# Principal Resolution

Source: https://apereo.github.io/cas/development/installation/Configuring-Principal-Resolution.html

Principal resolution converts information in the authentication credential into a security principal that commonly contains additional attributes (i.e. user details such as affiliations, group membership, email, display name). A CAS principal contains a unique identifier by which the authenticated user will be known to all requesting services. A principal also contains optional [attributes that may be released](https://apereo.github.io/cas/development/integration/Attribute-Release.html) to services to support authorization and personalization. Principal resolution is a requisite part of the authentication process that happens after credential authentication.

The attribute retrieval and resolution process that is carried out by principal resolution is [discussed here](https://apereo.github.io/cas/development/integration/Attribute-Resolution.html).

## Configuration

CAS uses the Person Directory library to provide a flexible principal resolution services against a number of data sources. The key to configuring `PersonDirectoryPrincipalResolver` is the definition of an `PersonAttributeDao` object.

The following settings and properties are available from the CAS configuration catalog:

- `cas.person-directory.principal-transformation.groovy.location` (Required; type `Resource`): The location of the resource. Resources can be URLs, or files found either on the classpath or outside somewhere in the file system. In the event the configured resource is a Groovy script, especially if the script is set to reload on changes, you may need to adjust the total number of `inotify` instances. On Linux, you may need to add the following line to `/etc/sysctl.conf`: `fs.inotify.max_user_instances = 256`. You can check the current value via `cat /proc/sys/fs/inotify/max_user_instances`. In situations and scenarios where CAS is able to automatically watch the underlying resource for changes and detect updates and modifications dynamically, you may be able to specify the following setting as either an environment variable or system property with a value of `false` to disable the resource watcher: `org.apereo.cas.util.io.PathWatcherService`.
- `cas.person-directory.active-attribute-repository-ids` (Optional; type `String`; default `*`): Activated attribute repository identifiers that should be used for fetching attributes if attribute resolution is enabled. The list here may include identifiers separated by comma.
- `cas.person-directory.attribute-repository-selection` (Optional; type `Map<String,String>`): Control the behavior of the attribute repository selection by authentication method or handler. The map here is keyed by the authentication handler name, and the value is the attribute repository identifiers separated by comma. When the authentication handler is executed, the attribute repositories assigned to this handler will be selected to fetch attributes. Note that the resolution engine will always favor attribute repositories assigned to the service definition, if any and as part of its authentication policy, over this global setting.
- `cas.person-directory.attribute-resolution-enabled` (Optional; type `TriStateBoolean`; default `undefined`): Whether attribute repositories should be contacted to fetch person attributes. Defaults to true if not set. Available values are as follows: - `TRUE`: Constant to represent the true state. - `FALSE`: Constant to represent the false state. - `UNDEFINED`: Constant to represent the undefined state.
- `cas.person-directory.principal-attribute` (Optional; type `String`): Attribute name to use to indicate the identifier of the principal constructed. If the attribute is blank or has no values, the default principal id will be used determined by the underlying authentication engine. The principal id attribute usually is removed from the collection of attributes collected, though this behavior depends on the schematics of the underlying authentication strategy.
- `cas.person-directory.principal-resolution-conflict-strategy` (Optional; type `String`; default `last`): In the event that the principal resolution engine resolves more than one principal, (especially if such principals in the chain have different identifiers), this setting determines strategy by which the principal id would be chosen from the chain. Accepted values are: `last`, `first`.
- `cas.person-directory.principal-resolution-failure-fatal` (Optional; type `TriStateBoolean`; default `undefined`): When true, throws an error back indicating that principal resolution has failed and no principal can be found based on the authentication requirements. Otherwise, logs the condition as an error without raising a catastrophic error. Available values are as follows: - `TRUE`: Constant to represent the true state. - `FALSE`: Constant to represent the false state. - `UNDEFINED`: Constant to represent the undefined state.
- `cas.person-directory.principal-transformation.blocking-pattern` (Optional; type `String`): A regular expression that will be used against the username to match for blocking/forbidden values. If a match is found, an exception will be thrown and principal transformation will fail.
- `cas.person-directory.principal-transformation.case-conversion` (Optional; type `PrincipalTransformationProperties.CaseConversion`; default `NONE`): Indicate whether the principal identifier should be transformed into upper-case, lower-case, etc. Available values are as follows: - `NONE`: No conversion. - `LOWERCASE`: Lowercase conversion. - `UPPERCASE`: Uppercase conversion.
- `cas.person-directory.principal-transformation.pattern` (Optional; type `String`): A regular expression that will be used against the provided username for username extractions. On a successful match, the first matched group in the pattern will be used as the extracted username.
- `cas.person-directory.principal-transformation.prefix` (Optional; type `String`): Prefix to add to the principal id prior to authentication.
- `cas.person-directory.principal-transformation.suffix` (Optional; type `String`): Suffix to add to the principal id prior to authentication.
- `cas.person-directory.return-null` (Optional; type `TriStateBoolean`; default `undefined`): Return a null principal object if no attributes can be found for the principal. Available values are as follows: - `TRUE`: Constant to represent the true state. - `FALSE`: Constant to represent the false state. - `UNDEFINED`: Constant to represent the undefined state.
- `cas.person-directory.use-existing-principal-id` (Optional; type `TriStateBoolean`; default `undefined`): Uses an existing principal id that may have already been established in order to run person directory queries. This is generally useful in situations where authentication is delegated to an external identity provider and a principal is first established to then query an attribute source. Available values are as follows: - `TRUE`: Constant to represent the true state. - `FALSE`: Constant to represent the false state. - `UNDEFINED`: Constant to represent the undefined state.

**Groovy scripting**

CAS takes advantage of Apache Groovy in forms of either embedded or external scripts that allow one to, by default, dynamically build constructs, attributes, access strategies and a lot more. To activate the functionality described here, you may need to prepare CAS to support and integrate with Apache Groovy.

Please [review this guide](https://apereo.github.io/cas/development/integration/Apache-Groovy-Scripting.html) to configure your build.

**Principal transformation**

Authentication handlers that generally deal with username-password credentials can be configured to transform the user id prior to executing the authentication sequence. Each authentication strategy in CAS provides settings to properly transform the principal. Refer to the relevant settings for the authentication strategy at hand to learn more.

Authentication handlers as part of principal transformation may also be provided a path to a Groovy script to transform the provided username. The outline of the script may take on the following form:

```groovy
String run(final Object... args) {
    def (providedUsername,logger) = args
    return providedUsername.concat("SomethingElse")
}
```

To prepare CAS to support and integrate with Apache Groovy, please [review this guide](https://apereo.github.io/cas/development//integration/Apache-Groovy-Scripting.html).

## Custom

You may also design and register your own principal resolution strategy by supplying an implementation of the `PrincipalResolver` component and registering it with the runtime context:

```java
@AutoConfiguration
public static class MyConfiguration {

    @Bean
    public PrincipalResolver myPrincipalResolver() {
        return new MyPrincipalResolver();
    }

    @Bean
    public PrincipalResolutionExecutionPlanConfigurer myExecutionPlanConfigurer(
        @Qualifier("myPrincipalResolver")
        final PrincipalResolver myPrincipalResolver) {
        return plan -> {
            plan.registerPrincipalResolver(myPrincipalResolver);
        };
    }
}
```

[See this guide](https://apereo.github.io/cas/development/configuration/Configuration-Management-Extensions.html) to learn more about how to register configurations into the CAS runtime.