It’s the biggest tech case of the year, and maybe the decade. Taking place in a California federal court, the case pits Apple against the US government over control of the iPhone, with terrorism and privacy as the backdrop. The outcome will ripple across the entire technology sector and influence governments around the world. Does the FBI, which wants to force Apple AAPL -0.31% to override the iPhone’s encryption features, have a legal right to do so? The government is frustrated with new iPhone security features that make it near impossible for the FBI or Apple or anyone else (except the phone owner) to crack the password. Now, the feds are asking a federal judge in California to force Apple to write special software that will override those encryption features in order to peer into the iPhone used by one of the San Bernardino terrorists. The technical facts of the case are important because the FBI wants to go beyond what it has done before. It’s different than forcing a phone company to place a wiretap or making an Internet company hand over emails. Also note the Justice Department isn’t fighting over just this iPhone. Law enforcement agencies across the country are being thwarted by Apple’s encrypted devices, and the FBI likely chose this case—which involves an infamous terrorist—as its best chance to force Apple to change course. The Justice Department already has a search warrant, but the warrant is doing little good. That’s because Apple says the iPhone’s encryption features mean it can’t access the phone or provide the information the FBI is seeking. In response, the government is citing a 1789 law called the “All Writs Act” to force Apple to comply. The law itself isn’t as strange as it sounds. According to Stephen Scott, a constitutional law professor at McGill University, the original point of the law was to ensure courts in colonial America had the same traditional powers as those in England. Many of those powers came in the form of old writs (“royal commands”) we still use today: writs of habeas corpus, certiorari, and so on. The Justice Department does not refer to a specific type of writ in its court petition (it just cites the All Writs Act), but that doesn’t really matter. The feds just want the court, like courts have on many other occasions, to use its power under the Act to get Apple to comply with the search warrant. But there are two catches. Apple hadn’t filed its legal brief yet, but we can anticipate its objections. The first of these has to do with limits put on the All Writs Act by the Supreme Court. Specifically, the court won’t order Apple to act if it concludes the company is “so far removed from the underlying controversy” or it would place an “unreasonable burden” on Apple. It’s a safe bet Apple will make the “burden” argument but, according to lawyer Alexander Abdo of the ACLU, the company could also claim it’s too far removed for the investigation. “The All Writs Act doesn’t allow government to conscript a company into service if the company doesn’t have the information … If the FBI is doing an investigation, it can’t force the local locksmith to help it break into a house,” said Abdo. So a court might find the limits on the All Writs Act apply to Apple. Or it might conclude the Act doesn’t apply at all—that’s what a US magistrate judge in Brooklyn suggested last year in another case involving iPhone encryption. What About the Constitution? No matter what the All Writs Act means, the Justice Department will not be able to make Apple comply if doing so would violate the Constitution. This opens another can of worms. First, there is the Fourth Amendment’s search and seizure rules. While the FBI obtained a valid warrant, it must still execute the search in a reasonable manner—and forcing Apple to break encryption might not qualify. According to Abdo of the ACLU, Apple has a claim based on the Fifth Amendment too: it could argue that the process would deprive it of liberty without due process of law. Finally, law professor Ryan Calo of the UW School of Law suggested on Twitter that the FBI’s proposed order violates free speech rights. The reason is that forcing Apple to write new software may amount to “compelled speech” that contravenes the First Amendment. So who will win? It’s way too early to say. For now, it’s clear that Apple has compelling legal arguments to support its claim that it shouldn’t be forced to decrypt the phone, but that doesn’t mean it will prevail. Meanwhile, the Justice Department, as it points out in its brief, has used the All Writs Act to prevail in numerous other cases, including ones where companies had to turn over passwords.