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Tricks of the Java Programming Gurus

Chapter 22 -- Authentication, Encryption, and Trusted Applets

Chapter 22

Authentication, Encryption, and Trusted Applets


The security that Java provides against malicious code is one of the language's biggest selling points. You can download programs over the network, automatically, perhaps even without realizing that it's happening, without serious risk of loss or theft of valuable information. Think of the possibilities!

Unfortunately, once you start thinking about it, the possibilities are somewhat limited unless the security can be relaxed somewhat. An applet that can't do anything dangerous can't do much that's useful either. In previous chapters, you learned ways of loosening security restrictions in controlled ways, but how do you decide which applets get the special privileges? How do you know whom you can trust, and how much?

Other problems hinder the development of really useful applets. Many useful applications, for instance, need to send sensitive information across the Internet. Even if the applet and its provider are trusted, having some way of protecting that information from eavesdroppers on the network is important.

Fortunately, you can use cryptography to solve these problems. Even more fortunately, a package of useful classes that provide the cryptographic building blocks for solutions is being developed by Sun and will probably be a part of the core Java library at some point, so applets and applications can rely on its availability in any Java environment. By the time you read this chapter, the package may be available.

In this chapter I discuss some of the security problems in more detail, with suggested strategies for solving them. I do discuss the package, but because the package interface is still unstable as I write this chapter, I'll stay away from details in favor of more general discussions of capabilities and the way the package will interact with and augment the Java security architecture of security managers and class loaders.

Cryptography Basics

Before I go into the details of cryptographic security as it relates to Java, you need to know a few basics about cryptography in general. Because this book isn't about cryptography, I won't go into great depth, and I will certainly stay far away from the complex math involved. The package hides all these details anyway, so the level of discussion presented here is sufficient for most developers.

Encryption is the process of transforming a message in such a way that it cannot be read without authorization. With the proper authorization (the message's key), the message can be decrypted and read in its original form. The theories and technologies of encryption and decryption processes are called cryptography.

Modern cryptography has its basis in some pretty heavy mathematics. Messages are treated as very large numbers, and an original, readable message (the plaintext) is transformed into an encrypted message (the ciphertext) and back again by means of a series of mathematical operations using the appropriate keys. The keys are also large numbers. All this math means that cryptography is a somewhat specialized field, but it also means that computers are good cryptographic tools. Because computers treat everything as numbers (at some level), cryptography and computers go together well.

The obvious use for encryption is to keep secrets. If you have a message that you need to save or send to a friend, but you don't want anyone else to be able to read it, you can encrypt it and give the key only to the people you want to trust with the secret message.

Less obvious, but just as important, is the possibility of using cryptography for authentication: verifying someone's identity. After you know how to keep secrets, authentication comes naturally. For centuries, people have proved their identities to each other by means of shared secrets: secret handshakes, knocks, or phrases, for example. If you were to meet someone who claimed to be a childhood friend, but who had changed so much that you didn't recognize him, how would he go about convincing you? Probably by telling you details of memorable experiences that you shared together, alone. The more personal, the better-the more likely that both of you would have kept the secret through the years. Cryptographic authentication works the same way: Alice and Bob share a key, which is their shared secret. To prove her identity, Alice encrypts an agreed-upon message using that key and passes on the encrypted message. When Bob decrypts it successfully, it is proof that the message originated from someone who shares the secret. If Bob has been careful to keep the secret and trusts Alice to do the same, then he has his proof.

You may have noticed in the preceding two paragraphs that keeping secrets and proving identity both depend on keeping other secrets: the keys. If some enemy can steal a key, he or she can read the secret messages or pretend to be someone else. Thus, key security is very important. Worse still, for most uses of cryptography, keys must be traded between people who want to communicate securely; this key exchange represents a prime opportunity for the security of the keys to be compromised.

Conventional cryptographic algorithms are symmetric: that is, the same key is used for both encryption and decryption. More recently, researchers have developed asymmetric public-key cryptographic algorithms that use key pairs: if a message is encrypted with one key, it must be decrypted with the other key in the pair. The two keys are related mathematically, but in such a complex way that it's infeasible (too costly or time consuming) to derive one key from the other, given sufficiently long keys.

Public-key cryptography simplifies key management immensely. You can treat one of the keys in the pair as your public key and distribute it widely, keeping the other as your secret key, known only to you. If Bob wants to create a message that only Alice can read, he can encrypt it using her public key. Because the public key can't be used to decrypt the message, others who also know Alice's public key can't read it, but Alice, using her secret key, can. Then, if Alice wants to prove her identity to Bob, she can encrypt an agreed-upon message with her secret key. Bob (or anyone else) can decrypt it with her public key, thus demonstrating that it must have been encrypted with her secret key. Because only Alice knows her secret key, the message must really have come from her.

