The present invention relates to a method, a device and a system for preventing unauthorized introduction of content items in a network containing compliant devices and enabling users in the network to be anonymous. A basic idea of the present invention is to provide a CA (206) with a fingerprint of a content item to be introduced in a network at which the CA is arranged. Further, the CA is provided with an identifier of a content introducer (201), which introduces the particular content item in the network. The CA compares the fingerprint to a predetermined set of fingerprints, and content item intro not duction is allowed if the content itemfingerprint cannot be found among the fingerprints comprised in the set. On introduction of the content item, the CA generates a pseudonym for the content introducer and creates a signed content ID certificate comprising at least said fingerprint and a unique content identifier for the content item and the pseudonym of the content introducer.