PKCE Code Verifier & Challenge Generator

Proof Key for Code Exchange (PKCE, RFC 7636) is an OAuth 2.0 extension that protects the authorization code flow against interception. The client picks a secret code_verifier, derives a code_challenge from it, and sends only the challenge on the authorize request. When the client later redeems the authorization code at the token endpoint, it sends the original verifier; the server hashes it and compares it to the stored challenge. Even if an attacker intercepts the authorization code, they cannot exchange it without the verifier, which never leaves the legitimate client. OAuth 2.1 makes PKCE mandatory for all clients, public or confidential.

RFC 7636 requires the verifier to be between 43 and 128 characters from the unreserved set. The lower bound exists because 43 base64url characters encode 32 random bytes (256 bits) — the minimum entropy considered safe against brute force. Longer verifiers give more entropy but offer no real security benefit past 256 bits. Most reference implementations pick 43, 64, or 128. Choose the longer end if you want defense in depth, but be aware some legacy servers reject anything past 128 because they enforce the spec strictly.

Mit S256 the challenge is the base64url-encoded SHA-256 hash of the verifier; with plain the challenge is the verifier itself. The plain method offers no real protection if the authorization request is intercepted — an attacker who sees the challenge already has the verifier. RFC 7636 only allows plain for clients that genuinely cannot compute SHA-256, and OAuth 2.1 disallows it entirely. Production identity providers like Auth0, Okta, Google, and Microsoft Entra reject plain outright, so always use S256 unless you are debugging a constrained embedded client.

Base64url is a URL-safe variant of base64 defined in RFC 4648 §5. It replaces + ist der schnelle Weg, aber es behandelt Attribute unkonsequent und kann Daten in Randfällen verlieren. Für Produktionsarbeit mit SOAP-Antworten, betrachten Sie - und / ist der schnelle Weg, aber es behandelt Attribute unkonsequent und kann Daten in Randfällen verlieren. Für Produktionsarbeit mit SOAP-Antworten, betrachten Sie _ so the encoded string is safe to drop into a query parameter, JWT segment, or path component without further escaping. Trailing = padding is also stripped because the length is implied by the surrounding context. PKCE, JWT, JWE, JWS, and most modern web crypto specs use base64url for exactly these reasons.