Uniden Corp. of America, 623 F. Supp. 1485 (D. Minn. 1985). a ‘barker code’ had to be copied to achieve compatibility,Id. 1494. however (among other similarities) three unnecessary lines of code were found in the same place in both programs.Id. 1495. The court noted that “[t]he existence of the identical unnecessary instructions in both codes is strong proof of substantial similarity.”Id. 1496. Conversely, in Vault Corp. v. Quaid Software Ltd.Vault Corp. v. Quaid Software Ltd., 847 F.2d 255 (5th. Cir. 1988). just 30 characters of source code were copied for functionality. This was found to be neither quantitatively nor qualitatively substantial enough to constitute copying.Id. 267–268.
Of course, if copyright infringement is found, it may be justifiable under the fair use doctrine, of which there are four factors to consider.
Under the first factor, one must also ask whether the reverse engineered code
merely supersedes the objects of the original creation, or instead adds something new, with a further purpose or different character, altering the first with new expression, meaning, or message; it asks, in other words, whether and to what extent the new work is “transformative.”Sony Computer Entertainment, Inc. v. Connectix Corp., 203 F.3d 596, 606 (9th Cir. 2000), citing Campbell v. Acuff-Rose Music, Inc., 510 US 569, 579 (1994) (internal quotation marks and citations omitted).
Lewis Galoob Toys, Inc. v. Nintendo of America, Inc.,Lewis Galoob Toys, Inc. v. Nintendo of America, Inc., 964 F.2d 965 (9th Cir. 1992). which concerned a device (the Game Genie) that allowed a user to alter three feature of a Nintendo game, may be an appropriate case to consider. The court found that “Game Genie users are engaged in a non-profit activity. Their use of the Game Genie to create derivative works therefore is presumptively fair.”Id. 970. Perhaps more importantly, the court held
a party who distributes a copyrighted work cannot dictate how that work is to be enjoyed. Consumers may use a Betamax to view copyrighted works at a more convenient time. They similarly may use a Game Genie to enhance a Nintendo Game cartridge’s audiovisual display in such a way as to make the experience more enjoyable.Id. 972.
While the Game Genie is a separate device that uses no code from the games it alters and leaves no lasting derivative works or copies, functionally, decompiled source code has a similar effect insofar as allowing for user modifications.
Courts are also to free consider the public benefit of a use.Sega Enterprises Ltd. v. Accolade, Inc., 997 F.2d 1510, 1523 (9th Cir. 1992). It should go without saying that modifying a piece of software allows users to get more enjoyment or usage out of it—in some cases allowing for creative expression. Modification of software has been common practice for decades, and is even encouraged by some software developers.
The second factor asks what kind of work is being used. Factual works, like news reports, are less protected than creative works.Campbell v. Acuff-Rose Music, Inc., 510 US 569, 586 (1994). Generally, software is considered to be sufficiently creative to qualify for copyright protection.Oracle America, Inc. v. Google Inc., 750 F.3d 1339, 1356 (Fed. Cir. 2014).
One may also wish to note that the second factor “typically has not been terribly significant in the overall fair use balancing.”Dr. Seuss Enterprises, LP v. Penguin Books, 109 F.3d 1394, 1402 (9th Cir. 1997).
This factor may not necessarily be a big issue, as making entire copies of a work can be a fair use some circumstances. Sony Corp. of America v. Universal City Studios, Inc.Sony Corp. of America v. Universal City Studios, Inc., 464 US 417 (1984). would suggest that recording a television broadcast for private home use is a fair use, as such copying “merely enables a viewer to see such a work which he had been invited to witness in its entirety free of charge.”Id. 449–450. This reasoning was applied in Lewis Galoob Toys, Inc. v. Nintendo of America, Inc.Lewis Galoob Toys, Inc. v. Nintendo of America, Inc., 964 F.2d 965 (9th Cir. 1992). where it was found to apply both to works that have been purchased (as opposed to invited to view for free),Id. 972. and to the creation of derivative works (as opposed to exact copies).Id. 971.
Such reasoning only applies to private home use, and while that definitely applies to large swathes reverse engineering, distributing derivative code is certainly not that. Although, SonySony Corp. of America v. Universal City Studios, Inc., 464 US 417 (1984). holds that the “copying of [an] entire work does not preclude fair use.”Sony Computer Entertainment, Inc. v. Connectix Corp., 203 F.3d 596, 606 (9th Cir. 2000).
The third factor depends on if the amount used is “reasonable in relation to the purpose of the copying,”Campbell v. Acuff-Rose Music, Inc., 510 US 569, 586 (1994). however “the extent of permissible copying varies with the purpose and character of the use.”Id. 586–587. Generally, “[i]f the secondary user only copies as much as is necessary for his or her intended use, then this factor will not weigh against him or her.”Kelly v. Arriba Soft Corp., 336 F.3d 811, 820–821 (9th Cir. 2003). This will vary from project to project.
The fourth statutory factor is considered to be the most important,Harper & Row, Publishers, Inc. v. Nation Enterprises, 471 US 539, 566 (1985) (citing 3 Nimmer on Copyright §13.05[A], at 13-81). as “a use that has no demonstrable effect upon the potential market for, or the value of, the copyrighted work need not be prohibited.”Sony Corp. of America v. Universal City Studios, Inc., 464 US 417, 450 (1984). One should consider market harm and what the effects of widespread conduct of similar actions would be.Id. 451.
Despite the relatively few cases concerning reverse engineering software, there is a clear understanding by courts that, in and of itself, reverse engineering is a fair use when the purpose is to better understand software. What you need to worry about is what you do after.