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introduction.tex
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introduction.tex
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\chapter{Introduction}
WISI Norden AB, previously A2B Electronics, is a Swedish company
founded in 1997. The company is a developer of head-end cable-TV
distribution systems. WISI Norden develops and designs both hardware
and software, with the purpose of providing Digital TV solutions.
The purpose of this thesis has been to find a replacement to the
currently implemented scrambler, located in the head-end solutions. The
previous scrambler needed to be replaced, since it was designed in
1994 and was supposed to last for ten years. The scrambler is used to
render the digital television streams unreadable if the user does not
subscribe to the encoded channels.
The task was to evaluate and analyze a few potential scrambling
algorithms, and then choose one algorithm to implement in WISI
Norden's devices.
\section{Background}
The formerly used \emph{common scrambling algorithm} (CSA) has due to
recent progresses in television broadcasting become obsolete. CSA was
designed to make software descrambling hard, if possible, while making
hardware descrambling fast.
There are two suggested replacements of CSA. The first one is named
after the CSA, and is called CSA3. The reason as to why it is not
called CSA2 is since there already exists an algorithm which is
called CSA2. CSA2 is the same algorithm as CSA, just with a different
key-length. The second algorithm is the software-friendly descrambling
algorithm CISSA. Both of them are based on the public Advanced
Encryption Standard - 128 (commonly known as the AES-128). There are
three versions of the AES, with varying numbers. The number depicts
what key-length the AES uses.
WISI Norden wanted to evaluate the replacement algorithms, even
though the CSA is still used in the DVB world. This was done to make
sure that there was an alternative to the CSA, when other companies
would start to switch scrambling methods. WISI Norden has also had some
requests to implement other scrambling methods from clients.
\section{Problem specification}
The task was to analyze the possible replacements for the common
scrambling algorithm, and decide which one was the most suitable
replacement. After choosing an algorithm, that algorithm was to be
implemented from scratch, making decisions to minimize the hardware
usage while achieving the frequency used by the rest of the system.
The decisions made were to be motivated either through simulations or
reference litterature.
There were two proposed replacements to be compared and analyzed
to find what made one of them software-friendly and the other one
hardware-friendly.
\section{Constraints}
The thesis has been limited to implement the scrambing algorithm
chosen in consent between the author and the supervisor at WISI
Norden. The algorithm that was chosen, after analysis of the two
proposed algorithms, was the AES128 algorithm in CBC-mode (chapter
\ref{sec:BlockCipher}) with a set IV, according to the CISSA standard.
This corresponds to the CISSA algorithm \citep{DVB:2013}.The
implementation is focused on minimizing the hardware usage, while
achieving a throughput of at least 1 Gbits/s.
\section{Methodology}
The project was split into a set of tasks, to be performed in the
order written below. Performing the tasks in this order was done to
decrease the complexity of the seperate tasks.
\begin{itemize}
\item Litterature study
\item Choosing an algorithm
\item Design and test of entities
\item Implementation
\item Optimization
\end{itemize}
To gain some knowledge about cryptography, a litterature study was
first conducted. This provided some insight into what the strenghts
and weaknesses of the algorithms actually were. The AES cipher was
chosen as an initial algortihm, since both of the proposed algorithms
used the AES as a base. The other parts of the cipher, which ever was
chosen, were to be added after the AES was finished. Using the
gathered background information about how the algorithm worked made
design and testing of the entities rather easy. The lower level
entities were designed first, which allowed for easier testing of
seperate parts of the system. Knowing that the functionality, of low
level entities, was already present allowed for easier merging of
entities. This led to the system being implemented through bottom-up
design.