ISO 8583 Concepts

This guide covers the fundamental concepts.

Message Structure

An ISO 8583 message consists of three main parts:

┌─────────┬──────────┬────────────────┐
│   MTI   │  Bitmap  │  Data Fields   │
│ (4 hex) │ (16 hex) │  (variable)    │
└─────────┴──────────┴────────────────┘

MTI (Message Type Indicator)

The MTI is a 4-digit code that identifies the message type:

Position	Meaning	Values
1	ISO Version	0=1987, 1=1993, 2=2003
2	Message Class	1=Auth, 2=Financial, 4=Reversal
3	Message Function	0=Request, 1=Response, 2=Advice
4	Message Origin	0=Acquirer, 2=Issuer

Common MTIs:

MTI	Description
0100	Authorization Request
0110	Authorization Response
0200	Financial Request
0210	Financial Response
0400	Reversal Request
0410	Reversal Response

Bitmap

The bitmap indicates which fields are present in the message. It’s a 64-bit (8-byte) or 128-bit (16-byte) field:

Primary Bitmap: Fields 1-64
Secondary Bitmap: Fields 65-128 (if bit 1 is set)

# Example: Bitmap 7024058020C09000
# Binary: 0111 0000 0010 0100 0000 0101 1000 0000 ...
# Fields present: 2, 3, 4, 7, 11, 14, 22, 24, 25, ...

Data Fields

ISO 8583 defines 128 standard fields. Each field has:

Field Number: 1-128
Data Type: Numeric, Alpha, Alphanumeric, Binary
Length: Fixed or Variable (LLVAR/LLLVAR)

Field Types

Fixed-Length Fields

# Field 3: Processing Code - 6 numeric digits
processing_code = "000000"  # Purchase

# Field 4: Amount - 12 numeric digits
amount = "000000001000"  # $10.00 (in cents)

Variable-Length Fields

Variable fields have a length prefix:

LLVAR: 2-digit length prefix (max 99 characters)
LLLVAR: 3-digit length prefix (max 999 characters)

# Field 2: PAN (LLVAR)
# "16" prefix + 16-digit PAN
pan_field = "164111111111111111"

# Field 35: Track 2 Data (LLLVAR)
# "037" prefix + 37 characters
track2 = "037411111111111111=2512101123400001"

Key Fields

Field	Name	Type	Description
2	PAN	LLVAR	Primary Account Number (card number)
3	Processing Code	n6	Transaction type (000000=purchase)
4	Amount	n12	Transaction amount in minor units
7	Transmission Date/Time	n10	MMDDhhmmss
11	STAN	n6	System Trace Audit Number
22	POS Entry Mode	n3	How card was read
35	Track 2 Data	LLVAR	Magnetic stripe data
38	Authorization Code	an6	Approval code
39	Response Code	an2	00=Approved, 05=Declined
41	Terminal ID	ans8	Card acceptor terminal ID
42	Merchant ID	ans15	Card acceptor ID
55	EMV Data	LLLVAR	Chip card ICC data (TLV format)

Response Codes (Field 39)

Code	Meaning
00	Approved
01	Refer to issuer
05	Do not honor
12	Invalid transaction
13	Invalid amount
14	Invalid card number
51	Insufficient funds
54	Expired card
55	Incorrect PIN

Processing Codes (Field 3)

The 6-digit processing code has three parts:

Digits	Meaning
1-2	Transaction Type
3-4	Account Type (From)
5-6	Account Type (To)

Transaction Types:

Code	Type
00	Purchase/Goods & Services
01	Cash Withdrawal
09	Purchase with Cash Back
20	Return/Refund
30	Balance Inquiry

EMV Data (Field 55)

Field 55 contains chip card data in TLV (Tag-Length-Value) format:

┌───────┬────────┬───────────────┐
│  Tag  │ Length │     Value     │
│ (1-3) │  (1-3) │  (variable)   │
└───────┴────────┴───────────────┘

Common EMV Tags:

Tag	Name
9F26	Application Cryptogram
9F27	Cryptogram Information Data
9F10	Issuer Application Data
9F37	Unpredictable Number
9F36	Application Transaction Counter
82	Application Interchange Profile
9C	Transaction Type

Card Networks

Different card networks have specific requirements:

VISA: Field 62 (VISA private data), specific response codes
Mastercard: Field 48 (additional data), DE127 subfields
AMEX: Different field formats, 15-digit PANs
Discover: Similar to VISA
JCB: Japan-specific requirements
UnionPay: China-specific requirements

See the Networks Guide for details.

Next Steps

Parser Guide - Parse ISO 8583 messages
Builder Guide - Build messages
EMV Data Guide - Work with chip card data

Get started

Core

Card networks

Tools and interfaces

LLM features

Architecture

Operations

Message Structure

MTI (Message Type Indicator)

Bitmap

Data Fields

Field Types

Fixed-Length Fields

Variable-Length Fields

Key Fields

Response Codes (Field 39)

Processing Codes (Field 3)

EMV Data (Field 55)

Card Networks

Next Steps

​Message Structure

​MTI (Message Type Indicator)

​Bitmap

​Data Fields

​Field Types

​Fixed-Length Fields

​Variable-Length Fields

​Key Fields

​Response Codes (Field 39)

​Processing Codes (Field 3)

​EMV Data (Field 55)

​Card Networks

​Next Steps

Message Structure

MTI (Message Type Indicator)

Bitmap

Data Fields

Field Types

Fixed-Length Fields

Variable-Length Fields

Key Fields

Response Codes (Field 39)

Processing Codes (Field 3)

EMV Data (Field 55)

Card Networks

Next Steps