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\section*{Handout 5 (Protocols)}+
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The protocols we are interested here are generic descriptions+
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of how to exchange messages in order to achieve a goal, be it+
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establishing a mutual secure connection or being able to+
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authenticate to a system. Our notion of protocol is+
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deliberately quite general: it includes situations like the+
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messages send between a key fob and a car in order to open+
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doors or the messages that participants need to exchange in+
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order to mine Bitcoins (which is often already called Bitcoin+
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\emph{protocol}).+
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Unlike the distant past where for example we had to meet a+
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person in order to authenticate him or her (via a passport for+
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example), the problem we are facing is that on the Internet we+
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cannot easily be sure who we are ``talking'' to. The obvious+
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reason is that only some electrons arrive at our computer; we+
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do not see the person, or computer, behind the incoming+
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electrons. Often there are is also no person behind the+
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messages, rather than a computer system.+
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+
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Keyfobs - protocol+
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+
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{\small+
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\url{http://www.cs.ru.nl/~rverdult/Gone_in_360_Seconds_Hijacking_with_Hitag2-USENIX_2012.pdf}}+
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attack such protocols because they use weak ciphers (Oyster+
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card)+
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