DES

1. Strength of DES
The Concerns about the level of security
provided by DES. These concerns, by and large,
fall into two areas:
1. key size
2.nature of the algorithm.

2. 1. The Use of 56-Bit Keys
• With a key length of 56 bits, there are 256 possible
keys, which is approximately 7.2 * 1016 keys. Thus, on
the face of it, a brute-force attack
appears impractical. Assuming that, on average, half th
e key space has to be searched, a single machine
performing one DES encryption per microsecond would
take more than a thousand years to break the cipher.

3. 2. The Nature of the DES Algorithm
Another concern is the possibility that cryptanalysis is possible
by exploiting the characteristics of the DES algorithm. The focus
of concern has been on the eight substitution tables, or S-boxes,
that are used in each iteration. Because the design
criteria for these boxes, and indeed for the entire algorithm, were
not made public,
there is a suspicion that the boxes were constructed in such a way
that cryptanalysis
is possible for an opponent who knows the weaknesses in the S-
boxes.

4. Timing Attacks
In essence, a timing attack is one in which infor
mation about the key or the plaintext
is obtained by observing how long it takes a give
implementation to perform decryptions on variou
s ciphertexts. A timing attack exploits the fact tha
t an encryp- tion or decryption algorithm often
takes slightly different
amounts of time on different inputs.

5. Block Cipher Design Principles
Block ciphers are built in the Feistel cipher structure. Block
cipher has a specific number of rounds and keys for generating
ciphertext. For defining the complexity level of an algorithm
few design principles are to be considered.
These are following:-
1. Number of Rounds
2. Design of function F
3. Key schedule algorithm

6. 1.Number of Rounds
The number of Rounds is regularly considered in
design criteria, it just reflects the number of
rounds to be suitable for an algorithm to make it
more complex, in DES we have 16 rounds
ensuring it to be more secure while in AES we
have 10 rounds which makes it more secure.

7. 2. Design of function F
• The core part of the Feistel Block cipher structure is the
Round Function. The complexity of cryptanalysis can be
derived from the Round function i.e. the increasing level of
complexity for the round function would be greatly
contributing to an increase in complexity.
• To increase the complexity of the round function, the
avalanche effect is also included in the round function, as
the change of a single bit in plain text would produce a
mischievous output due to the presence of avalanche effect.

8. 3.Key schedule algorithm
• In Feistel Block cipher structure, each round
would generate a sub-key for increasing the
complexity of cryptanalysis. The Avalanche
effect makes it more complex in deriving sub-
key. Decryption must be done very carefully to
get the actual output as the avalanche effect is
present in it.