here's a long-accepted notion in the publishing world that between the right of an author to control the uses of his book and the right of a reader to engage in free speech is the safety valve known as "fair use."

The fair use doctrine under copyright law permits uncompensated use of copyrighted works in some circumstances, such as in teaching, research and news gathering. Thanks to fair use, a reporter can quote portions of a newsworthy letter in an article and a scholar can use parts of a poem in a dissertation.

But there's a related question that has never been settled by the courts: Does fair use, which has its roots in the First Amendment, entitle the scholar, reporter or others to gain access to the copyrighted work in the first place -- -- especially when the material is guarded by a technological device designed to prevent digital piracy?

The riddle is not some academic musing. It's at the heart of a closely-watched copyright and First Amendment case winding its way through the federal appeals maze. A panel of judges from the United States Court of Appeals for the Second Circuit, in Manhattan, is scheduled to hear arguments in the case, "Universal City Studios v. Reimerdes," on May 1.

In what is sure to be a packed courtroom, Kathleen M. Sullivan, the dean of Stanford Law School and a celebrated First Amendment scholar, will argue for the defendant Eric Corley, a Long Island-based editor and publisher of 2600, a hacker publication and Web site.

Arguing for the other side, a group of movie studios, will be Charles Sims, a New York-based lawyer. A lawyer for the Department of Justice, Daniel S. Alter, will also address the panel. Whatever the Second Circuit decides, legal experts say it's likely the case will be heard eventually in the United States Supreme Court.

The case revolves around a piece of software devised by a Norwegian teenager called DeCSS. The underground code, available on the Internet, decrypts or unlocks the information in an encrypted DVD movie disk and allows a user to view the film on an unauthorized Linux DVD player or place it on a hard drive, potentially for copying or mass distribution.

Not surprisingly, DeCSS became Hollywood's nightmare. Last year eight major studios filed suit in federal court against Corley for posting the software on his own Web site and linking to other sites that also contained the code. The studios argued that Corley's actions in providing and "trafficking" in the software violated a federal law, the Digital Millennium Copyright Act of 1998, which prohibits distribution of a device that is primarily designed to circumvent a technological barrier guarding a copyrighted work. (Shawn Reimerdes, another defendant, was earlier dropped from the case).

Following a trial last summer, Judge Lewis A. Kaplan of the United States District Court for the Southern District, in Manhattan, agreed with the studios that Corley's distribution of the code violated the law. He issued an order banning Corley from posting or linking to the software.

Significantly, Judge Kaplan also found that the anti-trafficking law passed constitutional muster even though it does not include any fair use safety valve. According to Judge Kaplan, anyone who distributes DeCSS code violates the law, even if he distributes it to someone who merely wishes to employ it to 'unlock' a movie and make a fair use of it.

"This case poses the most important constitutional issues of the first part of the 21st Century," said Eben Moglen, a law professor at Columbia Law School and one of the co-authors of a friend-of-the-court brief supporting Corley. "The case points up the intrinsic First Amendment conflict with the new law of copyright," he said.

On appeal, the legal arguments will consider whether Congress may pass a law that, in effect, allows movie studios not only to place a digital "lock" on their films but also to sue anyone who distributes the digital key. Such powers would prevent pirates from copying films but fair users as well.

Various civil liberties groups, some law professors and the defendant in the Universal lawsuit all argued in briefs and friend-of-the-court briefs that the First Amendment absolutely requires that would-be fair users have the right to use certain information -- such as decryption software -- to gain access to copyrighted works.

They argue that the doctrine of fair use becomes meaningless in a digital world where any publisher, movie studio or record company can place an electronic wrapper around a work of art making it impossible to make a fair-use copy.

In the past, when a company published a book, the fair use rights of readers limited its control over the work. But if the same company issues a book today and encrypts it, its control over readers is far greater -- in fact, almost unlimited -- unless there is a right of access to the material.

The plaintiffs in the lawsuit, along with law professors and the Department of Justice, argued in a flurry of briefs and friend-of-the-court briefs that the First Amendment does not require that would-be fair users gain access to books and movies.

"There is no right to use [decryption] software" so as to use a protected work for fair use purposes, Rodney A. Smolla, a law professor at the University of Richmond and co-author of a friend-of-the-court brief, said in an interview.

"You don't have a fair use right to view an HBO televised fight and make a copy of it," he said. "Similarly, a movie theater can restrict access by charging admission -- even charging a movie critic," he said. In fact, he added, there is nothing preventing an author from hoarding a work and not sharing it with the public, including would-be fair users.

In addition, said the movie studios in legal papers, using encryption software to prevent mass digital piracy would be impossible if consumers could freely distribute and study decryption software. "This is an important, practical issue," Charles Sims, the lawyer representing the movie studios, said in an interview. "One reason why e-books have been so delayed is that the publishers are concerned about the security of their works and their ability to earn a fair profit without the books getting pirated the first day out there," he said. "The ability of technological protection schemes to be backed up by . . . some enforcement is vital," he added.

According to Peter Jazsi, a law professor at American University's Washington College of Law and a co-author of a friend-of-the-court brief supporting Corley, the Second Circuit has three choices with respect to the First Amendment/Fair Use debate.

They can accept, as Judge Kaplan did, the "unforgiving nature" of the Digital Millennium Copyright Act and uphold it against the First Amendment challenge mounted by Corley. Or they can disagree with Judge Kaplan's analysis and say that the anti-trafficking provision "is too much in tension with free expression to stand," said Jazsi. Alternately, the court may "save" the anti-trafficking statute from unconstitutionality by interpreting it as containing a fair use loophole.

"That's where fair use originally came in," said Jazsi. "The judges made it up. It's been around in the American case law since the middle of the 19th Century, and only made it into federal law in 1976," he said.

Jazsi said that a second major issue that the Second Circuit will grapple with is the constitutionality of Judge Kaplan's order banning Corley from linking to sites that provide the DeCSS code.

"The whole question of liability for linking . . . is still in its infancy," he said. "I would be very concerned about the broad precedential impact of the Second Circuit's embrace of [Judge] Kaplan's rule."

But Sims, the movie studio's lawyer, said the linking ban was narrow in scope and justified because Corley had been previously told by the court to stop distributing DeCSS. Corley's subsequent list of links, which he termed an act of electronic civil disobedience, frustrated the will of the court, Sims said.