\documentclass[dvipsnames,14pt,t]{beamer}
\usepackage{../slides}
\usepackage{../graphics}
\usepackage{../langs}
\usepackage{../data}

\usetikzlibrary{shapes}

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\begin{document}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{%
  \begin{tabular}{@ {}c@ {}}
  \\
  \LARGE Security Engineering\\[-3mm] 
  \end{tabular}}\bigskip\bigskip\bigskip

  \normalsize
  \begin{center}
  \begin{tabular}{ll}
  Email:  & christian.urban at kcl.ac.uk\\
  Office: & N7.07 (North Wing, Bush House)\\
  Slides: & KEATS (also homework is there)\\
  \end{tabular}
  \end{center}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%     

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% student prticipation
%\begin{frame}
%\frametitle{Bitcoins}
%
%P2P, 21 Mio, $10^-8$, Mt~Gox, value decreased?, anonymity, 
%Silk Road, verification incentive, puzzle, SHA-256, slow 
%transactions, 
%
%\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Recall: Bitcoins}

\begin{itemize}
\item a crypto currency by Satoshi Nakamoto
\item mined by solving special puzzles involving hashes
\item transaction history (ledger/blockchain) is P2P distributed (12 GB)
\item surely a scam/ponzi scheme!
\end{itemize}

\begin{textblock}{7}(6.5,8.5)
\includegraphics[scale=0.26]{../pics/bitcoin_ledgers.png}
\end{textblock}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Bitcoins for Real}

\begin{itemize}
\item you need a public-private key (the hash of the 
public key to determines your bitcoin address) 

\item if you want to receive bitcoins, you publicise 
this address

\item there are $2^{160}$ possibilities\\ 
(no check for duplicates)\bigskip\pause

\item transactions contain ``payment scripts''
 (non-Turing-complete scripting language)\bigskip
 
 \small simplest script: pay-to-public-key
\end{itemize}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Bitcoin Keys}

\begin{center}
\includegraphics[scale=0.9]{../pics/bitcoin-keys.png}
\end{center}

\begin{itemize}
\item \bl{k} private key: 256 bits (randomly chosen)
\item \bl{K} public key: generated from \bl{k}
\item \bl{A} bitcoin address: 160 Bit/20 Byte number: 
\[\bl{A \dn RIPEMD160(SHA256(K))}\]

\footnotesize RIPEMD160, SHA256 are hash functions
\end{itemize}


\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Bitcoin Addresses}

The ``human readable, checked version'' of \bl{A}:

\begin{center}
\begin{tikzpicture}[scale=1]
  \node (A) at (0,0)  [draw=black, rectangle, 
     very thick, minimum height=10mm, minimum width=20mm] 
     {\Large\bl{A}};
  \node (B) at (0,-2)  [draw=black, rectangle, 
     very thick, minimum height=10mm, minimum width=20mm] 
     {\Large\bl{A}};
  \node (B2) at (-2,-2)  [draw=black, rectangle, 
     very thick, minimum height=10mm, minimum width=20mm] 
     {\large\bl{Prefix}};
  \node (B3) at (-4,-0.5) {1 for P2PKH};      
  \node (C) at (0,-4)  [draw=black, rectangle, 
     very thick, minimum height=10mm, minimum width=20mm] 
     {\Large\bl{A}};
  \node at (-2,-4)  [draw=black, rectangle, 
     very thick, minimum height=10mm, minimum width=20mm] 
     {\large\bl{Prefix}};
  \node (C2) at (3,-4)  [draw=black, rectangle, 
     very thick, minimum height=10mm, minimum width=40mm] 
     {\large\bl{CheckSum}}; 
  \node (D) at (0,-5.8)  [draw=black, rectangle, 
     very thick, minimum height=10mm, minimum width=20mm] 
     {\large\bl{Base58Check(Prefix + A + CheckSum)}};
 
  \draw[<-, line width=0.5mm] (B2.north west) -- (B3);
  \draw[->, line width=2mm] (A) -- (B);
  \draw[->, line width=2mm] (B) -- (C);
  \draw[->, line width=2mm] (C) -- (D);
  \path[->, line width=2mm] (B.east) edge[bend left] node[right] 
  {\small{}\bl{\begin{tabular}{l}SHA256\\
                             SHA256\\
                             first 4bytes\\[6mm]\end{tabular}}} (C2);
\end{tikzpicture}
\end{center}

