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\section*{Handout 5 (Protocols)}

The protocols we are interested here are generic descriptions
of how to exchange messages in order to achieve a goal, be it
establishing a mutual secure connection or being able to
authenticate to a system. Our notion of protocol is
deliberately quite general: it includes situations like the
messages send between a key fob and a car in order to open
doors or the messages that participants need to exchange in
order to mine Bitcoins (which is often already called Bitcoin
\emph{protocol}).

Unlike the distant past where for example we had to meet a
person in order to authenticate him or her (via a passport for
example), the problem we are facing is that on the Internet we
cannot easily be sure who we are ``talking'' to. The obvious
reason is that only some electrons arrive at our computer; we
do not see the person, or computer, behind the incoming
electrons. Often there are is also no person behind the
messages, rather than a computer system.



Keyfobs - protocol

attack such protocols because they use weak ciphers (Oyster
card)

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