305 such random numbers are often called \emph{nonce}. What is the |
305 such random numbers are often called \emph{nonce}. What is the |
306 purpose of this nonce? Well, if an attacker records $A$'s |
306 purpose of this nonce? Well, if an attacker records $A$'s |
307 answer, it will not make sense to replay this message, because |
307 answer, it will not make sense to replay this message, because |
308 next time this protocol is run, the nonce $B$ sends out will |
308 next time this protocol is run, the nonce $B$ sends out will |
309 be different. So if we run this protocol, what can $B$ infer? |
309 be different. So if we run this protocol, what can $B$ infer? |
310 It has send out an (unpredictable) nonce to $A$ and received |
310 It has sent out an (unpredictable) nonce to $A$ and received |
311 this challenge back, but encoded under the key $K_{AB}$. If |
311 this challenge back, but encoded under the key $K_{AB}$. If |
312 $B$ assumes only $A$ and $B$ know the key $K_{AB}$ and the |
312 $B$ assumes only $A$ and $B$ know the key $K_{AB}$ and the |
313 nonce is unpredictable, then $B$ is able to infer it must be |
313 nonce is unpredictable, then $B$ is able to infer it must be |
314 talking to $A$. Of course the implicit assumption on this |
314 talking to $A$. Of course the implicit assumption on this |
315 inference is that nobody else knows about the key $K_{AB}$ |
315 inference is that nobody else knows about the key $K_{AB}$ |
553 publish the key $K^{pub}$ which people can use to encrypt |
553 publish the key $K^{pub}$ which people can use to encrypt |
554 messages for me and I can use my private key $K^{priv}$ to be |
554 messages for me and I can use my private key $K^{priv}$ to be |
555 the only one that can decrypt them. While this sounds all |
555 the only one that can decrypt them. While this sounds all |
556 good, it relies on the ability that people can associate me |
556 good, it relies on the ability that people can associate me |
557 with my public key. That is not as trivial as it sounds. For |
557 with my public key. That is not as trivial as it sounds. For |
558 example, if I would be the government, say Cameron, and try to |
558 example, if I would be the government, say Theresa Mayhem, and try to |
559 find out who are the trouble makers in the country, I would |
559 find out who are the trouble makers in the country, I would |
560 publish an innocent looking webpage and say I am The Guardian |
560 publish an innocent looking webpage and say I am The Guardian |
561 newspaper (or alternatively The Sun for all the juicy |
561 newspaper (or alternatively The Sun for all the juicy |
562 stories), publish a public key on it, and then just wait for |
562 stories), publish a public key on it, and then just wait for |
563 incoming messages. |
563 incoming messages. |