DES (Data Encryption Standards)

DES (Data Encryption Standards/Algorithm)

DES was adopted in 1977 by NIST (National Institute of Science & Technology).

In the above figure the plain text is 64-bits in length and the key is 56-bits in length.

Now the left hand side of the figure shows that the processing of plain text can process in three phases-

                                                       i.            Phase :

The 64-bit plain text passes through an initial permutation that rearranges the bits to produce the permuted input.

                                                     ii.            Phase :

This phase consists of 16 iteration of the same function.

                                                  iii.            Phase :

The output of 16^th iteration consists of 64-bits. That is a combination of inputted plain text and the key.

The right hand portion of the figure shows the use of 56-bit key.

It has two steps-

                                i.            The key is passed through a permutation function.

                              ii.            For each iteration, a sub key K_i is produced by the combination of a left circular shift and permutation.

Now we can examine the algorithm for a single iteration-

                                i.            The 64-bit permuted input passes through 16 iteration producing an intermediate 64-bit value at the conclusion of each iteration.

                              ii.            The left & right halves of each 64-bit values are treated as separate 32-bit quantities and get labelled left (L) and right (R).

                            iii.            The overall processing for each iteration is in terms of equation-

i.e.:

L_i = R­­_i-1

R­­_{i =} L_{i-1  XOR}  F(R_i-1 , K_i)

Where XOR shows bitwise X-OR function so the left hand output of an iteration (L_i) is simply equal to the right hand input to that iteration (R_i-1).

            And right hand output R_i is the Exclusive OR of L_i-1 and a complex function F(R_i-1 , K_i)

            This complex function F(R_i-1 , K_i) involves both permutation and substitution operation. The substitution operations which are represented as tables called S-Boxes.

The Decryption process with DES is same as follows:-

                                                        i.            Now the cipher text will be treated as input of DES algorithm.

                                                      ii.            Keys K_i will be used in reverse order, means K₁₆ will be used for first iteration and K₁₅ is for second iteration and so on until K₁ is used for last iteration.

Triple DES (TDES):

It was first proposed by Tuchman and first used in 1985.

It follows two processes –

                                i.            TDES uses three key pairs and three execution of DES algorithm.

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

Ex:

So we have equation of Encryption

C= E­­_K3 [D_K2[E­­­­­_K1[P]]]

Where

C=Cipher Text

P=Plain Text

E_K[X]= Encryption of X using key K

D_K[X]= Decryption of X using key K

And equation of Decryption

P= D­­_K1 [E_K2[D­­­­­_K3[C]]]

If the encryption is done only for second level with tow set of  keys K1 & K2 then this type of encryption scheme is known as double DES and when these encryption are done in three level then this encryption scheme is known as triple DES which uses three set of keys.

There is no cryptographic significance to the use of decryption for the second stage of encryption with three different keys. TDES has an effective key length of 168 bits. FIPS (Federal Information Processing standards) also allows for the use of two keys in TDES with K1=K3. This provides a key length of 112 bits.

FIPS includes the following guidelines for TDES –

                             i.)            Original DES algorithm which uses a 56 bit key length is permitted under the standards of legacy systems only.

                           ii.)            Govt. Organisations are encouraged transaction to TDES.

                         iii.)            It is very complex to crack 168 bit key cipher text with the help of any brute-force attack.

Advantages of TDES –

                                i.            The concept of TDES is based on DES algorithm.

                              ii.            It is having 168 bit key length so it overcomes the vulnerability to brute-force attack of DES.

Disadvantages of TDES –

                                i.            TDES has three time of iteration (rounds) of 16 so it is very time consuming and slower.

                              ii.            It follows 64 bit block size as an input so it can be more efficient if the block size is increased.