84 \\ |
88 \\ |
85 \LARGE Access Control and \\[-3mm] |
89 \LARGE Access Control and \\[-3mm] |
86 \LARGE Privacy Policies (2)\\[-6mm] |
90 \LARGE Privacy Policies (2)\\[-6mm] |
87 \end{tabular}}\bigskip\bigskip\bigskip |
91 \end{tabular}}\bigskip\bigskip\bigskip |
88 |
92 |
89 %\begin{center} |
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90 %\includegraphics[scale=1.3]{pics/barrier.jpg} |
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91 %\end{center} |
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92 |
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93 \normalsize |
93 \normalsize |
94 \begin{center} |
94 \begin{center} |
95 \begin{tabular}{ll} |
95 \begin{tabular}{ll} |
96 Email: & christian.urban at kcl.ac.uk\\ |
96 Email: & christian.urban at kcl.ac.uk\\ |
97 Of$\!$fice: & S1.27 (1st floor Strand Building)\\ |
97 Office: & S1.27 (1st floor Strand Building)\\ |
98 Slides: & KEATS (also home work is there) |
98 Slides: & KEATS (also homework is there)\\ |
99 \end{tabular} |
99 \end{tabular} |
100 \end{center} |
100 \end{center} |
101 |
101 |
102 |
102 |
103 \end{frame}} |
103 \end{frame}} |
104 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
104 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
105 |
105 |
106 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
106 |
107 \mode<presentation>{ |
107 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
108 \begin{frame}[c] |
108 \mode<presentation>{ |
109 \frametitle{\begin{tabular}{c}Homework\end{tabular}} |
109 \begin{frame}[c] |
110 |
110 \frametitle{\begin{tabular}{c}This Course is about\\[-2mm] ``Satan's Computer''\end{tabular}} |
111 |
111 |
112 \ldots{} I have a question about the homework.\\[3mm] |
112 Ross Anderson and Roger Needham wrote:\bigskip |
113 Is it required to submit the homework before\\ |
113 |
114 the next lecture?\\[5mm] |
114 \begin{tikzpicture} |
115 |
115 \draw (0,0) node[inner sep=2mm,fill=cream, ultra thick, draw=red, rounded corners=2mm] |
116 Thank you!\\ |
116 {\normalsize\color{darkgray} |
117 Anonymous |
117 \begin{minipage}{10cm}\raggedright\small |
118 |
118 ``In effect, our task is to program a computer which gives |
119 \end{frame}} |
119 answers which are subtly and maliciously wrong at the most |
120 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
120 inconvenient possible moment\ldots{} we hope that the lessons |
121 |
121 learned from programming Satan's computer may be helpful |
122 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
122 in tackling the more common problem of programming Murphy's.'' |
123 \mode<presentation>{ |
123 \end{minipage}}; |
124 \begin{frame}[c] |
124 \end{tikzpicture}\\[30mm] |
125 |
125 |
126 \begin{center} |
126 \only<2>{ |
127 \begin{tabular}[t]{c} |
127 \begin{textblock}{11}(2,12) |
128 \includegraphics[scale=1.2]{pics/barrier.jpg}\\ |
128 \begin{tabular}{c} |
129 future lectures |
129 \includegraphics[scale=0.12]{pics/ariane.jpg}\\[-2mm] |
130 \end{tabular}\;\;\; |
130 \footnotesize Murphy's computer |
131 \onslide<2>{ |
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132 \begin{tabular}[t]{c} |
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133 \includegraphics[scale=0.32]{pics/trainwreck.jpg}\\ |
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134 today |
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135 \end{tabular} |
131 \end{tabular} |
136 } |
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137 \end{center} |
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138 |
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139 |
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140 \end{frame}} |
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141 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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142 |
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143 |
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144 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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145 \mode<presentation>{ |
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146 \begin{frame}[c] |
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147 \frametitle{\begin{tabular}{@ {}c@ {}}SmartWater\end{tabular}} |
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148 |
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149 \begin{textblock}{1}(1,3) |
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150 \begin{tabular}{c} |
132 \begin{tabular}{c} |
151 \includegraphics[scale=0.15]{pics/SmartWater} |
133 \includegraphics[scale=0.15]{pics/mobile.jpg}\; |
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134 \includegraphics[scale=0.06]{pics/pinsentry.jpg}\\[-2mm] |
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135 \footnotesize Satan's computers |
152 \end{tabular} |
136 \end{tabular} |
153 \end{textblock} |
137 \end{textblock}} |
154 |
138 |
155 |
139 \end{frame}} |
156 \begin{textblock}{8.5}(7,3) |
140 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
157 \begin{itemize} |
141 |
158 \item seems helpful for preventing cable theft\medskip |
142 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
159 \item wouldn't be helpful to make your property safe, because of possible abuse\medskip |
143 \mode<presentation>{ |
160 |
144 \begin{frame}[c] |
161 \item security is always a tradeoff |
145 \frametitle{\Large\begin{tabular}{c}User-Tracking Without Cookies\end{tabular}} |
162 \end{itemize} |
146 |
163 \end{textblock} |
147 Can you track a user {\bf without}: |
164 |
148 |
165 \end{frame}} |
149 \begin{itemize} |
166 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
150 \item Cookies |
167 |
151 \item Javascript |
168 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
152 \item LocalStorage/SessionStorage/GlobalStorage |
169 \mode<presentation>{ |
153 \item Flash, Java or other plugins |
170 \begin{frame}[c] |
154 \item Your IP address or user agent string |
171 \frametitle{\begin{tabular}{@ {}c@ {}}Plain-text Passwords at IEEE\end{tabular}} |
155 \item Any methods employed by Panopticlick\\ |
172 |
