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I sneakily edit a sneaky MITM attacker into your image: The problem with this protocol is that Bob does not authenticate with Alice. That means a man-in-the-middle attacker who can manipulate the data-flow between Alice and Bob can intercept the initial connection attempt from Alice and respond with their own certificate. Alice will then communicate ...


Anyone can respond to Alice. I can say I'm bob, and then we will communicate securely, Alice will send me important information, and she will believe that I'm bob. To prevent it, bob responds both certificate given to him by a trusted CA, proving he really is bob.


Thanks to comments from @Anders (thx!), I'm unsure if the password generator is a shared service or a personal authentication token like digipass or SecureID. Password generator is a shared service In this situation, Alice can only get the signed response H(R,K) by proving to the password generator that she is Alice by presenting her PIN. If Alice knew K, ...

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