\only<2->{
\begin{textblock}{2.5}(1,10)
\begin{bubble}[10cm]
Example address (Base58):\medskip

\bl{1DSrfJdB2AnWaFNgSbv3MZC2m74996JafV}\bigskip

\mbox{}\hfill\small{}(does not contain 0OlI)
\end{bubble}
\end{textblock}}
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Transaction Graph}

\mbox{}\hspace{3cm}older \hspace{3cm} current

\begin{center}
\includegraphics[scale=0.52]{../pics/blockchain.png}
\end{center}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Types of Transactions}

\begin{itemize}
\item pay-to-public-key-hash (so far: Alice pays Bob)\pause
\item pay-to-script-hash
\[\bl{RIPEMD160(SHA256(script))}\]\pause

\item Each transaction, including P2PKH, contains a
      \alert{locking} and an \alert{unlocking} script (locking
      from output; unlocking from input).
\item The scripts are written in a Forth-like language (stack
      based).
\item Running both scripts has to evaluate to True.
\end{itemize}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Pay-to-Public-Key-Hash}

\begin{itemize}
\item Alice pays Bob:

\begin{center}
\begin{tabular}{ll}
\scode{<Bob's signature>} & {\small(unlocking script from input)}\\
\scode{<Bob's PKey>}\medskip\\
\scode{OP_DUP} & {\small(locking script from output)}\\
\scode{OP_HASH160}\\
\scode{<Bob's PKey Hash>}\\
\scode{OP_EQUALVERIFY}\\
\scode{OP_CHECKSIG}
\end{tabular}
\end{center}
\end{itemize}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[c]
\frametitle{A Transaction Msg}
\small
\lstinputlisting[language=Scala,
                 numbers=none, 
                 xleftmargin=-6mm]{msg}
                 
\only<2->{
\begin{textblock}{2.5}(4.8,2)
\begin{bubble}[8cm]
Question: Sender and receiver are the same; same amount
(no time stamps).\medskip

Can 2 transactions be exactly the same?
\end{bubble}
\end{textblock}}                 
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[c]
\frametitle{Pay-to-Script-Hash}

Bob wants to implement a multi-key/signature scheme in his 
company:

\begin{itemize}
\item Bob tells Alice the hash of a locking script:
\item Alice sends the payment to this ``hash address''\bigskip
\item Bob has to supply the locking script matching this hash, 
and the unlocking script\pause\bigskip
\item Bob can use this payment to implement 2-out-of-3 
signature procedures
\end{itemize}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Blockchain (Public Ledger)}

\begin{center}
\includegraphics[scale=0.5]{../pics/bitcoinblockchain0.png}
\end{center}

\begin{itemize}
\item each block is hashed and contains a reference to
the earlier block; ``validates'' potentially more than one
transaction
\end{itemize}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Proof-of-Work}

The idea is counterintuitive and involves a combination of two
ideas:

\begin{bubble}[10cm]
\begin{itemize}

\item to (artificially) make it computationally costly for
      network users to validate transactions, and

\item to reward them for trying to help validate transactions
\end{itemize}
\end{bubble}\pause

\small
this is called mining: whoever validates a transaction will be awarded with
50 bitcoins --- this halves every 210,000 transactions or
roughly every 4 years (currently 25 BC); no new bitcoins after 2140 -- then only 
transaction fees
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Solving Puzzles}

Given a string, say \code{"Hello, world!"}, what is the 
\alert{salt} so the hash starts with a long run of 
zeros?\bigskip

\begin{bubble}[10cm]
\footnotesize\code{h("Hello, world!0") =}\\
\;\;\scriptsize\pcode{1312af178c253f84028d480a6adc1e25e81caa44c749ec81976192e2ec934c64}\\
\pause
\footnotesize\code{h("Hello, world!1") =}\\ 
\;\;\scriptsize\pcode{e9afc424b79e4f6ab42d99c81156d3a17228d6e1eef4139be78e948a9332a7d8}\\
\pause
\ldots\\
\footnotesize\code{h("Hello, world!4250") =}\\ 
\;\;\scriptsize\pcode{0000c3af42fc31103f1fdc0151fa747ff87349a4714df7cc52ea464e12dcd4e9}
\end{bubble}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Hardness}

If we want the output hash value to begin with 10 zeroes,
say, then we will need, on average, to try $16^{10} \approx
10^{12}$ different salts before we find a suitable nonce. 