156 \mbox{}\hfill $\rightarrow$ \textcolor{blue}{\url{https://panopticlick.eff.org/}} |
173 \small\textcolor{gray}{On 25 September 2012, a report on a data breach at IEEE:} |
157 \end{itemize} |
174 |
158 |
175 |
159 Even when you disabled cookies entirely, have Javascript turned off and use a VPN service.\\\pause |
176 \begin{itemize} |
160 And numerous sites already use it (Google). |
177 \item IEEE is a standards organisation (not-for-profit) |
161 |
178 \item many standards in CS are by IEEE\medskip |
162 \end{frame}} |
179 \item 100k plain-text passwords were recorded in logs |
163 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
180 \item the logs were openly accessible on their FTP server |
164 |
181 \end{itemize}\bigskip |
165 |
182 |
166 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
183 \begin{flushright}\small |
167 \mode<presentation>{ |
184 \textcolor{gray}{\url{http://ieeelog.com}} |
168 \begin{frame}[c] |
185 \end{flushright} |
169 \frametitle{\begin{tabular}{c}Web-Protocol\end{tabular}} |
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170 |
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171 \only<1->{ |
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172 \begin{textblock}{1}(2,2) |
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173 \begin{tikzpicture}[scale=1.3] |
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174 \draw[white] (0,0) node (X) {\includegraphics[scale=0.12]{pics/firefox.jpg}}; |
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175 \end{tikzpicture} |
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176 \end{textblock}} |
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177 |
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178 \only<1->{ |
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179 \begin{textblock}{1}(11,2) |
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180 \begin{tikzpicture}[scale=1.3] |
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181 \draw[white] (0,0) node (X) {\includegraphics[scale=0.15]{pics/servers.png}}; |
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182 \end{tikzpicture} |
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183 \end{textblock}} |
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184 |
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185 \only<1->{ |
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186 \begin{textblock}{1}(5,2.5) |
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187 \begin{tikzpicture}[scale=1.3] |
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188 \draw[white] (0,0) node (X) {}; |
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189 \draw[white] (3,0) node (Y) {}; |
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190 \draw[red, ->, line width = 2mm] (X) -- (Y); |
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191 \node [inner sep=5pt,label=above:\textcolor{black}{\small GET static.jpg}] at ($ (X)!.5!(Y) $) {}; |
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192 \end{tikzpicture} |
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193 \end{textblock}} |
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194 |
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195 \only<2->{ |
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196 \begin{textblock}{1}(5,6) |
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197 \begin{tikzpicture}[scale=1.3] |
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198 \draw[white] (0,0) node (X) {}; |
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199 \draw[white] (3,0) node (Y) {}; |
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200 \draw[red, <-, line width = 2mm] (X) -- (Y); |
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201 \node [inner sep=5pt,label=below:\textcolor{black}{\small ETag: 7b33de1}] at ($ (X)!.5!(Y) $) {}; |
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202 \node [inner sep=5pt,label=above:{\includegraphics[scale=0.15]{pics/tvtestscreen.jpg}}] at ($ (X)!.5!(Y) $) {}; |
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203 \end{tikzpicture} |
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204 \end{textblock}} |
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205 |
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206 \only<3->{ |
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207 \begin{textblock}{1}(4.2,11) |
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208 \begin{tikzpicture}[scale=1.3] |
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209 \draw[white] (0,0) node (X) {}; |
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210 \draw[white] (3,0) node (Y) {}; |
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211 \draw[red, ->, line width = 2mm] (X) -- (Y); |
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212 \node [inner sep=5pt,label=above:\textcolor{black}{\small GET static.jpg ETag: 7b33de1}] at ($ (X)!.5!(Y) $) {}; |
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213 \end{tikzpicture} |
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214 \end{textblock}} |
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215 |
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216 \only<4->{ |
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217 \begin{textblock}{1}(4.2,13.9) |
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218 \begin{tikzpicture}[scale=1.3] |
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219 \draw[white] (0,0) node (X) {}; |
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220 \draw[white] (3,0) node (Y) {}; |
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221 \draw[red, <-, line width = 2mm] (X) -- (Y); |
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222 \node [inner sep=5pt,label=below:\textcolor{black}{\small HTTP/1.1 304 (Not Modified)}] at ($ (X)!.5!(Y) $) {}; |
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223 \end{tikzpicture} |
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224 \end{textblock}} |
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225 |
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226 \end{frame}} |
