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ISO 8583

The foundational financial transaction message standard used by ATMs, POS terminals, card switches, and payment gateways worldwide. Every card payment you make passes through this protocol.

Message Structure

Every ISO 8583 message has three mandatory components: the Message Type Indicator (MTI), the bitmap, and the data elements. The bitmap acts as a presence map indicating which data elements follow.

Message Type Indicator (MTI)

The MTI is a 4-digit numeric field that uniquely identifies each message. Each digit carries distinct meaning:

PositionMeaningValues
1st digitISO version0=1987, 1=1993, 2=2003
2nd digitMessage class1=Auth, 2=Financial, 4=Reversal, 5=Recon, 8=Network mgmt
3rd digitMessage function0=Request, 1=Response, 2=Advice, 3=Advice response, 4=Notification
4th digitMessage origin0=Acquirer, 1=Acquirer repeat, 2=Issuer, 3=Issuer repeat, 4=Other

Example: MTI 0200 decoded

0ISO 1987
2Financial
0Request
0Acquirer

Result: Financial transaction request from acquirer (e.g., ATM cash withdrawal or PIN-based debit purchase)

Bitmap

The bitmap is a binary field where each bit position N indicates whether Data Element N is present in the message. The primary bitmap covers DE1 through DE64. Bit 1 set means a secondary bitmap follows, covering DE65 through DE128.

Byte 1
1
2
3
4
5
6
7
8
Byte 2
9
10
11
12
13
14
15
16
Byte 3
17
18
19
20
21
22
23
24
Byte 4
25
26
27
28
29
30
31
32
Byte 5
33
34
35
36
37
38
39
40
Byte 6
41
42
43
44
45
46
47
48
Byte 7
49
50
51
52
53
54
55
56
Byte 8
57
58
59
60
61
62
63
64
Bit 1: If set, a secondary bitmap (bytes 9-16) follows immediately after the primary bitmap, extending coverage to DE65-DE128.
Reading bitmaps: Convert each byte to binary. A 1 bit means the corresponding DE is present. DEs follow in ascending numerical order.

Bitmap example

Primary bitmap: F2 30 00 01 02 20 00 04

Binary: 1111 0010 0011 0000 0000 0000 0000 0001 ...

Present: DE1 (secondary bitmap), DE2, DE3, DE4, DE7, DE11, DE12, DE25, DE32...

Data Element Formats

FormatAbbrevDescription
NumericnBCD or ASCII numeric characters (0-9)
AlphabeticaASCII alpha characters (A-Z, a-z)
AlphanumericansASCII printable characters including special
BinarybRaw binary data (e.g., bitmaps, cryptograms)
Fixed length(no prefix)Field is always exactly N characters
LLVARLL2-digit length prefix followed by up to 99 chars
LLLVARLLL3-digit length prefix followed by up to 999 chars