Documentation Contents

# Java Authentication and Authorization Service (JAAS): LoginModule Developer's Guide

- Overview
  - Who Should Read This Document
  - Related Documentation
  - Introduction
- Steps to Implement a `LoginModule`
  - Step 1: Understand the Authentication Technology
  - Step 2: Name the `LoginModule` Implementation
  - Step 3: Implement the Abstract `LoginModule` Methods
  - Step 4: Choose or Write a Sample Application
  - Step 5: Compile the `LoginModule` and Application
  - Step 6: Prepare for Testing
  - Step 7: Test Use of the `LoginModule`
  - Step 8: Document Your `LoginModule` Implementation
  - Step 9: Make `LoginModule` JAR File and Documents Available

## Overview

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

JAAS provides subject-based authorization on authenticated identities. This document focuses on the authentication aspect of JAAS, specifically the `LoginModule` interface.

### Who Should Read This Document

This document is intended for experienced programmers who require the ability to write a `LoginModule` implementing an authentication technology.

### Related Documentation

This document assumes you have already read the following:

- JAAS Reference Guide

It also discusses various classes and interfaces in the JAAS API. Please reference the javadocs for the JAAS API specification for more detailed information:

- javax.security.auth Package
- javax.security.auth.callback Package
- javax.security.auth.kerberos Package
- javax.security.auth.login Package
- javax.security.auth.spi Package
- javax.security.auth.x500 Package
- com.sun.security.auth Package
- com.sun.security.auth.callback Package
- com.sun.security.auth.login Package
- com.sun.security.auth.module Package

The following tutorials for JAAS authentication and authorization can be run by everyone:

- JAAS Authentication Tutorial
- JAAS Authorization Tutorial

Similar tutorials for JAAS authentication and authorization, but which demonstrate the use of a Kerberos LoginModule and thus which require a Kerberos installation, can be found at

- JAAS Authentication
- JAAS Authorization

These two tutorials are a part of the Java GSS-API and JAAS sequence of tutorials that utilize Kerberos as the underlying technology for authentication and secure communication.

### Introduction

The `LoginModule` documentation describes the interface that must be implemented by authentication technology providers. `LoginModule`s are plugged in under applications to provide a particular type of authentication.

While applications write to the `LoginContext` Application Programming Interface (API), authentication technology providers implement the `LoginModule` interface. A `Configuration` specifies the `LoginModule`(s) to be used with a particular login application. Different `LoginModule`s can be plugged in under the application without requiring any modifications to the application itself.

The `LoginContext` is responsible for reading the `Configuration` and instantiating the specified `LoginModule`s. Each `LoginModule` is initialized with a `Subject`, a `CallbackHandler`, shared `LoginModule` state, and `LoginModule`-specific options.

The `Subject` represents the user or service currently being authenticated and is updated by a `LoginModule` with relevant `Principal`s and credentials if authentication succeeds. `LoginModule`s use the `CallbackHandler` to communicate with users (to prompt for user names and passwords, for example), as described in the login method description. Note that the `CallbackHandler` may be null. A `LoginModule` that requires a `CallbackHandler` to authenticate the `Subject` may throw a `LoginException` if it was initialized with a `null` `CallbackHandler`. `LoginModule`s optionally use the shared state to share information or data among themselves.

The `LoginModule`-specific options represent the options configured for this `LoginModule` in the login `Configuration`. The options are defined by the `LoginModule` itself and control the behavior within it. For example, a `LoginModule` may define options to support debugging/testing capabilities. Options are defined using a key-value syntax, such as debug=true. The `LoginModule` stores the options as a `Map` so that the values may be retrieved using the key. Note that there is no limit to the number of options a `LoginModule` chooses to define.

The calling application sees the authentication process as a single operation invoked via a call to the `LoginContext`'s `login` method. However, the authentication process within each `LoginModule` proceeds in two distinct phases. In the first phase of authentication, the `LoginContext`'s `login` method invokes the `login` method of each `LoginModule` specified in the `Configuration`. The `login` method for a `LoginModule` performs the actual authentication (prompting for and verifying a password for example) and saves its authentication status as private state information. Once finished, the `LoginModule`'s `login` method returns `true` (if it succeeded) or `false` (if it should be ignored), or it throws a `LoginException` to specify a failure. In the failure case, the `LoginModule` must not retry the authentication or introduce delays. The responsibility of such tasks belongs to the application. If the application attempts to retry the authentication, each `LoginModule`'s `login` method will be called again.

