# Meet in the middle attack - message and key

I clearly understand the story of Alice and Bob:

Alice and Bob are going to use double DES. They know the keys K1 and K2.

Alice sends to Bob the encrypted message c= EK1(EK2(m)). Bob decrypts the message m = DK2(DK1(c)). Alice and Bob believe that Eve (the hacker) will need to discover both keys K1 and K2 by brute force to decrypt the message.

Eve has intercepted the "message m" and "c = EK1(EK2(m))". She wants to find K1 and K2. She computes EK(m) for all possible keys and stores the results in a list. She computes DK(c) for all possible keys and stores the results in a list. She compares the two lists, and looks for a match. If she founds a match then Eve knows K1andK2.

Now my question (sounds might be silly) is: How did Eve already get about the message? And if Eve gets the message then why she wanted to have the Keys K1 and K2 ?

• Read about known-plaintext attacks. – D.W. Mar 18 '14 at 22:09

• In addition of 8 aligned bytes of known plaintext, this attacks requires the ability to check which of about $2^{48}$ candidate keys are correct. This can be accomplished with about 6 other bytes of known plaintext (not necessarily aligned). – fgrieu Mar 18 '14 at 17:48