Explain meet-in-the-middle attack in double DES

Subject: Information and Network Security
Semester: BE Computer Sem-7
Question: meet-in-the-middle attack in double DES

Answer :

This attack involves encryption from one end, decryption from the other and matching the results in the middle.

Suppose cryptanalyst known Pi and corresponding Ci

Now, the aim is to obtain the values of K1 and K2.

No. of encryptions and decryptions : 2^56 + 2^56 = 2^57

For double DES requires 2^57 operations for brute force attack.

For all possible values(2^56) of key K1 the cryptanalyst would encrypt the known plaintext by performing  E(K1,P).

The cryptanalyst would store output in a table.

Cryptanalyst  decrypt the known ciphertext with all possible values of K2.

In each case cryptanalyst will compare the resulting value with the all values in the table of ciphertext.