hastables on java card

Question

I'm new on Java Card applications. At this moment I would like to store a hash table (dictionary) that contains the configuration of a terminal that reads this type of cards. If the hash table has values, those must be retrieved to the terminal (I think using APDU's right?) but also if there are no values, the terminal must create a "default" initial configuration.

Is it possible to do this? If it is, how? Maybe there is an applet ready for that (like Musclecard for key generation and signing) but I haven't found any.

Any advice? Thanks!

Answer 1

Java Card is pretty limited regarding support for data structures. It has a few basic types such as byte and short and optionally int, which is not used anywhere in the classic API. For those types you can generate two types of transient (RAM) arrays using JCSystem.makeTransientByteArray() and friends. Furthermore, the default byte[], short[] and Object[] created using new are stored in EEPROM.

The Object class in Java Card has been stripped down as well. This means that there is no such thing as hashCode(). If it was present then you would run into problems as the Java SE version of hashCode() returns an integer, which is probably not present. All defined data containers are either smart card or security related (e.g. the APDU and Key classes).

So basically, if you want to create a HashMap - the common type of dictionary on Java SE - then you will have to create it yourself. It is in that case a good idea to define a Hashable interface that classes can implement to act as a key. The structures should be generated in the right type of memory. For the kind of application you specify you probably need persistent memory, which is kind of the default for object instances created using the new key word.

Personally, I would make very sure you need a hashCode() method for your solution. It is probably easier to create an Object array and simply iterate over the elements.

Answer 2

Since there is no hash table in the smart card, you can store the terminal configurations in byte arrays. The smart card only stores the configuration (and optionally protect the data), and the instruction to get stored configuration or to update it shall be sent by terminal via APDU command.

Suggestion 1

Put your configuration in a Linear Fixed EF, if the card supports file system. No applet needs to be created/installed. It's the terminal job to read all records of the file to determine whether a configuration exist or not, and to write configuration into the file using standard APDU (UPDATE RECORD, READ RECORD).

NOTE:

• set record length as number of terminal configuration bytes
• number of records denotes the number of configurations that can be stored
• you can put initial condition to indicate that the record is unused, e.g. 00...00

Suggestion 2

Create your own javacard applet. The applet must handle at least three proprietary APDUs:

1. Get list of terminal configurations
2. Update a record of terminal configuration
3. Delete a record of terminal configuration

NOTE:

• You need to handle how to store and return the bytes between APDU format and your storage