On Dec. 2 of last year, 14 people were killed and 22 were seriously injured in a terrorist attack in San Bernardino, California, carried out by Syed Farook and Tashfeen Malik. As one might expect, in the ensuing weeks, the FBI has been gathering as much evidence about the case as possible. One piece of recovered evidence is an iPhone issued to Farook by his employer, the San Bernardino County Department of Public Health.

The health department authorized the FBI to access any data contained on the phone, but there's a problem. All the data on the phone — on all iPhones, for that matter — are encrypted so that even Apple can't decrypt them. To access the data, one needs to know the phone's passcode.

There are, of course, computer programs that can guess passcodes in rapid succession, but iPhones are designed with a trio of additional security measures to prevent that:

1. An auto-erase function deletes an iPhone's data after 10 incorrect passcode attempts.
2. After a certain number of failed attempts, the iPhone enforces a mandatory delay before the passcode can be attempted again.
3. The iPhone knows the difference between a manual passcode entry and one attempted by a computer, and only allows the former.

The FBI currently has no way to bypass these security measures — and just as important, Apple doesn't either. Yet the FBI is convinced the phone holds important information. That brings us to the current showdown.