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% please enter your group number your names and matriculation numbers here
\newcommand{\groupnumber}{04}
\newcommand{\name}{Tobias Eidelpes, Mehmet Ege Demirsoy, Nejra Komic}
\newcommand{\matriculation}{01527193, 01641187, 11719704}

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\newcommand{\Title}{Smart Contracts}
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\fancyhead[L]{Cryptocurrencies - Project \projnumber\ - Analysing the Blockchain}
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\begin{document}
\thispagestyle{empty}
\noindent\framebox[\linewidth]{%
 \begin{minipage}{\linewidth}%
 \hspace*{5pt} \textbf{Cryptocurrencies (WS2021/22)} \hfill Prof.~Matteo Maffei \hspace*{5pt}\\

 \begin{center}
  {\bf\Large Project \projnumber~-- \Title}
 \end{center}

 \vspace*{5pt}\hspace*{5pt} \hfill TU Wien \hspace*{5pt}
\end{minipage}%
}
\vspace{0.5cm}
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\section*{Group \groupnumber}
Our group consists of the following members:
\begin{center}
\textbf{\name}

\matriculation
\end{center}

\section{Bad Parity}

For this challenge we were given two contracts: \texttt{Wallet} and
\texttt{WalletLibrary}. The second contract is used by the \texttt{Wallet}
contract to set the owner upon initialization, to get the current owner, to
change the owner and to withdraw funds from the wallet. These functions are
called from the \texttt{Wallet} contract through the use of the
\texttt{delegatecall} function. In contrast to a regular \texttt{call},
\texttt{delegatecall} executes the function in the context of the \emph{calling}
smart contract. This means that if there happens to be a variable in both
contracts with the same name and a function changes that variable, the
\emph{caller's} and not the \emph{callee's} variable is changed. If insufficient
care is exercised during programming, the semantics of \texttt{delegatecall} can
have serious security implications, as in this case with \texttt{Wallet} and
\texttt{WalletLibrary}.

The \texttt{fallback} function in \texttt{Wallet} is called when the smart
contract receives a transaction with empty call data or call data which does not
match any other function. The call data sent with the transaction is then passed
to the \texttt{WalletLibrary} contract via \texttt{delegatecall}. The
\texttt{WalletLibrary} contract has a function called \texttt{initWallet} which
sets the owner of the contract to the given address. Usually this function would
be called only upon initialization of the contract (in the constructor for
example). We can call this function at any time by supplying the correct call
data to the \texttt{fallback} function from the \texttt{Wallet} contract. Since
the function is then called via \texttt{delegatecall}, the owner of the
\texttt{Wallet} contract is changed to an address of our choosing.

To trigger the \texttt{initWallet} function, the call data must contain the
signature of the function and all parameters. The function signature is the
first four bytes of the keccak hash of the function name and the types of its
parameters. Any parameters are added to the signature in a padded form. Creating
the call data in python works as follows (where \texttt{address} is the address
of the new owner):
\begin{minted}[frame=lines,framesep=2mm,bgcolor=LightGray,fontsize=\footnotesize,linenos]{python}
sig = w3.keccak(text='initWallet(address)')[:4].hex() + address[2:].rjust(64, '0')
# sig = 0x9da8be21000000000000000000000000f9ac06BAeb6597511C22Dc7b03DA447cA893fb4e
\end{minted}

We can then send this call data to the contract (via the geth console):
\begin{minted}[frame=lines,framesep=2mm,bgcolor=LightGray,fontsize=\footnotesize,linenos]{python}
eth.sendTransaction({
  from: student,
  to: badparityAddress,
  data: "0x9da8be21000000000000000000000000f9ac06BAeb6597511C22Dc7b03DA447cA893fb4e", 
  gas: "80000"
});
\end{minted}
The owner of the \texttt{Wallet} contract is now our own address. Since we are
the owner, we can call the \texttt{withdraw} function from the \texttt{Wallet}
contract:
\begin{minted}[frame=lines,framesep=2mm,bgcolor=LightGray,fontsize=\footnotesize,linenos,breaklines]{python}
sig = w3.keccak(text='withdraw(uint256)')[:4].hex() + hex(30000000000000000000)[2:].rjust(64, '0')
# sig = 0x2e1a7d4d000000000000000000000000000000000000000000000001a055690d9db80000
eth.sendTransaction({
  from: student,
  to: badparityAddress,
  data: "0x2e1a7d4d000000000000000000000000000000000000000000000001a055690d9db80000", 
  gas: "80000"
});
\end{minted}
Our own balance has increased by 30 Ether.

To mitigate this vulnerability the contract should use \texttt{call} instead of
\texttt{delegatecall}.

\section{DAO Down}
% Fill here your answers for exercise B

\section{Fail Dice}
% Fill here your answers for exercise C

\section{Not A Wallet}
% Fill here your answers for exercise D

\section*{Work distribution}
%Fill in here an overview on which group member participated in which task and to which extent

\end{document}

