Digital Encryption Standard(DES)

Digital Encryption Standard(DES) :

• Proposed by IBM in 1975
• Standardized by US Govt in 1977
• It is a combination of permutation and substitution on blocks of
  64 bits. A message is broken up into 64 bit blocks and each
  block is separately encrypted.

#General idea used in DES

M = PLAINTEXT        01101100 11011000 11011010
K = KEY                      10101111 00101100 01011011
E = M ⊕ K                   11000011 11110100 10000001 encryption
M= E ⊕ K                    01101100 11011000 11011010 decryption

Digital Encryption Standard(DES) Algorithm :  Before applying DES the text is split up into the 64 bit blocks. DES applied on each 64 bit block.

Encryption method
Step 1: Apply an initial permutation on a block.Result is B=IP(P)
              where P is the 64 bit block IP Initial Permutation function and
               B the result.
Step 2: Split B into 32 bit blocks
              Li = leftmost 32 bits
              Ri = rightmost 32 bits.
Step 3: Pick a 56 bit key. Permute it
Step 4: Left circular shift it by 1 bit giving K1.
Step 5: Perform a complex sequence of operations and obtain
              X1 = F(R1,K1) (The complex set of operations include table look up
              and dropping bits).
              Step 6: Find R2 = L1 + X1
              Step 7: Set L2 = R1
              Repeat steps 2 to 7 16 times to get B16 = L16,R16
Step 8: Apply inverse of initial permutation on B16
              The result is the encrypted block
In summary the DES encryption applies the following transformation 16
times.
The i'th round transformation are
L i+1 = R i
R i+1 = L i ⊕ F(R i ,K i )
Each round has a different key K i
For Decryption the process of encryption is reversed. The encrypted block is
permuted using IP -1. On this transformations are applied starting with K 16
and going to K 1 last. The keys and F are same as those used in encryption
process.
The encryption process uses simple binary operations. They can thus be
released in hardware as an integrated circuit chip. DES chips are
inexpensive. Key is externally fed.