File Security Code
What it is
Security code for file-update operations.
DE 92 on the wire
Bitmap position
DE 92 is bit 28 of the secondary bitmap: byte 4, mask 0x10. A secondary bitmap with only DE 92 present reads 0000001000000000 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 92 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 92 is fixed length: always 2 characters, with no length prefix. A parser reads exactly that many and moves on, so shorter values are left-aligned and padded with trailing spaces.
Size in the message
At most 2 bytes when the message travels as text (ASCII or EBCDIC).
Format an
Format an means alphanumeric: letters and digits.
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.