Code Verifier (CloudMonk.io)
Code Verifier
The code verifier is a critical component of the Proof Key for Code Exchange (PKCE) protocol, which is defined in RFC 7636. PKCE was introduced as an enhancement to the OAuth 2.0 authorization code flow, specifically to protect against authorization code interception attacks. The code verifier serves as a dynamically generated secret that is used by the client to prove to the authorization server that it is the legitimate initiator of the authorization request. By including the code verifier in the token exchange process, the client ensures that an attacker who intercepts the authorization code cannot use it to obtain an access token without the original verifier.
The code verifier itself is a cryptographically random string that is generated by the client at the start of the authorization process. According to RFC 7636, the code verifier must be a string of 43 to 128 characters, consisting of characters from the unreserved set, which includes letters, digits, hyphens, periods, underscores, and tildes. This randomness and length ensure that the code verifier has sufficient entropy to be secure against brute force attacks. The string is generated once per authorization request and must be kept secret by the client.
Once the code verifier is generated, the client creates a corresponding code challenge, which is derived from the code verifier. Typically, the code challenge is the result of hashing the code verifier using the SHA-256 algorithm. This hashed value is then sent to the authorization server along with the initial authorization request. The authorization server stores the code challenge for later comparison, but it does not see the code verifier at this stage. This is a key security feature of PKCE—the code verifier remains on the client side until the token exchange.
During the token exchange phase, after the client has received the authorization code from the authorization server, the client sends the original code verifier back to the server as part of the token request. The authorization server hashes the code verifier using the same SHA-256 algorithm (or compares it directly if no hashing was used) and checks if it matches the stored code challenge. If the two values match, the server issues the access token to the client. If the values do not match, the server denies the token request, preventing an attacker from using a stolen authorization code without the corresponding code verifier.
The use of the code verifier in conjunction with the code challenge is what makes PKCE highly effective at preventing authorization code interception attacks. Even if an attacker intercepts the authorization code during the redirect from the authorization server to the client, they would not be able to successfully exchange the code for an access token without the original code verifier, which was never transmitted over the network.
One of the primary benefits of using the code verifier is that it removes the need for public clients (such as mobile apps or single-page applications) to store a client secret. Public clients cannot securely store secrets due to their deployment environments, where reverse engineering or inspection could reveal hardcoded secrets. PKCE, with its code verifier mechanism, allows these clients to securely use the authorization code flow without relying on client secrets, making it the recommended best practice for public clients.
The introduction of the code verifier in RFC 7636 has significantly improved the security of OAuth 2.0 implementations. Without the code verifier, an attacker who intercepts an authorization code could easily exchange it for an access token. With PKCE, the attacker is thwarted unless they also possess the dynamically generated code verifier, which is only known to the legitimate client.
In terms of implementation, generating a code verifier is straightforward for developers. Many modern libraries that support OAuth 2.0 include built-in functions to generate secure random strings that comply with the requirements of RFC 7636. Similarly, generating the code challenge from the code verifier is often handled automatically, making the implementation of PKCE relatively easy for developers to integrate into existing authorization workflows.
PKCE has now become a standard feature of OAuth 2.0 implementations across major platforms, including Google, Facebook, and GitHub. These platforms encourage or require developers to use PKCE in their applications to prevent unauthorized access to user resources. The simplicity and security provided by the code verifier and code challenge mechanism have made it a vital tool for securing public clients in modern application environments.
Conclusion
The code verifier, as defined in RFC 7636, is a core component of the PKCE protocol, which enhances the security of the OAuth 2.0 authorization code flow. By using a dynamically generated code verifier and code challenge, clients can prove their identity during the token exchange, preventing attackers from using intercepted authorization codes to gain unauthorized access. The introduction of the code verifier has significantly improved the security of public clients, where storing client secrets securely is not feasible. As a result, PKCE has become the recommended approach for securing OAuth 2.0 implementations across various platforms and applications.