Why when reading NFCtag with Android phone you get different tag ID then when reading with dedicated reader?

Question

I am using an Android Cilico F750 and the dedicated RFID reader is CF-RS103. The RFID tag type is MIFARE Ultralight type C.

When read with a dedicated card reader the id of tag is: 2054270212(10 digit).

But when read with Android phone the id is: 36139312876727556(17digit) and reversed id is: 1316602805183616 (16digit).

Does anyone know why this happens and if its possible to convert the 10digit id to 17digit id or vice versa.

I use intents to detect tag and to resolve intent I use this:

public void resolveIntent(Intent intent){

    String action = intent.getAction();

    if(NfcAdapter.ACTION_TAG_DISCOVERED.equals(action)
            ||NfcAdapter.ACTION_TECH_DISCOVERED.equals(action)
            ||NfcAdapter.ACTION_NDEF_DISCOVERED.equals(action))

    {
        Parcelable[] rawMsgs = intent.getParcelableArrayExtra(NfcAdapter.EXTRA_NDEF_MESSAGES);
        NdefMessage[] msgs;

        if(rawMsgs!=null)
        {
            msgs= new NdefMessage[rawMsgs.length];

            for(int i=0; i<rawMsgs.length; i++)
            {
                msgs[i]=(NdefMessage) rawMsgs[i];
            }
        }

        else
        {
            byte[] empty = new byte[0];
            byte[] id = intent.getByteArrayExtra(NfcAdapter.EXTRA_ID);
            Tag tag = (Tag) intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
            byte[] payload = dumpTagData(tag).getBytes();
            NdefRecord record = new NdefRecord(NdefRecord.TNF_UNKNOWN,empty,id,payload);
            NdefMessage msg = new NdefMessage(new NdefRecord[]{record});
            msgs= new NdefMessage[] {msg};
        }
        displayMsgs(msgs);
    }}

And this are my helper functions:

private void displayMsgs(NdefMessage[] msgs)
{
    if(msgs==null || msgs.length==0) {
        return;
    }

    StringBuilder builder = new StringBuilder();
    List<ParsedNdefRecord> records= NdefMessageParser.parse(msgs[0]);
    final int size = records.size();

    for(int i=0;i<size;i++)
    {
        ParsedNdefRecord record = records.get(i);
        String str = record.str();
        builder.append(str).append("\n");
    }

    text.setText(builder.toString());
}



private String dumpTagData(Tag tag) {
StringBuilder sb = new StringBuilder();
byte[] id = tag.getId();
sb.append("ID (hex): ").append(toHex(id)).append('\n');
sb.append("ID (reversed hex):").append(toReversedHex(id)).append('\n');
sb.append("ID (dec): ").append(toDec(id)).append('\n');
sb.append("ID (reversed dec):").append(toReversedDec(id)).append('\n');

String prefix = "android.nfc.tech.";
sb.append("Technologies: ");
for (String tech: tag.getTechList()) {
    sb.append(tech.substring(prefix.length()));
    sb.append(", ");
}

sb.delete(sb.length() - 2, sb.length());

for (String tech: tag.getTechList()) {
    if (tech.equals(MifareClassic.class.getName())) {
        sb.append('\n');
        String type = "Unknown";

        try {
            MifareClassic mifareTag = MifareClassic.get(tag);

            switch (mifareTag.getType()) {
                case MifareClassic.TYPE_CLASSIC:
                    type = "Classic";
                    break;
                case MifareClassic.TYPE_PLUS:
                    type = "Plus";
                    break;
                case MifareClassic.TYPE_PRO:
                    type = "Pro";
                    break;
            }
            sb.append("Mifare Classic type: ");
            sb.append(type);
            sb.append('\n');

            sb.append("Mifare size: ");
            sb.append(mifareTag.getSize() + " bytes");
            sb.append('\n');

            sb.append("Mifare sectors: ");
            sb.append(mifareTag.getSectorCount());
            sb.append('\n');

            sb.append("Mifare blocks: ");
            sb.append(mifareTag.getBlockCount());
        } catch (Exception e) {
            sb.append("Mifare classic error: " + e.getMessage());
        }
    }

    if (tech.equals(MifareUltralight.class.getName())) {
        sb.append('\n');
        MifareUltralight mifareUlTag = MifareUltralight.get(tag);
        String type = "Unknown";
        switch (mifareUlTag.getType()) {
            case MifareUltralight.TYPE_ULTRALIGHT:
                type = "Ultralight";
                break;
            case MifareUltralight.TYPE_ULTRALIGHT_C:
                type = "Ultralight C";
                break;
        }
        sb.append("Mifare Ultralight type: ");
        sb.append(type);
    }
}

