ISO 8583 · Data fields

Field 128MAC

pyiso8583 starter profilesIndependent explanation

What this field describes

Field 128 is another MAC container in the selected profiles, located in the secondary bitmap range.

Do not infer interchangeability with field 64. The security profile determines the placement, algorithm and bytes covered.

Compare the two profiles

Container settings from the pinned pyiso8583 source.
Setting	`default`	`default_ascii`
Value encoding	Binary	ASCII
Length limit	Exactly 8 bytes	Exactly 16 bytes
Prefix	None	None

Length limits describe encoded values. They exclude any length prefix. The profiles can be customized; these settings do not establish the required fields or accepted values for a live connection.

Read the field boundary

Field 12816 bytesASCII value

Read exactly 16 value bytes. The next field begins immediately afterwards under this profile; no length prefix belongs to this field.

Find it in the bitmap

Bitmap: Secondary
Position: 64
Byte: 8
Bit in byte: 8

Positions start at one. Bits are counted from the most significant bit of each bitmap byte. This calculation locates the field’s presence indicator, not its value.