Format: bLength: 8 bytesFixed length
DE 64
Message Authentication Code (MAC)
What it is
MAC over the message content when only the primary bitmap is present — integrity protection between nodes.
The MAC proves the message was not altered in transit: an ISO 9797 (X9.19 retail) MAC computed over the message under a MAC key shared between the two nodes. When a secondary bitmap is present the MAC moves to DE 128 so it stays the last 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.