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227 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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228 |
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229 |
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230 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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231 \mode<presentation>{ |
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232 \begin{frame}[c] |
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233 \frametitle{Today's Lecture} |
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234 \begin{center} |
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235 \begin{tabular}{cc} |
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236 \large online banking & \hspace{6mm}\large e-voting\\ |
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237 \textcolor{gray}{solved} & \hspace{6mm}\textcolor{gray}{unsolved}\\ |
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238 \end{tabular} |
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239 \end{center} |
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240 |
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241 |
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242 \end{frame}} |
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243 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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244 |
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245 |
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246 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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247 \mode<presentation>{ |
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248 \begin{frame}[t] |
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249 \frametitle{\begin{tabular}{@ {}c@ {}}Voting as Security Problem\end{tabular}} |
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250 |
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251 What are the security requirements of a voting system?\bigskip |
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252 |
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253 \begin{itemize} |
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254 \item<2->Integrity |
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255 \item<3->Ballot Secrecy |
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256 \item<5->Voter Authentication |
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257 \item<6->Enfranchisement |
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258 \item<7->Availability |
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259 \end{itemize} |
186 |
260 |
187 \only<2>{ |
261 \only<2>{ |
188 \begin{textblock}{11}(3,2) |
262 \begin{textblock}{5.5}(8,5) |
189 \begin{tikzpicture} |
263 \begin{tikzpicture} |
190 \draw (0,0) node[inner sep=2mm,fill=white, ultra thick, draw=red, rounded corners=2mm] |
264 \draw (0,0) node[inner sep=2mm,fill=cream, ultra thick, draw=red, rounded corners=2mm, text centered] |
191 {\normalsize\color{darkgray} |
265 {\small |
192 \begin{minipage}{7.5cm}\raggedright\small |
266 \begin{minipage}{5cm}\raggedright |
193 \includegraphics[scale=0.6]{pics/IEEElog.jpg} |
267 \begin{center} |
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268 \begin{minipage}{4.5cm} |
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269 \begin{itemize} |
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270 \item The outcome matches with the voters' intend. |
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271 \item There might be gigantic sums at stake and need to be defended against. |
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272 \end{itemize} |
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273 \end{minipage} |
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274 \end{center} |
194 \end{minipage}}; |
275 \end{minipage}}; |
195 \end{tikzpicture} |
276 \end{tikzpicture} |
196 \end{textblock}} |
277 \end{textblock}} |
197 |
278 |
198 \end{frame}} |
279 \only<4>{ |
199 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
280 \begin{textblock}{5.5}(8,5) |
200 |
281 \begin{tikzpicture} |
201 |
282 \draw (0,0) node[inner sep=2mm,fill=cream, ultra thick, draw=red, rounded corners=2mm, text centered] |
202 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
283 {\small |
203 \mode<presentation>{ |
284 \begin{minipage}{5cm}\raggedright |
204 \begin{frame}[c] |
285 \begin{center} |
205 \frametitle{\begin{tabular}{@ {}c@ {}}Virgin Mobile (USA)\end{tabular}} |
286 \begin{minipage}{4.5cm} |
206 |
287 \begin{itemize} |
207 \begin{flushright}\small |
288 \item Nobody can find out how you voted. |
208 \textcolor{gray}{\url{http://arstechnica.com/security/2012/09/virgin-mobile-password-crack-risk/}} |
289 \item (Stronger) Even if you try, you cannot prove how you voted. |
209 \end{flushright} |
290 \end{itemize} |
210 |
291 \end{minipage} |
211 \begin{itemize} |
292 \end{center} |
212 \item for online accounts passwords must be 6 digits |
293 \end{minipage}}; |
213 \item you must cycle through 1M combinations (online)\pause\bigskip |
294 \end{tikzpicture} |
214 |
295 \end{textblock}} |
215 \item he limited the attack on his own account to 1 guess per second, \alert{\bf and} |
296 |
216 \item wrote a script that cleared the cookie set after each guess\pause |
297 \only<5>{ |
217 \item has been fixed now |
298 \begin{textblock}{5.5}(8,5) |
218 \end{itemize} |
299 \begin{tikzpicture} |
219 |
300 \draw (0,0) node[inner sep=2mm,fill=cream, ultra thick, draw=red, rounded corners=2mm, text centered] |
220 |
301 {\small |
221 |
302 \begin{minipage}{5cm}\raggedright |
222 \end{frame}} |
303 \begin{center} |
223 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
304 \begin{minipage}{4.5cm} |
224 |
305 \begin{itemize} |
225 |
306 \item Only authorised voters can vote up to the permitted number of votes. |
226 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
307 \end{itemize} |
227 \mode<presentation>{ |
308 \end{minipage} |
