export declare function save(key: string, value: string | null): void;
export declare function load(key: string): string | null;
export declare function once(key: string): boolean;
export declare function removeItem(key: string): void;
export declare class PreferenceKey {
    static PN_OPEN_SET_PAYMENT_PASSWORD: string;
    static PN_OPEN_SET_MASTER_PASSWORD: string;
}