JWT Service Tickets

JSON Web Tokens are an open, industry standard RFC 7519 method for representing claims securely between two parties. CAS may also be allowed to fully create signed/encrypted JWTs and pass them back to the application in form of service tickets.

JWTs are entirely self-contained and contain the authenticated principal as well as all authorized attributes in form of JWT claims.

:information_source: JCE Requirement

Make sure you have the proper JCE bundle installed in your Java environment that is used by CAS, specially if you need to use specific signing/encryption algorithms and methods. Be sure to pick the right version of the JCE for your Java version. Java versions can be detected via the java -version command.

Overview

JWT-based service tickets are issued to application based on the same semantics defined by the CAS Protocol. CAS having received an authentication request via its /login endpoint will conditionally issue back JWT service tickets to the application in form of a ticket parameter via the requested http method.

All JWTs are by default signed and encrypted by CAS based on keys generated and controlled during deployment. Such keys may be exchanged with client applications to unpack the JWT and access claims.

Flow Diagram

CAS Web flow JWT diagram

Note that per the above diagram, the JWT request by default internally causes CAS to generate an ST for the application and immediately then validate it in order to get access to the authenticated principal and attributes per policies associated with the application registration record in the CAS service registry. This response is transformed into a JWT that is then passed onto the client application.

In other words, the responsibility of receiving a service ticket (ST) and validating it is all moved into and handled internally by CAS. The application only needs to learn how to decipher and unpack the final JWT and ensure its validity.

The expiration time of the generated JWT is controlled by the length of the assertion returned as part of the validation event. If the assertion validity length is not specified, then the expiration time is controlled by the length of the SSO session defined as part of SSO expiration policy of the CAS server.

:warning: Not OpenID Connect

Remember that you are just receiving a ticket in form of a JWT, thereby removing the need from your client to validate a normal service ticket. The ticket is internally validated by CAS and you as the client are only left in charge of validating the JWT itself. Do not confuse this with OpenID Connect. While a JWT, the token itself is not an ID token, cannot be refreshed and must be obtained again once you deem it expired. If you need more, consider using the OpenID Connect protocol instead. Note that the responsibility of validating the JWT is pushed onto the client and NOT the CAS server itself.

Actuator Endpoints

The following endpoints are provided by CAS:

 Get public key for signing operations.


Configuration

JWT support is enabled by including the following dependency in the WAR overlay:



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<dependency>
    <groupId>org.apereo.cas</groupId>
    <artifactId>cas-server-support-token-tickets</artifactId>
    <version>${cas.version}</version>
</dependency>





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implementation "org.apereo.cas:cas-server-support-token-tickets:${project.'cas.version'}"





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dependencyManagement {
    imports {
        mavenBom "org.apereo.cas:cas-server-support-bom:${project.'cas.version'}"
    }
}

dependencies {
    implementation "org.apereo.cas:cas-server-support-token-tickets"
}





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dependencies {
    /*
        The following platform references should be included automatically and are listed here for reference only.

        implementation enforcedPlatform("org.apereo.cas:cas-server-support-bom:${project.'cas.version'}")
        implementation platform(org.springframework.boot.gradle.plugin.SpringBootPlugin.BOM_COORDINATES)
        
    */
    implementation "org.apereo.cas:cas-server-support-token-tickets"
}

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

The configuration settings listed below are tagged as Required in the CAS configuration metadata. This flag indicates that the presence of the setting may be needed to activate or affect the behavior of the CAS feature and generally should be reviewed, possibly owned and adjusted. If the setting is assigned a default value, you do not need to strictly put the setting in your copy of the configuration, but should review it nonetheless to make sure it matches your deployment expectations.

The configuration settings listed below are tagged as Optional in the CAS configuration metadata. This flag indicates that the presence of the setting is not immediately necessary in the end-user CAS configuration, because a default value is assigned or the activation of the feature is not conditionally controlled by the setting value. In other words, you should only include this field in your configuration if you need to modify the default value or if you need to turn on the feature controlled by the setting.

This CAS feature is able to accept signing and encryption crypto keys. In most scenarios if keys are not provided, CAS will auto-generate them. The following instructions apply if you wish to manually and beforehand create the signing and encryption keys.

Note that if you are asked to create a JWK of a certain size for the key, you are to use the following set of commands to generate the token:



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wget https://raw.githubusercontent.com/apereo/cas/master/etc/jwk-gen.jar
java -jar jwk-gen.jar -t oct -s [size]

The outcome would be similar to:



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{
  "kty": "oct",
  "kid": "...",
  "k": "..."
}

The generated value for k needs to be assigned to the relevant CAS settings. Note that keys generated via the above algorithm are processed by CAS using the Advanced Encryption Standard (AES) algorithm which is a specification for the encryption of electronic data established by the U.S. National Institute of Standards and Technology.

