Master Key or Breaking Smartphone Encryption
Imagine the situation where every house lock is made by one of three manufactures - Ambroid, Microlock, and Orange. All of the locks take your fingerprint to open, but each analyzes your fingerprint a little differently. Now the police need to enter your home (with justification). They do not have a fingerprint match, so they require Orange to create a keyhole for every lock that when using the proper key, the door will open. The key that fits in this hole is easily copied, and the same key works for all Orange locks. This is a simple analogy to what the Department of Justice wants Apple to do to the locked iPhone.
Apple and the US Department of Justice have been battling in and out of court for the last week. The DOJ wants Apple to provide them access to the contents of the phone that was in possession of the shooters in San Bernardino in December 2015. If you create an access code (aka PIN) on that model of an iPhone, the electronically stored contents of the phone are encrypted and cannot be accessed. You can try to guess the PIN; however, if there are 10 failures in a row, the system will erase the contents of the phone.
This has put the FBI in a tough spot. Normally the contents of an iPhone is automatically backed up to iCloud (Apple's Internet backup service). When the iCloud account's password is changed, the iPhone must be resynched with the iCloud service before backups will resume. The password was changed on the iCloud account at the request of the FBI. Apple has assisted government officials when presented with a valid court order to access material on iCloud. In this particular case, the material is not on iCloud and only exists on the phone.
The court order requires Apple to develop a means for accessing and retrieving the software. The DOJ insists that it will only apply to this phone. In my view, that statement either is a lie or made by someone who does not understand the entire situation. I am going to presume that the DOJ is not lying. All iPhones all you to create an access code. This is the only means to unlock your phone and unless someone else knows the code, they cannot unlock it.. The DOJ is requesting a means for unlocking the phone without knowing your code. Now all of the iPhones use the same type of lock, so the method for breaking into one is the same as breaking into any other iPhone. Since this is done in software, the method is the software code -- the single key that unlocks all iPhones.
Doorknob image from Wikihow. See http://realism.com/licensed/door-knob for details.