服务提供商视角下SSO系统实现相关技术问询

Great questions—let's unpack each part of your IdP-initiated SSO scenario clearly, since this is a common pattern in enterprise identity systems.

Let's start with role definitions tailored to your flow:

• System A is the IdP (Identity Provider): It's the system that already authenticated your identity—you logged into it first, so it holds the valid proof of who you are.
• System B is the SP (Service Provider): It's the third-party service you're trying to access, which relies on the IdP's authentication to grant you access without requiring a separate login.

Yes, server-to-server communication usually exists, but it's not mandatory in every scenario:

• Mandatory in production-grade setups: When the SP receives the authentication token/assertion from the IdP, it needs to verify the token is genuine (not forged) and still valid. To do this securely, the SP will make a direct server-to-server call to the IdP—for example, to fetch the IdP's public key metadata, check if the token has been revoked, or validate the signature against the IdP's official endpoint. This is non-negotiable for secure systems, as it blocks attackers from sending fake tokens to the SP.
• Optional in simplified, low-risk setups: For internal systems where trust is pre-established (like two apps in the same company network), the SP might skip the server-to-server call. Instead, it uses a pre-stored copy of the IdP's public key to locally verify the token's signature. This only works if you can guarantee the key doesn't change without the SP knowing, and the system has minimal security risks.

Let's walk through the exact sequence from your scenario:

1. User authenticates with System A (IdP)
   You visit System A, enter your credentials (username/password, MFA, etc.), and it validates your identity. It creates a local session for you and displays a link to System B—this link is pre-configured to trigger the SSO flow.
2. User clicks the SSO link
   The link is generated by System A and includes two key components: the redirect URL for System B (SP), and a signed authentication token/assertion (like a SAML Assertion or JWT). This token contains your user ID, session details, and a digital signature from the IdP to prove it's legitimate.
3. IdP redirects the user's browser to System B
   Your browser receives a 302 redirect response from System A, and it sends the token along to System B's SSO callback endpoint (e.g., /sso/accept).
4. System B (SP) validates the token
   • Secure production path: System B makes a server-to-server request to System A's validation endpoint. It checks if the token's signature matches the IdP's public key, if the token hasn't expired, and if it hasn't been revoked.
   • Simplified path: System B uses a pre-saved copy of System A's public key to locally verify the token's signature, skipping the server-to-server call.
5. System B creates a user session and grants access
   Once the token is validated, System B creates a local user session for you and redirects you to the actual resource you wanted to access—no need to log in again.

• SAML 2.0 IdP-Initiated Flow: The industry standard for enterprise SSO. The IdP sends a signed SAML Assertion to the SP, which typically uses server-to-server calls to fetch IdP metadata and validate the assertion.
• JWT-Based SSO: A lighter alternative where the IdP issues a signed JWT. The SP can either validate the signature locally (with a pre-stored key) or fetch the latest public keys from the IdP's JWKS endpoint via server-to-server communication.

内容的提问来源于stack exchange，提问作者Ranjit Singh