A decrypting apparatus for decrypting cryptography data included in a packet includes a receiver, a key generator, and a decrypting section. The receiver receives a packet transmitted from an encrypting apparatus that executes an encrypting process. The key generator generates a key used for the encrypting process. The decrypting section decrypts cryptography data included in the packet received by the receiver with using the key generated by the key generator. In the decrypting apparatus, the packet received by the receiver includes packet information used for generating the key. The key generator generates the key with using the packet information.