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JAAS Authentication Tutorial


The JavaTM Authentication and Authorization Service (JAAS) was introduced as an optional package to the JavaTM 2 SDK, Standard Edition (J2SDK), v 1.3. JAAS was integrated into the Java TM Standard Edition Development Kit starting with J2SDK 1.4.

JAAS can be used for two purposes:

This section provides a basic tutorial for the authentication component. The authorization component will be described in the JAAS Authorization Tutorial.

JAAS authentication is performed in a pluggable fashion. This permits Java applications to remain independent from underlying authentication technologies. New or updated technologies can be plugged in without requiring modifications to the application itself. An implementation for a particular authentication technology to be used is determined at runtime. The implementation is specified in a login configuration file. The authentication technology used for this tutorial is very basic, just ensuring that the user specifies a particular name and password.

The rest of this tutorial consists of the following sections:

  1. The Authentication Tutorial Code
  2. The Login Configuration
  3. Running the Code
  4. Running the Code with a Security Manager

If you want to first see the tutorial code in action, you can skip directly to Running the Code and then go back to the other sections to learn about coding and configuration file details.

The Authentication Tutorial Code

The code for this tutorial consists of three files:

SampleAcn.java

Our authentication tutorial application code is contained in a single source file, SampleAcn.java. That file contains two classes:

The SampleAcn Class

The main method of the SampleAcn class performs the authentication and then reports whether or not authentication succeeded.

The code for authenticating the user is very simple, consisting of just two steps:

  1. Instantiate a LoginContext.
  2. Call the LoginContext's login method.

First the basic code is shown, followed by a complete listing of the SampleAcn class, complete with the import statement it requires and error handling.

Instantiating a LoginContext

In order to authenticate a user, you first need a javax.security.auth.login.LoginContext. Here is the basic way to instantiate a LoginContext:

import javax.security.auth.login.*;
. . .
LoginContext lc = 
    new LoginContext(<config file entry name>,
           <CallbackHandler to be used for user interaction>);
and here is the specific way our tutorial code does the instantiation: 
import javax.security.auth.login.*;
. . .
LoginContext lc = 
    new LoginContext("Sample", 
          new MyCallbackHandler());

The arguments are the following:

  1. The name of an entry in the JAAS login configuration file

    This is the name for the LoginContext to use to look up an entry for this application in the JAAS login configuration file, described here. Such an entry specifies the class(es) that implement the desired underlying authentication technology(ies). The class(es) must implement the LoginModule interface, which is in the javax.security.auth.spi package.

    In our sample code, we use the SampleLoginModule supplied with this tutorial. The SampleLoginModule performs authentication by ensuring that the user types a particular name and password.

    The entry in the login configuration file we use for this tutorial (see sample_jaas.config) has the name "Sample", so that is the name we specify as the first argument to the LoginContext constructor.

  2. A CallbackHandler instance.

    When a LoginModule needs to communicate with the user, for example to ask for a user name and password, it does not do so directly. That is because there are various ways of communicating with a user, and it is desirable for LoginModules to remain independent of the different types of user interaction. Rather, the LoginModule invokes a javax.security.auth.callback.CallbackHandler to perform the user interaction and obtain the requested information, such as the user name and password.

    An instance of the particular CallbackHandler to be used is specified as the second argument to the LoginContext constructor. The LoginContext forwards that instance to the underlying LoginModule (in our case SampleLoginModule). An application typically provides its own CallbackHandler implementation. Two simple CallbackHandlers, TextCallbackHandler and DialogCallbackHandler, are provided in the com.sun.security.auth.callback package as sample implementations. For this tutorial we could use the TextCallbackHandler, which outputs information to and reads input from the command line. However, we instead demonstrate the more typical case of an application providing its own CallbackHandler implementation, described in The MyCallbackHandler Class.

Calling the LoginContext's login Method

Once we have a LoginContext lc, we can call its login method to carry out the authentication process:

lc.login();

The LoginContext instantiates a new empty javax.security.auth.Subject object (which represents the user or service being authenticated). The LoginContext constructs the configured LoginModule (in our case SampleLoginModule) and initializes it with this new Subject and MyCallbackHandler.

The LoginContext's login method then calls methods in the SampleLoginModule to perform the login and authentication. The SampleLoginModule will utilize the MyCallbackHandler to obtain the user name and password. Then the SampleLoginModule will check that the name and password are the ones it expects.

If authentication is successful, the SampleLoginModule populates the Subject with a Principal representing the user. The Principal the SampleLoginModule places in the Subject is an instance of SamplePrincipal, which is a sample class implementing the java.security.Principal interface.

