1. A decryption apparatus comprising: a rewritable non-volatile memory; an input unit which inputs encrypted data; a decryption unit which decrypts the input encrypted data to decrypted data using a first encryption key; a division unit which divides the decrypted data into a decrypted data block which has a size as an integer multiple of a bank size of said rewritable non-volatile memory, and authentication data; a comparison data generation unit which generates comparison data using a second encryption key for the decrypted data block; a determination unit which determines authenticity of the decrypted data block by comparing the comparison data with the authentication data; and a write unit which writes the decrypted data block, which is determined to be authentic, in said rewritable non-volatile memory; and a re-sending request unit which sends, when it is determined that the decrypted data block is unauthentic, a re-sending request of the encrypted data corresponding to only the unauthentic decrypted data block.