228 \begin{frame}[c] |
309 \end{center} |
229 \frametitle{\begin{tabular}{@ {}c@ {}}Smash the Stack for Fun \ldots\end{tabular}} |
310 \end{minipage}}; |
230 |
311 \end{tikzpicture} |
231 \begin{itemize} |
312 \end{textblock}} |
232 \item ``smashing the stack attacks'' or ``buffer overflow attacks'' |
313 |
233 \item one of the most popular attacks;\\ attack of the (last) decade\\ ($>$ 50\% of security incidents reported at CERT are related to buffer overflows) |
314 \only<6>{ |
234 \begin{flushright}\small |
315 \begin{textblock}{5.5}(8,5) |
235 \textcolor{gray}{\url{http://www.kb.cert.org/vuls}} |
316 \begin{tikzpicture} |
236 \end{flushright} |
317 \draw (0,0) node[inner sep=2mm,fill=cream, ultra thick, draw=red, rounded corners=2mm, text centered] |
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318 {\small |
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319 \begin{minipage}{5cm}\raggedright |
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320 \begin{center} |
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321 \begin{minipage}{4.5cm} |
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322 \begin{itemize} |
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323 \item Authorised voters should have the opportunity to vote. |
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324 \end{itemize} |
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325 \end{minipage} |
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326 \end{center} |
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327 \end{minipage}}; |
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328 \end{tikzpicture} |
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329 \end{textblock}} |
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330 |
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331 \only<7>{ |
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332 \begin{textblock}{5.5}(8,5) |
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333 \begin{tikzpicture} |
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334 \draw (0,0) node[inner sep=2mm,fill=cream, ultra thick, draw=red, rounded corners=2mm, text centered] |
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335 {\small |
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336 \begin{minipage}{5cm}\raggedright |
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337 \begin{center} |
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338 \begin{minipage}{4.5cm} |
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339 \begin{itemize} |
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340 \item The voting system should accept all authorised votes and produce results in a timely manner. |
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341 \end{itemize} |
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342 \end{minipage} |
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343 \end{center} |
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344 \end{minipage}}; |
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345 \end{tikzpicture} |
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346 \end{textblock}} |
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347 |
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348 \end{frame}} |
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349 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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350 |
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351 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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352 \mode<presentation>{ |
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353 \begin{frame}[t] |
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354 \frametitle{\begin{tabular}{@ {}c@ {}}Problems with Voting\end{tabular}} |
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355 |
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356 |
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357 \begin{center}\large |
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358 \begin{tabular}{rcl} |
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359 Integrity & vs. & Ballot Secrecy\bigskip\\ |
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360 Authentication & vs. &Enfranchisement |
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361 \end{tabular} |
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362 \end{center}\bigskip\bigskip\pause |
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363 |
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364 Further constraints: |
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365 |
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366 \begin{itemize} |
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367 \item costs |
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368 \item accessibility |
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369 \item convenience |
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370 \item intelligibility |
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371 \end{itemize} |
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372 \end{frame}} |
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373 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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374 |
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375 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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376 \mode<presentation>{ |
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377 \begin{frame}[t] |
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378 \frametitle{\begin{tabular}{@ {}c@ {}}Traditional Ballot Boxes\end{tabular}} |
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379 |
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380 |
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381 \begin{center} |
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382 \includegraphics[scale=2.5]{pics/ballotbox.jpg} |
