Understanding Java Card 2.0

Learn the inner workings of the Java Card architecture, API, and runtime environment

This article begins with an overview of smart cards and a brief review of ISO 7816, the smart card standard. Given the background on smart cards in previous Java Developer columns, this installment will begin with an answer to the question, "What is a Java Card?" and an overview of the Java Card system architecture. Next, we'll focus on the many issues specific to the Java Card, including the Java Card lifecycle; the Java Card 2.0 language subset and API library classes; and Java Card security. Then we'll discuss the Java Card runtime environment and show how a Java Card runs. We'll close with an illuminating example: An electronic wallet application written just for the Java Card.

From here on, all references to Java Card implicitly refer to the Java Card 2.0.

What is a smart card?

Identical to the size of a credit card, a smart card stores and processes information through the electronic circuits embedded in silicon in the plastic substrate of its body. There are two basic kinds of smart cards: An intelligent smart card contains a microprocessor and offers read, write, and calculating capability, like a small microcomputer. A memory card, on the other hand, does not have a microprocessor and is meant only for information storage. A memory card uses security logic to control the access of memory.

All smart cards contain three types of memory: persistent non-mutable memory; persistent mutable memory; and non-persistent mutable memory. ROM, EEPROM, and RAM are the most widely-used memory for the three respective types in the current smart cards. Persistent memory is also called non-volatile memory. We will use the terms persistent and non-volatile interchangeably in this article.

ISO 7816 part 1-7, defined by International Standard Organization, contains a set of standards that covers various aspects of smart cards. ISO 7816 consists of:

  • Physical characteristics (part 1)

  • Dimensions and location of the contacts (part 2)

  • Electronic signals and Transmission protocols (part 3)

  • Inter-industry commands for interchange (part 4)

  • Application identifiers (Part 5)

  • Inter-industry data elements (Part 6)

  • Inter-industry commands for SCQL (Part 7)

The following diagram illustrates the physical characteristics of a smart card, which are defined in ISO 7816, part 1.

For more on ISO 7816 and smart cards, see "Smart cards: A primer."

Normally, a smart card does not contain a power supply, a display, or a keyboard. It interacts with the outside world using the serial communication interface via its eight contact points. The dimensions and location of the contacts are covered in part 2 of ISO 7816. This diagram shows the contacts on a smart card.

A smart card is inserted into a Card Acceptance Device (CAD), which may connect to another computer. Other terms used for the Card Acceptance Device are terminal, reader, and IFD (interface device). They all provide the same basic functions, namely to supply the card with power and to establish a data-carrying connection.

When two computers communicate with each other, they exchange data packages, which are constructed following a set of protocols. Similarly, smart cards speak to the outside world using their own data packages -- called APDU (Application Protocol Data Units). APDU contains either a command or a response message. In the card world, the master-slave model is used whereby a smart card always plays the passive role. In other words, a smart card always waits for a command APDU from a terminal. It then executes the action specified in the APDU and replies to the terminal with a response APDU. Command APDUs and response APDUs are exchanged alternatively between a card and a terminal.

The following tables illustrate command and response APDU formats, respectively. APDU structure is described in ISO 7816, part 4.

Command APDU
Mandatory HeaderConditional Body
CLAINSP1P2LcData fieldLe

The header codes the selected command. It consists of four fields: class (CLA), instruction (INS), and parameters 1 and 2 (P1 and P2). Each field contains 1 byte:

  • CLA: Class byte. In many smart cards, this byte is used to identify an application.

  • INS: Instruction byte. This byte indicates the instruction code.

  • P1-P2: Parameter bytes. These provide further qualification to the APDU command.

Lc denotes the number of bytes in the data field of the command APDU; Le denotes the maximum number of bytes expected in the data field of the following response APDU.

Response APDU
Conditional BodyMandatory Trailer
Data fieldSW1SW2

Status bytes SW1 and SW2 denote the processing status of the command APDU in a card.

What is a Java Card?

A Java Card is a smart card that is capable of running Java programs. The Java Card 2.0 specification was published at http://www.javasoft.com/javacard. It contains detailed information for building the Java Card virtual machine and application programming interface (API) in smart cards. The minimum system requirement is 16 kilobytes of read-only memory (ROM), 8 kilobytes of EEPROM, and 256 bytes of random access memory (RAM).

The system architecture on the Java Card is illustrated in the following figure.

As shown in the figure, the Java Card VM is built on top of a specific integrated circuit (IC) and native operating system implementation. The JVM layer hides the manufacturer's proprietary technology with a common language and system interface. The Java Card framework defines a set of Application Programming Interface (API) classes for developing Java Card applications and for providing system services to those applications. A specific industry or business can supply add-on libraries to provide a service or to refine the security and system model. Java Card applications are called applets. Multiple applets can reside on one card. Each applet is identified uniquely by its AID (application identifier), as defined in ISO 7816, part 5.

An important point to keep in mind is what smart cards are not: They are not personal computers. They have limited memory resources and computing power. Users should not think of Java Card 2.0 as simply a stripped-down version of the JDK.

The lifetime of a Java Card

The Java Card lifetime starts when the native OS, Java Card VM, API classes libraries and optionally, applets are burned into ROM. This process of writing the permanent components into the non-mutable memory of a chip for carrying out incoming commands is called masking.

