HID Prox
The beige 125 kHz badge on half the office lanyards in North America. It shouts one fixed number and does no crypto at all.
HID Prox is the classic office access badge: a thin white card, a 125 kHz coil inside, and a long run of market dominance behind it. HID introduced the Prox family in the early 1990s, and it became the default for building access across North America. There's a decent chance the badge in your pocket is one.
#How it works
A Prox card is about as simple as RFID gets. Hold it to the reader, the reader's field powers it up, and the card plays back a fixed number — then plays it again, and again, for as long as it's in the field. No handshake, no challenge, no key. The card doesn't prove anything; it just recites.
That number is laid out in a format, most commonly the 26-bit H10301:
A facility code says which site, a card number says which badge, and a couple of parity bits catch read errors. The reader hands all of that to the controller to decide. There's genuinely nothing else on the card — no memory you'd use, no cryptography anywhere.
#Why it's showing its age
Because the card only ever says a number, and says it in the clear, the number can be read and repeated. This isn't a flaw that got patched; it's the entire design. Prox predates the idea that a badge should have to prove itself (see Why old cards are a bad idea), and swapping to higher-bit formats or longer facility codes doesn't change that — a longer fixed number is still a fixed number.
#What replaced it
HID's own answer is Seos (and the iCLASS SE platform): real 13.56 MHz cards with AES (and custom keys, if you ask for them). Phone-based credentials land in the same place. If ripping out readers isn't on the table this year, the cheapest real improvement is moving the wire to OSDP so the number at least isn't travelling the cable in the clear too.
With Proxmark3
lf search powers the antenna and identifies an LF card — it'll tell you if
it's HID Prox, Indala, EM410x and so on, and print the format and number for a
card you own.
With Flipper Zero
125 kHz RFID → Read, held against the back of the Flipper, names the protocol and shows the data on your own badge.