Common Scrambling Algorithm

The Common Scrambling Algorithm (or CSA) is the encryption algorithm used in the DVB digital television broadcasting for encrypting video streams.

CSA was specified by ETSI and adopted by the DVB consortium in May 1994. It is being succeeded by CSA3, based on a combination of 128-bit AES and a confidential block cipher, XRC. However, CSA3 is not yet in any significant use, so CSA continues to be the dominant cipher for protecting DVB broadcasts.

CSA was largely kept secret until 2002. The patent papers gave some hints, but important details, like the layout of the so-called S-boxes, remained secret. Without these, free implementations of the algorithm were out of question. Initially, CSA was to remain implemented in hardware only, and this would have made it difficult to reverse engineer existing implementations.

In 2002 FreeDec was released, implementing CSA in software. Though released as binary only, disassembly revealed the missing details and allowed reimplementation of the algorithm in higher-level programming languages.

With CSA now publicly known in its entirety, cryptanalysts started looking for weaknesses.

The CSA algorithm is composed of two distinct ciphers: a block cipher and a stream cipher.

When used in encryption mode the data are first encrypted using the 64 bits block cipher in CBC mode, starting from packet end. The stream cipher is then applied from packet start.

The block cipher process 64 bits blocks in 56 rounds. It uses 1 byte from expanded key on each round.

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