CerberAuth
Authentication Methods in OpenID Connect and OAuth 2.0
OpenID Connect (OIDC) supports a variety of mechanisms for authenticating clients to its endpoints, including the Token Endpoint, Introspection Endpoint, and Revocation Endpoint. These mechanisms ensure that the client, whether a confidential or public application, can securely prove its identity to the OpenID Provider (OP). The choice of authentication mechanism depends on the security requirements and the capabilities of the client. Below is a comprehensive explanation of the authentication mechanisms supported by OpenID Connect.
Basic Authentication
One of the simplest and most widely supported authentication mechanisms in OIDC is HTTP Basic Authentication. In this method, the client ID and client secret are combined into a single string, separated by a colon, and then Base64-encoded. The resulting string is included in the Authorization header of the HTTP request.
For example, the header would look like this:
POST /token HTTP/1.1
Host: server.example.com
Authorization: Basic <Base64(client_id:client_secret)>
This method is primarily used when interacting with the Token Endpoint, Introspection Endpoint, and Revocation Endpoint. While it is easy to implement and compatible with most systems, its security heavily relies on the use of HTTPS to encrypt the transmitted credentials.
Post Authentication
In situations where Basic Authentication is not supported or preferred, the client credentials can be sent in the body of the HTTP request. The client includes the client_id and client_secret as form parameters, along with other required parameters for the endpoint being accessed.
For example, the request body might include:
POST /token HTTP/1.1
Host: server.example.com
Content-Type: application/x-www-form-urlencoded
grant_type=client_credentials&client_id=<client_id>&client_secret=<client_secret>
This method offers functional parity with Basic Authentication but is generally considered less elegant. It is supported by the same endpoints: Token, Introspection, and Revocation.
Private Key JWT Authentication
Private Key JWT is a robust authentication method that enhances security by eliminating the need to transmit a static client secret. Instead, the client creates a JSON Web Token (JWT) signed with its private key. This JWT is sent to the OpenID Provider, which verifies the signature using the corresponding public key.
The JWT contains several claims, including:
iss(issuer): The client ID.sub(subject): The client ID.aud(audience): The token endpoint URL.exp(expiration time): The token's validity period.
This method is particularly useful for clients that need a high level of security, as it avoids the risks associated with static secrets. It is commonly used with the Token Endpoint.
Mutual TLS (mTLS) Authentication
Mutual TLS (mTLS) is a client authentication mechanism where the client presents an X.509 certificate during the TLS handshake. The OpenID Provider validates the certificate to authenticate the client. This mechanism provides strong security by leveraging cryptographic identities and ensures that the communication channel is secure.
mTLS is typically used for high-security applications and can also bind tokens to the client certificate, preventing token misuse if intercepted. It is supported by the Token, Introspection, and Revocation endpoints.
Client Assertion Authentication
Similar to Private Key JWT, Client Assertion Authentication allows a client to authenticate by presenting a signed assertion. However, instead of relying on a private key, the assertion might use other credentials such as client secrets. The assertion is sent as part of the token request and contains claims similar to those in Private Key JWT.
This mechanism is highly flexible and is particularly useful in federated or multi-party systems where traditional client credentials may not be suitable.
No Client Authentication (Public Clients)
For public clients, such as single-page applications or mobile apps, client authentication may not be required. These clients typically do not use confidential credentials and instead rely on flows like the Authorization Code flow with PKCE (Proof Key for Code Exchange). While the client itself is not authenticated, mechanisms like PKCE ensure the security of the authorization process.
