NFC读卡器无法读取加密卡仿真数据

Question

I'm currently using Seeed Studio's NFC Shield v2 and Arduino Uno R3. Here's the code from the sketch that I'm using to read. It based on the example provided by Seeed Studio Android HCE libraries:

void loop() {
    ...
    uint8_t response[510];
    uint8_t responseLen = sizeof(response);
    success = nfc.inDataExchange(selectApdu, sizeof(selectApdu), 
              response, &responseLen);

    if(success) {
      Serial.print("Response Length: "); Serial.println(responseLen);
      // nfc.PrintHexChar(response, responseLen);
      printResponse(response, responseLen);
    } else Serial.println("Failed sending SELECT AID"); 
  } else Serial.println("Didn't find anything!");
  delay(1000);
}

On the Android app that I have, I'm extending Android Developer's HCE example by adding digital signatures and AES encryption. I know for a fact that the app is readable if I don't add the AES encryption, and can verify the contents using a Python script. Once I add the encryption, the Arduino reads nothing from the app even though the Android device outputs logs that show the APDU was requested.

There seems to be two culprits:

1. Response[510] is not the right size.
2. The encrypted data being sent to Arduino is too big.

I changed the Response[] to various values such as 80, 1022, 2046, 8196, etc. However, the higher the values, the more freaked out Arduino becomes. Too low of a value, and nothing happens. 510 seems to be the magic number. I've logged a length count of my data being transferred:

• Without AES encryption, data length was 197 bytes which outputted fine on the Arduino serial console.
• With AES encryption, data length was 268 bytes and resulted in an output of "Failed sending SELECT AID", which is a false error since I was able to retrieve the AID request and APDU header.

What seems to be the problem and how can I go about fixing it, or at least have some workaround?

---

UPDATE: If I were to make the data being sent less than 245 bytes, the Arduino would output the data on the Serial console. Once it made it 246 bytes, it fails and outputs "Failed sending SELECT AID".

Answer 1

I still don't know how to work around this "limitation" on the Arduino but I've found a workaround. The key is to keep the data less than 245 bytes .

What I did is, instead of encrypting both the data and the signature together, I've decided to just encrypt the data and appended the signature to the end of the data along with something that I can use to delimit with (ie. "::").

This combination resulted to be a total 206 bytes , and it is being read and passed on as serial data to my Python script.