SSO with CAS or OAuth

In today’s interconnected digital landscape, seamless access to multiple applications without repeated logins is not just a convenience; it’s a necessity for both security and productivity. This is the promise of Single Sign-On (SSO), a powerful authentication scheme that allows users to log in once with a single set of credentials to gain access to multiple applications. However, organizations often face a critical decision when implementing SSO: which underlying protocol to choose? The debate often centers on solutions like CAS (Central Authentication Service) and OAuth (Open Authorization), leading to the crucial question: which is the better choice for your environment when considering SSO with CAS or OAuth? Understanding the nuances of each, their primary use cases, and how they align with your specific security and integration requirements is paramount for a successful identity management strategy. This deep dive will explore both protocols, helping you navigate their complexities and make an informed decision.

Understanding the Foundation of Single Sign-On (SSO)

Single Sign-On (SSO) fundamentally transforms the user experience by reducing login fatigue and enhancing overall security. Instead of managing separate credentials for every application, users authenticate once with an Identity Provider (IdP), which then grants them access to various Service Providers (SPs) without requiring additional logins. This streamlined process not only saves time but also mitigates the risk associated with users reusing weak passwords across different services or forgetting complex ones.

The core principle behind SSO involves a trust relationship established between the IdP and the SPs. Once a user authenticates with the IdP, a secure token or assertion is issued. This token is then presented to any connected SP, which verifies its authenticity with the IdP and grants access. This mechanism significantly simplifies user management for IT administrators, as they only need to manage user identities in a central directory, rather than across numerous disparate systems. According to a Statista report, the global SSO market size is projected to grow substantially, underscoring its increasing adoption and importance in modern enterprise architectures.

Beyond convenience, SSO plays a vital role in strengthening an organization’s security posture. By centralizing authentication, it enables consistent application of security policies, such as multi-factor authentication (MFA), across all integrated services. This reduces the attack surface by eliminating the need for users to remember and manage multiple passwords, which often leads to poor password hygiene. Implementing a robust SSO solution is a cornerstone of effective identity management and a critical component of a Zero Trust security model.

Diving into CAS (Central Authentication Service)

Central Authentication Service (CAS) is an open-source SSO protocol designed specifically for web applications. Originating from Yale University, CAS provides a simple yet effective mechanism for users to authenticate once and gain access to multiple web services within a specific trust domain. It operates on a ticket-based system, making it particularly well-suited for traditional browser-based applications where the client is a web browser.

The CAS protocol typically involves three main components: the user’s web browser, the CAS server (the Identity Provider), and the service application (the Service Provider). When a user attempts to access a protected application, they are redirected to the CAS server for authentication. Upon successful login, the CAS server issues a Ticket Granting Ticket (TGT) to the user’s browser, which is then used to obtain Service Tickets (STs) for specific applications. These STs are single-use and short-lived, ensuring that they cannot be replayed for unauthorized access. This design makes CAS a robust choice for intranet environments and educational institutions that need a straightforward, secure web SSO solution.

How CAS Works: A Step-by-Step Flow

Understanding the CAS flow helps illustrate its effectiveness in providing web-based SSO:

1. A user attempts to access a protected web application (Service Provider).
2. The Service Provider detects no existing CAS session and redirects the user’s browser to the CAS Server.
3. The CAS Server prompts the user for credentials (if not already logged in).
4. Upon successful authentication, the CAS Server issues a Ticket Granting Ticket (TGT) to the user’s browser via a cookie and redirects the browser back to the Service Provider with a unique Service Ticket (ST) in the URL.
5. The Service Provider validates the received ST with the CAS Server.
6. If the ST is valid, the CAS Server confirms the user’s identity to the Service Provider.
7. The Service Provider grants access to the user, who is now authenticated. Subsequent access to other CAS-enabled applications within the same session will reuse the TGT to obtain new STs without re-entering credentials.

CAS excels in environments where applications are primarily web-based and within a controlled enterprise boundary. Its simplicity and robust security features for this specific use case have made it a popular choice for many organizations seeking a dedicated web SSO solution.

Exploring OAuth (Open Authorization) and OpenID Connect

OAuth, or Open Authorization, serves a fundamentally different purpose than CAS. While often discussed in the context of SSO, OAuth is primarily an authorization framework, not an authentication protocol. It allows a user to grant a third-party application limited access to their resources on another service (e.g., granting a photo editing app access to your Google Photos) without sharing their credentials. This delegation of authority is crucial for modern internet services and mobile applications.

When people refer to OAuth for authentication or SSO, they are typically referring to OAuth 2.0 combined with OpenID Connect (OIDC). OIDC is an identity layer built on top of the OAuth 2.0 framework. It enables clients to verify the identity of the end-user based on the authentication performed by an authorization server, as well as to obtain basic profile information about the end-user in an interoperable REST-like manner. This combination provides both authentication and authorization, making it suitable for federated identity scenarios and consumer-facing applications.

