Abstract: The present invention relates to a data communication system that prevents a timing at which digital content data is made usable or a timing at which a billing process is performed from being known by a malicious client, to an information processing device and method therefor, to a recording medium therefor, and to a program therefor. A client terminal encrypts a signal TAG1 and a random number NON1 in a linked manner and transmits these to a content sales server. The content sales server decrypts the received encrypted data sequence and sends back the random number NON1 contained in the decrypted result to the client terminal. The client terminal confirms that the random number NON1 is contained in this response. Then, after this series of processes is repeated a random number of times, the digital content is set to the effective state. The present invention can be applied to services which sell digital content via the Internet.