Shahzad Bhatti Welcome to my ramblings and rants!

December 27, 2009

Building Security Systems

Filed under: Computing — admin @ 11:20 pm

Being software developer for over eighteen years, I have observed a number of recurring problems and one of those recurring problems is security system. Most systems you build will require some kind of security so in this post I will go over core concepts when adding security to your system.

User Registration

A pre-requisite for any security system is to allow users to register to the system and store those users in some database, LDAP, Active Directory, or storage system. Though, for an internal application this step may be unnecessary.


The authentication allows systems to validate users based on password or other form of verification. For internal applications within a company, users may have to use multiple applications with their own authentication and each external website would also require unique authentication. This quickly becomes burdensome for both users and applications as users have to remember the passwords and systems have to maintain them. Thus, many companies employ some form of Single-Sign-On and I have used many solutions such as SiteMinder, IChain, Kerberos, Open SSO, Central Authentication Service (CAS), or other home built solutions. These Single-Sign-On systems use reverse proxy servers that sit in front of the application and intercepts all requests and automatically redirects users to login page if the users are not authenticated. When an internal system consists of multiple tiers such as services, it is often required to pass authentication tokens to those services. In J2EE systems, you can Common Secure Interoperability (CSIv2) protocol to pass the authentication to other tiers, which uses Security Attribute Service (SAS) protocol to perform client authentication and impersonation.

For external systems, Open ID is a way to go and I have used RPX to integrate Open ID for a number of sites I have developed such as,, etc.

There are a number of factors that make authentication a bit tricky such as when part of your system does not require authentication, you have to ensure the authentication policy is being used correctly. Also, in general authentication requires https instead of http, so you have to ensure that the site use those protocols consistently. In generaly, static contents such as css, javascript and images do not require authentication but often they are also put behind authentication by mistake.

Another factor related to authentication is session management. A session determines how long the user can access the system without login. Though, many systems provide remember-me feature, but often sessions require system resources on the server. It’s essential to keep the session short as it can effect scalability if it’s stored on the server. I generally prefer keeping the session very short and storing only user-id and a couple of other database-ids such as shopping-cart-id, request-id, etc. If they are short, they can also be stored in cookies that makes a stateless system so you can scale easily.


Not all users are same in most systems, thus authorization allows you to provide access control to limit the usage based on permissions and access control. There are a number of ways to define authorization such as Access control list, Role-based access control, Capability-based security, etc. In most systems, I have used J2EE/EJB Security, Java Web Security, JAAS, Acegi, which is now part of Spring and home built systems. As security is a cross cutting concern, I prefer to define those declaratively in a common security file or with annotations. There is nothing worse than sporadic security code mixed with your business logic.

One of feature I have found lacked in most of open source and commercial tools is support for instance based security or dynamic security that verifies runtime properties. For example, in most RBAC systems you can define rule that a purchase order can be approved by a role “POApprover”, but it does not allow you to say that “POApprover” can only approve if the user is from the same department or if amount is less than $10,000, etc.

UI or Resource Protection

When users have various level of access, it is essential to hide the UI elements and resources that are not accessible. Though, I have seen some systems employ security by obscurity that only hide the resources without actually enforcing the permissions, but it’s a bad idea. This can be complicated when the access level is very fine grained such as when a single form has fields based on role and permissions.

Database Security

The security must be enforced in depth, ranging from the UI, business and database tier. The database operations must use security to prevent access to unauthorized data. For example, let’s assume a user can post and edit blogs, it is essential that the database only allows the user to modify his/her blog. Also, it is critical that any kind of sensitive data such as passwords or personal identification with encryption. This is another reason I like OpenId or SSO solution because you don’t need to maintain them.

Method/Message Security

The message security ensures that a user only invokes the operations that he/she is authorized. For example, Acegi provides an annotation based mechanism to protect unauthorized methods.

Data Integrity

Any communication based systems may need to use message authentication check (MAC) to detect changes to the data.


