FBI May Demand iOS Source Code, Signing Key
In a truly cryptographically secure system, there wouldn't be a master key, and a signing key would not be able to enable access to private data. Security would be guaranteed by mathematics, not by the UI. The data would be encrypted in a fashion that even a modified OS would not be able to decrypt without a user-generated high-entropy passphrase or key that Apple does not have. If iPhones were truly secure, Apple would technically and mathematically be unable to help, and the FBI wouldn't have a case to begin with.