A system and method to watermark a compressed content encrypted by at least one content key, said content key as well as pre-marking data forming Conditional Access System (CAS) data, said CAS Data being encrypted by a transmission key and comprising at least one signature to authenticate all or part of the CAS Data, said compressed content being received by a client device comprising: a Descrambler having an input to receive the encrypted compressed content and an output to produce an compressed content, a Watermark (WM) inserter directly connected to the output of the Descrambler, said Descrambler and said WM inserter being connected with a Conditioner, said Conditioner executing the following steps: receiving the CAS Data, decrypting the CAS Data with the transmission key, verifying the signature of the CAS Data, and if the signature is valid, transferring the content key to the descrambler and the pre-marking data to the WM inserter, and watermarking by the WM inserter, the decrypted content received by the descrambler using the pre-marking data and the identifier.