return sb.toString();
}

private String toHex(byte[] bytes) {
    StringBuilder sb = new StringBuilder();
    for (int i = bytes.length - 1; i >= 0; --i) {
        int b = bytes[i] & 0xff;
        if (b < 0x10)
            sb.append('0');
        sb.append(Integer.toHexString(b));
        if (i > 0) {
            sb.append(" ");
        }
    }
    return sb.toString();
}

private String toReversedHex(byte[] bytes) {
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < bytes.length; ++i) {
        if (i > 0) {
            sb.append(" ");
        }
        int b = bytes[i] & 0xff;
        if (b < 0x10)
            sb.append('0');
        sb.append(Integer.toHexString(b));
    }
    return sb.toString();
}

private long toDec(byte[] bytes) {
    long result = 0;
    long factor = 1;
    for (int i = 0; i < bytes.length; ++i) {
        long value = bytes[i] & 0xffl;
        result += value * factor;
        factor *= 256l;
    }
    return result;
}

private long toReversedDec(byte[] bytes) {
    long result = 0;
    long factor = 1;
    for (int i = bytes.length - 1; i >= 0; --i) {
        long value = bytes[i] & 0xffl;
        result += value * factor;
        factor *= 256l;
    }
    return result;
}`

EDIT: I managed to resolve this issue by truncating the 7-byte HEX ID to 4-bytes. And then formating the decimal ID if its total lenght is less than 10 digits with this statement that basically adds zeroes from left side if DEC ID is smaller than 10 digits:

String strFinal=String.format("%010d", Long.parseLong(str));

This document that describes how the ID is converted from HEX8 TO DEC10 helped me alot aswell: https://www.batag.com/download/rfidreader/LF/RAD-A200-R00-125kHz.8H10D.EM.V1.1.pdf

And a huge thanks to @Andrew and @Karam for helping me resolve this!

Answer 1

I don't know why are you trying always to convert to decimal? and please try to explain more about the code you use to read the UID.

about your numbers and to convert 17 digits to 10 digits; I convert both of them to Hex:

36139312876727556(17digit) in Hex : 8064837A71AD04.

2054270212(10 digit) in Hex: 7A71AD04

as you notice you can just tirm first three bytes to get the 10 digits.

and I do belive the both of them are not the UID. but the 7bytes as sayed Andrew, and you already read it in the your photo : (04:B5:71:7A:83:64:80)

Answer 2

So I think the answer is that because you are converting a 7 byte ID to decimal you are getting variable lengths of numbers because of the conversion to decimal.

"The byte data type is an 8-bit signed two's complement integer. It has a minimum value of -128 and a maximum value of 127 (inclusive)."

From https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

Could generate a decimal number with 1,2 or 3 characters thus as decimal the id can vary in length.

It also looks like that conversion is going wrong as in theory it should have negative numbers in there as well.

It is much better to handle it as a hex string if you want it to be human readable.

The correct method in Java to convert the 2 pages of the ID to hex is

StringBuilder Uid;
for (int i = 0; i < result.length; i++) {
                        // byte 4 is a check byte
                        if (i == 3) continue;
                        Uid.append(String.format("%02X ", result[i]));
                    }

Note as per the spec sheet of the card https://www.nxp.com/docs/en/data-sheet/MF0ICU2_SDS.pdf (Section 7.3.1)

There is check byte that is part of the ID, while this will always be the same on the same card and will still give you a unique ID it is technically not part of the ID.

Or if not reading at a low level then

https://developer.android.com/reference/android/nfc/Tag#getId()

will get you the id.

Note that the "36139312876727556(17digit) and reversed id" when converted to hex and reversed actual is 7 bytes and start with the right number.

The 10 digit just looks like the first 4 bytes of the 7 byte number also reversed.

Answer 3

The card reader on the PC is configured wrong, it is configured by default to display the ID as 10 digit decimal number (4 byte) when the card has a 7 byte ID.

It thus has to loose some data, it is doing this by truncating the ID to the first 4 bytes of the 7 byte ID

Use the software on the PC change the output format to something suitable for the ID size on the Mifare Ultralight C cards (8 Hex?)

or

Use Mifare Classic cards instead as these had 4 byte ID

or

truncate the 7 byte ID to 4 bytes eg change bytes.length to 4 (a hard coding to the first 4 bytes in the 7 byte ID) in your code and handle the fact that there is a very large number (around 16.7 million) of Mifare Ultralight C cards that will seem to have the same "ID" as you want to display it

This is because the spec's give by a seller on Amazon https://www.amazon.co.uk/Chafon-CF-RS103-Multiple-Support-Compatible-Black/dp/B017VXVZ66 (I cannot find any details on the manufacturer's site)

It says "Default output 10 digit Dec, control output format through software. "

"Support with windows,linux and android system, but can only set output format in windows pcs.No programming and software required, just plug and play. "

The only sensible answer is move everything to use a 7 byte ID.