Harvard Law professor Susan Crawford and former special assistant to President Obama has written a blog post setting out the reason why she believes it is legally impossible for the FBI to win its case. The piece is entitled ‘The Law is Clear: The FBI Cannot Make Apple Rewrite its OS.’

While the FBI is relying on an extremely broad interpretation of the All Writs Act, Crawford points out that it is an accepted principle that specific laws take precedence over more general ones – and there is a specific law which outlaws what the FBI is asking for …

That law is the the Communications Assistance for Law Enforcement Act (CALEA). CALEA grants the government a lot of wiretapping powers, she says, but also clearly sets out the limits to those powers.

The government won an extensive, specific list of wiretapping assistance requirements in connection with digital communications. But in exchange, in Section 1002 of that act, the Feds gave up authority to “require any specific design of equipment, facilities, services, features or system configurations” from any phone manufacturer.

That wording, she argues, means that the government is specifically prohibited from requiring Apple to create a compromised version of iOS.

The government is aware of this, and the blog piece describes how it is attempting to argue its way around the issue, but Crawford says the FBI’s brief uses a circular argument. CALEA has, she writes, “no gaps; no interpretive sunlight: CALEA stops the government from doing what it wants to do to Apple.”

The next hearing takes place at the U.S. District Courthouse in Riverside, California, on 22nd March. Protest group Fight for the Future is inviting Apple supporters to post online messages which it will display outside the court.

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