Issue in parsing TLV data in java, how to get value,length

Question

I have tried many sample codes to parse APDU response to TLV format. I am able to parse it properly if the response length is less but facing issue if length is more(how calculate length of a tag without any libraries)

NOTE: I am using predefined tags in Constants

code:

private HashMap<String, String> parseTLV(String apduResponse) {

    HashMap<String, String> tagValue = new HashMap<>();
    String remainingApdu = apduResponse.replaceAll(" ", "");
    if (remainingApdu.endsWith(ResponseTags._SUCCESS_STATUSWORDS)) {
        remainingApdu = remainingApdu.substring(0, remainingApdu.length() - 4);
    }
    while (remainingApdu != null && remainingApdu.length() > 2) {
        remainingApdu = addTagValue(tagValue, remainingApdu);
    }
    return tagValue;
}

addTagValue method

   private String addTagValue(HashMap<String, String> tagValue, String apduResponse) {
        String tag = "";
        String length = "";
        String value = "";
        int tagLen = 0;

        if (tagUtils.isValidTag(apduResponse.substring(0, 2))) {
            tagLen = readTagLength(apduResponse.substring(3));
            // tagLen = 2;
            tag = apduResponse.substring(0, 2);
        } else if (tagUtils.isValidTag(apduResponse.substring(0, 4))) {
            tagLen = 4;
            tag = apduResponse.substring(0, 4);
        } else {
            return "";
        }
        Log.e("TAG_LEN","tag: "+tag+"taglen: "+tagLen);
        if (tagUtils.shouldCheckValueFor(tag)) {
            length = apduResponse.substring(tagLen, tagLen + 2);
            int len = tagUtils.hexToDecimal(length);
            value = apduResponse.substring(tagLen + 2, (len * 2) + tagLen + 2);
            tagValue.put(tag, value);
            if (ResponseTags.getRespTagsmap().containsKey(tag)) {
                //logData = logData + "\nKEY:" + tag + " TAG:" + ResponseTags.getRespTagsmap().get(tag)/* + " VALUE:" + value + "\n "*/;
            }
            if (tagUtils.isTemplateTag(tag)) {
              //  logData = logData + "\n\t-->";
                return addTagValue(tagValue, value) + apduResponse.substring(tag.length() + value.length() + length.length());
            } else {
                return apduResponse.substring(tag.length() + value.length() + length.length());
            }
        } else {
            value = apduResponse.substring(2, 4);
            tagValue.put(tag, value);
//            logData = logData + "\n\t\tKEY:" + tag + " TAG:" + ResponseTags.getRespTagsmap().get(tag) /*+ " VALUE:" + value + "\n "*/;
            return apduResponse.substring(tag.length() + value.length() + length.length());
        }
    }

readTagLength :

private int readTagLength(String apduResponse) {
    int len_bytes = 0;
    if (apduResponse.length() > 2) {
        len_bytes = (apduResponse.length()) / 2;
    }
    Log.e("tlv length:", "bytes:" + len_bytes);
    if (len_bytes < 128) {
        return 2;
    } else if (len_bytes > 127 && len_bytes < 255) {
        return 4;
    } else {
        return 6;
    }
}

I cannot able to get length properly for few cards(if apdu response is long) Please help

Answer 1

First be sure the input data is proper before you go into the code. Take the full data and try it on https://www.emvlab.org/tlvutils/ .

Once its confirmed the data is proper, go through in EMV 4.3 Book 3, Annex B Rules for BER-TLV Data Objects sections B1, B2, B3 - with utmost attention.

If you follow this precisely, then you wouldn't need to store a static list of tags; will save time in future.

Answer 2

Below sample has an assumption that TLV array is ending with special 0x00 tag but for sure you can ignore it.

Pojo class :

public class Tlv {

    private short tag;

    private byte[] value;

    public Tlv(short tag) {
        this.tag = tag;
    }
    public short getTag() {
        return tag;
    }
    public byte[] getValue() {
        return value;
    }
    public void setValue(byte[] valueBytes) {
        this.value = valueBytes;
    }
}

Utility method :

public static Map<Byte, Tlv> parse(ByteBuffer bb) throws TlvException {

    Map<Byte, Tlv> tlvs = null;
    tlvs = new HashMap<Byte, Tlv>();
    try {
        while (bb.remaining() > 0) {
            byte tag = bb.get();
            if(tag == 0x00)
                continue;
            int length = bb.get();
            byte[] value = new byte[length];
            bb.get(value, 0, length);
            Tlv tlv = new Tlv(tag);
            tlv.setValue(value);
            tlvs.put(tag, tlv);
        }
    } catch (IndexOutOfBoundsException e) {
        throw new TlvException("Malformed TLV part: " + bb.toString() + ".", e);
    }
    return tlvs;
}