Common Scrambling Algorithm

The common scrambling algorithm (CSA ), the encryption method which is used in the digital television DVB to encrypt the video data stream.

CSA was kept secret for several years. Some notes about the patent came into the public spotlight, important details remained secret, for example, the construction of the so-called S-boxes. Without these details, a free implementation of the algorithm was not possible. CSA was originally to be implemented only in hardware, which it seemed impossible to determine the necessary details by reverse engineering of existing implementations, such as Conditional Access Module (CAM).

In 2002, a program appeared called FreeDec, which implemented the CSA in software. The program was only as a binary version (also: Executable) available. Hackers disassembled the software and thus determined the missing details. This has made it possible to realize an implementation of CSA in a high level language.

Since then, the algorithm for CSA is fully known, cryptanalyst looking for weak points in the process. As with other encryption methods, a point of attack is produced in that part of the plaintext is assumed to be known or at least as very probable (for example, MPEG header ). From the length of the key ( here: Control Word) 64-bit result 264 possibilities of encryption. If one were to try all possible keywords with the help of a computer and this will need 1 microseconds for each attempt lasted Search over 500,000 years. The assumption of certain plaintext bytes, conclusions can be drawn on the key used to reduce the total number of possible keys clearly.

Under the current state of cryptanalysis of the CSA is vulnerable and at least sequentially decodable by rainbow table. However, this is due to the artificially shortened key length of 48 bits instead of 64 bits, and provided to the slow, cyclical key changes that are predictable. Therefore, the use of the full 64 -bit key length and the subsequent migration to AES is recommended. A similar program to calculate the rainbow table has also been published.