Hardness can be controlled by setting a \alert{target} (maximum
number).

\begin{center}
\includegraphics[scale=0.37]{../pics/blockchainsolving.png}
\end{center}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{How to Adjust the Target?}

\begin{itemize}
\item every 2016 blocks the hardness is adjusted\\ (app 2 weeks)
\end{itemize}

\begin{center}
\includegraphics[scale=0.37]{../pics/blockchainsolving.png}
\end{center}

\small
\[
\bl{\begin{array}{@{}l@{}}\text{New}\\ \text{Hardness}\end{array} \dn
\begin{array}{@{}l@{}}\text{Old}\\ \text{Hardness}\end{array} 
* \frac{\text{Actual time for the last 2016 blocks}}{20160}}
\]

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Hardness}

\begin{itemize}
\item for example block \#277,316 has the hardness 

\[\scode{0x1903a30c}\]
\end{itemize}

where \scode{19} is the exponent and \scode{03a30c} is the 
coefficient.

\[
\bl{target \dn coeffcient * 2^{8 * (exponent - 3)}}
\]

in this example the hash has to be smaller that

\footnotesize
\[
\begin{array}{l}
\scode{0x0000000000000003A30C000000000000}\\
\qquad\qquad\scode{00000000000000000000000000000000}
\end{array}
\]\pause

It is fun to see that nowadays mining equipment is so 
efficient that the hardness is closely related to the 
cost of electricity.
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Bitcoin Schedule}

\begin{itemize}
\item every 210000 blocks the amount of bitcoins to be 
  mined halves (``reward era'')
\end{itemize}

\begin{center}
\begin{tikzpicture}
\begin{axis}[
    xlabel={\footnotesize year},
    ylabel={\footnotesize \% of total bitcoins},
    ylabel style={yshift=0.0em},
    enlargelimits=false,
    xtick={2009,2011,...,2025},
    xmin=2009,
    xmax=2026,
    ymax=105,
    ymin=0,
    ytick={0,20,...,100},
    scaled ticks=false,
    axis lines=left,
    width=9cm,
    height=6cm,
    legend entries={\footnotesize plan,\footnotesize in reality 2\% ahead},  
    legend pos=south east,
    legend cell align=left,  
    y tick label style={font=\footnotesize}, 
    x tick label style={font=\footnotesize,/pgf/number format/1000 sep={}} 
    ]
\addplot
  table {bitcoinestimate.data};
\only<2>{\addplot[red] 
  table {bitcoinactual.data};}
\end{axis}
\end{tikzpicture}
\end{center}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Order of Transactions}

If we don’t have such an ordering at any given moment 
then it may not be clear who owns which Bitcoins.

\begin{center}
\includegraphics[scale=0.38]{../pics/bitcoin_unconfirmed.png}
\end{center}

\small
Say, miner David is lucky and finds a suitable salt
to confirm the transactions. Celebration!\pause \hspace{5mm}??

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Forks}

Typically the blockchain will look as follows

\begin{center}
\includegraphics[scale=0.65]{../pics/block_chain1.png}
\end{center}

\pause
But every so often there is a fork

\begin{center}
\includegraphics[scale=0.65]{../pics/block_chain_fork.png}
\end{center}

\small
\ldots{}bugger this is exactly what we are trying to avoid
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]

The tie is broken if another block is solved

\begin{center}
\includegraphics[scale=0.4]{../pics/bitcoin_blockchain_branches.png}
\end{center}

\small
The rule is: if a fork occurs, people on the network keep
track of all forks. But at any given time, miners only work
to extend whichever fork is longest in their copy of the block
chain.

