1.It is the most widely used algorithm for encryption and it is known as data encryption algorithm. (DEA)

2.In DES, data are encrypted in 64 blocks bits using a 56 bit key.

3.The algorithm transforms the 64 bit input in a series of steps into a 64 bit iutput using 56 bit key.

4.The same steps are used to reverse encryption using the same key.

5.The overall scheme of DES encryption is illustrated led into te adjoining figure.

$$\text{Figure 4.7 Overall Scheme of DES algorithm}$$

6.There are 2 inputs to the encryption function, the plain text of 64 bits ( to be encrypted)

7.The key of 64 bits (actually 56 bits + 8 parity bits)

8.The processing of plaintext (LHS) of above figure proceeds in three phases.

9.First the 64 bit plaintext passes through an initial permutation that rearranges the bits to produce permitted output.

10.Output of last round consists of 64 bits that are a function of input plain text and key.

11.The left and right halves of input are swapped to produce pre-output.

12.Finally the pre output is passed through inverse permutation (initially used) to produce 64 bit cipher text.

13.The R.H.S of figure shows the way in which 56 bit key is used.

14.Then for each round, a subkey Ri is produced by combination of left circular shift and permutation.

15.The permutation function is same for each round but a different subkey is produced because of repeated shift of key bits.

$$\text{Single Round of DES algorithm}$$

• The following figure shows the internal structure of a single round.
• The left and right halves of each 64 bit intermediate value are treated as separate 32 bit quantities labeled left (L) and Right (R)

$$\text{Figure 4.8 Single Round of DES algorithm}$$

The overall processing of each channel summarized as:

$L_i = R_i -1 \\ R_i = L_i -1 XOR F (R_i -1 ,k_i )$