1 \documentclass[dvipsnames,14pt,t]{beamer}

     2 \usepackage{../slides}

     3 \usepackage{../graphics}

     4 \usepackage{../langs}

     5 

     6 \usetikzlibrary{shapes}

     7 

     8 % beamer stuff 

     9 \renewcommand{\slidecaption}{APP 08, King's College London}

    10 \newcommand{\bl}[1]{\textcolor{blue}{#1}}

    11 

    12 \newcommand{\DOWNarrow}[3]{%

    13 \begin{textblock}{0}(#2,#3)%

    14 \onslide<#1>{%

    15 \begin{tikzpicture}%

    16 \node at (0,0) [single arrow, shape border rotate=270, fill=red,text=red]{a};%

    17 \end{tikzpicture}}%

    18 \end{textblock}}

    19 \newcommand{\LEFTarrow}[3]{%

    20 \begin{textblock}{0}(#2,#3)%

    21 \onslide<#1>{%

    22 \begin{tikzpicture}%

    23 \node at (0,0) [single arrow, shape border rotate=180, fill=red,text=red]{a};%

    24 \end{tikzpicture}}%

    25 \end{textblock}}

    26 

    27 

    28 \begin{document}

    29 

    30 

    31 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    32 \begin{frame}[t]

    33 \frametitle{%

    34   \begin{tabular}{@ {}c@ {}}

    35   \\

    36   \LARGE Access Control and \\[-3mm] 

    37   \LARGE Privacy Policies (8)\\[-6mm] 

    38   \end{tabular}}\bigskip\bigskip\bigskip

    39 

    40   \normalsize

    41   \begin{center}

    42   \begin{tabular}{ll}Ch

    43   Email:  & christian.urban at kcl.ac.uk\\

    44   Office: & S1.27 (1st floor Strand Building)\\

    45   Slides: & KEATS (also homework is there)\\

    46   \end{tabular}

    47   \end{center}

    48 

    49 \end{frame}

    50 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%     

    51 

    52 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    53 % student prticipation

    54 \begin{frame}

    55 \frametitle{Bitcoins}

    56 

    57 \begin{center}

    58 \Huge\bf ?

    59 \end{center}

    60 

    61 \end{frame}

    62 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

    63 

    64 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    65 \begin{frame}[t]

    66 \frametitle{Bitcoins from 10,000m}

    67 

    68 \begin{itemize}

    69 \item a crypto ``currency'' by Satoshi Nakamoto (likely a pen name)

    70 \item a digital resource designed to be scarce 

    71   (max 21 Mio bitcoins---deflationary currency) 

    72 \item mined by solving special puzzles involving hashes

    73 \item transaction history (ledger/blockchain) is P2P distributed (12 GB)

    74 \item two ``mining pools'' produce\\ currently more than 50\% 

    75   of bitcoins 

    76 \item can be stolen and also lost

    77 \item anoynmous?\pause

    78 \item a ponzi scheme

    79 \end{itemize}

    80 

    81 \begin{textblock}{7}(11.5,10)

    82 \includegraphics[scale=0.21]{../pics/bitcoin_ledgers.png}

    83 \end{textblock}

    84 

    85 \end{frame}

    86 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

    87 

    88 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    89 \begin{frame}[t]

    90 \frametitle{Bitcoins}

    91 

    92 \begin{itemize}

    93 \item you create a public-private key pair

    94 \item you have a wallet which can be

    95 \begin{itemize}

    96 \item electronic (on you computer, passwords)

    97 \item cloud-based (passwords)

    98 \item paper-based

    99 \end{itemize}

   100 

   101 \item Mt.~Gox: hacked $\Rightarrow$ insolvent

   102 \item no form of dispute resolution

   103 \end{itemize}

   104 

   105 \end{frame}

   106 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   107 

   108 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   109 \begin{frame}

   110 \frametitle{Underlying Ideas}

   111 

   112 Establishing trust in a completely 

   113 untrusted environment\medskip

   114 

   115 \begin{itemize}

   116 \item public-private key encryption\medskip

   117 \item digital signatures\medskip

   118 \item cryptographic hashing (SHA-256)

   119 \end{itemize}

   120 

   121 \begin{center}

   122 If Alice sends you: \bl{$msg, \{msg\}_{K^{priv}_{Alice}}$}

   123 \ldots

   124 \end{center}

   125 

   126 \end{frame}

   127 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   128 

   129 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   130 \begin{frame}[t]