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383 \end{center}\pause\bigskip |
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384 |
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385 they need a ``protocol'' |
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386 |
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387 |
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388 \end{frame}} |
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389 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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390 |
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391 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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392 \mode<presentation>{ |
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393 \begin{frame}[t] |
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394 \frametitle{\begin{tabular}{@ {}c@ {}}E-Voting\end{tabular}} |
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395 |
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396 |
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397 \begin{itemize} |
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398 \item The Netherlands between 1997 - 2006 had electronic voting machines\\ |
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399 \textcolor{gray}{(hacktivists had found: they can be hacked and also emitted radio signals revealing how you voted)} |
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400 |
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401 \item Germany had used them in pilot studies\\ |
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402 \textcolor{gray}{(in 2007 a law suit has reached the highest court and it rejected electronic voting |
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403 on the grounds of not being understandable by the general public)} |
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404 |
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405 \item UK used optical scan voting systems in a few polls |
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406 \end{itemize} |
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407 \end{frame}} |
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408 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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409 |
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410 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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411 \mode<presentation>{ |
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412 \begin{frame}[t] |
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413 \frametitle{\begin{tabular}{@ {}c@ {}}E-Voting\end{tabular}} |
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414 |
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415 \mbox{}\\[-12mm] |
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416 \begin{itemize} |
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417 \item US used mechanical machines since the 30s, later punch cards, now DREs and |
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418 optical scan voting machines |
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419 |
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420 \item Estonia used in 2007 the Internet for national elections |
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421 \textcolor{gray}{(there were earlier pilot studies in other countries)} |
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422 |
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423 \item India uses e-voting devices since at least 2003\\ |
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424 \textcolor{gray}{(``keep-it-simple'' machines produced by a government owned company)} |
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425 |
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426 \item South Africa used software for its tallying in the 1993 elections (when Nelson Mandela was elected) |
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427 \textcolor{gray}{(they found the tallying software was rigged, but they were able to tally manually)} |
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428 \end{itemize} |
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429 \end{frame}} |
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430 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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431 |
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432 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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433 \mode<presentation>{ |
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434 \begin{frame}[t] |
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435 \frametitle{\begin{tabular}{@ {}c@ {}}A Brief History of Voting\end{tabular}} |
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436 |
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437 |
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438 \begin{itemize} |
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439 \item Athenians |
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440 \begin{itemize} |
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441 \item show of hands |
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442 \item ballots on pieces of pottery |
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443 \item different colours of stones |
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444 \item ``facebook''-like authorisation |
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445 \end{itemize}\bigskip |
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446 |
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447 \textcolor{gray}{problems with vote buying / no ballot privacy}\bigskip |
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448 |
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449 |
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450 \item French Revolution and the US Constitution got things ``started'' with |
