We propose a stream cipher that provides confidentiality, traceability and renewability in the context of broadcast encryption
assuming that collusion-resistant watermarks exist. We prove it to be as secure as the generic pseudo-random sequence on which
it operates. This encryption approach, termed fingercasting, achieves joint decryption and fingerprinting of broadcast messages
in such a way that an adversary cannot separate both operations or prevent them from happening simultaneously. The scheme
is a combination of a known broadcast encryption scheme, a well-known class of fingerprinting schemes and an encryption scheme
inspired by the Chameleon cipher. It is the first to provide a formal security proof and a non-constant lower bound for resistance
against collusion of malicious users (i.e.) a minimum number of content copies needed to remove all fingerprints. To achieve
traceability, the scheme fingerprints the receivers’ key tables such that they embed a fingerprint into the content during
decryption. The scheme is efficient and includes parameters that allow, for example, to trade-off storage size for computation
cost at the receiving end.
Keywords Chameleon encryption - stream cipher - spread-spectrum watermarking - fingerprinting - collusion resistance - frame-proofness - broadcast encryption
An extended abstract of this paper appeared in the Proceedings of the Tenth Australasian Conference on Information Security
and Privacy (ACISP 2006) [1].