Apple has vowed to defy a court order to give the FBI access to files on a phone owned by one of the San Bernadino shooters. Tim Cook says that the move would threaten the security of all customers.

The order involves the phone of Syed Rizwan Farook, which the FBI believes could house information about Farook’s motives and the involvement of others in planning the mass shooting.

Apple is unable to decrypt the handset, an iPhone 5C, because it’s deliberately designed so that nobody — not even the manufacturers — can do so without the unlock code. That in itself is an issue that’s hotly debated among politicians.

While a court order demanding decryption would thus be meaningless, the FBI has instead successfully applied for a court to force Apple to modify the phone’s operating system. They want the removal of the safeguard that wipes the phone after 10 failed attempts to unlock it.

The idea is that the FBI could then simply brute force its way into finding the correct code, although that would involve carefully taking the phone apart to find the hardware key. That would allow the phone to be hooked up to a supercomputer to crack the code rather than doing it by hand.

The court agreed with the request, which bizarrely was made under the All Writs Act of 1789. That’s a very general legislative tool which in principle means federal courts can tell anyone to do anything that’s necessary in the interests of justice. However, Supreme Court rulings have put limits on its application, most notably that it can’t be used to force somebody to break other laws, and it can’t make people do something that would be “unreasonably burdensome.”

Apple chief Tim Cook says that to date the company has always complied with FBI requests when they have the ability to do so and what they are being asked to do is legal. However, he now says that Apple is being asked “for something we simply do not have, and something we consider too dangerous to create. They have asked us to build a backdoor to the iPhone.”

Although the FBI application to the court says that only Farook’s handset would be affected, Cook says there’s no way to guarantee that the modified version of the operating system wouldn’t get into the wrong hands: “this software — which does not exist today — would have the potential to unlock any iPhone in someone’s physical possession.”

Cook also argues that complying with the order would create a dangerous precedent. “If the government can use the All Writs Act to make it easier to unlock your iPhone, it would have the power to reach into anyone’s device to capture their data. The government could extend this breach of privacy and demand that Apple build surveillance software to intercept your messages, access your health records or financial data, track your location, or even access your phone’s microphone or camera without your knowledge.”