Single Sign-On Protocol Overview

Single Sign-On (SSO) has become essential for organizations managing multiple applications and user identities. This authentication process allows users to access multiple applications with one set of credentials, delivering improved user experience, enhanced security, and simplified user management.

The challenge lies in choosing the right SSO protocol for your environment. Each protocol serves different purposes and excels in specific scenarios. This guide provides a comprehensive overview and comparison of the four most common SSO protocols: Security Assertion Markup Language (SAML), OAuth 2.0, OpenID Connect (OIDC), and Kerberos.

Understanding SSO Fundamentals

SSO operates through a trust relationship between an Identity Provider (IdP) and Service Providers (SPs). The IdP authenticates users and issues proof of identity, while SPs rely on the IdP for authentication rather than managing credentials directly.

This system hinges on the secure exchange of digital documents—tokens or assertions—that prove identity or authorization. Understanding the distinction between authentication (“who you are”) and authorization (“what you can do”) is crucial, as different protocols focus on one or both aspects.

Security Assertion Markup Language (SAML)

SAML is an XML-based open standard for exchanging authentication and authorization data between an Identity Provider and Service Provider. It enables federated identity management across organizational boundaries.

How SAML Works

The common SP-initiated Web-Browser SSO flow follows these steps:

1. User requests access to a Service Provider
2. SP redirects user to IdP for authentication 
3. User authenticates with the IdP
4. IdP creates a digitally signed SAML Assertion containing user identity, attributes, and authentication method
5. IdP sends the signed SAML Assertion back to the SP via the user’s browser
6. SP validates the SAML Assertion by checking signature, issuer, and expiration before granting access

Key Features

SAML uses XML-based assertions with digital signatures for security. It supports federated identity management and represents a mature, widely-adopted standard in enterprise environments.

Primary Focus

SAML handles both authentication and authorization, though it’s primarily used for authentication in most implementations.

Common Use Cases

Organizations deploy SAML for enterprise SSO across web applications, particularly in B2B scenarios and cloud application integration. It excels in federated identity management between organizations.

Advantages

SAML offers wide enterprise adoption, strong security through digital signatures, and reduces password management burden for Service Providers. Its maturity provides stability and extensive vendor support.

Considerations

XML verbosity can impact performance, and SAML’s reliance on browser redirects makes it less suitable for mobile applications compared to modern alternatives.

OAuth 2.0 (Open Authorization)

OAuth 2.0 is an authorization framework that allows third-party applications to obtain limited access to user resources without exposing credentials. It operates by issuing access tokens rather than sharing passwords.

How OAuth 2.0 Works

A common and secure flow for web applications is the Authorization Code Flow, which follows this pattern:

1. Client application requests authorization from the Authorization Server
2. User is redirected to Authorization Server to grant consent
3. Authorization Server issues an Authorization Code to the client
4. Client exchanges Authorization Code for an Access Token through secure server-to-server communication
5. Client uses Access Token to access protected resources on behalf of the user

Key Features

OAuth 2.0 provides authorization delegation through various grant types for different scenarios. It uses access tokens and supports scopes for granular permission control.

Primary Focus

OAuth 2.0 specifically handles authorization—delegating access to resources rather than verifying identity.

Common Use Cases

The “Login with Google/Facebook” pattern for third-party applications, API access delegation, and mobile application access to user data all rely on OAuth 2.0.

Advantages

OAuth 2.0 offers flexibility through multiple grant types, wide adoption for API security, and granular control over delegated access through scopes.

Considerations

OAuth 2.0 wasn’t designed for authentication alone and requires an identity layer for complete SSO functionality.

OpenID Connect (OIDC)

OpenID Connect builds an authentication layer on top of OAuth 2.0. It allows clients to verify user identity based on Authorization Server authentication while obtaining basic profile information through REST-like interfaces.

How OIDC Works

Building on OAuth, the Authentication Code Flow with OIDC expands as follows:

1. User attempts to log in to a Relying Party (client application)
2. RP redirects user to OpenID Provider for authentication
3. User authenticates with the OpenID Provider
4. OP issues both an Authorization Code and an ID Token (JWT) to the RP
5. RP exchanges Authorization Code for Access Token and verifies the ID Token
6. RP can use Access Token to call the UserInfo Endpoint for additional profile data

Key Features

OIDC uses JSON Web Tokens (JWT) for ID Tokens, provides a standardized UserInfo Endpoint, supports standardized scopes, and offers lightweight, mobile-friendly implementation.