   131 \frametitle{Lets Start with Infocoins}

   132 

   133 \begin{center}

   134 \bl{$\{\text{I, Alice, am giving Bob one infocoin.}\}_{K^{priv}_{Alice}}$}

   135 \end{center}\bigskip

   136 

   137 \begin{itemize}

   138 \item no-one else could have created that message

   139 \item Alice cannot deny the ``intend''\pause\bigskip

   140 \item forgery possible only after Alice created the string

   141 \item Q: What is money? --- A: Well string above

   142   (or later messages like that)

   143 \end{itemize}

   144 

   145 

   146 

   147 \end{frame}

   148 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   149 

   150 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   151 \begin{frame}[t]

   152 \frametitle{Double Spend}

   153 

   154 \begin{center}

   155 \bl{$\{\text{I, Alice, am giving Bob one infocoin.}\}_{K^{priv}_{Alice}}$}

   156 \end{center}

   157 

   158 \begin{itemize}

   159 \item Alice could keep sending Bob this message over and over

   160       again\pause

   161 \item we need to have a serial number

   162 \begin{center}

   163 \bl{$\{\text{I, Alice, am giving Bob infocoin \#1234567.}\}_{K^{priv}_{Alice}}$}

   164 \end{center}\pause\medskip

   165 

   166 \item but then we need a trusted source of serial numbers (e.g.~bank)

   167 \end{itemize}

   168 

   169 \end{frame}

   170 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   171 

   172 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   173 \begin{frame}[t]

   174 \frametitle{No Banks Please}

   175 

   176 With banks we could implement:

   177 

   178 \begin{itemize}

   179 \item Bob asks the bank whether the infocoin with that serial

   180       number belongs to Alice and

   181 \item Alice hasn’t already spent this infocoin. 

   182 \item If yes, then Bob tells the bank he accept the infocoin

   183 \item the bank updates the records to show that the infocoin

   184       with that serial number is now in Bob’s possession and

   185       no longer belongs to Alice\bigskip\pause

   186 \end{itemize}

   187 

   188 \end{frame}

   189 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   190 

   191 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   192 \begin{frame}[t]

   193 \frametitle{Blockchain}

   194 

   195 The solution for double spend: 

   196 

   197 \begin{itemize}

   198 \item make everybody the bank, everybody has the entire

   199       transaction history --- will be called

   200       \alert{blockchain}\medskip

   201 \item Bob checks whether infocoin belongs to Alice and then

   202       broadcasts the message to anybody else 

   203 \end{itemize}

   204 

   205 \begin{center}

   206 \includegraphics[scale=0.21]{../pics/bitcoin_ledgers.png}

   207 \end{center}

   208 

   209 \end{frame}

   210 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   211 

   212 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   213 \begin{frame}[t]

   214 \frametitle{Blockchain}

   215 

   216 \begin{center}

   217 \includegraphics[scale=0.5]{../pics/bitcoinblockchain0.png}

   218 \end{center}

   219 

   220 \end{frame}

   221 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   222 

   223 

   224 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   225 \begin{frame}[t]

   226 \frametitle{Transaction Graph}

   227 

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

   229 

   230 \begin{center}

   231 \includegraphics[scale=0.5]{../pics/blockchain.png}

   232 \end{center}

   233 

   234 \end{frame}

   235 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   236 

   237 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   238 \begin{frame}[t]

   239 \frametitle{Double Spend Again}

   240 

   241 \begin{bubble}[10cm]\addtolength{\leftmargini}{5mm}

   242 \begin{itemize}

   243 \item I , Alice, am giving Bob one infocoin, with serial

   244       number 1234567.

   245 \item I, Alice, am giving \alt<2->{\alert{Alice}}{Charlie} 

   246       one infocoin.

   247 \end{itemize}

   248 \end{bubble}\bigskip

   249 

   250 How should other people update their blockchain (public

   251 register)?\pause

   252 

   253 

   254 \begin{center}

   255 \includegraphics[scale=0.3]{../pics/bitcoindisagreement.png}

   256 \end{center}

   257 

   258 

   259 Once enough people have broadcast that message, everyone

   260 updates their block chain to show that infocoin 1234567 now

   261 belongs to Bob, and the transaction is complete.

   262 

   263 \end{frame}

   264 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   265 

   266 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   267 \begin{frame}[t]

   268 \frametitle{Creating Agreement}

   269 

   270 \begin{bubble}[10cm]

   271 Once \alert{enough} people have broadcast that message, 

   272 everyone updates their block chain to show that infocoin 

   273 1234567 now belongs to Bob, and the transaction is accepted.