See Subjects, Principals, Authentication, and Credentials for information on these terms.

The calling application can subsequently retrieve the authenticated Subject by calling the LoginContext's getSubject method, although doing so is not necessary for this tutorial.

The Complete SampleAcn Class Code

Now that you have seen the basic code required to authenticate the user, we can put it all together into the full class in SampleAcn.java, which includes relevant import statements and error handling:

package sample;

import javax.security.auth.login.*;
// . . . other import statements needed by MyCallbackHandler . . .

/**
 * This Sample application attempts to authenticate a user
 * and reports whether or not the authentication was 
 * successful.
 */
public class SampleAcn {

   /**
    * Attempt to authenticate the user.
    *
    * @param args input arguments for this application.  
    * These are ignored.
    */
    public static void main(String[] args) {

      // Obtain a LoginContext, needed for authentication. 
      // Tell it to use the LoginModule implementation 
      // specified by the entry named "Sample" in the 
      // JAAS login configuration file and to also use the 
      // specified CallbackHandler.
      LoginContext lc = null;
      try {
          lc = new LoginContext("Sample", 
                          new MyCallbackHandler());
      } catch (LoginException le) {
          System.err.println("Cannot create LoginContext. "
              + le.getMessage());
          System.exit(-1);
      } catch (SecurityException se) {
          System.err.println("Cannot create LoginContext. "
              + se.getMessage());
          System.exit(-1);
      } 

      // the user has 3 attempts to authenticate successfully
      int i;
      for (i = 0; i < 3; i++) {
          try {

              // attempt authentication
              lc.login();

              // if we return with no exception, 
              // authentication succeeded
              break;

          } catch (LoginException le) {

              System.err.println("Authentication failed:");
              System.err.println("  " + le.getMessage());
              try {
                  Thread.currentThread().sleep(3000);
              } catch (Exception e) {
                  // ignore
              } 
      
          }
      }

      // did they fail three times?
      if (i == 3) {
          System.out.println("Sorry");
          System.exit(-1);
      }

      System.out.println("Authentication succeeded!");

    }
}

The MyCallbackHandler Class

In some cases a LoginModule must communicate with the user to obtain authentication information. LoginModules use a javax.security.auth.callback.CallbackHandler for this purpose. An application can either use one of the sample implementations provided in the com.sun.security.auth.callback package or, more typically, write a CallbackHandler implementation. The application passes the CallbackHandler as an argument to the LoginContext instantiation. The LoginContext forwards the CallbackHandler directly to the underlying LoginModules.

The tutorial sample code supplies its own CallbackHandler implementation, the MyCallbackHandler class in SampleAcn.java.

CallbackHandler is an interface with one method to implement:

     void handle(Callback[] callbacks)
         throws java.io.IOException, UnsupportedCallbackException;

The LoginModule passes the CallbackHandler handle method an array of appropriate javax.security.auth.callback.Callbacks, for example a NameCallback for the user name and a PasswordCallback for the password, and the CallbackHandler performs the requested user interaction and sets appropriate values in the Callbacks.

The MyCallbackHandler handle method is structured as follows:

public void handle(Callback[] callbacks)
  throws IOException, UnsupportedCallbackException {
      
  for (int i = 0; i < callbacks.length; i++) {
    if (callbacks[i] instanceof TextOutputCallback) {
      
      // display a message according to a specified type
      . . .

    } else if (callbacks[i] instanceof NameCallback) {
  
      // prompt the user for a username
      . . .
 
    } else if (callbacks[i] instanceof PasswordCallback) {
  
      // prompt the user for a password
      . . .

    } else {
        throw new UnsupportedCallbackException
         (callbacks[i], "Unrecognized Callback");
    }
  }
}

A CallbackHandler handle method is passed an array of Callback instances, each of a particular type (NameCallback, PasswordCallback, etc.). It must handle each Callback, performing user interaction in a way that is appropriate for the executing application.

MyCallbackHandler handles three types of Callbacks: NameCallback to prompt the user for a user name, PasswordCallback to prompt for a password, and TextOutputCallback to report any error, warning, or other messages the SampleLoginModule wishes to send to the user.

