How to parse DE55

DE 55, also called Field 55, in ISO-8583 is Integrated Chip Data; so, it's easy to deduce that the EMV tags of an online transaction should be put in DE 55 when creating the corresponding ISO-8583 financial transaction message. But how?

Well, firstly, one needs to know that EMV tags are BER-TLV encoded in Field 55. Secondly, once you know the encoding itself, it's just a matter of putting all tags together one after another and you get your DE 55---no delimiters and no meta information; it's as simple as that. The only extra stuff that you might need will be the length/ encoding required by the ISO-8583 variant you are implementing. For example, if DE 55 is an LLVAR field, you'll need to add the overall length of the EMV data as LL in DE 55 before putting in all the EMV data.

Below is an example of DE 55 in an ISO-8583 message.

01495F2A0201245F34010182021C008407A0000000031010950580000000009A031102249B0268009C01009F02060000000000009F03060000000000009F0607A00000000310109F0802008C9F0902008C9F100706010A039000009F1A0201249F2608423158936ED6C38F9F2701809F3303E0B0C89F34034103029F3501229F360200019F3704ACAC66E89F5800DF0100DF0200DF0400

Here, 0149 is the LL-part of the LLVAR field, which indicates that the data is 149 bytes (or 298 hex characters), starting from 5F and ending at 00. You can copy this data, excluding the starting 0149, into EMV Lab's TLV Utilities and get this decoding by just a click of the button:

Tag	Value
5F2A	0124
5F34	01
...and so on

BER-TLV Encoding of EMV Tags

ISO-8583 messages encode EMV tags using BER-TLV scheme in Field 55 of the message. The formal name of the encoding scheme is ASN.1 Basic Encoding Rule (ISO 8825).

The term TLV stands for Tag Length and Value (sometimes also referred to as Type, Length and Value). For EMV tags, the tag is usually 2-4 bytes in size; the length part is again 2-4 bytes in size,

Valid EMV Tag Names

Some sample EMV tag names are 81, 9F02, 9F26, 4F and 5A. You can find a complete list of tags and their description on EMV Lab's EMV Tags page. BER-TLV encoding rules divide the tag name (sometimes called tag type) into following parts: class (2 bits), primitive/ constructed identifier (1 bit) and tag name (5 bits followed by 0 or more bytes).

Take tag 9F26, for example. In binary, it's equivalent to byte1 = 1001 1111, and byte2 = 0010 0110. The first two bits are 10, which indicate that its class is "Context-Specific". The next bit is 0, which indicates that it's a primitive tag. The next 5 bits of the first byte are all 1, which indicates that the tag name is of the "long form" and we need to look at the next byte as well. All bits in byte 2 form part of the tag name. However, the highest bit of byte 2 (and all following bytes) must be turned on if there are more bytes included in the tag name. In our case, 9F26 is just two bytes; hence, the second byte has it's first bit turned off.

Constructed EMV Tags

The term "constructed" in the context of BER-TLV encoding means that the tag's value is a set of EMV tags itself; i.e., the tag has recursive BER-TLV encoding structure. Let' use Tag 71 as an example.

The hex value of 71 is represented as 0111 0001 in binary. The first two bits are 01 which indicate that it has an "Application" class. The third bit 1 indicates that the tag's value is a set of EMV tags itself. The rest of the bits 10001 indicate the actual tag identifier.

Now, one of the valid values of Tag 71 would be "9F06021234", i.e., the value of Tag 71 is the tag 9F06 along with it's length and content. However, the value "1234" will be incorrect as "1234" is not valid BER-TLV data.