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Double Spending}

So if Alice wants to fake it, she needs to produce
a longer chain:

\begin{center}
\includegraphics[scale=0.4]{../pics/bitcoin_blockchain_double_spend.png}
\end{center}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}
\frametitle{Racing Against the World}

\begin{columns}
\begin{column}{4.5cm}
\includegraphics[scale=0.3]{../pics/bitcoin_doublespend_blockchain_race.png}
\end{column}
\begin{column}{5.5cm}
\includegraphics[scale=0.3]{../pics/bitcoin_transaction_order_race.png}
\end{column}
\end{columns}\bigskip\bigskip\pause

\small
A transaction is ``confirmed'' if:\smallskip

(1) it is part of a block in the longest fork, and (2) at
least 5 blocks follow it in the longest fork. In this case we
say that the transaction has ``6 confirmations''.\bigskip

\footnotesize (might take 1h+\ldots but for creditcards you have 
6 months chargeback)
\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[c]
\frametitle{Mining Pools}

\begin{bubble}[10cm]
On average, it would take several years for a typical computer
to solve a block, so an individual’s chance of ever solving
one before the rest of the network, which typically takes 10
minutes, is negligibly low.
\end{bubble}\pause

\small
Many people join groups called mining pools that collectively
work to solve blocks, and distribute rewards based on work
contributed. These act somewhat like lottery pools among
co-workers, except that some of these pools are quite large,
and comprise more than 20\% of all the computers in the
network.\medskip

\footnotesize
BTCC, the largest mining pool, has limited its members to 
not solve more than 6 blocks in a row.
\hfill\url{https://blockchain.info/pools}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Multi-Signature Addresses}

\begin{itemize}
\item \ldots Bitcoin Improvement Proposal\bigskip\pause
\item pay-to-public-key (explained so far)
\item pay-to-script-hash (since 2012)\bigskip\pause

can specify: requires \bl{M} out of \bl{N} signatures\medskip

{\small for example\\
1-of-2: me and my wife, or\\
2-of-2 in banking/companies}

\end{itemize}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Dispute Mediation}

\begin{itemize}
\item say, client and (online) merchant do not trust 
  each other\bigskip\pause

\item 2-of-3: mutually trusted escrow service
  \begin{enumerate}
  \item client sends money to 2-of-3 transaction
  \item merchant sends out goods
  \item if goods are OK, client sends signed transaction
    to merchant, merchant can sign and receive the money
    (publish in blockchain)
  \item if goods are defective, merchant sends signed transaction
    to client, client can sign and receive the money back
  \item if client and merchant disagree, then they ask escrow
  servive who signs a transaction and sends it to ``winning''
  party 
 \end{enumerate}
\end{itemize}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{A Block in the Blockchain}

\begin{center}
\includegraphics[scale=0.38]{../pics/bitcoin_unconfirmed.png}
\end{center}

\begin{itemize}
\item each block is hashed and contains a reference to
the earlier block
\item contains the ``salt'' and address of whoever solved the 
puzzle
\end{itemize}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}
\frametitle{Transaction History}

you can follow back the transaction history until
you reach either

\begin{itemize}
\item the genesis block (a transaction without input of
50 bitcoins), or
\item a coinbase transaction (this is the reward of the
miner who validated a block of transactions in the blockchain)

\end{itemize}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Lost Bitcoins?}

\begin{itemize}
\item somebody needs to be able to generate a key-pair
  for the signature (for this you need the private 
  key)\bigskip

\item somebody spends your bitcoins fraudulently
  (you cannot charge them back)\ldots{} bad luck

\item you can send bitcoins to a ``non-existing'' address
  
\item you send them to a script that can never be satisfied  
\end{itemize}

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Good Points}

An attacker can't:

\begin{itemize}
\item reverse other people's transactions
\item change the number of coins generated per block
\item create coins out of thin air
\item send coins that never belonged to an attacker
\item you cannot meddle with the ``history''
\end{itemize}\bigskip

The system can be scaled to all world transactions. 

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{frame}[t]
\frametitle{Take Home Points}


\begin{itemize}
  \item Don't gamble! I am not a first mover in such things.
  \item Cool idea, but I am sure there will be a Bitcoin~2.0.
  (can for example mining pools force to be paid more?)
  \item It still depends on a lot of old-fashioned security 
  (e.g.~keeping private-keys secret)\bigskip

  \item Having now the knowledge how it works, go back
  and listen to what people/media make of it. 
\end{itemize} 

\end{frame}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 


\end{document}

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