Double DES

In this approach, we use two instances of DES ciphers for encryption and two instances of reverse ciphers for decryption.

• Each instances use a different key.
• The size of the key is doubled.
• There are issues of reduction to a single stage.
• However, double DES is vulnerable to meet in the middle attack

Given a plaintext P and two encryption keys $K_1$ and $K_2$, a cipher text can be generated as,

$C = E(K_2, E(K_1, P))$

Decryption requires that the keys be applied in reverse order,

$P = D(K_1, D(K_2, C))$

Meet in the middle attack

The middle text, the text created by the first encryption or the first decryption, M, should be same

$M = EK_1(P) \quad $ and $ \quad M = DK_2(C)$

• Encrypt P using all possible values of $K_1$ and records all values obtained for M.

• Decrypt C using all possible values of $K_2$ and records all values obtained for M.

• Create two tables sorted by M values.

• Now compares the values for M until we finds those pairs of $K_1$ & $K_2$ for which the value of M is same in both tables

  

• Instead of using $2^{112}$ key search tests, we have to use $2^{56}$ key search tests two times.

• Moving from a Single DES to Double DES, we have to increase the strength from $2^{56}$ to $2^{57}$ .