\documentclass[dvipsnames,14pt,t]{beamer}\usepackage{../slides}\usepackage{../graphics}\usepackage{../langs}\usepackage{../data}\usetikzlibrary{shapes}% beamer stuff \renewcommand{\slidecaption}{SEN 08, King's College London}\newcommand{\bl}[1]{\textcolor{blue}{#1}}\newcommand{\DOWNarrow}[3]{%\begin{textblock}{0}(#2,#3)%\onslide<#1>{%\begin{tikzpicture}%\node at (0,0) [single arrow, shape border rotate=270, fill=red,text=red]{a};%\end{tikzpicture}}%\end{textblock}}\newcommand{\LEFTarrow}[3]{%\begin{textblock}{0}(#2,#3)%\onslide<#1>{%\begin{tikzpicture}%\node at (0,0) [single arrow, shape border rotate=180, fill=red,text=red]{a};%\end{tikzpicture}}%\end{textblock}}\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: & S1.27 (1st floor Strand Building)\\ 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).\medskipCan 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 tothe earlier block; ``validates'' potentially more than onetransaction\end{itemize}\end{frame}%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%\begin{frame}[t]\frametitle{Proof-of-Work}The idea is counterintuitive and involves a combination of twoideas:\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\smallthis is called mining: whoever validates a transaction will be awarded with50 bitcoins --- this halves every 210,000 transactions orroughly 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 $16^{10} \approx10^{12}$ different salts before we find a suitable nonce. Hardness can be controlled by setting a \alert{target} (maximumnumber).\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\[\begin{array}{l}\scode{0x0000000000000003A30C000000000000}\\\qquad\qquad\scode{00000000000000000000000000000000}\end{array}\]\pauseIt 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}\smallSay, miner David is lucky and finds a suitable saltto 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}\pauseBut 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}\smallThe rule is: if a fork occurs, people on the network keeptrack of all forks. But at any given time, miners only workto extend whichever fork is longest in their copy of the blockchain.\end{frame}%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%\begin{frame}[t]\frametitle{Double Spending}So if Alice wants to fake it, she needs to producea 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\smallA transaction is ``confirmed'' if:\smallskip(1) it is part of a block in the longest fork, and (2) atleast 5 blocks follow it in the longest fork. In this case wesay 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 computerto solve a block, so an individual’s chance of ever solvingone before the rest of the network, which typically takes 10minutes, is negligibly low.\end{bubble}\pause\smallMany people join groups called mining pools that collectivelywork to solve blocks, and distribute rewards based on workcontributed. These act somewhat like lottery pools amongco-workers, except that some of these pools are quite large,and comprise more than 20\% of all the computers in thenetwork.\medskip\footnotesizeBTCC, 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\pausecan 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 tothe 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 untilyou reach either\begin{itemize}\item the genesis block (a transaction without input of50 bitcoins), or\item a coinbase transaction (this is the reward of theminer 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}\bigskipThe 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}%%% Local Variables: %%% mode: latex%%% TeX-master: t%%% End: