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Ciphers Interview Questions

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A block cipher transforms a fixed-length block of plaintext data into a block of ciphertext data of the same length. This transformation takes place under the action of a user-provided secret key. Decryption is performed by applying the reverse transformation to the ciphertext block using the same secret key. The fixed length is called the block size, and for many block ciphers, the block size is 64 bits. 

An iterated block cipher is one that encrypts a plaintext block by a process that has several rounds. In each round, the same transformation or round function is applied to the data using a subkey. The set of subkeys are usually derived from the user-provided secret key by a key schedule. 

Feistel ciphers are a special class of iterated block ciphers where the ciphertext is calculated from the plaintext by repeated application of the same transformation or round function. Feistel ciphers are also sometimes called DES-like ciphers.

In a Feistel cipher, the text being encrypted is split into two halves. The round function f is applied to one half using a subkey and the output of f is exclusive-ored with the other half. The two halves are then swapped. Each round follows the same pattern except for the last round where there is no swap. 

Exhaustive key search, or brute-force search, is the basic technique of trying every possible key in turn until the correct key is identified. To identify the correct key it may be necessary to possess a plaintext and its corresponding ciphertext, or if the plaintext has some recognizable characteristic, ciphertext alone might suffice. Exhaustive key search can be mounted on any cipher and sometimes a weakness in the key schedule of the cipher can help improve the efficiency of an exhaustive key search attack.

Advances in technology and computing performance will always make exhaustive key search an increasingly practical attack against keys of a fixed length. When DES was designed, it was generally considered secure against exhaustive key search without a vast financial investment in hardware. Over the years, this line of attack will become increasingly attractive to a potential adversary. 

Differential cryptanalysis is a type of attack that can be mounted on iterative block ciphers. These techniques were first introduced by Murphy [Mur90] in an attack on FEAL-4, but they were later improved and perfected by Biham and Shamir who used them to attack DES. Differential cryptanalysis is basically a chosen plaintext attack and relies on an analysis of the evolution of the differences between two related plaintexts as they are encrypted under the same key. By careful analysis of the available data, probabilities can be assigned to each of the possible keys and eventually the most probable key is identified as the correct one. 

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