HID Proximity Cards: Decoded Versus Undecoded Outputs?

["Will" <westes-usc@xxxxxxxxxxxxxx>]

Can someone explain the difference between an HID proximity card's decoded
and undecoded outputs?    My guess is that number printed on the card is an
undecoded output, and it's just there to make it easier for humans to type
in a number to a software application.    Probably the real number is on the
card as is longer or more complex format?    How many digits are there and
in what format (e.g., alphanumeric only).

I saw a demo on TV recently of some guy who using a home made circuit board
was able to swipe any person in his vicinity's prox cards, then record that
and play it back to get access through any prox reader.   Pretty scary
stuff, and it's obviously not a very secure architecture if they are sending
out numbers in a way that doesn't use some kind of private and public key
exchange.

We are thinking of using the proximity cards as part of a two factor
authentication system to login to computers, which is why I would like to
understand the length and structure of the number on the card.   We would be
using PCPROX readers.

--
Will