RSA Keys

Certain CAS features such as the ability to produce JWTs as CAS tickets may allow you to use the RSA algorithm with public/private keypairs for signing and encryption. This behavior may prove useful generally in cases where the consumer of the CAS-encoded payload is an outsider and a client application that need not have access to the signing secrets directly and visibly and may only be given a half truth vis-a-vis a public key to verify the payload authenticity and decode it. This particular option makes little sense in situations where CAS itself is both a producer and a consumer of the payload.

:information_source: Remember

Signing and encryption options are not mutually exclusive. While it would be rather nonsensical, it is entirely possible for CAS to use AES keys for signing and RSA keys for encryption, or vice versa.

In order to enable RSA functionality for signing payloads, you will need to generate a private/public keypair via the following sample commands:



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openssl genrsa -out private.key 2048
openssl rsa -pubout -in private.key -out public.key -inform PEM -outform DER

The private key path (i.e. file:///path/to/private.key) needs to be configured for the signing key in CAS properties for the relevant feature. The public key needs to be shared with client applications and consumers of the payload in order to validate the payload signature.

:information_source: Key Size

Remember that RSA key sizes are required to be at least 2048 and above. Smaller key sizes are not accepted by CAS and will cause runtime errors. Choose wisely.

In order to enable RSA functionality for encrypting payloads, you will need to essentially execute the reverse of the above operations. The client application will provide you with a public key which will be used to encrypt the payload and whose path (i.e. file:///path/to/public.key) needs to be configured for the encryption key in CAS properties for the relevant feature. Once the payload is submitted, the client should use its own private key to decode the payload and unpack it.

Certain authentication handlers are allowed to determine whether they can operate on the provided credential and as such lend themselves to be tried and tested during the authentication handler selection phase. The credential criteria may be one of the following options:

  • A regular expression pattern that is tested against the credential identifier.
  • A fully qualified class name of your own design that looks similar to the below example:


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import java.util.function.Predicate;
import org.apereo.cas.authentication.Credential;

public class PredicateExample implements Predicate<Credential> {
    @Override
    public boolean test(final Credential credential) {
        // Examine the credential and return true/false
    }
}
  • Path to an external Groovy script that looks similar to the below example:


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import org.apereo.cas.authentication.Credential
import java.util.function.Predicate

class PredicateExample implements Predicate<Credential> {
    @Override
    boolean test(final Credential credential) {
        // test and return result
    }
}

To prepare CAS to support and integrate with Apache Groovy, please review this guide.

Configuration Metadata

The collection of configuration properties listed in this section are automatically generated from the CAS source and components that contain the actual field definitions, types, descriptions, modules, etc. This metadata may not always be 100% accurate, or could be lacking details and sufficient explanations.

Be Selective

This section is meant as a guide only. Do NOT copy/paste the entire collection of settings into your CAS configuration; rather pick only the properties that you need. Do NOT enable settings unless you are certain of their purpose and do NOT copy settings into your configuration only to keep them as reference. All these ideas lead to upgrade headaches, maintenance nightmares and premature aging.

YAGNI

Note that for nearly ALL use cases, declaring and configuring properties listed here is sufficient. You should NOT have to explicitly massage a CAS XML/Java/etc configuration file to design an authentication handler, create attribute release policies, etc. CAS at runtime will auto-configure all required changes for you. If you are unsure about the meaning of a given CAS setting, do NOT turn it on without hesitation. Review the codebase or better yet, ask questions to clarify the intended behavior.

Naming Convention

Property names can be specified in very relaxed terms. For instance cas.someProperty, cas.some-property, cas.some_property are all valid names. While all forms are accepted by CAS, there are certain components (in CAS and other frameworks used) whose activation at runtime is conditional on a property value, where this property is required to have been specified in CAS configuration using kebab case. This is both true for properties that are owned by CAS as well as those that might be presented to the system via an external library or framework such as Spring Boot, etc.

:information_source: Note

When possible, properties should be stored in lower-case kebab format, such as cas.property-name=value. The only possible exception to this rule is when naming actuator endpoints; The name of the actuator endpoints (i.e. ssoSessions) MUST remain in camelCase mode.

