Extending the Server

The architecture of the OA4MP server (all versions, both based on OAuth 1.0a and OAuth 2.0) is designed to accommodate extensions. The delegation exchanges should, in general, never be altered since there is a high risk of making the system insecure, so hooks have been added in all the places that should let a developer extend the system as safely as possible. There are two main mechanisms involved. These are

  • Transaction filtering -- intercepting and changing the state of a transaction
  • Presentation -- changing the look and feel of any user-facing servlet.

These will be discussed in turn. To use this you should check out the most current maven module of oa4mp-webapp and create your own package under src/main/java which will hold any custom classes you need to write. Specifying these in the deployment descriptor should have them compiled and deployed automatically.

Transaction Filtering

Internally, the state of the service is kept in a transaction. This has all the information available to the service on each leg of the delegation. Each servlet implements the TransactionFilter interface, which has two methods:

    public void preprocess(TransactionState t);

    public void postprocess(TransactionState t);

The lifecycle is straightforward: A request is received by the servlet and parsed. The transaction is retrieved from storage and validity checks are made. Once a token is issued, the preprocess method is called. This allows you to intercept the state of the transaction before anything is written to the output stream. Once the output stream is written, postprocess is invoked.

The transactionState contains the original servlet request and response, any parsed parameters (this may be null) and the currently active transaction. To save a transaction, call it t, you must call the store:

getTransactionStore().save(t);

Presentation

Servlets that present something to the user implement the presentation interface. This is extremely similar to the transaction filtering above. The difference being that it is assumed you will be customizing the response for any JSP pages, and an additional handleException method that is invoked on any error that lets you control what is displayed to the user. The entire API then is

    public void prepare(PresentationState p);

    public void present(PresentationState p);

    public void handleException(PresentationState p, Throwable t);

Transaction filtering is still active, so if you just need some special processing and do not need to change the presentation layer, use that.

Table 1: A table the summarizes what each servlet does, and when transaction filtering/presentation is invoked

Servlet name Purpose Transaction filtering Presentation state
RegistrationServlet Handles registration request for the service. preprocess:invoked immediately before prepare. Note there is no parameter map or transaction for this, so these are null prepare:invoked before anything else. First call in the servlet
postprocess:invoked immediately before present. There is no parameter map or transaction either. present:invoked immediately before displaying a page.
InitServlet Starts the delegation process. Sets initial state and issues a token. preprocess: invoked after the request for a token has been made internally. --
postprocess: invoked immediately before return, after writing to the output stream. --
AuthorizationServlet Authentication with MyProxy. The cert is fetched. The user is redirected to the callback url at the end. preprocess:invoked after getting the username and password, immediately before fetching the cert prepare:invoked as soon as the state is determined and the pending transaction has been retrieved. State is set depending upon user action -- see the documentation for state codes if this is of interest.
postprocess:invoked immediately before invoking present present:invoked immediately before page display. This mostly just forwards to the correct JSP page.
AccessTokenServlet Issues an access token so the client can retrieve a certificate. preprocess:invoked as soon as the transaction is found to be valid, but before the request is parsed, so the paramter map is null --
postprocess:invoked after the access token has been issued and the transaction state has been updated. This is the last call in the servlet. The stream has been written at this point. --
CertServlet Retrieves the certificate that has been previously fetched and returns it to the client. preprocess:invoked after parsing the servlet request since this is needed to retrieve the transaction, but before the response is written --
postprocess:invoked immediately before returning, after the response stream is written. --

Example: replacing the cert request.

Sometimes a service need to customize the cert request. Here is a complete example of how to do that in OA4MP OAuth 1.0a. You would want to intercept the request in the AuthorizationServlet, right before the cert is actually retrieved. Simply override the postprocess method:


package my.package;

import edu.uiuc.ncsa.security.util.pkcs.CertUtil;
import edu.uiuc.ncsa.security.util.pkcs.KeyUtil;
import edu.uiuc.ncsa.myproxy.oa4mp.server.OA4MPServiceTransaction;
import edu.uiuc.ncsa.myproxy.oa4mp.server.servlet.AuthorizationServlet;
import java.security.KeyPair;

public void MyAuthServlet extends AuthorizationServlet{
    @Override
    public void preprocess(transactionState state) throws Throwable{
       super.preprocess(state); // required to get certain security headers set in response!
       OA4MPServiceTransaction t = (OA4MPServiceTransaction) state.getTransaction(); // Here is how to use the utilities to create a new cert request
       KeyPair kp = KeyUtil.generateKeyPair();
       t.setCertReq(CertUtil.createCertRequest(kp)); // Note that if you will need the keypair later, you will have to store it at this point.
       // You can use other information, such as the username or lifetime of the cert
       getTransactionStore().save(t);
    }
}

To use this, deploy this in place of the standard AuthorizationServlet in the deployment descriptor (web.xml file).


Last modified 10/25/18.
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