Issue 97, August 1998
Designing for Smart Cards - Part 2: Practical Implementation

ISO COMMANDS

The ISO commands are explained in ISO 7816-4, which is similar to other data-transmission specifications like those for OSI 802.2 and 802.3. Due to the relatively simple environment of smart cards, 7816 is easy to understand.

The types of commands contained in the standard include file selection, file reading and writing, file searching, file operations, identification, authentication, file management, program execution, and special instructions only implemented in an individual application. Obviously, not all commands are necessary for every application.

Careful thought should be given to the transaction process, as well as a system of checks that the process transpired properly. Transactions concern user identification, verification of the user’s account or access privileges, the actual transaction, verification that the transaction was successful, and verification that a history entry was made of the transaction.

One important point is that EEPROM bytes are cleared to their erased state prior to being written (this may be improved by only writing or erasing bits that need changing). Intruders shouldn’t be allowed to remove power in the middle of an EEPROM write sequence and thus alter error counters or other sensitive variables. So, any EEPROM updates shouldn’t be signaled to the reader until after they complete.