This is a three (MasterCard, Visa, Discover) or four (American Express, called the "CID") code printed on the card to help prevent against fraud. The idea behind it, is it proves possession of the card and is required when the card is not present. On American Express cards, this value is printed on the front, while MasterCard, Visa, and Discover cards have it printed on the back, usually in the signature box.The magnetic stripe on the back of the card is broken up horizontally into three "tracks", the tracks often contain duplicate data and most times track 3 doesn't really contain any data.ISO/IEC 7813 Defines the attributes of this data, and these sites describe the data available on each track in detail:To read the data, you'll need a magstripe reader that will support reading all three tracks. Most magstripe readers emulate a HID (human input device), so its surprisingly easy to read from them - just open a text capturing program and swipe a card.The reader I'm using is the MagTek SureSwipe 21040145.So lets see what data we'll get from the magstripe:%B5XXXXXXXXXXXXXX2^ANTONIEWICZ/BRAD^1103101000000001000000003000000? 5XXXXXXXXXXXXXX2=1103101000000300001?So you can see that Track 2 actually contains much data already present in track 1. You'll also notice that there is a difference in the discretionary data between track 1 and 2.But where the heck is that Track 3 data? Well.
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