Before it lands in your wallet, a Java Card needs to go through initialization and personalization. Initialization refers to loading general data into a card's non-volatile memory. This data is identical across a large number of cards and is not specific to an individual; an example might be the issuer or manufacture's name.

The next step, personalization, involves assigning a card to a person. It can occur through physical personalization or through electronic personalization. Physical personalization refers to embossing or laser engraving your name and card number on the plastic surface of a card. Electronic personalization refers to loading personal data into a card's non-volatile memory, for example, your personal key, name, and pin number.

Initialization and Personalization vary from vendor to vendor and issuer to issuer. In both, EEPROM (a type of non-volatile memory) is often used for storing data.

At this point, the Java Card is ready for use. You can get a Java Card from an issuer or buy it from a retailer. Cards sold by a retailer are general-purpose, in which case personalization is often omitted.

Now you can insert your Java Card into a reader and send APDU commands to the applets residing on the card or download more applets or data onto the card.

A Java Card remains active until it is expired or blocked due to an unrecoverable error.

Lifetime of a Java Card virtual machine

Unlike the Java virtual machine (JVM) in a PC or workstation, the Java Card virtual machine runs forever.

Most of the information stored on the card must be preserved even when the power is removed -- that is, when the card is removed from the reader. The Java Card VM creates objects in EEPROM to hold the persistent information. The execution lifetime of the Java Card VM is the lifetime of the card. When the power is not provided, the VM runs in an infinite clock cycle.

The lifetime of Java Card applets and objects

An applet's life starts when it is properly installed and registered with the system's registry table and ends when it is removed from the table. The space of a removed applet may or may not be reused, however, depending on whether garbage collection is implemented on the card. An applet on a card is in an inactive stage until it is explicitly selected by the terminal.

Objects are created in the persistent memory (for example, EEPROM). They could be lost or garbage-collected if other persistent objects do not reference them. However, it's a thousand times slower to write to EEPROM than to RAM.

Some objects are accessed frequently, and the contents of their fields need not be persistent. The Java Card supports transient (temporary) objects in RAM. Once an object has been declared as transient, its contents can not be moved back to the persistent memory.

Java Card 2.0 language subset

Java Card programs are, of course, written in Java. They are compiled using common Java compilers. Due to limited memory resources and computing power, not all the language features defined in the Java Language Specification are supported on the Java Card. Specifically, the Java Card does not support:

  • Dynamic class loading

  • Security manager

  • Threads and synchronization

  • Object cloning

  • Finalization

  • Large primitive data types (float, double, long, and char)

It's no surprise that keywords that support those features are also omitted from the language. VM implementors may decide to support 32-bit integer type or native methods for post-issuance applets if they are working on a more advanced smart card with more memory. Post-issuance applets are those applets that are installed on a Java Card after the card is issued to a card holder.

The Java Card 2.0 framework

Smart cards have been in the market for 20 years, and most of them are generally compatible with ISO 7816 Parts 1-7 and/or EMV. We've already looked at ISO 7816. What's EMV? The EMV standard, defined by Europay, MasterCard, and Visa, is based on the ISO 7816 series of standards with additional proprietary features to meet the specific needs of the financial industry. The Java Card Framework is designed to easily support smart card systems and applications. It hides the details of the smart card infrastructure and provides Java Card application developers with a relatively easy and straightforward programming interface.

The Java Card framework contains four packages:

Package NameDescription
javacard.frameworkThis is the core package on the card. It defines classes such as and , which are the fundamental building blocks for Java Card programs and , and , which provide runtime and system service to Java Card programs, such as APDU handling and object sharing
javacardx.framework This package provides an object-oriented design for an ISO 7816-4 compatible file system. It supports elementary files (EF), dedicated files (DF) and file-oriented APDUs as specified in ISO7816
javacardx.crypto and javacardx.cryptoEnc Those two packages support cryptographic functionality required in smart cards

Conforming to the Java naming convention, Java Cardx packages are extensions to the Java Card framework. It's not required that you support them on the card.

Java Card security

Java applets are subject to Java security restrictions, however, the security model of Java Card systems differs from standard Java in many ways.

The Security Manager class is not supported on Java Card. Language security policies are implemented by the virtual machine.

Java applets create objects that store and manipulate data. An object is owned by the applet that creates it. Even though an applet may have the reference to an object, it cannot invoke the object's methods, unless it owns the object or the object is explicitly shared. An applet can share any of its objects with a particular applet or with all applets.

An applet is an independent entity within a Java Card. Its selection, execution, and functionality are not affected by other applets residing on the same card.

How things work together inside a Java Card

Inside a Java Card, JCRE (Java Card Runtime Environment) refers to the Java Card virtual machine and the classes in the Java Card Framework. Each applet within a Java Card is associated with unique AID assigned by JCRE.

After an applet is correctly loaded into the card's persistent memory and linked with the Java Card Framework and other libraries on the card, JCRE calls the applet's install method as the last step in the applet installation process. A public static method, install, must be implemented by an applet class to create an instance of the applet and register it with JCRE. Because memory is limited, it's good programming practice, at this point, to create and initialize the objects the applet will need during its lifetime.

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