In order to perform cross-origin code exchange, a proof key is required. This proof key helps protect against certain types of security threats like CSRF (Cross-Site Request Forgery). The proof key is typically generated and verified by both the client (request sender) and the server (request receiver) to establish trust and prevent unauthorized access to sensitive data.
One common implementation of proof key for code exchange is the OAuth 2.0 authorization code flow with PKCE (Proof Key for Code Exchange). PKCE is an extension to OAuth 2.0 that adds an extra step to the authorization process.
Here’s a brief example of how it works:
- The client generates a random secret code called the “code verifier”. This code is typically a random string with a high entropy.
- The client creates a “code challenge” derived from the code verifier. This is usually done by applying a cryptographic transformation, like SHA-256 hashing, to the code verifier.
- The client initiates the authorization process by sending a request to the authorization server, including the code challenge.
- The authorization server validates the code challenge and redirects the user to a login page if necessary. After the user authenticates, the server issues an authorization code.
- The client exchanges the received authorization code for an access token and/or ID token by including the original code verifier in the token request. The server verifies the code verifier to ensure its authenticity.
This additional step adds a layer of security by preventing an attacker from intercepting the authorization code and using it to obtain unauthorized access. Since the code challenge is derived from the code verifier and sent in the initial authorization request, the server can verify the authenticity of the subsequent token request using the original code verifier.