Audio CoFIP (contents fingerprinting) robust against collusion attack

Digital fingerprinting method allows individualization of all content purchased by users. In a multi-cast network, however, distributed contents must be identical among all users. Therefore, the process of individualizing content should be given on the user side while the original signal should not be delivered to users for security reasons related to copyright protection. Content fingerprinting (CoFIP) for audio signals was proposed to meet both requirements. In CoFIP, a problem might occur when users collude. Synchronous addition of several individualized audio signals makes it difficult to identify each individualized contents composed in it. This article proposes a countermeasure against such collusion, providing imperceptible phase shift in the individualization. Listening tests and computer simulations show that the fingerprint can be detected correctly while the sound quality of the colluded signal is effectively degraded.