OIDC vs SAML vs OAuth: Which to choose for SSO and why?
Choosing between OIDC, SAML, and OAuth is one of the most important identity decisions in modern distributed systems. These three protocols look similar but solve different problems. Understanding how each one works helps you design secure and smooth Single Sign On for any web, mobile, or enterprise environment.
Why It Matters
Identity choices affect login experience, enterprise integration, security, performance, and long term scalability. In system design interviews this topic frequently appears when building dashboards, SaaS products, API platforms, or any architecture that handles user sessions. A clear explanation of these protocols shows maturity in security design.
How It Works
OIDC
OIDC adds authentication on top of OAuth. It lets your application confirm user identity with a trusted identity provider.
Steps
- The user clicks Sign in in your app.
- Your app sends the user to the identity provider with an OIDC request.
- The user authenticates with the identity provider.
- The identity provider redirects back with an authorization code.
- Your backend exchanges the code for tokens.
- You validate the ID token signature and claims.
- Your app creates a session for the user.
Key idea
The ID token is the cryptographically signed proof of user identity.
SAML
SAML is an XML based enterprise standard for SSO.
Steps
- The user accesses your protected resource.
- Your app creates a SAML authentication request.
- The identity provider authenticates the user.
- The identity provider sends a signed SAML assertion.
- Your app validates the XML signature and conditions.
- Your app creates a session.
Key idea
SAML assertions provide identity in XML format. It is widely used in older corporate SSO stacks.
OAuth
OAuth is an authorization framework for delegated access to APIs. It is not an authentication protocol.
Steps
- Your app sends the user to authorize with another provider.
- The user grants permission for specific scopes.
- The provider returns an authorization code.
- Your backend exchanges the code for an access token.
- Your app uses the token to call the provider API.
Key idea
OAuth identifies what a client can do but not who the user is. That is why OIDC was created.
Real World Example
A SaaS platform often supports both small teams and enterprise customers. Small teams usually prefer OIDC based SSO with Google or GitHub. Large enterprises may require SAML integration because their identity stack uses older XML based systems. A flexible design supports both protocols with per tenant configuration and a shared internal identity abstraction.
Common Pitfalls
Misusing OAuth as login
OAuth alone cannot confirm user identity and leads to insecure login flows.
Ignoring token validation
Not checking signatures, audience, or expiration lets attackers replay tokens.
Using only one identity protocol
Hard coding only OIDC or only SAML makes onboarding new customers much harder.
Using SAML for mobile and API heavy apps
XML based SAML flows are painful for native apps or microservice APIs.
Using OIDC when customers require SAML
Many large enterprises still rely on SAML for compliance and auditing.
Interview Tip
Interviewers want you to distinguish authentication from authorization. A strong answer says that OIDC is ideal for modern clients, SAML is preferred for enterprise SSO, and OAuth is for delegated API access. Mention token validation and per tenant metadata to show real world experience.
Key Takeaways
- OIDC is the best choice for modern Single Sign On across web, mobile, and APIs.
- SAML is still dominant in enterprise environments and corporate identity systems.
- OAuth alone is not authentication and must be paired with OIDC when used for login.
- A scalable SaaS platform usually supports both OIDC and SAML for customer flexibility.
- In interviews always clarify the difference between identity and authorization.
Table of Comparison
| Aspect | OIDC | SAML | OAuth |
|---|---|---|---|
| Main purpose | Authentication | Authentication | Delegated authorization |
| Data format | JSON tokens | XML assertions | Provider specific access tokens |
| Best fit | Modern web, mobile, APIs | Enterprise SSO | Third party API access |
| Developer experience | Clean and modern | Verbose XML | Simple for scopes and API tokens |
| Best choice for new SSO | Yes | Only when required | Not for login without identity layer |
FAQs
Q1. Which protocol is best for Single Sign On?
OIDC is usually the best for new applications because it uses JSON tokens and integrates well with mobile and API based architectures.
Q2. When should SAML be used instead of OIDC?
Use SAML when integrating with large enterprise customers that rely on older corporate identity infrastructure built around SAML.
Q3. Can OAuth be used as a login protocol?
Not safely. OAuth does not define how to verify user identity. It must be paired with OIDC when used for authentication.
Q4. Why do enterprises still use SAML?
Because many compliance checks, auditing tools, and internal identity systems were built around SAML and have not fully migrated to modern standards.
Q5. Is OIDC better for mobile apps?
Yes. OIDC supports modern flows like PKCE and uses lightweight JSON tokens that work cleanly with mobile clients.
Q6. Should a SaaS product support both SAML and OIDC?
Yes, especially if it serves enterprise customers. A flexible identity layer helps onboard any customer with minimal friction.
Further Learning
If you want to practice complete Single Sign On design inside full system architecture problems, explore the advanced interview patterns inside Grokking the System Design Interview.
If you prefer a simpler and more visual introduction to authentication, authorization, sessions, and tokens, start with Grokking System Design Fundamentals.
For deeper coverage of distributed identity and API security in scalable architecture, continue learning inside Grokking Scalable Systems for Interviews
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