### DES Operates on blocks of 64 bits of plaintext, encrypted with a block of 56 key bits. It is constructed as successive rounds comprised of the most popular crypto primitives: transposition, substitution, XORing, Concatenation.
Three successive elements:
Key Independent transposition
Key bit-wise operation
Key Independent transposition
The 56 bits key is expressed as 8 7bits strings. Usually each string is trailed with a parity bit, combining into a 64 bits block.
Deciphering DES works the same as encrypting with the one difference that the 16 48bits extracted keys are used in reverse: the 16th key is used first, and the first key is used last.
DES comes with slight modifications for international use, sometimes known as DEA: data encryption algorithm (the final permutation in the f function is omitted).
### The First Key Independent Transposition:
The 64 plaintext block is transposed by a speicifc Transposition operator:
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T=58, 50, 42, 34, 26, 18, ..., 23, 15, 7,
namely: in the transposed list the original 58th bit becomes the first, the 50s bit becomes the second,...
### The Second Key Independent Transposition:
Reverse transposition of the starting action, T^{-1} |