Settings and properties that are controlled by the CAS platform directly always begin with the prefix cas. All other settings are controlled and provided to CAS via other underlying frameworks and may have their own schemas and syntax. BE CAREFUL with the distinction. Unrecognized properties are rejected by CAS and/or frameworks upon which CAS depends. This means if you somehow misspell a property definition or fail to adhere to the dot-notation syntax and such, your setting is entirely refused by CAS and likely the feature it controls will never be activated in the way you intend.

Validation

Configuration properties are automatically validated on CAS startup to report issues with configuration binding, specially if defined CAS settings cannot be recognized or validated by the configuration schema. Additional validation processes are also handled via Configuration Metadata and property migrations applied automatically on startup by Spring Boot and family.

Indexed Settings

CAS settings able to accept multiple values are typically documented with an index, such as cas.some.setting[0]=value. The index [0] is meant to be incremented by the adopter to allow for distinct multiple configuration blocks.

Register Clients

Signal the relevant application in CAS service registry to produce JWTs for service tickets:



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{
  "@class" : "org.apereo.cas.services.CasRegisteredService",
  "serviceId" : "^https://.*",
  "name" : "Sample",
  "id" : 10,
  "properties" : {
    "@class" : "java.util.HashMap",
    "jwtAsServiceTicket" : {
      "@class" : "org.apereo.cas.services.DefaultRegisteredServiceProperty",
      "values" : [ "java.util.HashSet", [ "true" ] ]
    }
  }
}

Configure Keys Per Service

By default, the signing and encryption keys used to encode the JWT are global to the CAS server and can be defined via CAS settings. It is also possible to override the global keys on a per-service basis, allowing each application to use its own set of signing and encryption keys. To do so, configure the service definition in the registry to match the following:



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{
  "@class" : "org.apereo.cas.services.CasRegisteredService",
  "serviceId" : "^https://.*",
  "name" : "Sample",
  "id" : 10,
  "properties" : {
    "@class" : "java.util.HashMap",
    "jwtAsServiceTicket" : {
      "@class" : "org.apereo.cas.services.DefaultRegisteredServiceProperty",
      "values" : [ "java.util.HashSet", [ "true" ] ]
    },
    "jwtAsServiceTicketSigningKey" : {
       "@class" : "org.apereo.cas.services.DefaultRegisteredServiceProperty",
       "values" : [ "java.util.HashSet", [ "..." ] ]
    },
    "jwtAsServiceTicketEncryptionKey" : {
         "@class" : "org.apereo.cas.services.DefaultRegisteredServiceProperty",
         "values" : [ "java.util.HashSet", [ "..." ] ]
    },
    "jwtAsServiceTicketCipherStrategyType" : {
         "@class" : "org.apereo.cas.services.DefaultRegisteredServiceProperty",
         "values" : [ "java.util.HashSet", [ "ENCRYPT_AND_SIGN" ] ]
    }
  }
}

The following cipher strategy types are available:

Type Description
ENCRYPT_AND_SIGN Default strategy; encrypt values, and then sign.
SIGN_AND_ENCRYPT Sign values, and then encrypt.

The following properties are available and recognized by CAS for various modules and features:

JWT Validation - AES

The following example code snippet demonstrates how one might go about validating and parsing the CAS-produced JWT that is created using shared secrets via AES:



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import org.apache.commons.codec.binary.Base64;
import org.jose4j.jwe.JsonWebEncryption;
import org.jose4j.jwk.JsonWebKey;
import org.jose4j.jws.JsonWebSignature;
import org.jose4j.keys.AesKey;

import java.nio.charset.StandardCharsets;
import java.security.Key;

...

var signingKey = "...";
var encryptionKey = "...";

var key = new AesKey(signingKey.getBytes(StandardCharsets.UTF_8));

var jws = new JsonWebSignature();
jws.setCompactSerialization(secureJwt);
jws.setKey(key);
if (!jws.verifySignature()) {
    throw new Exception("JWT verification failed");
}

var decodedBytes = Base64.decodeBase64(jws.getEncodedPayload().getBytes(StandardCharsets.UTF_8));
var decodedPayload = new String(decodedBytes, StandardCharsets.UTF_8);

var jwe = new JsonWebEncryption();
var jsonWebKey = JsonWebKey.Factory
    .newJwk("\n" + "{\"kty\":\"oct\",\n" + " \"k\":\"" + encryptionKey + "\"\n" + "}");

jwe.setCompactSerialization(decodedPayload);
jwe.setKey(new AesKey(jsonWebKey.getKey().getEncoded()));
System.out.println(jwe.getPlaintextString());