Public-key cryptography sounds unlikely and almost magical when you first encounter it, but it's not such an uncommon idea. Your own handwritten signature is somewhat like a key pair. Many of the people and organizations you deal with regularly might recognize your signature (or have a copy on file for comparison), making the appearance of your signature a sort of public key. Actually placing your signature on a new piece of paper, however, is a skill that only you have: that's the secret key. Of course, signatures can be forged, but the point is that for all but one person, creating the signature is pretty difficult, whereas having anyone verify it is easy. Public-key cryptography makes possible the creation of digital signatures that work in much the same way, except that forging a digital signature is much more difficult.

If Alice wants to apply a digital signature to a document before sending it to Bob, a simple way for her to do so is to encrypt the document with her secret key. Because many people know her public key, the document isn't private-anyone with Alice's public key can decrypt it and read the contents (applying another layer of encryption with another key is possible, to produce a document that is both signed and private). When Bob successfully decrypts the message with Alice's public key, that action indicates that the message must have originally been encrypted with her secret key. What makes this effective as a signature is that, because only Alice knows her secret key, only she could have encrypted it in the first place.

Many other details enter into practical use of cryptography, of course. For several reasons, practical digital signatures are not as simple as the preceding example. Even with public-key cryptography, key management and security are important (and tricky) issues. Furthermore, public-key cryptography is much more complicated (and thus much slower) than symmetric cryptography, so symmetric cryptography still has an important role to play. One serious complication is that, unlike most computer algorithms, most good cryptographic algorithms come with legal entanglements. Many are protected by patents, so they must be licensed from the patent holders. The United States government considers implementations of strong encryption algorithms to be in the same category as munitions, and it places heavy restrictions on their export (even though many of the best algorithms were invented outside the U.S.). Some other governments prohibit the use of strong cryptography except for purposes of authentication, and a few governments ban it entirely. There are bills currently pending in the U.S. Congress to lift the export restrictions, but those bills haven't become law yet, and the U.S. government's cryptography export policy is one of the factors currently delaying the release of the package.

Fortunately, the package will hide most of the technical complications, and the Java license will explain all the legal and political details. The rest of this chapter covers the basics of how you can use the package with the rest of the Java library to make it possible for applets to do really useful work.

Security Mechanisms Provided by

The package provides five separate but related services: encrypted data, digital signatures, secure channels, key exchange, and key management.

Encrypted Data

Using either symmetric or public-key encryption algorithms, Java programs can use the package to encrypt and decrypt data buffers using specified keys. The encryption facilities can also be used in filtered I/O streams, so files or sockets can be encrypted or decrypted transparently during input and output operations (see Chapter 5, "Building Special-Purpose I/O Classes" for more information). When encrypted two-way communication is necessary, creating a secure channel may be better, as described later in this chapter.

Digital Signatures

Signatures are used as proof that a communication-whether legal or personal-came from a particular individual or organization. You can apply digital signatures to any kind of electronic document, whether they are text files, binary data files, or even short tokens used for authentication. In many cases, digital signatures can provide much stronger guarantees than conventional signatures: a digital signature can show not only that a particular entity signed a document but also that the document has not been modified by a third party since it was signed. The package provides facilities for applying digital signatures and for verifying them.

Secure Channels

A secure channel is a communication channel that is both authenticated and encrypted. The authentication ensures that the party on the other end of the communication is genuine, and not some impostor; the encryption ensures that a third party eavesdropping on the channel cannot understand the communication. Establishing a secure channel involves trading proof of identity (using small messages with digital signatures), after which the two parties can agree on a key to be used to encrypt all the subsequent communication on the channel. After the channel is successfully established, communications are automatically encrypted before transmission and decrypted upon reception. Facilities for easily establishing secure channels with other entities are provided as a part of

Key Exchange

Effective use of the facilities mentioned in the preceding section requires that encryption keys be exchanged between two parties. Secure channels, in particular, require that two parties exchange a conventional, symmetric encryption key (the session key), which is used to encrypt the communication on the channel and is then thrown away when the channel is destroyed. The key must be exchanged on an open channel, however, because the channel can't be secured until the key has been exchanged. Cryptographers have developed mechanisms for two parties to exchange keys securely on open channels. The secure channel implementation uses these mechanisms transparently, but the package also makes the key exchange mechanism available for direct use by programmers.

