This new BatFly messaging service solves the public key distribution problem,” announces Watson cheerfully. “Suppose Ali

[answerhappygod]

This new BatFly messaging service solves the public key
distribution problem,” announces Watson cheerfully. “Suppose Alice
wants to send a message m to Bob, but she’s not sure what public
key Bob is using at the moment. Alice sends the message to the
BatFly server, encrypted with the server’s public key to keep the
message secret. Alice specifies that she wants to send the message
to Bob, and signs the whole thing with her private key to prove
that the message really is from her: A → S : A, Ea- [ B, Es+ [ m ]
] The BatFly server knows Bob’s up-to-date public key, and uses it
to re-encrypt the message m. Then the server signs everything for
integrity, and sends it back to Alice: S → A : S, Es- [ B, Eb+ [ m
] ] Now Alice sends this message on to Bob using whatever app she
likes. The only public key that Alice has to know is the key for
the server. And when Bob changes his public key, he only has to
tell the BatFly server, not everyone else. The server never knows
anybody else’s private key, and revocation is trivial!” Holmes has
been examining the protocol closely. “Doesn’t anything worry you
about the protocol at all, Watson?” he asks. “Well, now that you
mention it,” replies Watson, “I’m a little uneasy about encrypting
and then signing. I was thinking of recommending to them that it
should be the other way round: A → S : B, Es+ [ A, Ea- [ m ] ] S →
A : B, Eb+ [ S, Es- [ m ] ] What do you think, Holmes?” Holmes
shakes his head. “I think that if I were Moriarty I’d be using
BatFly to revoke my public key right now,” he replies
enigmatically.q)Suggest and discuss some ways that Moriarty might
attack the Batfly server. Propose a revised version of the protocol
and discuss the strengths and weaknesses of your revised version.
State any assumptions carefully.