In the second phase, if the `LoginContext`'s overall authentication succeeded (calls to the relevant required, requisite, sufficient and optional `LoginModule`s' `login` methods succeeded), then the `commit` method for each `LoginModule` gets invoked. (For an explanation of the `LoginModule` flags required, requisite, sufficient and optional, please consult the `javax.security.auth.login.Configuration` documentation and Appendix B: Login Configuration Files in the JAAS Reference Guide.) The `commit` method for a `LoginModule` checks its privately saved state to see if its own authentication succeeded. If the overall `LoginContext` authentication succeeded and the `LoginModule`'s own authentication succeeded, then the `commit` method associates the relevant `Principal`s (authenticated identities) and credentials (authentication data such as cryptographic keys) with the `Subject`.

If the `LoginContext`'s overall authentication failed (the relevant REQUIRED, REQUISITE, SUFFICIENT and OPTIONAL `LoginModule`s' `login` methods did not succeed), then the `abort` method for each `LoginModule` gets invoked. In this case, the `LoginModule` removes/destroys any authentication state originally saved.

Logging out a `Subject` involves only one phase. The `LoginContext` invokes the `LoginModule`'s `logout` method. The `logout` method for the `LoginModule` then performs the logout procedures, such as removing `Principal`s or credentials from the `Subject`, or logging session information.

## Steps to Implement a `LoginModule`

The steps required in order to implement and test a `LoginModule` are the following:

- Step 1: Understand the Authentication Technology
- Step 2: Name the `LoginModule` Implementation
- Step 3: Implement the Abstract `LoginModule` Methods
- Step 4: Choose or Write a Sample Application
- Step 5: Compile the `LoginModule` and Application
- Step 6: Prepare for Testing
  - Step 6a: Place Your `LoginModule` and Application Code in JAR Files
  - Step 6b: Decide Where to Store the JAR Files
  - Step 6c: Set `LoginModule` and Application JAR File Permissions
  - Step 6d: Create a Configuration Referencing the LoginModule
- Step 7: Test Use of the `LoginModule`
- Step 8: Document Your `LoginModule` Implementation
- Step 9: Make `LoginModule` JAR File and Documents Available

The steps required to implement and test a new `LoginModule` follow. Please reference the SampleLoginModule and other files described in the JAAS Reference Guide for examples of what may be done for the various steps.

### Step 1: Understand the Authentication Technology

The first thing you need to do is understand the authentication technology to be implemented by your new `LoginModule` provider, and determine its requirements.

One thing you will need to determine is whether or not your `LoginModule` will require some form of user interaction (retrieving a user name and password, for example). If so, you will need to become familiar with the `CallbackHandler` interface and the `javax.security.auth.callback` package. In that package you will find several possible `Callback` implementations to use. (Alternatively, you can create your own `Callback` implementations.) The `LoginModule` will invoke the `CallbackHandler` specified by the application itself and passed to the `LoginModule`'s `initialize` method. The `LoginModule` passes the `CallbackHandler` an array of appropriate `Callback`s. See the login method in Step 3.

Note that it is possible for `LoginModule` implementations not to have any end-user interactions. Such `LoginModule`s would not need to access the `callback` package.

Another thing you should determine is what configuration options you want to make available to the user, who specifies configuration information in whatever form the current Configuration implementation expects (for example, in files). For each option, decide the option name and possible values. For example, if a `LoginModule` may be configured to consult a particular authentication server host, decide on the option's key name ("auth_server", for example), as well as the possible server hostnames valid for that option ("server_one.example.com" and "server_two.example.com", for example).

### Step 2: Name the `LoginModule` Implementation

Decide on the proper package and class name for your `LoginModule`.

For example, a `LoginModule` developed by IBM might be called `com.ibm.auth.Module` where `com.ibm.auth` is the package name and `Module` is the name of the `LoginModule` class implementation.