The Synergy of OAuth 2.0 and OpenID Connect

The power of OAuth 2.0 and OpenID Connect lies in their flexibility and broad adoption across the internet. OAuth 2.0 defines various “flows” or “grant types” to suit different client types (web applications, mobile apps, single-page applications) and security requirements. OIDC then layers identity information on top, returning an ID Token (a signed JSON Web Token or JWT) that contains claims about the authenticated user, such as their name, email, and user ID. This token can be cryptographically verified by the relying party (Service Provider) to confirm the user’s identity.

Key advantages of using OAuth/OIDC include:

• Flexibility: Supports a wide range of client types and use cases, from web to mobile to IoT.
• Industry Standard: Widely adopted by major identity providers like Google, Facebook, Microsoft, and others, facilitating federated identity.
• Granular Authorization: OAuth’s core strength is allowing users to grant specific permissions to applications.
• API-Centric: Designed for modern API-driven architectures, making it ideal for microservices and cloud-native applications.

For organizations looking to implement SSO across a diverse ecosystem of applications, including internal systems, external SaaS services, and mobile apps, the OAuth 2.0/OpenID Connect stack provides a modern, scalable, and highly interoperable solution. It supports complex identity management scenarios and is foundational to many cloud-based identity services.

CAS vs. OAuth: Which Protocol is Right for You? -----------------------------------------------

Choosing between CAS and OAuth (specifically with OpenID Connect) for your SSO strategy boils down to understanding your specific environment, application types, and future identity management goals. While both facilitate a single sign-on experience, their architectures, strengths, and ideal use cases diverge significantly. This section aims to clarify the distinctions to help you make an informed decision when considering SSO with CAS or OAuth?.

CAS is often described as a “traditional” web SSO solution. Its strength lies in its simplicity and effectiveness for securing web applications within a Question & Answer :

I wonder if I should use the CAS protocol or OAuth + some authentication provider for single sign-on.

Example Scenario:

1. A User tries to access a protected resource, but is not authenticated.
2. The application redirects the user to the SSO server.
3. If beeing authenticated the user gets a token from the SSO server.
4. The SSO redirects to the original application.
5. The original application checks the token against the SSO server.
6. If the token is ok, access will be allowed and the application knows of the user id.
7. The user performs a log-out and is logged out from all connected application at the same time (single sign-out).

As far as I understand that is exactly what was CAS invented for. CAS clients have to implement the CAS protocol to use the authentication service. Now I’m wondering about to use CAS or OAuth at the client (consumer) site. Is OAuth a replacement for that part of CAS? Should OAuth as a new de-facto standard be preferred? Is there an easy to use (not Sun OpenSSO!) replacement for the authentication part of CAS supporting different methods like username/password, OpenID, TLS certifactes …?

Context:

• Different applications should rely on the authentication of the SSO server and should use something session-like.
• The applications can be GUI web applications or (REST) serivces.
• The SSO server must be provide a user id, which is necessary to get more information about the user like roles, email and so on from a central user information store.
• Single Sign-out should be possible.
• Most clients are written in Java or PHP.

I’ve just discovered WRAP, which could become the OAuth successor. It is a new protocol specified by Microsoft, Google and Yahoo.

Addendum

I’ve learned that OAuth was not designed for authentication even it could be used to implement SSO, but only together with a SSO service like OpenID.

OpenID seems to me to be the “new CAS”. CAS has some features OpenID misses (like single sign-out), but it should not be to hard to add the missing parts in a particular scenario. I think OpenID has broad acceptance and it is better to integrate OpenID into applications or application servers. I know that CAS also supports OpenID, but I think CAS is dispensable with OpenID.

OpenID is not a ‘successor’ or ‘substitute’ for CAS, they’re different, in intent and in implementation.

CAS centralizes authentication. Use it if you want all your (probably internal) applications to ask users to login to a single server (all applications are configured to point to a single CAS server).

OpenID decentralizes authentication. Use it if you want your application to accept users login to whatever authentication service they want (the user provides the OpenID server address - in fact, the ‘username’ is the server’s URL).

None of the above handle authorization (without extensions and/or customization).

OAuth handles authorization, but it is not a substitute for the traditional ‘USER_ROLES table’ (user access). It handles authorization for third-parties.

For example, you want your application to integrate with Twitter: a user could allow it to tweet automatically when they update their data or post new content. You want to access some third-party service or resource on behalf of a user, without getting his password (which is obviously unsecure for the user). The application asks Twitter for access, the user authorizes it (through Twitter), and then the app may have access.

So, OAuth is not about Single Sign-On (nor a substitute for the CAS protocol). It is not about you controlling what the user can access. It is about letting the user to control how their resources may be accessed by third-parties. Two very different use-cases.

To the context you described, CAS is probably the right choice.

[updated]

That said, you can implement SSO with OAuth, if you consider the identity of the user as a secured resource. This is what ‘Sign up with GitHub’ and the likes do, basically. Probably not the original intent of the protocol, but it can be done. If you control the OAuth server, and restrict the apps to only authenticate with it, that’s SSO.

No standard way to force logout, though (CAS has this feature).