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     8 \section*{Handout 5 (Protocols)}

     9 

    10 The protocols we are interested here are generic descriptions

    11 of how to exchange messages in order to achieve a goal, be it

    12 establishing a mutual secure connection or being able to

    13 authenticate to a system. Our notion of protocol is

    14 deliberately quite general: it includes situations like the

    15 messages send between a key fob and a car in order to open

    16 doors or the messages that participants need to exchange in

    17 order to mine Bitcoins (which is often already called Bitcoin

    18 \emph{protocol}).

    19 

    20 Unlike the distant past where for example we had to meet a

    21 person in order to authenticate him or her (via a passport for

    22 example), the problem we are facing is that on the Internet we

    23 cannot easily be sure who we are ``talking'' to. The obvious

    24 reason is that only some electrons arrive at our computer; we

    25 do not see the person, or computer, behind the incoming

    26 electrons. Often there are is also no person behind the

    27 messages, rather than a computer system.

    28 

    29 

    30 

    31 Keyfobs - protocol

    32 

    33 attack such protocols because they use weak ciphers (Oyster

    34 card)

    35 

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