Key Management

Session keys for secure channels are used once and thrown away, but other keys, especially secret keys and the public keys of other people or groups, must be stored and used repeatedly. Such keys are useless unless you keep track of whom they belong to, and they are also useless if they aren't stored securely. If someone can steal your secret key, then he or she can read your private communications and impersonate you, and if someone can modify a public key that you hold, then he or she can substitute his or her own public key, making it easy to impersonate others in interactions with you. The package provides key management facilities that help to maintain this key security.

Enabling Trusted Applets

Applets and applications can use all these facilities to perform secure operations across the network in fairly straightforward ways. But I previously mentioned that features can be used to loosen the security boundaries for trusted applets so that they can do useful work. How can you accomplish that task?

Chapter 21, "Creating a Security Policy," explains that Java security enforcement in Java is largely the responsibility of the security manager, with help from class loaders. The class loaders keep track of the source of each class currently loaded into the virtual machine, and the security manager uses that information to decide whether a class is allowed access to particular resources.

In early Java applications, the "source" of a class meant the Internet host from which the class was loaded. But that criterion is not a particularly useful one on which to base trust. Classes can be copied from site to site, and because many sites are insecure, even classes from trusted machines might not really be trustworthy. And because that's such a poor way to determine the origin of a class, current applications are (justifiably) paranoid: if a class was loaded from a local directory in the CLASSPATH, the class is trusted; otherwise, it isn't.

If you want to trust a dynamically loaded class, either partially or completely, the really important information isn't where the class resides on the network, it's who wrote the class (or, in a more general sense, who takes responsibility for it). Digital signatures provide a way to determine the real source of a class with some degree of confidence.

It's easier to see how this might work using a concrete example. Assume that you trust the kind folks at GoodGuys, Inc. (GGI) to write well-written, trustworthy software that doesn't steal or destroy your data (I examine whether such blanket trust is reasonable later in this chapter). From GGI, you get their public key, and using some application-specific configuration mechanism, you give the public key from GoodGuys to your Java-based Web browser and inform it that GGI is an entity you trust completely. Meanwhile, hard-working GGI programmers have just finished a terrific applet, and a "signature officer" at GGI uses their secret key to sign the Useful.class file, which contains the applet's code, and places it on their Web server.

Next, the class loader in your browser can load the digitally signed class file, verifying the signature against the list of known entities using the facilities. If the signature turns out to be a valid signature from GoodGuys, the class loader can be certain that the class really came from that company, and because digital signatures provide strong guarantees, the class loader can be sure that nobody else has modified the class because it was signed by a GoodGuys employee.

Later, when that class requests access to a secured local resource (such as your financial database), the security manager will ask the class loader where the class came from, and the class loader can confidently report to the security manager that the class came from GoodGuys, Inc. Then, when the security manager sees in the configuration database that you trust that company, it allows the access.

Cryptographic Security Solves Everything, Right?

The package provides many useful building blocks. Those building blocks, coupled with the Java security model, permit you to build Java applications that make better use of network resources than before, enlisting applets to extend the basic application functionality. You also can use these building blocks to build applications that participate in electronic commerce-perhaps agents that carry out transactions on behalf of a user, or downloadable applications that are rented for a small per-use fee instead of purchased. These capabilities are possible without encryption-based security features, but they simply aren't practical because abusing them is too easy, and the potential for loss is too great.

Careful use of cryptographic facilities can raise serious security barriers against thieves and vandals-serious enough to make many new kinds of applications feasible. But the security still won't be perfect, just as the security at your bank, although hopefully very good, isn't perfect. Having realistic expectations about cryptographic security is important.

Even in nonelectronic life, security has its price; it doesn't happen magically. Locking your door when you leave the house is a bit of an inconvenience, and unlocking it again when you return is a little more trouble. Unless you're fortunate enough to live someplace that is relatively idyllic, you've probably cultivated the habit of locking your door, so you don't really notice the inconvenience any more. But if you happen to lose your key or lock it inside your house, the inconvenience of security measures comes back to you full force.

Electronic security is no different. It requires a little bit of trouble, and a little vigilance, for users (whether they be individuals or organizations) to secure their systems, and it requires thought and discretion to decide who to trust and how much.