### Step 3: Implement the Abstract `LoginModule` Methods

The `LoginModule` interface specifies five abstract methods that require implementations:

- initialize
- login
- commit
- abort
- logout

See below and the `LoginModule` API for more information on each method above.

In addition to these methods, a `LoginModule` implementation must provide a public constructor with no arguments. This allows for its proper instantiation by a `LoginContext`. Note that if no such constructor is provided in your `LoginModule` implementation, a default no-argument constructor is automatically inherited from the `Object` class.

#### LoginModule.initialize Method

    public void initialize (
      Subject subject,
      CallbackHandler handler,
      Map<java.lang.String, ?> sharedState,
      Map<java.lang.String, ?> options) { ... }

The `initialize` method is called to initialize the `LoginModule` with the relevant authentication and state information.

This method is called by a `LoginContext` immediately after this `LoginModule` has been instantiated, and prior to any calls to its other public methods. The method implementation should store away the provided arguments for future use.

The `initialize` method may additionally peruse the provided sharedState to determine what additional authentication state it was provided by other `LoginModule`s, and may also traverse through the provided options to determine what configuration options were specified to affect the `LoginModule`'s behavior. It may save option values in variables for future use.

Note: JAAS LoginModules may use the options defined in PAM (`use_first_pass`, `try_first_pass`, `use_mapped_pass`, and `try_mapped_pass`) to achieve single-sign on. See Making Login Services Independent from Authentication Technologies for further information.

Below is a list of options commonly supported by LoginModules. Note that the following is simply a guideline. Modules are free to support a subset (or none) of the following options.

- `try_first_pass` - If `true`, the first LoginModule in the stack saves the password entered, and subsequent LoginModules also try to use it. If authentication fails, the LoginModules prompt for a new password and retry the authentication.
- `use_first_pass` - If `true`, the first LoginModule in the stack saves the password entered, and subsequent LoginModules also try to use it. LoginModules do not prompt for a new password if authentication fails (authentication simply fails).
- `try_mapped_pass` - If `true`, the first LoginModule in the stack saves the password entered, and subsequent LoginModules attempt to map it into their service-specific password. If authentication fails, the LoginModules prompt for a new password and retry the authentication.
- `use_mapped_pass` - If `true`, the first LoginModule in the stack saves the password entered, and subsequent LoginModules attempt to map it into their service-specific password. LoginModules do not prompt for a new password if authentication fails (authentication simply fails).
- `moduleBanner` - If `true`, then when invoking the CallbackHandler, the LoginModule provides a TextOutputCallback as the first Callback, which describes the LoginModule performing the authentication.
- `debug` - If `true`, instructs a LoginModule to output debugging information.

The `initialize` method may freely ignore state or options it does not understand, although it would be wise to log such an event if it does occur.

Note that the `LoginContext` invoking this `LoginModule` (and the other configured `LoginModule`s, as well), all share the same references to the provided `Subject` and `sharedState`. Modifications to the `Subject` and `sharedState` will, therefore, be seen by all.

#### LoginModule.login Method

    boolean login() throws LoginException;

The `login` method is called to authenticate a `Subject`. This is phase 1 of authentication.

This method implementation should perform the actual authentication. For example, it may cause prompting for a user name and password, and then attempt to verify the password against a password database. Another example implementation may inform the user to insert their finger into a fingerprint reader, and then match the input fingerprint against a fingerprint database.

If your `LoginModule` requires some form of user interaction (retrieving a user name and password, for example), it should not do so directly. That is because there are various ways of communicating with a user, and it is desirable for `LoginModule`s to remain independent of the different types of user interaction. Rather, the `LoginModule`'s `login` method should invoke the `handle` method of the `CallbackHandler` passed to the `initialize` method to perform the user interaction and set appropriate results, such as the user name and password. The `LoginModule` passes the `CallbackHandler` an array of appropriate `Callback`s, 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 `Callback`s. For example, to process a `NameCallback`, the `CallbackHandler` may prompt for a name, retrieve the value from the user, and call the `NameCallback`'s `setName` method to store the name.

