Private Key JWT Authentication

Private Key JWT (JSON Web Token) Authentication is a method where a client uses a private key to sign a JWT that authenticates it to a server or an API. This approach is commonly used in OAuth 2.0 and OpenID Connect scenarios, especially for securely authenticating machine-to-machine (M2M) and server-to-server interactions.

How It Works

  1. Client Creates and Signs a JWT:
    • The client generates a JWT that includes claims (information) about the request, such as the client ID, issuer, audience (the server or API it wants to access), subject (the identity of the client), expiration time, and a unique identifier for the token.
    • The client signs the JWT using its private key, ensuring it cannot be tampered with and can only be verified by a server with the corresponding public key.
  2. Client Sends the JWT to the Server:
    • The client includes the signed JWT in the authentication request to the server (often an authorization server or an API).
    • This is typically done by specifying the client_assertion (the JWT) and client_assertion_type (usually "urn:ietf:params:oauth:client-assertion-type:jwt-bearer") parameters in the request.
  3. Server Verifies the JWT:
    • The server retrieves the public key associated with the client (e.g., from a trusted certificate authority or a pre-shared key setup).
    • The server verifies the JWT’s signature using this public key, ensuring the token was indeed issued by the client and hasn’t been altered.
    • If the JWT passes verification and all claims are valid (e.g., the token hasn’t expired, the audience is correct), the server authenticates the client.
  4. Server Grants Access:
    • Once the JWT is verified, the server grants access to the requested resources or issues an access token for use with further requests.

Where Private Key JWT Authentication is Used

  • OAuth 2.0 Client Authentication: Commonly used in OAuth 2.0 scenarios where clients (like backend services, microservices, or M2M applications) need to securely authenticate to an authorization server.
  • OpenID Connect (OIDC): Used as an alternative to client secrets for authenticating clients in OIDC flows, especially useful for confidential clients (e.g., web applications, APIs) where storing a static client secret might not be secure or feasible.
  • M2M and B2B Applications: Frequently used in machine-to-machine and business-to-business applications where services and APIs need to authenticate to each other without involving user interaction.
  • Secure Environments: Suitable for secure environments that require strong authentication and where sensitive data is exchanged, such as financial services, healthcare, and government APIs.

This method is advantageous because it’s more secure than client secrets (passwords) and is suited to situations where a client cannot be easily modified but can safely store a private key (e.g., in hardware security modules or dedicated storage).