The Common Scrambling Algorithm (or CSA) is the encryption algorithm used in the DVB digital television broadcasting for encrypting video streams. This article is about algorithms for encryption and decryption. ... DVB, short for Digital Video Broadcasting, is a suite of internationally accepted, open standards for digital television maintained by the DVB Project, an industry consortium with more than 300 members, and published by a Joint Technical Committee (JTC) of European Telecommunications Standards Institute (ETSI), European Committee for Electrotechnical Standardization (CENELEC... Digital television (DTV) refers to the sending and receiving of moving images and sound by means of discrete (digital) signals, in contrast to the analog signals used by analog TV. Introduced in the late 1990s, this technology appealed to the television broadcasting business and consumer electronics industries as offering new... Video coding is the field in computer science that deals with finding efficient coding formats for digital video. ...

CSA was specified by ETSI and adopted by the DVB consortium in May 1994. CSA was largely kept secret until 2002. The patent papers gave some hints, but important details remained secret, like the layout of the so-called S-boxes. Without these, free implementations of the algorithm were out of question. Initially, CSA was to remain implemented in hardware only. This would have made it difficult to reverse engineer existing implementations. The European Telecommunications Standards Institute (ETSI) is a standardization organization of the telecommunications industry (equipment makers and network operators) in Europe, with worldwide projection. ... Year 1994 (MCMXCIV) The year 1994 was designated as the International Year of the Family and the International Year of the Sport and the Olympic Ideal by the United Nations. ... Also see: 2002 (number). ... In cryptography, a substitution box (or S-box) is a basic component of symmetric key algorithms. ... For other uses, see Hardware (disambiguation). ... Reverse engineering (RE) is the process of taking something (a device, an electrical component, a software program, etc. ...

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. Also see: 2002 (number). ... Computer software (or simply software) refers to one or more computer programs and data held in the storage of a computer for some purpose. ... An executable or executable file, in computer science, is a file whose contents are meant to be interpreted as a program by a computer. ... A disassembler is a computer program that translates machine language into assembly language — the inverse operation to that of an assembler. ... A programming language is an artificial language that can be used to control the behavior of a machine, particularly a computer. ...

With CSA now publicly known in its entirety, cryptanalysts started looking for weaknesses. Like in other encryption algorithms a weak spot arises inasmuch that parts of the message are known or at least easily predictable, like MPEG headers. The length of the key of 64 bits allows 2⁶⁴ different possibilities of encryption. A brute force attack taking 1 μs for each try through all possible key words would take around 300,000 years, on average. This can be reduced by using the predictable parts of the encrypted message to rule out potential keys. Cryptanalysis (from the Greek kryptós, hidden, and analýein, to loosen or to untie) is the study of methods for obtaining the meaning of encrypted information without access to the secret information which is normally required to do so. ... The Moving Picture Experts Group or MPEG is a working group of ISO/IEC charged with the development of video and audio encoding standards. ... A key is a piece of information that controls the operation of a cryptography algorithm. ... This article is about the unit of information. ... The EFFs US$250,000 DES cracking machine contained over 1,800 custom chips and could brute force a DES key in a matter of days — the photograph shows a DES Cracker circuit board fitted with several Deep Crack chips. ...

While CSA algorithm uses 64-bit keys, in reality only 48 bits of key are unknown, since bytes 3 and 7 are used as checksum bytes and may be easily recalculated. This fact allows practical space-time tradeoff attack when 32 bits are brute-forced, 16 bits are calculated with memory tables built from ciphertext and 16 bits calculated as checksum with running time O(2¹⁶)+O(2³²) which can be less than a second if implemented in FPGA hardware or on scalable architecture like cell processor.

Were CSA to be broken, encrypted DVB transmissions would be decipherable, regardless of any proprietary conditional access system used. This could seriously compromise paid digital television services, as DVB has been standardised on for digital terrestrial television in Europe and elsewhere, and is used by many satellite television providers. No attack has yet been published, however. Digital Terrestrial Television (DTTV or DTT) is an implementation of digital technology to provide a greater number of channels and/or better quality of picture and sound using aerial broadcasts to a conventional antenna (or aerial) instead of a satellite dish or cable connection. ...

External links

• Analysis of the DVB Common Scrambling Algorithm
• CSA: known facts and speculations

References

• Kai Wirt, Fault attack on the DVB Common Scrambling Algorithm, November 2003, Cryptology ePrint Archive: Report 2004/289 [1].