Instead, it's the digital signature that Apple would use to sign that code that is the key to Apple's First Amendment argument, say legal experts who spoke with WIRED. "The human equivalent of the company signing code is basically saying, 'We believe that this code is safe for you to run,'" says Jennifer Granick, director of civil liberties for the Center for Internet and Society at Stanford Law School. "So I think that when you force Apple to cryptographically sign the software, it has a communicative aspect to it that I think is compelled speech to force them to do it."
"[B]ecause what's so expressive, necessarily, about that? But to me, the signing is expressive—very clearly so," she says. "That's kind of what the code signing is—it's saying 'I'm Apple Computer and we support this software and we think this software is safe for you to run'...So a forced signature to me is compelled speech."If Apple were to use this defense it could set some interesting precedents for digital signatures.
Other than the First Amendment, Apple could take the 5th in this case. Not refuse to testify, of course, but apply that other pesky part of the 5th Amendment - due process. David Kravets at Ars Technica explains how "conscripting Apple to build something that it doesn't want to do...is a breach of its 'substantive due process.'"
There is also the question of whether Congress has already had its say in this matter. The All Writs Act that the FBI is using to compel Apple only gives courts a tool to enforce existing statutes. As Albert Gidari shows, Congress in 1994 passed the Communications Assistance for Law Enforcement Act (CALEA) which "did not prohibit a [telecommunications] carrier from deploying an encryption service for which it did not retain the ability to decrypt communications for law enforcement access" (emphasis mine). So, CALEA could trump AWA. It is yet to be seen, however, if the courts view Apple as a "telecommunications carrier."