Patent ID: 8190874

Claim:
A data transmission system comprising: a server; and a terminal which communicates with said server, said server comprising: an arithmetic processing unit; a key storage unit for storing a plurality of digital keys; and a memory, wherein the arithmetic processing unit is configured to execute a program loaded into the memory, the program comprising the following processes: a key generation process for generating a key having a limited valid period, outside of which said key is expired; an encryption process for reading out, from the plurality of keys stored in said key storage unit, a set of plural unexpired keys, and encrypting unencrypted data with each of the keys in the set of plural unexpired keys to generate a plurality of encrypted data, wherein each encrypted data in the plurality represents the unencrypted data encrypted using one of the keys in the set of plural unexpired keys, wherein said encryption process concatenates the plurality of encrypted data, and adds to a start of the resultant concatenation, a header containing encrypted-data start information serving as information representing a start position of each encrypted data to thereby generate concatenated encrypted data, a key transmission process for transmitting in advance, to said terminal, a key from the set of plural unexpired keys; and an encrypted-data transmission process for transmitting, to said terminal, the concatenated encrypted data generated by said encryption unit, and said terminal comprising a processor configured to decrypt the encrypted data transmitted from said server with the key transmitted in advance from said server, wherein said processor decrypts, on the basis of the encrypted-data start information contained in the concatenated encrypted data, encrypted data corresponding to the key transmitted in advance from said server out of the plurality of encrypted data concatenated as the concatenated encrypted data.