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.

You have to know what the system defines. You've not provided enough information for us to be able to help you. There are many possible systems for TLV-encoding. Does 'EMV' mean "EuroPay, MasterCard, Visa"? BER is presumably the ASN.1 Basic Encoding Rules. But we shouldn't be having to guess. — Jonathan Leffler, Mar 21 '20 at 19:09
sorry, Im working with EMV(Europay,mastercard, Visa) card data. — Alexandra Jimenez, Mar 21 '20 at 19:13
I tried to find out in google but the practice of using tags that are 3 byte length seems no common in fact. — Alexandra Jimenez, Mar 21 '20 at 19:19
That suggests that there probably aren't 3 byte tags, then. Why do you think that's what you're getting? — Jonathan Leffler, Mar 21 '20 at 19:20
Searching on 'emv specification' comes up with versions 4.2 and 4.3 (the latter seems to be from November 2011, so version 4.2 shouldn't be very relevant), and a site https://www.emvco.com/ and links to PDF files. There's a Book 3 — I'm not sure what might be in Book 1 or Book 2. There's mention of BER-TLV in Book 3. The PDF URL includes a date in 2017. You still need to clarify what your problem is — where you think you're getting an undocumented encoding from. I can't help much more. — Jonathan Leffler, Mar 21 '20 at 19:27
BER-TLV encoding is documented in EMV Book 3, Annex B "Rules for BER-TLV Data Objects". You might find [this answer](https://stackoverflow.com/a/36644826/5128464) interesting...Good luck! — vlp, Mar 22 '20 at 08:56

score 4 · Answer 1 · edited Mar 22 '20 at 15:36

4

Go through EMV 4.3 Book 3, Annex B - Rules for BER-TLV Data Objects sections B1, B2, B3. If you follow this precisely, then you wouldn't need to store a static list of tags; it says clearly how to code and interpret tag length and values.

edited Mar 22 '20 at 15:36

Jonathan Leffler

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answered Mar 22 '20 at 12:10

Adarsh Nanu

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score 1 · Accepted Answer · answered Mar 22 '20 at 18:01

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.

I've found some EMV Tags that I think are excepted from 0x1F rule! Based on the EMV Book 4.3 Annex B, B1~B5 = 1 means there is an subsequent Byte Then why this rule is not imply to "628C Security attribute in compact format" or any other similar tags? — Mahmoud Hosseinipour, Mar 09 '22 at 11:40
You seem to be taking all 8 bits of the extended tag bytes, but aren't only the lowest 7 bits part of the tag, according to the very spec you mentioned? — Fabio A., Jun 07 '23 at 16:17

how to parse 3 byte length BER-TLV TAG

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