The handle method handles a TextOutputCallback by extracting the message to be reported and then printing it to System.out, optionally preceded by additional wording that depends on the message type. The message to be reported is determined by calling the TextOutputCallback's getMessage method and the type by calling its getMessageType method. Here is the code for handling a TextOutputCallback:

if (callbacks[i] instanceof TextOutputCallback) {
      
  // display the message according to the specified type
  TextOutputCallback toc = (TextOutputCallback)callbacks[i];
  switch (toc.getMessageType()) {
     case TextOutputCallback.INFORMATION:
        System.out.println(toc.getMessage());
        break;
     case TextOutputCallback.ERROR:
        System.out.println("ERROR: " + toc.getMessage());
        break;
     case TextOutputCallback.WARNING:
        System.out.println("WARNING: " + toc.getMessage());
        break;
     default:
        throw new IOException("Unsupported message type: " +
            toc.getMessageType());
   }

The handle method handles a NameCallback by prompting the user for a user name. It does this by printing the prompt to System.err. It then sets the name for use by the SampleLoginModule by calling the NameCallback's setName method, passing it the name typed by the user:

} else if (callbacks[i] instanceof NameCallback) {
  
  // prompt the user for a username
  NameCallback nc = (NameCallback)callbacks[i];

  System.err.print(nc.getPrompt());
  System.err.flush();
  nc.setName((new BufferedReader
      (new InputStreamReader(System.in))).readLine());

Similarly, the handle method handles a PasswordCallback by printing a prompt to System.err to prompt the user for a password. It then sets the password for use by the SampleLoginModule by calling the PasswordCallback's setPassword method, passing it the password typed by the user, which it reads by calling the readPassword method (shown below):

} else if (callbacks[i] instanceof PasswordCallback) {
  
  // prompt the user for sensitive information
  PasswordCallback pc = (PasswordCallback)callbacks[i];

  System.err.print(pc.getPrompt());
  System.err.flush();
  pc.setPassword(readPassword(System.in));

The readPassword method is a basic method for reading a password from an InputStream:

private char[] readPassword(InputStream in) throws IOException {
    
    char[] lineBuffer;
    char[] buf;
    int i;

    buf = lineBuffer = new char[128];

    int room = buf.length;
    int offset = 0;
    int c;

  loop:    
    while (true) {
      switch (c = in.read()) {
         case -1:
         case '\n':
            break loop;

         case '\r':
             int c2 = in.read();
             if ((c2 != '\n') && (c2 != -1)) {
                 if (!(in instanceof PushbackInputStream)) {
                     in = new PushbackInputStream(in);
                 }
                 ((PushbackInputStream)in).unread(c2);
             } else
                 break loop;

         default:
             if (--room < 0) {
                 buf = new char[offset + 128];
                 room = buf.length - offset - 1;
                 System.arraycopy(lineBuffer, 0, buf, 0, offset);
                 Arrays.fill(lineBuffer, ' ');
                 lineBuffer = buf;
             }
             buf[offset++] = (char) c;
             break;
        }
    }

    if (offset == 0) {
        return null;
    }

    char[] ret = new char[offset];
    System.arraycopy(buf, 0, ret, 0, offset);
    Arrays.fill(buf, ' ');

    return ret;
  }
}

SampleLoginModule.java and SamplePrincipal.java

SampleLoginModule.java implements the LoginModule interface. SampleLoginModule is the class specified by the tutorial's login configuration file as the class implementing the desired underlying authentication. SampleLoginModule's user authentication consists of simply verifying that the name and password specified by the user have specific values. This SampleLoginModule is specified by the tutorial's login configuration file as the LoginModule to use because (1) It performs a basic type of authentication suitable for any environment and thus is appropriate for a tutorial for all users, and (2) It provides an example LoginModule implementation for experienced programmers who require the ability to write a LoginModule implementing an authentication technology.

SamplePrincipal.java is a sample class implementing the java.security.Principal interface. If authentication is successful, the SampleLoginModule populates a Subject with a SamplePrincipal representing the user.

Important: If you are an application writer, you do not need to know how to write a LoginModule or a Principal implementation. You do not need to examine the SampleLoginModule or SamplePrincipal code. All you have to know is how to write your application and specify configuration information (such as in a login configuration file) such that the application will be able to utilize the LoginModule specified by the configuration to authenticate the user. You need to determine which LoginModule(s) you want to use and read the LoginModule's documentation to learn about what options you can specify values for (in the configuration) to control the LoginModule's behavior.

Any vendor can provide a LoginModule implementation that you can use. Some implementations are supplied with the JRE from Sun Microsystems, as listed in the JAAS Login Configuration File document.

Information for programmers who want to write a LoginModule can be found in the JAAS LoginModule Developer's Guide.