The authentication process may also involve communication over a network. For example, if this method implementation performs the equivalent of a kinit in Kerberos, then it would need to contact the KDC. If a password database entry itself resides in a remote naming service, then that naming service needs to be contacted, perhaps via the Java Naming and Directory Interface (JNDI). Implementations might also interact with an underlying operating system. For example, if a user has already logged into an operating system like Solaris, Linux, Mac OS X, or Windows NT, this method might simply import the underlying operating system's identity information.

The `login` method should

1. Determine whether or not this `LoginModule` should be ignored. One example of when it should be ignored is when a user attempts to authenticate under an identity irrelevant to this `LoginModule` (if a user attempts to authenticate as root using NIS, for example). If this `LoginModule` should be ignored, `login` should return `false`. Otherwise, it should do the following:
2. Call the `CallbackHandler` `handle` method if user interaction is required.
3. Perform the authentication.
4. Store the authentication result (success or failure).
5. If authentication succeeded, save any relevant state information that may be needed by the `commit` method.
6. Return `true` if authentication succeeds, or throw a `LoginException` such as `FailedLoginException` if authentication fails.

Note that the `login` method implementation should not associate any new `Principal` or credential information with the saved `Subject` object. This method merely performs the authentication, and then stores away the authentication result and corresponding authentication state. This result and state will later be accessed by the `commit` or `abort` method. Note that the result and state should typically not be saved in the sharedState `Map`, as they are not intended to be shared with other `LoginModule`s.

An example of where this method might find it useful to store state information in the sharedState `Map` is when `LoginModule`s are configured to share passwords. In this case, the entered password would be saved as shared state. By sharing passwords, the user only enters the password once, and can still be authenticated to multiple `LoginModule`s. The standard conventions for saving and retrieving names and passwords from the sharedState `Map` are the following:

- `javax.security.auth.login.name` - Use this as the shared state map key for saving/retrieving a name.
- `javax.security.auth.login.password` - Use this as the shared state map key for saving/retrieving a password.

If authentication fails, the `login` method should not retry the authentication. This is the responsibility of the application. Multiple `LoginContext` `login` method calls by an application are preferred over multiple login attempts from within `LoginModule.login()`.

#### LoginModule.commit Method

    boolean commit() throws LoginException;

The `commit` method is called to commit the authentication process. This is phase 2 of authentication when phase 1 succeeds. It is called if the `LoginContext`'s overall authentication succeeded (that is, if the relevant REQUIRED, REQUISITE, SUFFICIENT and OPTIONAL `LoginModule`s succeeded.)

This method should access the authentication result and corresponding authentication state saved by the `login` method.

If the authentication result denotes that the `login` method failed, then this `commit` method should remove/destroy any corresponding state that was originally saved.

If the saved result instead denotes that this `LoginModule`'s `login` method succeeded, then the corresponding state information should be accessed to build any relevant `Principal` and credential information. Such `Principal`s and credentials should then be added to the `Subject` stored away by the `initialize` method.

After adding `Principal`s and credentials, dispensable state fields should be destroyed expeditiously. Likely fields to destroy would be user names and passwords stored during the authentication process.

The `commit` method should save private state indicating whether the commit succeeded or failed.

The following chart depicts what a `LoginModule`'s `commit` method should return. The different boxes represent the different situations that may occur. For example, the top-left corner box depicts what the `commit` method should return if both the previous call to `login` succeeded and the `commit` method itself succeeded.

LoginModule.commit method return values
| COMMIT: SUCCESS | COMMIT: FAILURE
LOGIN: SUCCESS | return TRUE | throw EXCEPTION
LOGIN: FAILURE | return FALSE | return FALSE

#### LoginModule.abort Method

    boolean abort() throws LoginException;

The `abort` method is called to abort the authentication process. This is phase 2 of authentication when phase 1 fails. It is called if the `LoginContext`'s overall authentication failed.

This method first accesses this `LoginModule`'s authentication result and corresponding authentication state saved by the `login` (and possibly `commit`) methods, and then clears out and destroys the information. Sample state to destroy would be user names and passwords.

If this `LoginModule`'s authentication attempt failed, then there shouldn't be any private state to clean up.

