/**
 * Validates a VAPID subject
 * @param subject - Subject string to validate
 * @throws Error if subject is invalid
 */
export declare function validateSubject(subject: string): void;
/**
 * Validates a VAPID public key
 * @param publicKey - Public key to validate
 * @throws Error if public key is invalid
 */
export declare function validatePublicKey(publicKey: string): void;
/**
 * Validates a VAPID private key
 * @param privateKey - Private key to validate
 * @throws Error if private key is invalid
 */
export declare function validatePrivateKey(privateKey: string): void;
/**
 * Calculates a future expiration timestamp
 * @param numSeconds - Number of seconds to add to current time
 * @returns Future expiration in seconds since epoch
 */
export declare function getFutureExpirationTimestamp(numSeconds: number): number;
/**
 * Validates that an expiration time is valid
 * @param expiration - Expiration timestamp in seconds since epoch
 * @throws Error if expiration is invalid
 */
export declare function validateExpiration(expiration: number): void;