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451 paper ballots (you first had to bring your own; later they were pre-printed by parties) |
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452 \end{itemize} |
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453 \end{frame}} |
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454 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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455 |
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456 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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457 \mode<presentation>{ |
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458 \begin{frame}[t] |
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459 \frametitle{\begin{tabular}{@ {}c@ {}}Ballot Boxes\end{tabular}} |
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460 |
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461 Security policies involved with paper ballots: |
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462 |
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463 \begin{enumerate} |
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464 \item you need to check that the ballot box is empty at the start of the poll / no false bottom (to prevent ballot stuffing) |
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465 \item you need to guard the ballot box during the poll until counting |
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466 \item tallied by a team at the end of the poll (independent observers) |
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467 \end{enumerate} |
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468 |
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469 \begin{center} |
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470 \includegraphics[scale=1.5]{pics/ballotbox.jpg} |
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471 \end{center} |
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472 |
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473 |
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474 \end{frame}} |
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475 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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476 |
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477 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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478 \mode<presentation>{ |
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479 \begin{frame}[t] |
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480 \frametitle{\begin{tabular}{@ {}c@ {}}Paper Ballots\end{tabular}} |
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481 |
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482 What can go wrong with paper ballots? |
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483 |
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484 \only<2>{ |
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485 \begin{center} |
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486 \includegraphics[scale=0.8]{pics/tweet.jpg}\\ |
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487 \footnotesize William M.~Tweed, US Politician in 1860's\\ |
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488 ``As long as I count the votes, what are you going to do about it?'' |
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489 \end{center}} |
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490 |
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491 \only<3>{ |
237 \medskip |
492 \medskip |
238 \item made popular in an article by Elias Levy\\ (also known as Aleph One):\\ |
493 \begin{center} |
239 \begin{center} |
494 \begin{minipage}{10cm} |
240 {\bf ``Smashing The Stack For Fun and Profit''} |
495 {\bf Chain Voting Attack} |
241 \end{center}\medskip |
496 \begin{enumerate} |
242 |
497 \item you obtain a blank ballot and fill it out as you want |
243 \begin{flushright} |
498 \item you give it to a voter outside the polling station |
244 \small\textcolor{gray}{\url{http://www.phrack.org}, Issue 49, Article 14} |
499 \item voter receives a new blank ballot |
245 \end{flushright} |
500 \item voter submits prefilled ballot |
246 |
501 \item voter gives blank ballot to you, you give money |
247 \end{itemize} |
502 \item goto 1 |
248 |
503 \end{enumerate} |
249 |
504 \end{minipage} |
250 \end{frame}} |
505 \end{center} |
251 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
506 } |
252 |
507 |
253 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
508 \end{frame}} |
254 \mode<presentation>{ |
509 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
255 \begin{frame}[c] |
510 |
256 \frametitle{\begin{tabular}{c}The Problem\end{tabular}} |
511 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
257 |
512 \mode<presentation>{ |
258 \begin{itemize} |
513 \begin{frame}[c] |
259 \item The basic problem is that library routines in C look as follows: |
514 |
260 \begin{center} |
515 Which security requirements do paper ballots satisfy better than voice voting?\bigskip |
261 {\lstset{language=Java}\fontsize{8}{10}\selectfont% |
516 |
262 \texttt{\lstinputlisting{app5.c}}} |
517 \begin{itemize} |
263 \end{center} |
518 \item Integrity |
264 \item the resulting problems are often remotely exploitable |
519 \item Enfranchisement |
265 \item can be used to circumvents all access control |
520 \item Ballot secrecy |
266 (botnets for further attacks) |
521 \item Voter authentication |
267 \end{itemize} |
522 \item Availability |
268 |
523 \end{itemize} |
269 \end{frame}} |
524 |
270 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
525 \end{frame}} |
271 |
526 |
272 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
527 |
273 \mode<presentation>{ |
528 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
274 \begin{frame}[c] |
529 \mode<presentation>{ |
275 \frametitle{\begin{tabular}{c}Variants\end{tabular}} |
530 \begin{frame}[t] |
276 |
