Typically, one takes the ones compliment and then increment to find the inverse, so that when the numbers added together is "0". So the inverse of 0 is 0. This means that -0 == 0. So, zero has the ability to have both signs.

My guess is that the NSA can read any phone they want. The real issue is trying to get what they find admissible into court. On a different note, here is the IOS Security Guide: https://www.apple.com/business/docs/iOS_Security_Guide.pdf[^]