DATA ENCRYPTION ALGORITHM

Some points to points to ponder

An initial permutation P before round 1,and its inverse at the end
Halves are swapped after last round
A final permuatation(inverse of P) is applied to (R₁₆,L₁₆) to yield ciphertext

weak keys

A weak key is the one which after parity drop operation,consists either of all 0's,all 1's or half 0's and half 1's
Four out of the 2⁵⁶ keys are weak keys

what the key scheduling algorithms does it just pemutates and shift operations
so if we do with the shift operation and permutation for this bits it almost results the same.That means there is non-linearity in the key scheduling its just trasposition
so these keys are considered as the weak keys
If we encrypt a block with a weak key and subsequently encrypt the result with the same weak key,we get the original block

Semi weak keys

A semi weak key creates only two different round keys and each of them is repeated eight times
There are six key pairs that are called semi-weak keys
The round keys created from each pair are the same in diffferent order

The main disadvantage of DES is the Key size because the key size is only 56 bits and this DES can be vulnerable to atacks and they can break the Key by Brute force attack.so in order to improve this they have deveoped 2-DES and 3-DES

The major criticism against DES is the key length
So,we may try cascading several DES applications

Multiple DES

2 DES

Uses two applications of the DES cipher
The total Key Size is 56 * 2 = 112 bits
However 2 DES is vulnerable to a known Plaintext attack

Attacker performs a known plaintext attack.He collects (P,C) pairs.
Using 1^st relation,he encrypts p using all possible 2⁵⁶ keys,and records all values for M
Using 2^ndrelation,he decryptes C using all possible 2⁵⁶ keys,and records all values for M

M = E_k1(P) and M = D_k2(C)

At some position the attacker will get the Plaintext
So they extended there this 2-DES into Triple Des

Triple DES(3 DES)

In cryptography,Triple DES is the common name for the Triple Data Encryption Algorithm(TDEA or Triple DEA) block cipher,which applies the Data encryption Standard(DES) cipher algorithm three times data block

The original DES cipher's key size of 56 bits was generally sufficient when that algorithm was designed,but the availability of increasing computational power made Brute Force Attacks feasible.Triple DES provides a relatively simple method of increasing the key size of DES to product against such attacks,with ut the need to design a completely new block algorithm

Triple DES uses a "key bundle" which comprises three DES keys,K₂,K₂,K₃,eah of 56 bits (excluding parity bits).The encryption algorithm is

cipher text = E_K3(D_K2(E_K1(plaintext)))

DES encrypt with K₁,DES decrypt with K₂,then DES encrypt with k₃
Decryption is the reverse:

plaintext = D_K1(E_K2(D_K3(cipher text)))

Decrypt with K₃,encrypt with K₂,then decrypt with K₁
Each triple encryption encrypts one block of 64 bits of data
In each case the middle operation is the reverse of the first and last.This improves the strength of the algorithm when using Key option 2,and provides Backward Compatibility with DES with Keying option 3
There are many keying options but usually use KEYs which are independent from one another to provide better security
we can take K₁ = K₃,and K₂ as different.Here the size of the key would be 112 bits and security will be less when compared to the different keys
we can also take same keys K₁,K₂,K₃, but it will be similar to the DES
the most prefered option in 3-DES is to use 3 different indeendent keys

The next stage of 3-DES is AES

Advanced Encryption Standard

Brute force attack

In crytogrpahy a brute force attack is a cryptanalytic attack.
It is used against any encrypted data.It comes into play when te atacker can't take advantage of other weakness in an encryption system(if any exist) that wuld make the task easier
It consists of systematically checking all possible keys until the correct key is found
In the worst case,this would involve traversing the entire search space