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What is the effective key size of DES?
56 bits
256 bits
128 bits
64 bits
DES is the archetypal block cipher, an algorithm that takes a fixed-length string of plaintext bits and transforms it through a series of complicated operations into another cipher text bitstring of the same length. In the case of DES, the block size is 64 bits. DES also uses a key to customize the transformation, so that decryption can supposedly only be performed by those who know the particular key used to encrypt.
The key ostensibly consists of 64 bits; however, only 56 of these are actually used by the algorithm. Eight bits are used solely for checking parity, and are thereafter discarded. Hence the effective key length is 56 bits.
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