PKCE S256: Minimum code_verifier length enforcement in OAuth 2.0 authorization servers

The Required Minimum Length

Authorization servers should enforce a minimum length of 43 characters for the code_verifier. This is not merely a recommendation but a specification requirement defined in RFC 7636.

Cryptographic Rationale

The 43-character minimum is designed to ensure sufficient entropy to resist brute-force guessing attacks. The code_verifier uses a set of 65 unreserved characters (alphanumeric plus -, ., _, and ~). A 43-character string provides approximately 263 bits of entropy ($\log_2(65^{43})$), which far exceeds the security margin required to prevent an attacker from guessing the verifier after intercepting the authorization code.

Implementation Guidance

To maintain compliance with the PKCE standard and ensure security, servers should implement the following constraints during the token exchange phase:

• Minimum Length: Reject any code_verifier shorter than 43 characters.
• Maximum Length: Reject any code_verifier longer than 128 characters to prevent potential Denial of Service (DoS) attacks via oversized request bodies.
• Character Validation: Ensure the string contains only the allowed unreserved characters.

Verification Steps

To verify that your authorization server is correctly enforcing these bounds, perform the following tests at the token endpoint:

1. Under-length Test: Submit a code_verifier of 42 characters. The server must return an invalid_request or invalid_grant error.
2. Boundary Test: Submit a valid 43-character string that matches the previously stored code_challenge. The server should accept the request.
3. Over-length Test: Submit a code_verifier exceeding 128 characters. The server should reject the request.

Assumption and Diagnostic Note

This guidance assumes the use of the S256 challenge method. If your server still supports the plain method, the same length requirements apply, although the security benefit is significantly lower. To provide a more tailored recommendation, please specify if your server is intended to support legacy clients that predate RFC 7636.