The following charts depict what a `LoginModule`'s `abort` method should return. This first chart assumes that the previous call to `login` succeeded. For instance, the top-left corner box depicts what the `abort` method should return if both the previous call to `login` and `commit` succeeded, and the `abort` method itself also succeeded.

LoginModule.abort method return values; login succeeded
| ABORT: SUCCESS | ABORT: FAILURE
COMMIT: SUCCESS | return TRUE | throw EXCEPTION
COMMIT: FAILURE | return TRUE | throw EXCEPTION

The second chart depicts what a `LoginModule`'s `abort` method should return, assuming that the previous call to `login` failed. For instance, the top-left corner box depicts what the `abort` method should return if the previous call to `login` failed, the previous call to `commit` succeeded, and the `abort` method itself also succeeded.

LoginModule.abort method return values; login failed
| ABORT: SUCCESS | ABORT: FAILURE
COMMIT: SUCCESS | return FALSE | return FALSE
COMMIT: FAILURE | return FALSE | return FALSE

#### LoginModule.logout Method

    boolean logout() throws LoginException;

The `logout` method is called to log out a `Subject`.

This method removes `Principal`s, and removes/destroys credentials associated with the `Subject` during the `commit` operation. This method should not touch those `Principal`s or credentials previously existing in the `Subject`, or those added by other `LoginModule`s.

If the `Subject` has been marked read-only (the `Subject`'s `isReadOnly` method returns true), then this method should only destroy credentials associated with the `Subject` during the `commit` operation (removing the credentials is not possible). If the `Subject` has been marked as read-only and the credentials associated with the `Subject` during the `commit` operation are not destroyable (they do not implement the `Destroyable` interface), then this method may throw a `LoginException`.

The `logout` method should return `true` if logout succeeds, or otherwise throw a LoginException.

### Step 4: Choose or Write a Sample Application

Either choose an existing sample application for your testing, or write a new one. See JAAS Reference Guide for information about application requirements and a sample application you can use for your testing.

### Step 5: Compile the `LoginModule` and Application

Compile your new `LoginModule` and the application you will use for testing.

### Step 6: Prepare for Testing

#### Step 6a: Place Your `LoginModule` and Application Code in JAR Files

Place your `LoginModule` and application code in separate JAR files, in preparation for referencing the JAR files in the policy in Step 6c. Here is a sample command for creating a JAR file:

    jar cvf <JAR file name> <list of classes, separated by spaces>

This command creates a JAR file with the specified name containing the specified classes.

For more information on the jar tool, see jar (for Solaris, Linux, or Mac OS X) (for Microsoft Windows).

#### Step 6b: Decide Where to Store the JAR Files

The application can be stored essentially anywhere you like.

Your `LoginModule` can also be placed anywhere you (and other clients) like. If the `LoginModule` is fully trusted, it can be placed in the JRE's `lib/ext` (standard extension) directory.

You will need to test the `LoginModule` being located both in the `lib/ext` directory and elsewhere because in one situation your `LoginModule` will need to explicitly be granted permissions required for any security-sensitive operations it does, while in the other case such permissions are not needed.

If your `LoginModule` is placed in the JRE's `lib/ext` directory, it will be treated as an installed extension and no permissions need to be granted, since the default system policy file grants all permissions to installed extensions.

If your `LoginModule` is placed anywhere else, the permissions need to be granted, for example by `grant` statements in a policy file.

Decide where you will store the `LoginModule` JAR file for testing the case where it is not an installed extension. In the next step, you grant permissions to the JAR file, in the specified location.

#### Step 6c: Set `LoginModule` and Application JAR File Permissions

If your `LoginModule` and/or application performs security-sensitive tasks that will trigger security checks (making network connections, reading or writing files on a local disk, etc), it will need to be granted the required permissions if it is not an installed extension (see Step 6b) and it is run while a security manager is installed.

Since `LoginModule`s usually associate `Principal`s and credentials with an authenticated Subject, some types of permissions a `LoginModule` will typically require are AuthPermissions with target names "modifyPrincipals", "modifyPublicCredentials", and "modifyPrivateCredentials".

