/**
 * PKCE (RFC 7636) helpers for the authorization-code flow with S256.
 */
export declare function base64url(bytes: Uint8Array): string;
/**
 * Generate a high-entropy `code_verifier` (43 chars, url-safe).
 */
export declare function generateCodeVerifier(): string;
/**
 * Derive the S256 `code_challenge` for a `code_verifier`.
 */
export declare function deriveCodeChallenge(verifier: string): Promise<string>;