Patent ID: 8731196

Claim:
A decrypting apparatus for decrypting cryptography data included in a packet, comprising: a receiver for receiving a plurality of packets transmitted from an encrypting apparatus that executes an encrypting process on the packets; a key generator for generating a key for decrypting one of the packets, the key being the same as a key used for the encrypting process to encrypt the one packet; and a decrypting section for decrypting cryptography data included in the one packet received by the receiver with using the key generated by the key generator, wherein the one packet received by the receiver includes packet information to be used by the key generator for generating the key, the packet information being information for uniquely identifying the one packet relative to the others of the plurality of packets, and the key generator encrypts the packet information and uses the encrypted packet information in generating the key, the decrypting apparatus further comprising: a random number generator for generating random numbers with using the key generated by the key generator as a seed, wherein the decrypting section performs an XOR operation on the random numbers generated by the random number generator and the cryptography data included in the one packet, wherein: the encrypting apparatus transmits a first packet, . . . , an (M−1)-th packet, an M-th packet, . . . , and an N-th packet (1<M<N, N≧2) to the decrypting apparatus, the decrypting apparatus further comprising a random sequence setting section for setting a random sequence based on the random numbers generated by the random number generator according to a total of a number of communication data from the first packet to the (M−1)-th packet, and the decrypting section performs the XOR operation on the random sequence set by the random sequence setting section and the cryptography data included in the N-th racket.