Strength of des & block cipher principle
- 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.