alicebob

   The Story of Alice and Bob
   
   (Short extract from after dinner speech by John Gordon at The Zurich
   Seminar April 1984)
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   I go to lots of conferences on Coding Theory in which complicated
   protocols get discussed. You know the sort of thing:
   
     "A communicates with someone who claims to be B, So to be sure, A
     tests that B knows a secret number K. So A sends to B a random
     number X. B then forms Y by encrypting X under key K and sends Y
     back to A"
     
   and so on. Because this sort of thing is is quite hard to follow, a
   few years ago theorists stopped using the letters A and B to represent
   the main players, and started calling them Alice and Bob. So now we
   say
   
     "Alice communicates with someone claiming to be Bob, and to be
     sure, So Alice tests that Bob knows a secret number K. Alice sends
     to Bob a random number X. Bob then forms Y by encrypting X under
     key K and sends Y back to Alice" 
     
   It's supposed to make it easier to understand.
   
   Now there are hundreds and hundreds of papers written about Alice and
   Bob.
   
   Alice and Bob have been used to illustrate all sorts of protocols and
   bits of coding theory in scientific papers. Over the years Alice and
   Bob have tried to defraud insurance companies, they've exchanged
   secret messages over a tapped line, and the've played poker for high
   stakes by mail. Now if we put together all the little details from
   lots of papers - a snippet from here, a snippet from there - we get a
   facinating picture of their lives.
   
   This may be the first time in the history of coding theory that a
   definitive biography of Alice and Bob has been given.
   
   Take Bob. Bob is often selling securities to speculators so we can be
   pretty sure he's a stockbroker. But from his concern about
   eavesdropping he is probably into something subersive on the side too.
   
   Take Alice. From the number of times Alice tries to buy stock from him
   we can say she is probably a speculator. And she's also worried that
   her husband doesn't get to find out about her financial dealings.
   
   So Bob is a subversive stockbroker and Alice is a two-timing
   speculator.
   
   But Alice has a number of serious problems.
     * She and Bob only get to talk by telephone or by email.
     * And in the country where they live the phone service is very
       expensive.
       
   And Alice and Bob are cheapscates.
   
   So the first thing Alice must do is MINIMISE THE COST OF THE PHONE
   CALL.
   
   The telephone in their country is also pretty lousy. The interference
   is so bad that Alice and Bob can hardly hear each other. So the second
   thing Alice must do is to PROTECT HER MESSAGES AGAINST ERRORS in
   transmission.
   
   On top of that Alice and Bob have very powerful enemies.
   
   One of their enemies the is the Tax Authority. Another is the Secret
   Police.
   
   These enemies have almost unlimited resources. They always listen in
   to telephone conversations between Alice and Bob.
   
   This is a pity since Bob and Alice are always plotting tax frauds and
   overthrowing the government.
   
   So the third thing ALICE must do is PROTECT HER COMMUNICATIONS FROM
   EAVESDROPPING.
   
   And these enemies are very sneaky. One of their favourite tricks is to
   telephone Alice and pretend to be Bob.
   
   So the fourth thing Alice has to do is to BE SURE SHE IS COMMUNICATING
   WITH WHO SHE THINKS SHE IS.
   
   Well, you think, so all Alice has to do is listen very carefully to be
   sure she recognises Bob's voice.
   
   But no.
   
   You see Alice has never met Bob. She has no idea what his voice sounds
   like.
   
   All in all Alice has a whole bunch of problems.
   
   Oh yes, and there is one more thing I forgot so say
   
   - Alice doesn't trust Bob.
   
   Now most people in Alice's position would give up.
   
   Not Alice.She has courage which can only be described as awesome.
   
   Against all odds, over a noisy telephone line, tapped by the tax
   authorities and the secret police, Alice will happily attempt, with
   someone she doesn't trust, whom she can't hear clearly, and who is
   probably someone else, to fiddle her tax return and to organise a cout
   d'etat, while at the same time minimising the cost of the phone call.
   
   A coding theorist is someone who doesn't think Alice is crazy.
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                                                       © John Gordon 1984