The Login Configuration

JAAS authentication is performed in a pluggable fashion, so applications can remain independent from underlying authentication technologies. A system administrator determines the authentication technologies, or LoginModules, to be used for each application and configures them in a login Configuration. The source of the configuration information (for example, a file or a database) is up to the current javax.security.auth.login.Configuration implementation. The default Configuration implementation from Sun Microsystems reads configuration information from configuration files, as described in com.sun.security.auth.login.ConfigFile.html.

See JAAS Login Configuration File for information as to what a login configuration file is, what it contains, and how to specify which login configuration file should be used.

The Login Configuration File for This Tutorial

As noted, the login configuration file we use for this tutorial, sample_jaas.config, contains just one entry, which is

Sample {
  sample.module.SampleLoginModule required debug=true;
};

This entry is named "Sample" and that is the name that our tutorial application, SampleAcn, uses to refer to this entry. The entry specifies that the LoginModule to be used to do the user authentication is the SampleLoginModule in the sample.module package and that this SampleLoginModule is required to "succeed" in order for authentication to be considered successful. The SampleLoginModule succeeds only if the name and password supplied by the user are the one it expects ("testUser" and "testPassword", respectively).

The SampleLoginModule also defines a "debug" option that can be set to true as shown. If this option is set to true, SampleLoginModule outputs extra information about the progress of authentication. A LoginModule can define as many options as it wants. The LoginModule documentation should specify the possible option names and values you can set in your configuration file.

Running the Code

To execute our JAAS authentication tutorial code, all you have to do is

  1. Place the following file into a directory:
  2. Create a subdirectory named "sample" of that top-level directory, and place the following into it (note the SampleAcn and MyCallbackHandler classes, both in SampleAcn.java, are in a package named sample):
  3. Create a subdirectory of the "sample" directory and name it "module". Place the following into it (note the SampleLoginModule class is in a package named sample.module):
  4. Create another subdirectory of the "sample" directory and name it "principal". Place the following into it (note the SamplePrincipal class is in a package named sample.principal):
  5. While in the top-level directory, compile SampleAcn.java, SampleLoginModule.java, and SamplePrincipal.java: 
    javac sample/SampleAcn.java sample/module/SampleLoginModule.java 
sample/principal/SamplePrincipal.java
    (Type all that on one line.)
  6. Execute the SampleAcn application, specifying
    • by -Djava.security.auth.login.config==sample_jaas.config that the login configuration file to be used is sample_jaas.config.

The full command is below.

java -Djava.security.auth.login.config==sample_jaas.config sample.SampleAcn

You will be prompted for your user name and password, and the SampleLoginModule specified in the login configuration file will check to ensure these are correct. The SampleLoginModule expects "testUser" for the user name and "testPassword" for the password.

You will see some messages output by SampleLoginModule as a result of the debug option being set to true in the login configuration file. Then, if your login is successful, you will see the following message output by SampleAcn:

Authentication succeeded!
If the login is not successful (for example, if you misspell the password), you will see 
Authentication failed:
followed by a reason for the failure. For example, if you mistype the password, you may see a message like the following: 
Authentication failed:
  Password Incorrect

SampleAcn gives you three chances to successfully log in.

Running the Code with a Security Manager

When a Java program is run with a security manager installed, the program is not allowed to access resources or otherwise perform security-sensitive operations unless it is explicitly granted permission to do so by the security policy in effect. In Java platforms that are compatible with J2SE v 1.2 and later, the permission must be granted by an entry in a policy file.

Most browsers install a security manager, so applets typically run under the scrutiny of a security manager. Applications, on the other hand, do not, since a security manager is not automatically installed when an application is running. Thus an application, like our SampleAcn application, by default has full access to resources.

To run an application with a security manager, simply invoke the interpreter with a -Djava.security.manager argument included on the command line.

If you try invoking SampleAcn with a security manager but without specifying any policy file, you will get the following (unless you have a default policy setup elsewhere that grants the required permissions or grants AllPermission):

% java -Djava.security.manager \
 -Djava.security.auth.login.config==sample_jaas.config sample.SampleAcn
Exception in thread "main" java.security.AccessControlException: 
  access denied (
  javax.security.auth.AuthPermission createLoginContext.Sample)

As you can see, you get an AccessControlException, because we haven't created and used a policy file granting our code the permission that is required in order to be allowed to create a LoginContext.

Here are the complete steps required in order to be able to run our SampleAcn application with a security manager installed. You can skip the first five steps if you have already done them, as described in Running the Code.

