export interface DecodedJwtClaims {
    exp: number;
    iat: number | null;
}
/**
 * Validates JWT token structure, header (presence of alg), and parses the expiration and issued-at claims.
 * Does not verify signature or issuer.
 * @returns The exp and iat claims (iat is null if absent), or null if invalid structure
 */
export declare function validateJwt(token: string): DecodedJwtClaims | null;