   274 \end{bubble}\bigskip\bigskip

   275 \pause

   276 

   277 \small

   278 But what if Alice sets up a large number of separate

   279 identities, let’s say a billion, on the Infocoin network. When

   280 Bob asks the network to validate the transaction, Alice’s

   281 puppet identities say ``Yes his transaction is validated'',

   282 while actually the rest network says Alice's transaction 

   283 is OK?

   284 \end{frame}

   285 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   286 

   287 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   288 \begin{frame}[t]

   289 \frametitle{Proof-Of-Work}

   290 

   291 The idea is counterintuitive and involves a combination of two

   292 ideas:\bigskip

   293 

   294 \begin{bubble}[10cm]

   295 \addtolength{\leftmargini}{5mm}

   296 \begin{itemize}

   297 

   298 \item to (artificially) make it computationally costly for

   299       network users to validate transactions, and

   300 

   301 \item to reward them for trying to help validate transactions

   302 \end{itemize}

   303 \end{bubble}\pause\bigskip

   304 

   305 \small

   306 this is called mining: whoever validates a transaction will be awarded with

   307 50 bitcoins --- this halves every 210,000 transactions or

   308 roughly every 4 years; no new bitcoins after 2140 -- then only 

   309 transaction fees

   310 \end{frame}

   311 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   312 

   313 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   314 \begin{frame}[t]

   315 \frametitle{Solving Puzzles}

   316 

   317 Given a string, say \code{"Hello, world!"}, what is the 

   318 \alert{salt} that the hash starts with a long run of 

   319 zeros?\bigskip

   320 

   321 \begin{bubble}[10cm]

   322 \begin{tabular}{l}

   323 \footnotesize\code{h("Hello, world!0") =}\\

   324 \;\;\scriptsize\code{1312af178c253f84028d480a6adc1e25e81caa44c749ec81976192e2ec934c64}\\

   325 \pause

   326 \footnotesize\code{h("Hello, world!1") =}\\ 

   327 \;\;\scriptsize\code{e9afc424b79e4f6ab42d99c81156d3a17228d6e1eef4139be78e948a9332a7d8}\\

   328 \pause

   329 \ldots\\

   330 \footnotesize\code{h("Hello, world!4250") =}\\ 

   331 \;\;\scriptsize\code{0000c3af42fc31103f1fdc0151fa747ff87349a4714df7cc52ea464e12dcd4e9}

   332 \end{tabular}

   333 \end{bubble}

   334 

   335 \end{frame}

   336 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   337 

   338 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   339 \begin{frame}[t]

   340 \frametitle{Hardness}

   341 

   342 If we want the output hash value to begin with 10 zeroes,

   343 say, then we will need, on average, to try $16^{10} \approx

   344 10^{12}$ different salts before we find a suitable nonce. 

   345 

   346 Hardness can be controlled by setting a \alert{target} (maximum

   347 number).

   348 

   349 \begin{center}

   350 \includegraphics[scale=0.37]{../pics/blockchainsolving.png}

   351 \end{center}

   352 

   353 \begin{textblock}{7}(7,10)

   354 10 mins

   355 \end{textblock}

   356 

   357 \end{frame}

   358 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   359 

   360 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   361 \begin{frame}[t]

   362 \frametitle{Order of Transactions}

   363 

   364 If we don’t have such an ordering then at any given moment it

   365 may not be clear who owns which infocoins.

   366 

   367 \begin{center}

   368 \includegraphics[scale=0.38]{../pics/bitcoin_unconfirmed.png}

   369 \end{center}

   370 

   371 \small

   372 Say, miner David is lucky and finds a suitable salt

   373 to confirm the transactions. Celebration!\pause \hspace{5mm}??

   374 

   375 \end{frame}

   376 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   377 

   378 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   379 \begin{frame}[t]

   380 \frametitle{Forks}

   381 

   382 Typically the blockchain will look as follows

   383 

   384 \begin{center}

   385 \includegraphics[scale=0.65]{../pics/block_chain1.png}

   386 \end{center}

   387 

   388 \pause

   389 But every so often there is a fork

   390 

   391 \begin{center}

   392 \includegraphics[scale=0.65]{../pics/block_chain_fork.png}

   393 \end{center}

   394 

   395 \small

   396 \ldots{}bugger this is exactly what we are trying to avoid

   397 \end{frame}

   398 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   399 

   400 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   401 \begin{frame}[t]

   402 

   403 The tie is broken if another block is solved

   404 

   405 \begin{center}

   406 \includegraphics[scale=0.4]{../pics/bitcoin_blockchain_branches.png}

   407 \end{center}

   408 

   409 \small

   410 The rule is this: if a fork occurs, people on the network keep

   411 track of all forks. But at any given time, miners only work

   412 to extend whichever fork is longest in their copy of the block

   413 chain.

