More than 20 years ago, a surreptitious recording of a cellphone conversation arrived at a Pennsylvania radio station. A federal law made it illegal to broadcast such recordings, but the radio station aired it anyway.

The case reached the United States Supreme Court, which in 2001 ruled for the radio station. Legal experts say the decision, Bartnicki v. Vopper, powerfully undermines Donald J. Trump’s claim that The New York Times can be held liable for publishing parts of his tax returns.

Any effort to punish “publication of truthful information of public concern,” Justice John Paul Stevens wrote for the majority, “implicates the core purposes of the First Amendment.”

That is so even if someone broke the law in providing information to a news organization, he wrote. “A stranger’s illegal conduct does not suffice to remove the First Amendment shield from speech about a matter of public concern.”