Data Encryption Standard DES ,Advantages Disadvantages algorithm

Introduction

DES is the block cipher which takes a fixed-length string of plaintext bits and
transforms it through a series of complicated operations into another cipher
text bit string of the same length. It is a symmetric encryption technique which
means both sender and receiver use a shared key to encrypt and/or decrypt the
data. The only problem with this technique is that if the key is known to others
the entire conversation is compromised. In this, the block size is 64 bits it also
uses a key to customize the transformation, so that decryption can supposedly
only be performed by those who know the particular key used to encrypt. The
key basically consists of 64 bits however, only 56-bits of these are actually
used by the algorithm. Eight bits are used solely for checking parity, and are
thereafter discarded. Hence the effective key length is 56-bits, and it is always
quoted as such. Every 8th bit of the selected key is discarded i.e., positions 8,
16, 24, 32, 40, 48, 56, 64 are removed from the 64-bit key leaving behind only
the 56-bit key.

 

The DES algorithm

The main parts of the algorithm are as follows:

• Fractioning of the text into 64-bit (8 octet) blocks;
• Initial permutation of blocks;
• Breakdown of the blocks into two parts: left and right, named L and R;
• Permutation and substitution steps repeated 16 times (called rounds);
• Re-joining of the left and right parts then inverse initial permutation.

Encryption algorithm

The overall structure of encryption steps are as follows:

1. A block of 64 bits is permuted by an initial permutation called IP.
2. Resulting 64 bits are divided in two halves of 32 bits, left and right.
3. Right half goes through a function F(Feistel function)
4. Left half is XOR-ed with output from F function above.
5. Left and right are swapped(except last round).
6. If last round, apply an inverse permutation IP-1 on both halves and that’s the output else, goto step 3.

Steps 3-5 constitute a round. DES has 16 identical rounds. Note the two halves are processed alternately.

Advantages and disadvantages of DES

Advantages

1. DES has been around a long time (since 1977), even now no real weaknesses
have been found: the most efficient attack is still brute force.

2. DES is an official United States Government standard; the Government is
required to re-certify, DES every five years and ask it be replaced if necessary.
DES has been re-certified in 1983, 1987, 1992.

3. DES is also an ANSI and ISO standard - anybody can learn the details and
implement it.

4. Since DES was designed to run on 1977 hardware, it is fast in hardware and
relatively fast in software.

Disadvantages

1. The 56-bit key size is the biggest defect of DES. Chips to perform one
million of DES encrypt or decrypt operations a second are available (in 1993).
A $1 million DES cracking machine can search the entire key space in about 7
hours.

2. Hardware implementations of DES are very fast; DES was not designed for
software and hence runs relatively slowly.

3. As the technology is improving lot more day by day so there is a possibility
to break the encrypted code, so AES is preferred than DES.

4. As we know in DES only one private key is used for encryption as well as for
decryption because it is symmetric encryption technique so if we lost that key to
decrypt the data then we cannot get the readable data at the receiving end.