Explaining ISO 8583 Messages

ISO 8583 is a messaging standard for systems that do electronic transactions. For instance, when you buy a coffee and pay for it with your credit card the POS device and bank communicate using this standard. Hence, ISO 8583 standard provides communication of different systems, and process financial transaction securely.

ISO 8583 Message Structure

It’s composed of 3 main areas which are MTI, bitmap, and data elements.

MTI Message Type Indicator

As the name applies, this area defines the message type. Therefore, the system can understand what type of message is received. This field has 4 numeric indicators.

The first indicator defines the ISO Message version,

0xxx — ISO 8583:1987

1xxx — ISO 8583:1993

2xxx — ISO 8583:2003

The second indicator defines message class,

x1xx — Authorization Message

x2xx — Financial Message

x4xx — Reversal & Chargeback Message

x8xx — Network Management Message etc.

The third indicator defines the message function,

xx0x — Request Message

xx1x — Response Message

xx2x — Advice Message

The fourth indicator defines who began the communication

xxx0 — Acquirer

xxx1— Acquirer Repeat

xxx2— Issuer

Bitmap

This area shows us to which data element fields are filled in the message. A message contains at least one bitmap called the Primary Bitmap that shows the presence of Data Elements 1 up to 64. Additionally, a secondary bitmap may exist which shows the presence of Data Elements 65 up to 128. Last but not least, the message can contain 3rd bitmap as well.

The bitmap may be transmitted as 8 bytes (binary), or sometimes with the 8 bytes unpacked into 16 hexadecimal characters 1–9, A-F (ASCII)

— Let’s look at an example to better understanding,

8 bytes long primary bitmap:

2038000000200002

Let’s transform the given bitmap into BCD format

20 → 0010 0000

As you can see here there are 8 bits only the third one value is 1. Hence, the third field is filled in the message.

38 → 0011 1000

The 11th,12th, and 13th fields are filled.

00 → 0000 0000

00 → 0000 0000

00 → 0000 0000

20 → 0010 0000

The 43rd field is filled.

00 → 0000 0000

02 → 0000 0010

The 63rd field is filled.

Data Element

Data Elements are fields carrying the information of the transaction itself. As I mentioned data elements presence is given in the bitmap area. Each Data Element has a specified meaning and format. ISO 8583 also includes some general purpose Data Elements and system-specific Data Elements that are used in different ways by different standards derived from ISO 8583.

Field Type Attributes

a — Alphabetic, including Blanks

n — Numeric Values

s — Special Characters only

an — Alphanumeric

as — Alphabetic & Special Characters only

ns — Numeric and Special Characters only

ans — Alphabetic, numeric & Special Characters

b — Binary Data

z — Tracks 2 & 3 code set as defined in ISO 7811 & ISO 7813

h — Hex Data

LL, LLL — Length of variable field that follows. ‘LL’ — Two-digit length indicator (1 byte BCD) ‘LLL — 3-digit length indicator (2 bytes BCD)

Example:

Let's parse the given sample,

08002038000000200002810000000001084909052253415630305A303537363331202020205341564E47583130303131303032303030302020202020200011010008B9F3F723CA3CD2F8

0800 MTI — 2 bytes 2038000000200002 Bitmap — Fields that present in message 3,11,12,13,43,63 810000 Processing Code — 3 bytes 000001 Stan No — 3 bytes 084909 Time — 3 bytes 0522 Date — 2 bytes 53415630305A303537363331202020205341564E4758313030313130303230303030202020202020 Field 43 — 40 bytes fixed length 0011 Field 63 Length, its 11 byte 010008B9F3F723CA3CD2F8 Field 63

I hope this article can give you some information about the ISO 8583 message standard. Enjoy!