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Ciphers Interview Questions

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For some time it has been common practice to protect and tansport a key for DES encryption with triple-DES. This means that the plaintext is, in effect, encrypted three times. There are, of course. a variety of ways of doing this; we will explore these ways below. See Question 85 for a discussion of multiple encryption in general.

A number of modes of triple-encryption have been proposed:

DES-EEE3: Three DES encryptions with three different keys.
DES-EDE3: Three DES operations in the sequence encrypt-decrypt-encrypt with three different keys.
DES-EEE2 and DES-EDE2: Same the previrous formats except that the first and third operations use the same key.

Attacks on two-key triple-DES have been proposed by Merkle and Hellman [MH81] and Van Oorschot and Wiener [VW91], but the data requirements of these attacks make them impractical. 

Until recently, the most significant use of triple-DES was for the encryption of single DES keys, and there was really no need to consider how one might implement various block cipher modes when the block cipher in question is actually one derived from multiple encryption. However, as DES nears the end of its useful lifetime, more thought is being given to an increasingly widespread use of triple-DES. 

G-DES was devised by Schaumuller-Bichl to improve on the performance of DES by defining a cipher based on DES with a larger block size, but without an increase in the amount of computation required. It was claimed that G-DES was as secure as DES since the cipher was based on DES. However, Biham and Shamir showed that G-DES with the recommended parameter sizes is easily broken and that any alterations of G-DES parameters that result in a cipher faster than DES are less secure than DES. 

DES is the Data Encryption Standard, an encryption block cipher defined and endorsed by the U.S. government in 1977 as an official standard; the details can be found in the latest official FIPS (Federal Information Processing Standards) publication concerning DES. It was originally developed at IBM. DES has been extensively studied since its publication and is the most well-known and widely used cryptosystem in the world.

DES is a symmetric cryptosystem: when used for communication, both sender and receiver must know the same secret key, which is used both to encrypt and decrypt the message. DES can also be used for single-user encryption, such as to store files on a hard disk in encrypted form. In a multi-user environment, secure key distribution may be difficult; public-key cryptography provides an ideal solution to this problem 

No easy attack on DES has been discovered, despite the efforts of many researchers over many years. The obvious method of attack is brute-force exhaustive search of the key space; this takes 255 steps on average. Early on it was suggested that a rich and powerful enemy could build a special-purpose computer capable of breaking DES by exhaustive search in a reasonable amount of time. Later, Hellman [Hel80] showed a time-memory trade-off that allows improvement over exhaustive search if memory space is plentiful, after an exhaustive precomputation. These ideas fostered doubts about the security of DES. There were also accusations that the NSA had intentionally weakened DES. Despite these suspicions, no feasible way to break DES faster than exhaustive search was discovered. The cost of a specialized computer to perform exhaustive search (requiring 3.5 hours on average) has been estimated by Wiener at one million dollars. 

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