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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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