BUILD

PKCE

Proof Key for Code Exchange protects the authorization code when the browser cannot keep a client secret.

S256 outline

const verifier = createHighEntropyRandomString();
const challenge = base64url(sha256(verifier));
// Send challenge with /oauth/authorize. Keep verifier server-side
// until the matching /oauth/token exchange.

Do

Use S256.
Generate a new verifier per request.
Keep the verifier secret until token exchange.
Reject a response whose state does not match.