And, as mentioned previously, security still won't be perfect. Just as a determined criminal can ultimately find a way to defeat any physical lock, people will find ways to get around cryptographic security measures. Some of the most effective ways to do so are decidedly low-tech. A classic example is the so-called "social engineering" attack, in which people call employees on the phone and pretend to be other employees, asking questions that would be innocuous coming from a real employee but that give the attackers valuable information. Often the attack progresses in stages-the first encounters might yield only harmless information, but even that information can help the attackers to be more convincing in later probes. Eventually, the attackers might learn passwords or other important network security information from an unsuspecting employee who is convinced that the caller is a highly placed company employee.

No technological security solution is completely effective against such attacks. Key management is currently the weakest point in many encryption-based security systems. Researchers will surely find ways to improve security over time, and as people become more familiar with computers, they might become more sophisticated in their responses to people who call and ask for information on the telephone, but perfect security will never come.

In the earlier section "Enabling Trusted Applets," I gave an example showing how an application can grant trust to an applet, based on digital signatures and configuration options specified by the application's user. The user in the example decided to trust any applet that came from GoodGuys, Inc. Does granting a company complete trust like that make sense?

The fact is, people do that all the time today. We tend, rightly or wrongly, to think of software vendors as reasonably trustworthy. If, while browsing in a computer store, I see a new program for sale and it looks as though it does something that would be useful to me (or maybe I just think it looks cool), I might be tempted to buy it, bring it home, and install it on my computer. I might ask myself a lot of questions first: "Do I really need this? Is it worth the price? Do I have enough disk space?" But I'm not accustomed to asking "Is this company trustworthy? Is this thing going to reformat my hard drive? Is it going to steal my private e-mail archive and send it to the company headquarters the next time I connect to the Internet?"

Occasionally, a software company does something that does weaken the trust people place in them. Several companies have carelessly shipped viruses to their customers on their software installation disks. Many consumers lost some of their trust in Microsoft when it was revealed that The Microsoft Network software shipped with early beta versions of Windows 95 would collect information about software packages installed on users' computers and send the information back to Microsoft. Where applets are concerned, similar adjustments can occur. A user might start out trusting a familiar company, but if an applet from that company does something unscrupulous (or just careless), the trust could vanish. Furthermore, in the Internet environment, word can spread among users more easily: "Don't trust applets from ShadyCo-you'll get burned like I did!"

The important point is that security doesn't have to be perfect. Flaws in physical security (for example, the ability to hot-wire cars to start them without a key) cause everyone problems from time to time, and people are always working to improve the situation, but by and large, we get along well, even with the flaws. The reason is that we adapt our security measures to fit the value of the things that are being protected and the inconvenience we can live with, so that the more valuable an item, the more difficult it is to subvert the security barriers surrounding it. In addition, we establish penalties for those who break and enter, or steal, or destroy what is not their own. We are always struggling to get the balance right, but in most cases our property and well-being are successfully protected by the combination of troublesome barriers and the risk of penalty in case of failure.

So it is with computer security. You probably have data on your computer that is so important to you, so valuable, that you trust nobody with it. (Your personal secret key might be a good example.) You are very careful with that data, and you don't mind some inconvenience associated with using it, if security accompanies that inconvenience. Other data, though, might be less valuable, and less stringent security measures apply in the interest of getting more work done. A reasonable level of security doesn't have to be a serious inconvenience, and as you learn to understand computer security better, you should be able to achieve higher levels of security before it starts to get in the way. Nevertheless, truly strong security will always take work, and it will never be free.


Applets that display animated coffee cups are fun, and they have definitely helped to make the Web more interactive and interesting, but developers and users alike are clamoring for applets that do more, that can actually help users be productive by doing some of the things applications do today. Especially when it comes to small tasks that might not be needed very often, such as specialized effects in an image processing program, the idea of using an applet that can be downloaded on demand and then thrown away is attractive. But for applets to do such useful things, users have to grant them some privileges, and before wise users grant those privileges, they need some way of knowing where the applet came from. That information is important because users don't want to trust applets from people or organizations they know nothing about, and also because they want to know who to be angry with should their trust turn out to be misplaced.

Additionally, applets need some way of communicating securely with servers and some way to know that they are communicating with the right server. Such capabilities are necessary for many simple client-server applications, as well as more advanced applications, such as those involving electronic commerce.

The package, currently being developed by JavaSoft and soon to be a part of the core Java library, provides basic security facilities that are necessary to solve these problems. Using both symmetric and public-key encryption technology, the package provides primitives for encryption and decryption, applying and verifying digital signatures, establishing secure communication channels, and exchange and management of encryption keys.

The facilities in the package are just building blocks, but they are essential ones. Using these facilities, combined with the Java security model and the information in the other chapters in this section, you can build applets that perform valuable services worth paying for and the applications that can host them.