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How many distinct stages are there in $DES$ algorithm, which is parameterized by a $56$-bit key ?

1. $16$
2. $17$
3. $18$
4. $19$
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DES(Data Encryption Standard) algorithm which is parameterized by a 56 bit key has 19 distinct stages including 16 rounds or repetition so ans is D
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Please explain how to solve this
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The answer should be A.16 since, first, one before last and the last are just transpositions of bits and independent of keys.
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19 stages
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Why 19 stages? I know about 16 of them only
 In DES, plain text (here, password) is encrypted in block of 64 bits, yielding 64 bits of cipher text. The algorithm which is parameterized by 56 bit key has 19 distinct stages.

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