Triple DES with 2-keys

It uses three stages of DES for encryption and decryption. The 1st, 3rd stage use $K_1$ key and 2nd stage use $K_2$ key.

To make triple DES compatible with single DES, the middle stage uses decryption in the encryption side and encryption in the decryption side.

It’s much stronger than double DES.

The function follows an encrypt-decrypt-encrypt (EDE) sequence.

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

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

By the use of triple DES with 2-key encryption, it raises the cost of the meet-in-the-middle attack to $2^{112}$ It has the drawback of requiring a key length of $56 \times 3 = 168$ bits which may be somewhat unwieldy.

Triple DES with 3-key

Although the attacks just described appear impractical, anyone using two key 3DES may feel some concern. Thus, many researchers now feel that 3-key 3DES is the preferred alternative.

It uses three stages of DES for encryption and decryption with three different keys. 3-key 3DES has an effective key length of 168 bits and is defined as,

$C = E(K_3, D(K_2, E(K_1, P)))$

$ P = D(K_1, E(K_2, D(K_3, C)))$