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Add report for Exercise C

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Tobias Eidelpes 2022-02-08 10:45:47 +01:00
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@ -136,7 +136,42 @@ address even though the timelock is way in the future. \textbf{This exercise is
considered solved if your UTXO for exercise C is spent in a transaction with one considered solved if your UTXO for exercise C is spent in a transaction with one
output, which gives Dave 4000 satoshis as P2PKH.} output, which gives Dave 4000 satoshis as P2PKH.}
\begin{minted}[frame=lines,framesep=2mm,bgcolor=LightGray,fontsize=\footnotesize,linenos]{text}
OP_DUP OP_HASH160
OP_PUSHBYTES_20 <dave_pk_hash>
OP_EQUALVERIFY OP_CHECKSIGVERIFY
OP_IF
OP_PUSHBYTES_3 fbd42f OP_CLTV
OP_DROP OP_DUP OP_HASH160
OP_PUSHBYTES_20 <dave_pk_hash>
OP_ELSE
OP_SHA256
OP_PUSHBYTES_32 <hash_lock>
OP_EQUAL OP_2DUP OP_HASH160
OP_PUSHBYTES_20 <irrelevant_hash>
OP_2ROT OP_DUP OP_DUP
OP_ENDIF
OP_EQUALVERIFY OP_CHECKSIGVERIFY
OP_2DROP OP_DROP OP_NOT
\end{minted}
The script provided to us contains a bug where the \texttt{OP\_EQUAL} opcode is
used but the return value is never checked (line 11). Unlocking the script
before the locktime has expired is thus possible without knowing the preimage of
the hash in line 10. The value on the stack is hashed with \texttt{SHA256} and
compared with the hash lock. Execution of the script continues regardless of the
outcome of this comparison. If the \texttt{OP\_EQUAL} opcode is replaced with
\texttt{OP\_EQUALVERIFY}, the script will halt if the comparison fails,
restoring intended behavior.
The following unlocking script allows successful spending of the output:
\begin{minted}[frame=lines,framesep=2mm,bgcolor=LightGray,fontsize=\footnotesize,linenos]{text}
<sig_dave>
<id_dave.pk.to_hex()>
OP_0
OP_0
<sig_dave>
<id_dave.pk.to_hex()>
\end{minted}
\section*{Work distribution} \section*{Work distribution}
%Fill in here an overview on which group member participated in which task and %Fill in here an overview on which group member participated in which task and