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How to Read Data from an RFID Card: What’s Really Stored on That Thing?

Cykeo News RFID FAQ 80

You swipe your office badge every day. That beep means something. But what? If you’re trying to integrate a system, recover lost data, or just understand the tech in your wallet, learning how to read data from an RFID card is your first step. Forget the marketing fluff. I’ll tell you what works, what doesn’t, and why you’ll probably get stuck at an authentication wall.

First, identify your target. Not all cards are created equal.

  • The Dumb Ones (125 kHz LF): Most hotel keycards, basic gym tags. These are beep-and-forget. They broadcast a fixed, factory-lasered ID number—that’s it. No other data. Reading it gives you a string like 0007894542. Done. If you need more, you’re out of luck.
  • The Smart Ones (13.56 MHz HF – MIFARE/NFC): Your corporate ID, transit card, university pass. These have actual memory—organized in sectors and blocks, like a tiny, paranoid hard drive. This is where the real game is.

I learned this the hard way on a client job. They needed to audit old access cards. We bought a fancy “universal” reader. It beeped happily on their new cards but was stone-cold silent on the old ones. Why? The old cards were LF, the reader was HF. A $300 paperweight lesson. Match the tool to the card.

The Toolbox: What You Actually Need

Don’t waste money.
For LF cards, grab an RDM6300 module. Hook it to an Arduino’s serial pins, and it’ll spit out the ID. It’s boring but reliable.
For HF cards, you need a reader that speaks the MIFARE language. The PN532 (as a USB dongle or board) is the Swiss Army knife. The cheaper RC522 works but chokes on some card types. For the serious work—the kind that involves figuring out what’s in those memory blocks—you use a Proxmark3 or a dedicated handheld tool like a CYKEO diagnostic reader. The Proxmark isn’t for beginners; its learning curve looks like a cliff.

The Process: It’s Not a Simple Scan

Let’s say you have a common MIFARE Classic 1K card and a PN532. Here’s the real sequence, with the frustrations included.

  1. Plug In and Hope: Fire up software like “MIFARE Classic Tool” or “ProxGrind”. Hold the card to the reader. If you’re lucky, it detects the card and shows the UID (e.g., A1 B2 C3 D4). This is the card’s serial number, not your data. It’s like reading the cover of a book.
  2. Hit the Authentication Wall: This is where most people stop. Every sector (there are 16) is locked. You need one of two 6-byte secret keys (Key A or Key B) to unlock it. Many cheap or old systems use factory defaults like FF FF FF FF FF FF or A0 A1 A2 A3 A4 A5. Your software will brute-force these first. If it succeeds, you’re in. If the system admin changed the keys (as they should), you’re done. That sector is a black box.
  3. Dump the Memory: After authentication, you can “dump” the sector. The software shows a grid of 16-byte blocks in hexadecimal. This is the raw data.Making Sense of the Hex Jungle:
    That block of hex 48 65 6C 6C 6F isn’t gibberish. It’s ASCII. It decodes to “Hello”.
    But it could also be a number: 00 00 27 10 is the decimal 10000.
    Or, it could be proprietary encoding—a timestamp, a facility code, an encrypted credential. Without the system designer’s map, you’re just guessing.I once dumped a transit card and found a block that changed after each ride. It was the “remaining trip” counter. The hex didn’t spell it out; I had to reverse-engineer the pattern by taking trips and dumping after each one.

Why You’ll Fail (And What to Do)

  1. “No Card Found”: Wrong frequency. Verify your card type.
  2. “Authentication Failed”: The keys are custom. You might try a dictionary attack with common key sets, but if it’s a secured system, give up. This is a feature, not a bug.
  3. “Garbled Data”: You’re reading the wrong block, or the data is formatted in a non-standard way. Start with a blank card you’ve written to yourself, so you know what you put in.
  4. “Card is Magic”: Modern cards like MIFARE DESFire don’t allow this low-level memory dump. They use a secure channel. You’ll only get what the card’s application decides to give you, which is often nothing without the proper cryptographic credentials.

The real skill in knowing how to read data from an RFID card isn’t in running the software—it’s in knowing what you’re looking at after the dump. It’s digital archaeology. Start with a blank MIFARE Classic card. Write a known phrase to a block. Read it back. See the hex. That’s your baseline. Then try it on an old, disposable card from a non-critical system. You’ll learn more from one successful decode and ten failures than from any tutorial. Remember, just because you can read the bytes doesn’t mean you understand them. That’s where the real work begins.

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