Apache CXF STS documentation - part I

The forthcoming Apache CXF 2.5 release will have an STS (Security Token Service) implementation which has been donated by Talend. This is the first in a series of blog posts where I will be going into the STS implementation in detail. In this post I will be explaining what an STS is and talking about the STS provider framework in CXF.

1) What is a Security Token Service?

An informal description of a Security Token Service is that it is a web service that offers some or all of the following services (amongst others):

• It can issue a Security Token of some sort based on presented or configured credentials.
• It can say whether a given Security Token is valid or not
• It can renew (extend the validity of) a given Security Token
• It can cancel (remove the validity of) a given Security Token
• It can transform a given Security Token into a Security Token of a different sort.

Offloading this functionality to another service greatly simplifies client and service provider functionality, as they can simply call the STS appropriately rather than have to figure out the security requirements themselves. For example, the WSDL of a service provider might state that a particular type of security token is required to access the service. A client of the service can ask an STS for a Security Token of that particular type, which is then sent to the service provider. The service provider could choose to validate the received token locally, or dispatch the token to an STS for validation. These are the two most common use-cases of an STS.

A client can communicate with the STS via a protocol defined in the WS-Trust specification. The SOAP Body of the request contains a "RequestSecurityToken" element that looks like:

<wst:RequestSecurityToken Context="..." xmlns:wst="...">
    <wst:TokenType>...</wst:TokenType>
    <wst:RequestType>...</wst:RequestType>
    <wst:SecondaryParameters>...</wst:SecondaryParameters>
    ...
</wst:RequestSecurityToken>

The Apache CXF STS implementation supports a wide range of parameters that are passed in the RequestSecurityToken element. The SOAP Body of the response from the STS will contain a "RequestSecurityTokenResponse(Collection)" element, e.g.:

<wst:RequestSecurityTokenResponseCollection xmlns:wst="...">
    <wst:RequestSecurityTokenResponse>
    ...
    </wst:RequestSecurityTokenResponse>
</wst:RequestSecurityTokenResponseCollection>

1.1 A sample request/response for issuing a Security Token

A sample client request is given here, where the client wants the STS to issue a SAML 2.0 token for the "http://cxf.apache.org:8080/service" service:

<wst:RequestSecurityToken Context="..." xmlns:wst="...">
    <wst:TokenType>
       http://docs.oasis-open.org/wss/oasis-wss-saml-token-profile-1.1#SAMLV2.0
    </wst:TokenType>
    <wst:RequestType>
        http://docs.oasis-open.org/ws-sx/ws-trust/200512/Issue
    </wst:RequestType>
    <wsp:AppliesTo>http://cxf.apache.org:8080/service</wsp:AppliesTo>
</wst:RequestSecurityToken>

The STS responds with:

<wst:RequestSecurityTokenResponseCollection xmlns:wst="...">
    <wst:RequestSecurityTokenResponse>
         <wst:TokenType>
         http://docs.oasis-open.org/wss/oasis-wss-saml-token-profile-1.1#SAMLV2.0
         </wst:TokenType>
         <wst:RequestedSecurityToken>
             <saml2:Assertion xmlns:saml2="..." ... />
         </wst:RequestedSecurityToken>
    </wst:RequestSecurityTokenResponse>
</wst:RequestSecurityTokenResponseCollection>

2) The STS provider framework in Apache CXF

The first support for an STS in Apache CXF appeared in the 2.4.0 release with the addition of an STS provider framework in the WS-Security module. This is essentially an API that can be used to create your own STS implementation. As the STS implementation shipped in CXF 2.5 is based on this provider framework, it makes sense to examine it in more detail.

The SEI (Service Endpoint Interface) is available here. It contains the following methods that are relevant to the STS features discussed above:

• RequestSecurityTokenResponseCollectionType issue(RequestSecurityTokenType request) - to issue a security token
• RequestSecurityTokenResponseType issueSingle( RequestSecurityTokenType request) - to issue a security token that is not contained in a "Collection" wrapper (for legacy applications).
• RequestSecurityTokenResponseType cancel(RequestSecurityTokenType request) - to cancel a security token
• RequestSecurityTokenResponseType validate(RequestSecurityTokenType request) - to validate a security token
• RequestSecurityTokenResponseType renew(RequestSecurityTokenType request) - to renew a security token

The SEI implementation handles each request by delegating it to a particular operation, which is just an interface that must be implemented by the provider framework implementation. Finally, a JAX-WS provider is available, which dispatches a request to the appropriate operation.

Significant updates to the STS Provider framework after the CXF 2.4.0 release include support for SOAP 1.2, a major bug fix to support operations other than issue,  better exception propagation, and adding support for the WS-Trust 1.4 schema. These features are all available in the Apache CXF 2.4.3 release onwards.