# Symmetric-Key Based authentication [closed]

So i've managed to answer all the other cryptography questions, but I have not come across this exam style question before, and need help.

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## migration rejected from security.stackexchange.comMay 26 '14 at 18:55

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## closed as off-topic by e-sushi♦, rath, DrLecter, Gilles, Stephen TousetMay 26 '14 at 18:55

• This question does not appear to be about cryptography within the scope defined in the help center.
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@Neelam, I'm having a hard time seeing what your question is. Are you just wanting one of us to answer i) and ii) for you or are you having trouble understanding the protocol? –  mikeazo May 22 '14 at 11:57
Hint for i): Assume Trudy not knowing $K_A$ could impersonate as Alice to the NW with sizable probability, and prove Trudy could distinguish the cipher with unknown key $K_A$ from a random (permutation) oracle with sizable probability. Note for ii): IMHO the question is wrong and Eve does not even need to perform a man-in-the-middle attack, passive eavesdropping is enough. If you nevertheless want to answer the question as given, do the obvious: perform as NW vs Alice, and as Alice vs NW. –  fgrieu May 22 '14 at 13:18
This question appears to be off-topic because it is about the style of the exam paper and not cryptography. –  rath May 25 '14 at 10:49
Bleh. Posting a scan is lazy and rude. If you can't even be bothered to type a question, why should we be bothered to read it? And as for doing your homework, what have you tried? –  Gilles May 25 '14 at 19:43