The 4 Different Wires in iphone cable

Hello,

I am trying to build a wall charger using a 5 volt phone charger (that belonged to a different phone) and an iphone usb cable. I am attempting to do this for an iphone 4s.

I read online how to do this, and it seems like it might be doable for the average person to try this out. However, I am having trouble finding the black ground wire in the cable! All the sources on the internet said there will be one red wire for the voltage, two data +/- cables and one black ground wire. I see all of the wires except for the black one. There is however, a bundle of stranded wires without any insulation. When taking the multimeter to that wire I find that it seems to connect to all 4 pins on the USB connector. Is this the black wire? I had assumed that this was the shielding for the cable, but I dont have much electrical knowledge.

I should mention that I am using an aftermarket ipod cable bought on ebay. Maybe that makes a difference in finding the black wire?

Also, how does the voltage on the cable? Do you not need a positive and negative to supply to voltage? Is the red the only wire supplying the phone with power?

Any help would be really appreciated.