   414 

   415 \end{frame}

   416 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   417 

   418 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   419 \begin{frame}[t]

   420 \frametitle{Double Spending Again}

   421 

   422 So if Alice wants to fake it she needs to produce

   423 a longer chain:

   424 

   425 \begin{center}

   426 \includegraphics[scale=0.4]{../pics/bitcoin_blockchain_double_spend.png}

   427 \end{center}

   428 

   429 \end{frame}

   430 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   431 

   432 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   433 \begin{frame}

   434 \frametitle{Racing Agianst the World}

   435 

   436 \begin{columns}

   437 \begin{column}{4.5cm}

   438 \includegraphics[scale=0.3]{../pics/bitcoin_doublespend_blockchain_race.png}

   439 \end{column}

   440 \begin{column}{5.5cm}

   441 \includegraphics[scale=0.3]{../pics/bitcoin_transaction_order_race.png}

   442 \end{column}

   443 \end{columns}\bigskip\bigskip\pause

   444 

   445 \small

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

   447 

   448 (1) it is part of a block in the longest fork, and (2) at

   449 least 5 blocks follow it in the longest fork. In this case we

   450 say that the transaction has ``6 confirmations''.\bigskip

   451 

   452 \footnotesize (might take 1h+\ldots but for creditcards you have 

   453 6 months chargeback)

   454 \end{frame}

   455 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   456 

   457 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   458 \begin{frame}[t]

   459 \frametitle{Mining Pools}

   460 

   461 \begin{bubble}[10cm]

   462 On average, it would take several years for a typical computer

   463 to solve a block, so an individual’s chance of ever solving

   464 one before the rest of the network, which typically takes 10

   465 minutes, is very low.

   466 \end{bubble}\bigskip\pause

   467 

   468 \small

   469 Many people join groups called mining pools that collectively

   470 work to solve blocks, and distribute rewards based on work

   471 contributed. These act somewhat like lottery pools among

   472 co-workers, except that some of these pools are quite large,

   473 and comprise more than 20\% of all the computers in the

   474 network.\medskip

   475 

   476 \footnotesize

   477 BTC, the largest mining pool, has limited its members to 

   478 not solve more than 6 blocks in a row.

   479 

   480 \end{frame}

   481 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   482 

   483 

   484 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   485 \begin{frame}[t]

   486 \frametitle{Bitcoins for Real}

   487 

   488 \begin{itemize}

   489 \item you need a public-private key (the hash of the 

   490 public key to determines your bitcoin address) 

   491 

   492 \item if you want to receive bitcoins, you publicise 

   493 this address

   494 

   495 \item there are $2^{160}$ possibilities\\ 

   496 (no check for duplicates)

   497 \end{itemize}

   498 

   499 \end{frame}

   500 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   501 

   502 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   503 \begin{frame}[c]

   504 \frametitle{A Transaction Msg}

   505 \small

   506 \lstinputlisting[language=Scala,

   507                  numbersep=3pt, 

   508                  xleftmargin=-6mm]{msg}

   509 

   510 \DOWNarrow{2}{3.5}{1.6}

   511 \LEFTarrow{3}{3.5}{3}

   512 \LEFTarrow{4}{4.7}{4}

   513 \LEFTarrow{4}{5.4}{4.8}

   514 \LEFTarrow{5}{5.4}{5.6}

   515 \LEFTarrow{6}{5}{6.4}

   516 \DOWNarrow{7}{6}{8.2}

   517 \LEFTarrow{8}{5}{9.7}

   518 \DOWNarrow{9}{7}{9.7}

   519 \DOWNarrow{9}{10}{9.7}

   520 \LEFTarrow{10}{9}{12}

   521 \DOWNarrow{11}{12.5}{12}

   522 

   523 

   524 \begin{textblock}{0}(7,3)%

   525 \small

   526 \onslide<2,4,7,8,9,10,11,12>{

   527 \begin{bubble}[6cm]%

   528 \only<2>{the hash of the msg that follows; kind of serial number}

   529 \only<4>{the transaction has one inout and one output (could be more)}

   530 \only<7>{the hash of the incoming transaction (incoming serial number)}

   531 \only<8>{use the 0th output of the incoming transaction}

   532 \only<9>{the public key and signature of the sender}

   533 \only<10>{use $x$ amount of the incoming money}

   534 \only<11>{public key of the receiver}

   535 \only<12>{you do not need a central authority to issue serial numbers\bigskip\\

