ISO 7816-4 File System Explorer

ISO 7816 File System Structure

ISO/IEC 7816-4 defines a hierarchical file system for smart cards. Understanding this structure is fundamental for smart card application development, file management, and data access operations.

File Hierarchy

The file system has three levels: MF (Master File, ID 3F00) is the root directory, always present after card reset. DF (Dedicated File) is a subdirectory that can contain other DFs or EFs. ADF (Application Dedicated File) is a special DF associated with an AID, providing application isolation. EF (Elementary File) stores actual data records.

Elementary File Types

Transparent EF: stored as a contiguous byte array, accessed by READ BINARY / UPDATE BINARY with an offset. This is the simplest type, used for small data like serial numbers. Linear Fixed EF: stores records of equal length, accessed by READ RECORD / UPDATE RECORD with a record number. Used for fixed-size records like phone book entries. Linear Variable EF: records have varying lengths, each prefixed with a length byte. Cyclic EF: circular buffer of equal-length records, oldest entries overwritten first. Used for transaction logs where only recent entries matter.

File Selection

Files are selected using the SELECT command (INS=A4). Selection can be by file ID (2 bytes), by AID (for application DFs), or by path (sequence of file IDs from MF). The short file ID (SFI) method allows selecting an EF within the current DF using a 5-bit identifier in the command's P2 parameter, useful for READ RECORD operations. Access to files is controlled by security attributes defined in the file's FCP (File Control Parameters), which may require PIN verification or cryptographic authentication before read or write operations.