531 \frametitle{\begin{tabular}{@ {}c@ {}}Mechanical Voting Machines\end{tabular}} |
277 There are many variants: |
532 |
278 |
533 \begin{itemize} |
279 \begin{itemize} |
534 \item<1-> Lever Voting Machines (ca.~1930 - 1990) |
280 \item return-to-lib-C attacks |
535 \only<1>{ |
281 \item heap-smashing attacks\\ |
536 \begin{center} |
282 \textcolor{gray}{\small(Slammer Worm in 2003 infected 90\% of vulnerable systems within 10 minutes)}\bigskip |
537 \includegraphics[scale=0.56]{pics/leavermachine.jpg} |
283 |
538 \end{center} |
284 \item ``zero-days-attacks'' (new unknown vulnerability) |
539 } |
285 \end{itemize} |
540 \item<2->Punch Cards (ca.~1950 - 2000) |
286 |
541 \only<2>{ |
287 \end{frame}} |
542 \begin{center} |
288 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
543 \includegraphics[scale=0.5]{pics/punchcard1.jpg}\;\; |
289 |
544 \includegraphics[scale=0.46]{pics/punchcard2.jpg} |
290 |
545 \end{center} |
291 |
546 } |
292 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
547 \end{itemize} |
293 \mode<presentation>{ |
548 |
294 \begin{frame}[c] |
549 |
295 |
550 |
296 \small |
551 \end{frame}} |
297 \texttt{my\_float} is printed twice:\bigskip |
552 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
298 |
553 |
299 {\lstset{language=Java}\fontsize{8}{10}\selectfont% |
554 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
300 \texttt{\lstinputlisting{C1.c}}} |
555 \mode<presentation>{ |
301 |
556 \begin{frame}[t] |
302 |
557 \frametitle{\begin{tabular}{@ {}c@ {}}Electronic Voting Machines\end{tabular}} |
303 \end{frame}} |
558 |
304 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
559 \begin{center} |
305 |
560 \begin{tabular}{c} |
306 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
561 \includegraphics[scale=0.45]{pics/dre1.jpg}\; |
307 \mode<presentation>{ |
562 \includegraphics[scale=0.40]{pics/dre2.jpg}\\\hline\\ |
308 \begin{frame}[c] |
563 \includegraphics[scale=0.5]{pics/opticalscan.jpg} |
309 |
564 \end{tabular} |
310 \begin{center} |
565 \end{center} |
311 \only<1>{\includegraphics[scale=0.9]{pics/stack1}\;\;} |
566 |
312 \only<2>{\includegraphics[scale=0.9]{pics/stack2}\;\;} |
567 \only<1->{ |
313 \only<3>{\includegraphics[scale=0.9]{pics/stack3}\;\;} |
568 \begin{textblock}{5.5}(1,4) |
314 \end{center} |
569 DREs |
315 |
570 \end{textblock}} |
316 |
571 \only<1->{ |
317 \end{frame}} |
572 \begin{textblock}{5.5}(1,11) |
318 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
573 Optical Scan |
319 |
574 \end{textblock}} |
320 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
575 |
321 \mode<presentation>{ |
576 \only<2>{ |
322 \begin{frame}[c] |
577 \begin{textblock}{5.5}(0.5,14.5) |
323 |
578 all are computers |
324 {\lstset{language=Java}\fontsize{8}{10}\selectfont% |
579 \end{textblock}} |
325 \texttt{\lstinputlisting{C2.c}}} |
580 |
326 |
581 \end{frame}} |
327 |
582 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
328 \end{frame}} |
583 |
329 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
584 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
330 |
585 \mode<presentation>{ |
331 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
586 \begin{frame}[c] |
332 \mode<presentation>{ |
587 \frametitle{\begin{tabular}{@ {}c@ {}}DREs\end{tabular}} |
333 \begin{frame}[c] |
588 |
334 |
589 Direct-recording electronic voting machines\\ |
335 \small |
590 (votes are recorded for example on memory cards) |
336 A programmer might be careful, but still introduce vulnerabilities:\bigskip |
591 |
337 |
592 typically touchscreen machines |
338 {\lstset{language=Java}\fontsize{8}{10}\selectfont% |
593 |
339 \texttt{\lstinputlisting{C2a.c}}} |
594 usually no papertrail |
340 |
595 |
341 |
596 \begin{center} |
342 \end{frame}} |
597 \includegraphics[scale=0.56]{pics/dre1.jpg} |
343 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
598 \end{center} |
344 |
599 |
345 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
600 |
346 \mode<presentation>{ |
601 \end{frame}} |
347 \begin{frame}[c] |
602 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
348 \frametitle{\begin{tabular}{c}Payloads\end{tabular}} |
603 |
349 |
604 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
350 \begin{itemize} |
605 \mode<presentation>{ |
351 \item the idea is you store some code as part to the buffer |
606 \begin{frame}[c] |
352 \item you then override the return address to execute this payload\medskip |
607 \frametitle{\begin{tabular}{@ {}c@ {}}Diebold Machines\end{tabular}} |
353 \item normally you start a root-shell\pause |
608 |
354 \item difficulty is to guess the right place where to ``jump'' |
609 The work by J.~Alex Halderman: |
355 \end{itemize} |
610 |
356 |
611 \begin{itemize} |
357 \end{frame}} |
612 \item acquired a machine from an anonymous source\medskip |
358 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
613 \item the source code running the machine was tried to be kept secret\medskip\pause |
359 |
614 |
360 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
615 \item first reversed-engineered the machine (extremely tedious) |
361 \mode<presentation>{ |
616 \item could completely reboot the machine and even install a virus that infects other Diebold machines |
362 \begin{frame}[c] |
617 \item obtained also the source code for other machines |
363 \frametitle{\begin{tabular}{c}Payloads (2)\end{tabular}} |
618 \end{itemize} |
364 |
619 |
365 \begin{itemize} |
620 \end{frame}} |
366 \item another difficulty is that the code is not allowed to contain \texttt{$\backslash$x00}: |
621 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
367 |
622 |
368 \begin{center} |
623 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
369 \texttt{xorl \%eax, \%eax} |
624 \mode<presentation>{ |
370 \end{center} |
625 \begin{frame}[c] |
371 \end{itemize}\bigskip\bigskip |
626 \frametitle{\begin{tabular}{@ {}c@ {}}Diebold Machines\end{tabular}} |
372 |
627 |
373 {\lstset{language=Java}\fontsize{8}{10}\selectfont% |
628 What could go wrong?\pause \;\;Failure-in-depth.\bigskip\pause |
374 \texttt{\lstinputlisting{app5.c}}} |
629 |
375 |
630 A non-obvious problem: |
376 \end{frame}} |
