Message Security Code
What it is
Security code for network management (e.g. key-exchange) messages.
DE 96 on the wire
Bitmap position
DE 96 is bit 32 of the secondary bitmap: byte 4, mask 0x01. A secondary bitmap with only DE 96 present reads 0000000100000000 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 96 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 96 is fixed length: always 8 bytes, with no length prefix. A parser reads exactly that many and moves on.
Size in the message
Exactly 8 bytes of binary data. Interfaces that carry binary data as text send each byte as two hex characters, so the value takes 16 characters there.
Format b
Format b means binary: raw bytes, often shown or sent as hexadecimal text.
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.