supplied from user is of 64 bits. The following operations are performed with it. Drop Parity bits Drop the bits of the grey positions (8x) to make 56 bit space for further operation for each round. After that bits are permuted according to the following table, The table is row major way, means, Actual Bit position = Substitute with the bit of row * 8 + column. Bits Rotation Before the round sub-key is selected, each half of the key schedule state is rotated left by a number of places. This table specifies the number of places rotated. • The key is divided into two 28-bit parts • Each part is shifted left (circular) one or two bits • After shifting, two parts are then combined to form a 56 bit temp-key again Key Compression • The compression P-box changes the 56 bits key to 48 bits key, which is used as a key for the corresponding round. The table is row major way, means, Actual Bit position = Substitute with the bit of row * 8 + column. After this return the Round-Key of 48 bits to the called function, i.e. the Round. References External links The DES Algorithm Illustrated Data Encryption Standard S-box