how to parse 3 byte length BER-TLV TAG

Question

I am working with some EMV tags which most of them have a length of 1 or 2 bytes for example 9F02, 81,.. I understand that there is a certain bit configuration to know how I can determine the tag length for determine if a tags is 2 byte length or larger, Im using:

unsigned char tags[]={0x9F,0x02};
if((tags[0]&0x1F)==0x1F){
  ...
}

but I dont know to do when I have some tag larger than this.

Im working with EMV Data, Im testing with certification card , I'm receiving these tags are: DF8111, DF8119, DF811E, DF812C they are relate to CVM.

Answer 1

According to

EMV 4.3 Book 3

, Annex B - Rules for BER-TLV Data Objects sections B1, B2 that was linked above, you should check the bit 8 in the current byte in order to know if there are more byte in tag, assuming that you are using c/c++ (as you tag it in this post) here is a code that could be taken in order to check that condition, I commented the code bellow where the condition is and could used by you.

        int start = 0;
        int end = start + len;
        while (start < end) {

            int tag = src[start++] & 0xFF;

            if (tag == 0x00 || tag == 0xFF) {
                continue;
            }

            if ((tag & 0x1F) == 0x1F) {
                if (start >=  len ) {
                    break;
                }

                tag = (tag << 8) | src[start++] & 0xFF;
                // tag has 3 bytes (0xFFFFFF)
                /// check this line it could what you need.
                if ((tag & 0x80) != 0) {
                    if (start >=  len ) {
                        break;
                    }
                    //// Append third byte to the Tag.
                    tag = (tag << 8) | src[start++] & 0xFF;
                } 
            }
       /// ...
} ///end while

I hope this help you.