Multiple Encryption and Triple DES

Multiple Encryption and Triple DES

Multiple Encryption increase the security of confidential data. This technique increase the data security in such a manner that unauthorized user can not access any part of information over wireless network such as internet. 

A. Double DES

In this case two encryption as well as two decryption are performed (as shown in Fig. 1). 

Fig. 1: Double Encryption
For Encryption
                    C = E(k2,E(k1,P))
For Decryption
                    P = D(k1,D(k2,C))

Disadvantage 😔 : 

Meet In Middle Attack 

This attack involves encryption from one end and decryption from another end and then "Matching the result in the middle", and hence, the name called as meet in middle attack or MIM. 

This attack requires knowing some plaintext / ciphertext pairs. 

Fig. 2: Meet In Middle Attack. 

The middle text created by first encryption and first decryption should be same or written as-
M = Ek1(P)  and   M = Dk1(C)

  • Encrypt P using all possible values of k1 and records all values obtained for M.

  • Decrypt C using all possible values of k2 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 k1& k2 for which the value of M is same in both tables. (as shown in Fig. 3).

Fig. 3: Table 
NOTE 1: Now more than 1 pair of keys may result in a match, but these number of pairs will be small. we should try each possible pair  of keys so it takes twice as long to break double DES using brute force. 
DES has 2^56 bit security. 
Double DES has 2 x 2^56 i.e. 2^57 bit security. 

B. Triple DES (3-DES) with Two Keys 
Since 3-DES overcome the drawback of 2 DES i.e., Meet in Middle attack but somehow it is unwieldy because it use the key length of 168 bits i.e. 3 x 56. 
Fig. 3: Triple DES

For Encryption
C = E(k1,D(k2,E(k1,P)

For Decryption
P = D(k1,E(k2,D(k1,C)))

Overcome the Limitation of MIM 😉: 

For every plaintext value that matches with P of shown fig. 4 create an entry in another table with the key value. Now sort the table according to the value of B.
For a given known (P,C) the probability of selecting the unique value of any text leads to success of 1/(2^64). 

prob(x) = Possible Outcomes / Total Outcomes
 = (2^64) / n 
So expected running time of attack is on the order of -
((2^56).(2^64)) / n 

Fig. 4: Known-Plaintext Attacks on Triple DES

C. Triple DES (3-DES) with Three Keys
In this case we just use total of 3 keys.
So,
        C = E(k3, D(k2, E(k1, P))

Note 2 : Many Researchers find attack with two keys are impractical so they use 3 keys instead of 2. 

NOTE 3 : Why 4DES is not used ??😑 You can use 4DES but should not be used. In case of 2DES there is an attack taking about 2^56 time and space complexity. In case of 4DES it will take 2^112.        so instead of using 4DES you can use AES because DES is slow as compared to AES. So if you repeat 4 times DES then this will kill you. 









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