Traitor tracing scheme with constant ciphertext rate against powerful pirates

Traitor tracing schemes are used to fight piracy when distributing securely some data to multiple authorized receivers: if some receivers collude and share their decryption keys to build some pirate decoder, a tracing procedure should be able to find at least one of these “traitors” from the pirate decoder. In this paper, we consider powerful pirate decoders, which may sometimes refuse to decrypt, or may try to detect when the tracing procedure is running. Most known traitor tracing schemes are not secure against this kind of pirate decoders: this is in particular the case of the schemes with constant ciphertext rate, which are the most efficient ones. We build then a new scheme, with constant ciphertext rate and security against powerful pirate decoders, using watermarking techniques. This scheme has the interesting feature that a receiver may decrypt the ciphertexts progressively, when it was not possible in previous schemes with constant ratio between ciphertext and plaintext.