A sample statement granting permissions to a `LoginModule` whose code is in `MyLM.jar` appears below. Such a statement could appear in a policy file. In this example, the `MyLM.jar` file is assumed to be in the `/localWork` directory.

    grant codeBase "file:/localWork/MyLM.jar" {
      permission javax.security.auth.AuthPermission "modifyPrincipals";
      permission javax.security.auth.AuthPermission "modifyPublicCredentials";
      permission javax.security.auth.AuthPermission "modifyPrivateCredentials";
    };

Note: Since a `LoginModule` is always invoked within an `AccessController.doPrivileged` call, it should not have to call `doPrivileged` itself. If it does, it may inadvertently open up a security hole. For example, a `LoginModule` that invokes the application-provided `CallbackHandler` inside a `doPrivileged` call opens up a security hole by permitting the application's `CallbackHandler` to gain access to resources it would otherwise not have been able to access.

#### Step 6d: Create a Configuration Referencing the LoginModule

Because JAAS supports a pluggable authentication architecture, your new `LoginModule` can be used without requiring modifications to existing applications. Only the login `Configuration` needs to be updated in order to indicate use of a new `LoginModule`.

The default `Configuration` implementation from Oracle reads configuration information from configuration files, as described in com.sun.security.auth.login.ConfigFile.html.

Create a configuration file to be used for testing. For example, to configure the previously-mentioned hypothetical IBM `LoginModule` for an application, the configuration file might look like this:

        AppName {
            com.ibm.auth.Module REQUIRED debug=true;
        };

where `AppName` should be whatever name the application uses to refer to this entry in the login configuration file. The application specifies this name as the first argument to the `LoginContext` constructor.

### Step 7: Test Use of the `LoginModule`

Finally, test your application and its use of the `LoginModule`. When you run the application, specify the login configuration file to be used. For example, suppose your application is named `MyApp`, it is located in `MyApp.jar`, and your configuration file is `test.conf`. You could run the application and specify the configuration file via the following:

    java -classpath MyApp.jar
     -Djava.security.auth.login.config=test.conf MyApp

Type all that on one line. Multiple lines are used here for legibility.

To specify a policy file named `my.policy` and run the application with a security manager installed, do the following:

    java -classpath MyApp.jar -Djava.security.manager
     -Djava.security.policy=my.policy
     -Djava.security.auth.login.config=test.conf MyApp

Again, type all that on one line.

You may want to configure the `LoginModule` with a debug option to help ensure that it is working correctly.

Debug your code and continue testing as needed. If you have problems, review the steps above and ensure they are all completed.

Be sure to vary user input and the `LoginModule` options specified in the configuration file.

Be sure to also include testing using different installation options (e.g., making the `LoginModule` an installed extension or placing it on the class path) and execution environments (with or without a security manager running). Installation options are discussed in Step 6b. In particular, in order to ensure your `LoginModule` works when a security manager is installed and the `LoginModule` and application are not installed extensions, you need to test such an installation and execution environment, after granting required permissions, as described in Step 6c.

If you find during testing that your `LoginModule` or application needs modifications, make the modifications, recompile (Step 5), place the updated code in a JAR file (Step 6a), re-install the JAR file (Step 6b), if needed fix or add to the permissions (Step 6c), if needed modify the login configuration file (Step 6d), and then re-run the application and repeat these steps as needed.

### Step 8: Document Your `LoginModule` Implementation

The next step is to write documentation for clients of your `LoginModule`. Example documentation you may want to include is:

- A README or User Guide describing
  - the authentication process employed by your `LoginModule` implementation.
  - information on how to install the `LoginModule`.
  - configuration options accepted by the `LoginModule`. For each option, specify the option name and possible values (or types of values), as well as the behavior the option controls.
  - the permissions required by your `LoginModule` when it is run with a security manager (and it is not an installed extension).
- An example `Configuration` file that references your new `LoginModule`.
- An example policy file granting your `LoginModule` the required permissions.
- API documentation. Putting javadoc comments into your source code as you write it will make the API javadocs easy to generate.

### Step 9: Make `LoginModule` JAR File and Documents Available

The final step is to make your `LoginModule` JAR file and documentation available to clients.

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