Any communication based systems may need to encrypt sensitive data with HTTPS.


The system must audit users action so that they cannot repudiate them.


As achieving high level of security can be difficult and expensive so you need to treat security as a risk and employ the level of security that suits the underlying system. Finally, as I have found most RBAC systems lack, I have started my own open source project PlexRBAC to provide instance based security. Of course if you hare interested in assisting with the effort, you are welcome to join the project.

December 13, 2009

Dynamic Inheritance and Composition

Filed under: Languages — admin @ 3:55 pm

Static Inheritance

Inheritance is a core feature of object oriented languages that has been used to simulate real world by modeling closely related objects and to build reusable code. The inheritance relationship is defined statically in class specifications and it comes in various flavors such as:

Single Inheritance

It allows a class to be extended by just one other class.

Multiple Inheritance

It allows a class to be derived from multiple classes and historically has been difficult to maintain and has been source of diamond inheritance in C++, though other languages use order such as Method Resolution Order (MRO) in Python to avoid those issues.


The interfaces are used in C# and Java to define methods without implementation and a class can implement multiple interfaces without the downsides of multiple inheritance.


The mixins are available in Ruby and D, that use mixins for code reuse. The mixins are similar to interfaces with implementations except they aggregate methods and attributes at runtime.


The traits are available in Squeak and Scala and are conceptually similar to Mixins except traits do not allow attributes.

Dynamic Inheritance

As opposed to static inheritance, dynamic inheritance can be added at runtime using Object Extension Pattern, which I first learned in Erich Gamma, et al’s Gof patterns. In late 90s, I used Voyager ORB for building distributed systems, which used this pattern. Following example shows how this pattern can be used:

Let’s define a marker interface Extension in Java such as:

 1 package ext;
 3 public interface Extension {
 5 }

Then create a factory class such as

 1 package ext;
 3 public class ExtensionsFactory {
 4     public void register(final Class subject, final Extension ... exts) {/* ... */}
 5     public <T> T get(final Object subject, final Class<T> extClass) { /* ... */ return …;}
 6 }

The subject is object that needs to extend extensions, e.g. let’s assume you have a User class and you need to add hobbies, you can do it as follows:

 1 package domain;
 3 public class User {
 4     //...
 5 }

And you then define Hobbies as follows:

 1 package domain;
 3 public class Hobbies implements ext.Extension {
 4     public Hobbies(User user) {
 5         // ...
 6     }
 7 }

At runtime, you can register Hobbies to User and use it as follows

  1 package test;
  3 public class Main {
  4     public static void main(String[] args) {
  5         ExtensionsFactory f = new ExtensionsFactory();
  7         f.register(User.class, Hobbies.class);
  9         //
 10         User user = new User();
 11         Hobbies hobbies = f.get(user, Hobbies.class);
 12     }
 14 }

The dynamic inheritance allows you to follow open-closed principle by extending classes without modifying existing classes and allows you to choose features that you need at runtime. Of course, dynamic languages such as Ruby make this a lot easier as you can extend classes or objects with modules at runtime, e.g.

  1 ### defining Hobbies extension
  2 module Hobbies
  3   def hobbies
  4   end
  5 end
  7 ### defining User class
  8 class User
  9 end
 11 user =
 13 puts user.singleton_methods   #["hobbies"]
 15 ## or
 16 ### binding Hobbies with User at runtime
 17 class << User
 18   include Hobbies
 19 end
 20 puts User.singleton_methods   # ["hobbies"]

In real life, the inheritance relationship can be difficult to get right and often you have to use Liskov Substitution Principle to ensure base class can be replaced by derived class in all uses of the base class. However, dynamic inheritance acts more like Composition feature so above technique can also be used to implement dynamic composition. The dynamic inheritance or composition allows you to mix and match features you need at runtime and build extendable systems. This technique has been success key of evolution of Eclipse IDE. Also, this technique goes nicely with the Adaptive Object Modeling technique I described in my last post to build easily extendable systems.

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