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Secret Key algorithms are based on elaborating a simple idea
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Caesar rotated alphabet in his cipher. An obvious extension of this is use a 1<-ɭ permutation of a group of N bits
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for DES N=64 and permutation is calculated from a 48 bit key
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To make decoding harder, this is done 16 times with different keys extracted from an original 56 bit secret
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Note secrets in real world are usually generated randomly
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This strategy is combined with (ad-hoc) transformations to further obfuscate the process
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The full message must be divided into blocks before applying this process and the method of running secret key cryptography on long messages is non trivial (but not very fundamental) as doing in 64 bit separate units would allow information to be freely shuffled!
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