Unconditional security techniques may represent an advantageous alternative to computational security techniques for smart card systems providing authentication or message authentication when the number of authentications needed during the lifetime of each key is limited. In this paper we propose simple algorithms based on modulo 2 scalar products among binary vectors which are easy to implement in so-called synchronous cards, require a reasonable amount of secret key bits (typically 10 to 20 bits per authentication), and provide provably secure authentication and message authentication. Applications mainly include low cost prepaid cards for telephone, parking meters, transportation, automatic vending machines, etc.