Data Encryption Standard DES):

• DES uses both transposition and substitution and for that reason sometimes referred as product cipher  its input,output and key are 64 bit long the set of 64 bits referred as blocks
• The Cipher consisting of 16 rounds or iteration . Each round uses separate key of 48 bits 
• DES encryption algorithm is the first 64 bits plain text uses through initial permutation(IP) that rearrange the bits to produce permitted input .

Then there is a phase consisting of 16 rounds of the same function which involves both permutation and substitution technique functions. The output of 16 round consisting of 64 bits that are function opf input plain text and key

• The left and right half of the are swapped to produce the pre-output atleast the pre-output is passed through a permutation.(IP) . i.e,the inverse of initial permutation to produce the 64 bit cipher text.

INITIAL PERMUTATION:

•  The initial permutation and its inverse the input to the table consists of 64 bits numbered from 1-64.
•  The 64 entries in the permutation table contain the permutation of the numbers from 1-64 each entry in the permutation  indicates the position of numbered input bit in the output which also contains 64-bits as the output.

• The over all processing at each round can be summarized in the following 

  Li=Ri-1

  Ri=li-F*(Ri-1,Ki)

• The left output li is the copy of right input Ri-1 
• The right output Ri is the XOR of left input Li-1 and right input Ri-1and key Ki
• The substitution and permutation both functions are used.

SUBSTITUTION box:

•  It is a basic component of symmetric key algorithms which performs substitution. In block ciphers, they are typically used to obscure the relationship between the key and the ciphertext
• The 48 bit input block is divided into 8-sub blocks and each sub block is given to a s-box . The S-box transforms the 6-bit input 4-bit output.
•  A two bit binary number to select one of the 4 substitutions defined by the four rows in the table the two bits can store any decimal between 0-3 this specifies row number the middle 4 bits selects one of the 16 columns.

Key Generation:

64 bit key is used as the input to the algorithm the initial 64-bit key is transformed into 56-bit the key by discarding every 8th bit of a initial key.