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What's the point of a Meet In The Middle attack while using, for example, a double AES encryption and using one time keys? You can recover the keys for a secret message already known and you can't use those keys to retrieve other secret messages. So why would an attacker use it?

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First off, a meet in the middle attack on AES is not even computationally feasible, so an attacker wouldn't use it period. –  mikeazo Aug 27 at 15:00

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up vote 4 down vote accepted

As mikeazo points out, a meet-in-the-middle (or any other known plaintext attack) against AES is believed to be infeasible; I'll generalize your question to "if we never reuse a key, why do we care about known plaintext attacks? If the attacker already knew the message, why do we care if he could decrypt it?"

The reason is that the attacker might be able to use that attack even if he has partial information about the plaintext. For example, suppose that the attacker knew (or guessed) that the text of the message that Alice sent Bob starts off with:

"Hi, Bob! The password to my bank account is "

In that case, the attacker may be able to use the known partial plaintext to recover the key; if he is able to do that, he can then decrypt the rest of the text, including the bank password.

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Never thought of that! So using an all or nothing transform will defeat any partial known plaintext attack? –  maincodebase Aug 27 at 18:00
    
@maincodebase: not quite any such attack; if the attacker has enough partial information to limit the number of potential messages to a small bound, as in "The first letter of my password is X", then an AON transform won't help you. On the other hand, we have ciphers that are immune to known plaintext (and chosen plaintext) attacks; hence we don't worry too much about how we might work with a cipher that isn't. –  poncho Aug 27 at 18:26

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