   536 there are no ``coins'', just a long series of transactions}

   537 \end{bubble}}

   538 \end{textblock}

   539 

   540 \end{frame}

   541 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   542 

   543 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   544 \begin{frame}

   545 \frametitle{Transaction History}

   546 

   547 you can follow back the transaction history until

   548 you reach either

   549 

   550 \begin{itemize}

   551 \item the genesis block (a transaction without input of

   552 50 bitcoins)

   553 \item coinbase transaction (this is the reward of the

   554 miner who validated a block of transactions in the blockchain)

   555 

   556 \end{itemize}

   557 

   558 \end{frame}

   559 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   560 

   561 

   562 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   563 \begin{frame}[t]

   564 \frametitle{Lost Bitcoins?}

   565 

   566 \begin{itemize}

   567 \item somebody needs to be able to generate a key-pair

   568   for the signature (for this you need the private 

   569   key)\bigskip

   570 

   571 \item somebody spends your bitcoins fraudulently

   572   (you cannot charge them back)\ldots{} bad luck

   573 

   574 \item you can send bitcoins to a ``non-existing'' address

   575   (Mt.~Gox)

   576 \end{itemize}

   577 

   578 \end{frame}

   579 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   580 

   581 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   582 \begin{frame}[t]

   583 \frametitle{Good Points}

   584 

   585 An attacker can't:

   586 

   587 \begin{itemize}

   588 \item reverse other people's transactions

   589 \item change the number of coins generated per block

   590 \item create coins out of thin air

   591 \item send coins that never belonged to an attacker

   592 \end{itemize}\bigskip

   593 

   594 The system can be scaled to all world transactions. 

   595 

   596 \end{frame}

   597 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   598 

   599 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   600 \begin{frame}[t]

   601 \frametitle{But I did not Inhale}

   602 

   603 \begin{center}

   604 \includegraphics[scale=0.04]{../pics/bitcointornetwork.png}

   605 \end{center}

   606 

   607 \end{frame}

   608 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   609 

   610 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   611 \begin{frame}[t]

   612 \frametitle{Anonymity}

   613 

   614 Bitcoins may not be ideal for money laundering because all

   615 transactions are public.[218] Authorities have expressed

   616 concerns, however. 

   617 

   618 How anonymous is Bitcoin? Many people claim that Bitcoin can be used

   619 anonymously. This claim has led to the formation of marketplaces such

   620 as Silk Road (and various successors), which specialize in illegal

   621 goods. However, the claim that Bitcoin is anonymous is a myth. The

   622 block chain is public, meaning that it’s possible for anyone to see

   623 every Bitcoin transaction ever. Although Bitcoin addresses aren’t

   624 immediately associated to real-world identities, computer scientists

   625 have done a great deal of work figuring out how to de-anonymize

   626 “anonymous” social networks. The block chain is a marvellous target

   627 for these techniques.

   628 

   629 \end{frame}

   630 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   631 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   632 \begin{frame}[t]

   633 \frametitle{Bitcoin vs Gov}

   634 

   635 Purported absence of potential government interference?

   636 \pause 

   637 Far from it:

   638   

   639 \begin{itemize}

   640   \item government could compel ``mayor players'' to blacklist

   641      bitcoins (exchanges)

   642   \item coerce developer community (e.g.~Lavabit)

   643   \item put pressure on mining pools, or be big a miner 

   644   itself

   645 \end{itemize} 

   646 

   647 \begin{center}

   648 \includegraphics[scale=0.04]{../pics/nsautah1.jpg}

   649 \hspace{3mm}

   650 \includegraphics[scale=0.031]{../pics/nsautah2.jpg}

   651 \end{center}

   652 

   653 \end{frame}

   654 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% 

   655 

   656 

   657 

   658 

   659 \end{document}

   660 

   661 %%% Local Variables:  

   662 %%% mode: latex

   663 %%% TeX-master: t

   664 %%% End: 

   665