codeverifier

A code verifier is a mechanism used in certain authentication and authorization protocols, most notably OAuth 2.0, to mitigate the threat of authorization code interception. Its primary function is to ensure that the client application that initiated an authorization request is the same client application that receives the authorization code and subsequently exchanges it for an access token.

The process begins when a client application requests authorization from an authorization server. As part of this request, the client generates a secret string called a code verifier and a corresponding transformed string called a code challenge. The code challenge is then sent to the authorization server along with the authorization request. The authorization server stores the code challenge.

Upon successful user authorization, the authorization server redirects the user back to the client application, including a temporary authorization code in the redirect. Crucially, the client application must then make a request to the authorization server's token endpoint to exchange this authorization code for an access token. As part of this token exchange request, the client must also send the original, un-transformed code verifier.

The authorization server then transforms the received code verifier using the same method it used to generate the code challenge initially and compares this transformed value with the stored code challenge. If the values match, the authorization server verifies that the client possesses the correct code verifier, confirming its identity and security. If they do not match, the request is rejected, preventing a malicious actor who may have intercepted the authorization code from obtaining an access token. This adds a critical layer of security to the OAuth 2.0 flow, especially in public client scenarios where the client secret cannot be securely stored.