631 |
377 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
632 \begin{itemize} |
378 |
633 \item you can nowadays get old machines, which still store old polls |
379 |
634 |
380 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
635 \item the paper ballot box needed to be secured during the voting until counting; |
381 \mode<presentation>{ |
636 e-voting machines need to be secured during the entire life-time |
382 \begin{frame}[c] |
637 \end{itemize} |
383 \frametitle{\begin{tabular}{c}Format String Vulnerability\end{tabular}} |
638 |
384 |
639 \end{frame}} |
385 \small |
640 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
386 \texttt{string} is nowhere used:\bigskip |
641 |
387 |
642 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
388 {\lstset{language=Java}\fontsize{8}{10}\selectfont% |
643 \mode<presentation>{ |
389 \texttt{\lstinputlisting{programs/C4.c}}}\bigskip |
644 \begin{frame}[c] |
390 |
645 \frametitle{\begin{tabular}{@ {}c@ {}}Paper Trail\end{tabular}} |
391 this vulnerability can be used to read out the stack |
646 |
392 |
647 Conclusion:\\ Any electronic solution should have a paper trail. |
393 \end{frame}} |
648 |
394 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
649 \begin{center} |
395 |
650 \begin{tabular}{c} |
396 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
651 \includegraphics[scale=0.5]{pics/opticalscan.jpg} |
397 \mode<presentation>{ |
652 \end{tabular} |
398 \begin{frame}[c] |
653 \end{center}\pause |
399 \frametitle{\begin{tabular}{c}Protections against BO Attacks\end{tabular}} |
654 |
400 |
655 You still have to solve problems about |
401 \begin{itemize} |
656 voter registration, voter authentification, guarding against tampering |
402 \item use safe library functions |
657 |
403 \item ensure stack data is not executable (can be defeated) |
658 \end{frame}} |
404 \item address space randomisation (makes one-size-fits-all more difficult) |
659 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
405 \item choice of programming language (one of the selling points of Java) |
660 |
406 |
661 |
407 \end{itemize} |
662 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
408 |
663 \mode<presentation>{ |
409 \end{frame}} |
664 \begin{frame}[c] |
410 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
665 \frametitle{\begin{tabular}{@ {}c@ {}}E-Voting in India\end{tabular}} |
411 |
666 |
412 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
667 Their underlying engineering principle is ``keep-it-simple'': |
413 \mode<presentation>{ |
668 |
414 \begin{frame}[c] |
669 \begin{center} |
415 \frametitle{\begin{tabular}{c}Security Goals\end{tabular}} |
670 \begin{tabular}{c} |
416 |
671 \includegraphics[scale=1.05]{pics/indiaellection.jpg}\;\; |
417 \begin{itemize} |
672 \includegraphics[scale=0.40]{pics/india1.jpg} |
418 \item Prevent common vulnerabilities from occurring (e.g. buffer overflows)\pause |
673 \end{tabular} |
419 \item Recover from attacks (traceability and auditing of security-relevant actions)\pause |
674 \end{center}\medskip\pause |
420 \item Monitoring (detect attacks)\pause |
675 |
421 \item Privacy, confidentiality, anonymity (to protect secrets)\pause |
676 Official claims: ``perfect'', ``tamperproof'', ``no need for technical improvements'' , ``infallible'' |
422 \item Authenticity (needed for access control)\pause |
677 \end{frame}} |
423 \item Integrity (prevent unwanted modification or tampering)\pause |
678 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
424 \item Availability and reliability (reduce the risk of DoS attacks) |
679 |
425 \end{itemize} |
680 |
426 |
681 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
427 \end{frame}} |
682 \mode<presentation>{ |
428 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
683 \begin{frame}[c] |
429 |
684 \frametitle{\begin{tabular}{@ {}c@ {}}Lessons Learned\end{tabular}} |
430 |
685 |
431 |
686 \begin{itemize} |
432 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
687 \item keep a paper trail and design your system to keep this secure\medskip |
433 \mode<presentation>{ |
688 \item make the software open source (avoid security-by-obscurity)\medskip |
434 \begin{frame}[c] |
689 \item have a simple design in order to minimise the attack surface |
435 \frametitle{\begin{tabular}{c}Homework\end{tabular}} |
690 \end{itemize} |
436 |
691 |
437 \begin{itemize} |
692 \end{frame}} |
438 \item Assume format string attacks allow you to read out the stack. What can you do |
693 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
439 with this information?\bigskip |
694 |
440 |
695 |
441 \item Assume you can crash a program remotely. Why is this a problem? |
696 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
442 \end{itemize} |
697 \mode<presentation>{ |
443 |
698 \begin{frame}[c] |
|
699 |
|
700 \begin{center} |
|
701 \includegraphics[scale=0.56]{pics/Voting1.png} |
|
702 \end{center} |
|
703 |
|
704 |
|
705 \end{frame}} |
|
706 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
|
707 |
|
708 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
|
709 \mode<presentation>{ |
|
710 \begin{frame}[c] |
|
711 |
|
712 \begin{center} |
|
713 \includegraphics[scale=0.56]{pics/Voting2.png} |
|
714 \end{center} |
|
715 |
|
716 |
|
717 \end{frame}} |
|
718 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
|
719 |
|
720 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
|
721 \mode<presentation>{ |
|
722 \begin{frame}[c] |
|
723 |
|
724 \begin{center} |
|
725 \includegraphics[scale=0.56]{pics/Voting3.png} |
|
726 \end{center} |
|
727 |
|
728 |
|
729 \end{frame}} |
|
730 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
|
731 |
|
732 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
|
733 \mode<presentation>{ |
|
734 \begin{frame}[c] |
|
735 |
|
736 \begin{center} |
|
737 \includegraphics[scale=0.56]{pics/Voting4.png} |
|
738 \end{center} |
|
739 |
|
740 |
444 \end{frame}} |
741 \end{frame}} |
445 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
742 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
446 |
743 |
447 |
744 |
448 \end{document} |
745 \end{document} |