514 The idea of public-private key encryption is that one can

   514 The idea of public-private key encryption is that one can make

   515 make public the key $P^{pub}$ which people can use to

   515 public the key $K^{pub}$ which people can use to encrypt

   516 encrypt messages for me. and I can use my key $P^{priv}$

   516 messages for me. and I can use my key $K^{priv}$ to be the

   517 to be the only one that can decrypt them. While this sounds

   517 only one that can decrypt them. While this sounds all good, it

   518 all good, it relies that people can associate me, for example,

   518 relies that people can associate me, for example, with my

   519 with my public key. That i snot so trivial as it sounds. 

   519 public key. That i snot so trivial as it sounds. For example,

   520 For example, if I would be the government, Obama for example, 

   520 if I would be the government, say Cameron, and try to find out

   521 and find out who are the trouble makers, I would publish an

   521 who are the trouble makers in the country, I would publish an

   522 innocent looking webpage and say I am the New York Times, for 

   522 innocent looking webpage and say I am The Guardian newspaper

   523 example, publish a public key, and then just wait for incoming 

   523 (or alternatively The Sun for all the juicy stories), publish

   524 messages. 

   524 a public key on it, and then just wait for incoming messages. 

   525 

   525 

   526 

   526 This problem is supposed to be solved by using certificates.

   527 

   527 The purpose of certification organisations is that they verify

   528 \bigskip\bigskip

   528 that a public key, say $K^{pub}_{Bob}$, really belongs to Bob.

   529 Keyfobs - protocol

   529 This is also the mechanism underlying the HTTPS protocol. The

   530 problem is that this system is essentially completely

   531 broken\ldots{}but this is a story for another time. Suffice

   532 to say for now that one of the main certification

   533 organisations, VeriSign, has limited its liability to \$100 in

   534 case it issues a false certificate. This is really a joke and

   535 really the wrong incentive for the certification organisations

   536 to clean up their mess.

   537 

   538 The problem we want to study closer here is that

   539 public-private key encryption is susceptible to

   540 person-in-the-middle attack. Consider the following protocol

   541 where $A$ and $B$ attempt to exchange secret messages using 

   542 public-private keys. 

   543 

   544 \begin{itemize}

   545 \item $A$ sends public key  to $B$

   546 \item $B$ sends public key  to $A$

   547 \item $A$ sends message encrypted with $B$'s public 

   548 key,\\ $B$ decrypts it with its private key

   549 \item $B$ sends message encrypted with $A$'s public 

   550 key,\\ $A$ decrypts it with its private key

   551 \end{itemize}

   552   

   553 

   554 \bigskip\bigskip Keyfobs - protocol