  1. Place the following file into a directory:
  2. Create a subdirectory named "sample" of that top-level directory, and place the following into it (note the SampleAcn and MyCallbackHandler classes, both in SampleAcn.java, are in a package named sample):
  3. Create a subdirectory of the "sample" directory and name it "module". Place the following into it (note the SampleLoginModule class is in a package named sample.module):
  4. Create another subdirectory of the "sample" directory and name it "principal". Place the following into it (note the SamplePrincipal class is in a package named sample.principal):
  5. While in the top-level directory, compile SampleAcn.java, SampleLoginModule.java, and SamplePrincipal.java: 
    javac sample/SampleAcn.java sample/module/SampleLoginModule.java 
sample/principal/SamplePrincipal.java
    (Type all that on one line.)
  6. Create a JAR file containing SampleAcn.class and MyCallbackHandler.class: 
    jar -cvf SampleAcn.jar sample/SampleAcn.class 
sample/MyCallbackHandler.class

    (Type all that on one line.) This command creates a JAR file, SampleAcn.jar, and places the SampleAcn.class and MyCallbackHandler.class files inside it.

  7. Create a JAR file containing SampleLoginModule.class and SamplePrincipal.class: 
    jar -cvf SampleLM.jar sample/module/SampleLoginModule.class 
sample/principal/SamplePrincipal.class
  8. Create a policy file granting the required permissions.

    The permission that is needed by code attempting to instantiate a LoginContext is a javax.security.auth.AuthPermission with target "createLoginContext.<entry name>". Here, <entry name> refers to the name of the login configuration file entry that the application references in its instantiation of LoginContext. The name used by our SampleAcn application's LoginContext instantiation is "Sample", as you can see in the code:

    LoginContext lc = 
    new LoginContext("Sample", 
          new MyCallbackHandler());

    Thus, the permission that needs to be granted to SampleAcn.jar is

    permission javax.security.auth.AuthPermission 
  "createLoginContext.Sample";

    The SampleLM.jar file also needs to be granted a permission. The documentation for a LoginModule should tell you what permissions it needs to be granted. In the case of SampleLoginModule, it needs a javax.security.auth.AuthPermission with target "modifyPrincipals" in order to populate a Subject with a Principal:

    permission javax.security.auth.AuthPermission 
  "modifyPrincipals";

    Copy the policy file sampleacn.policy to the same directory as that in which you stored SampleAcn.java, etc. The policy file contains the following grant statement to grant SampleAcn.jar (in the current directory) its required permission:

    grant codebase "file:./SampleAcn.jar" {
   permission javax.security.auth.AuthPermission 
                    "createLoginContext.Sample";
};

    The policy file also contains the following grant statement to grant SampleLM.jar (also in the current directory) its required permission:

    grant codebase "file:./SampleLM.jar" {
   permission javax.security.auth.AuthPermission 
                    "modifyPrincipals";
};

    Note: Policy files and the structure of entries within them are described in Default Policy Implementation and Policy File Syntax. Permissions are described here.

  9. Execute the SampleAcn application, specifying
    1. by an appropriate -classpath clause that classes should be searched for in the SampleAcn.jar and SampleLM.jar JAR files,
    2. by -Djava.security.manager that a security manager should be installed,
    3. by -Djava.security.policy==sampleacn.policy that the policy file to be used is sampleacn.policy, and
    4. by -Djava.security.auth.login.config==sample_jaas.config that the login configuration file to be used is sample_jaas.config.

    Below are the full commands to use for both Microsoft Windows and Unix systems. The only difference is that on Microsoft Windows systems you use semicolons to separate classpath items, while you use colons for that purpose on Unix systems.

    Here is the full command for Microsoft Windows systems:

    java -classpath SampleAcn.jar;SampleLM.jar 
 -Djava.security.manager 
 -Djava.security.policy==sampleacn.policy \
 -Djava.security.auth.login.config==sample_jaas.config \
 sample.SampleAcn

    Here is the full command for UNIX systems:

    java -classpath SampleAcn.jar:SampleLM.jar 
 -Djava.security.manager 
 -Djava.security.policy==sampleacn.policy \
 -Djava.security.auth.login.config==sample_jaas.config \
 sample.SampleAcn

    Type all that on one line. Multiple lines are used here for legibility. If the command is too long for your system, you may need to place it in a .bat file (for Microsoft Windows) or a .sh file (for UNIX) and then run that file to execute the command.

    Since the specified policy file contains an entry granting the code the required permissions, execution should proceed without any exceptions indicating a required permission was not granted. You will be prompted for a user name and password (use "testUser" and "testPassword"), and the SampleLoginModule specified in the login configuration file will check the name and password. If your login is successful, you will see the message "Authentication succeeded!" and if not, you will see "Authentication failed:" followed by a reason for the failure.


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