Format: bLength: 8Fixed length
DE 128
Message Authentication Code (MAC)
What it is
MAC over the message content when a secondary bitmap is present — always the final element.
Specification reference
Defined in ISO 8583:1987 (base). Card networks publish their own dialect tables (Visa BASE I / V.I.P., Mastercard CIS) that refine formats, lengths, and — for private-use elements — the entire layout. Always confirm against the interface specification you implement.