Primary Focus

OIDC handles authentication and identity verification, building on OAuth 2.0’s authorization capabilities.

Common Use Cases

Modern web and mobile applications use OIDC for SSO, consumer-facing identity services like “Sign in with Google/Microsoft,” and API-driven identity management.

Advantages

OIDC provides lightweight JSON-based tokens, mobile-friendly architecture, strong authentication capabilities, and standardizes areas left open by OAuth 2.0.

Considerations

OIDC adds complexity with ID Tokens and UserInfo Endpoints compared to basic OAuth 2.0 implementations.

Kerberos

Kerberos is a network authentication protocol using tickets to allow nodes on non-secure networks to prove identity securely. It employs symmetric-key cryptography and relies on a trusted Key Distribution Center (KDC).

How Kerberos Works

The Kerberos authentication process involves multiple steps:

1. Authentication Server (AS): The client authenticates with the AS component of the KDC using its credentials (e.g., a hash derived from the user’s password). The AS then issues an encrypted Ticket-Granting Ticket (TGT) back to the client.
2. Ticket-Granting Server (TGS): Client presents TGT to TGS to request service tickets for specific applications.
3. Service Ticket Presentation: Client presents service ticket to the target application or server.
4. Mutual Authentication: Both client and server verify each other’s identity using the tickets.

Key Features

Kerberos uses ticket-based authentication with symmetric-key cryptography, provides mutual authentication, and enables SSO within trusted domains.

Primary Focus

Kerberos delivers strong authentication and authorization within trusted, internal network domains.

Common Use Cases

Microsoft Windows Active Directory environments, internal enterprise networks, and highly secure internal systems rely on Kerberos for authentication.

Advantages

Kerberos provides extremely strong security without sending passwords over the network, mutual authentication capabilities, fast performance after initial ticket acquisition, and built-in Windows integration.

Considerations

Kerberos requires complex setup procedures, demands time synchronization across all systems, works primarily for internal networks, and creates a single point of failure at the KDC.

Comparative Analysis of SSO Protocols

Purpose

• SAML: Authentication and authorization for enterprise federation
• OAuth 2.0: Authorization and delegated access to resources 
• OIDC: Authentication and identity verification built on OAuth
• Kerberos: Authentication and SSO for internal network security

Data Format

• SAML: XML-based assertions with digital signatures
• OAuth 2.0: JSON-based tokens
• OIDC: JSON Web Tokens (JWT) for ID Tokens
• Kerberos: Encrypted tickets using symmetric cryptography

Primary Environment

• SAML: Enterprise-to-enterprise (B2B) web-based applications
• OAuth 2.0: APIs, mobile applications, third-party resource access
• OIDC: Modern web and mobile applications, consumer-facing identity
• Kerberos: Internal Windows-based enterprise networks

Complexity

• SAML: Moderate to high complexity due to XML parsing and metadata exchange
• OAuth 2.0: Variable complexity depending on chosen grant type
• OIDC: Moderate complexity adding authentication layer to OAuth
• Kerberos: High complexity requiring KDC setup, time synchronization, and SPN management

Security Strengths

• SAML: Digitally signed assertions provide robust federation security
• OAuth 2.0: Scoped access control without credential sharing
• OIDC: JWT-based strong authentication built on OAuth’s authorization foundation
• Kerberos: Symmetric key cryptography with mutual authentication and no network password transmission

Choosing the Right SSO Protocol

The choice of SSO protocol depends heavily on your specific environment, security requirements, and use cases. SAML remains the enterprise standard for web-based B2B federation scenarios. OIDC represents the modern choice for web and mobile authentication, leveraging OAuth 2.0’s proven authorization framework. Kerberos continues as the secure standard for internal Windows domain environments.

These protocols often complement each other in comprehensive identity management strategies. Organizations frequently implement multiple protocols to address different authentication scenarios across their technology stack. Understanding each protocol’s strengths and limitations enables informed decisions that balance security, usability, and technical requirements.