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Ciphers Interview Questions

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Since there are 256 possible DES keys the chance of picking a weak or semi-weak key at random is 2-52. As long as the user-provided key is chosen entirely at random, they can be safely ignored when DES is used for encryption. Despite this, some users prefer to test whether a key to be used for DES encryption is in fact a weak key. Such a test will have no significant impact on the time required for encryption. 

Export of DES, either in hardware or software, is strictly regulated by the U.S. State Department and the NSA. The government rarely approves export of DES, despite the fact that DES is widely available overseas; financial institutions and foreign subsidiaries of U.S. companies are exceptions. 

Over the years, various new block cipher algorithms have been designed as alternatives to DES. One is FEAL, a cipher for which numerous attacks have been discovered. IDEA is a cipher designed by Lai and Massey that seems much more promising and two more recent designs are RC5 and SAFER. In addition, the U.S. government announced in 1993 an algorithm called Skipjack as part of its Capstone project. Skipjack operates on 64-bit blocks of data, as does DES, but uses 80-bit keys, as opposed to the 56-bit keys in DES. However, the details of Skipjack are classified, so Skipjack is only available in hardware from government-authorized manufacturers. 

Linear cryptanalysis was first devised by Matsui and Yamagishi in an attack on FEAL. It was extended by Matsui to attack DES. Linear cryptanalysis is a known plaintext attack and uses a linear approximation to describe the behavior of the block cipher. Given sufficient pairs of plaintext and corresponding ciphertext, bits of information about the key can be obtained and increased amounts of data will usually give a higher probability of success. 

Weak keys are secret keys with a certain value for which the block cipher in question will exhibit certain regularities in encryption or, in other cases, a poor level of encryption. For instance, with DES there are four keys for which encryption is exactly the same as decryption. This means that if one were to encrypt twice with one of these weak keys, then the original plaintext would be recovered. For IDEA there is a class of keys for which cryptanalysis is greatly facilitated and the key can be recovered. However, in both these cases, the number of weak keys is such a small fraction of all possible keys that the chance of picking one at random is exceptionally slight. In such cases, they pose no significant threat to the security of the block cipher when used for encryption. 

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