Wiegand

The badge reader on the wall usually doesn't decide anything. It reads the card, then passes the number down a cable to an access controller in a locked closet somewhere. The controller checks its list and decides whether to unlock the door. For decades that cable has almost always spoken Wiegand.

#Where the name comes from

The name comes from the Wiegand effect: a specially treated wire that gives a sharp magnetic pulse when a field passes over it. Early access cards had strips of this wire embedded in them, and the reader-to-controller interface took its name from them. The cards are long gone. The interface survived.

#How it works

• Two data wires, DATA0 and DATA1, both normally held high.
• A brief pulse low on DATA0 means a 0; on DATA1 it means a 1.
• The reader sends the card's bits one after another, and that's it.

The bits follow a format: an agreed layout of how many bits there are and what each group means. Usually that's a site or facility code, a card number, and a couple of parity bits for catching errors. 26 bits is the classic, and there are dozens of others.

#Why it's showing its age

Wiegand was designed when the threat model was "someone might wiggle the cable loose".

• One-way. The controller can't talk back, ask questions, or check that the reader is still the reader it installed.
• Unsupervised. If a reader is unplugged or replaced, the controller has no protocol-level way to notice.
• Cleartext. The card number travels in the open. However secure the card is, the number that comes out the back of the reader is just a number.
• Short and point-to-point. Each reader needs its own run of cable, officially up to about 150 m.

The cleartext point is why a modern encrypted card wired into a Wiegand reader is only as strong as the cable behind the wall. The fix is OSDP.