An apparatus and method that reuse a pair of public and private keys. The method includes determining whether a pair of public and private keys that have already been used in a first encryption process are still usable; and reusing the pair of public and private keys in a second encryption process if the pair of public and private keys are determined as being reusable. Accordingly, it is possible to considerably reduce the amount of computation and time that is generally required to calculate a pair of public and private keys, by allowing the pair of public and private keys to be reused.