Implementing a symmetric cryptographic protocol : DES
The DES protocol allows encryption and decryption of 64-bits blocks, with a 56 bits key. This algorithm
uses various tables for substitution, expansion, permutations, and also uses a nonlinear operator : the
bitwise XOR operator.
The sequence of DES is the following
Let M be the original message (64 bits block to cipher)
Step 1
M is shuffled using an initial permutation (init_perm array)
The resulting block is splitted in two 32-bit blocks: L0 and R0
Step 2 is composed of 16 rounds described by the following picture:
2.1 expand Ri using the E table (expansion_table array)
2.2 generate 48 bits subkey keyi (see below)
2.3 operate a XOR between expanded version of Ri and keyi
2.4 input the 48 bit computed value at step 2.3 to the ith S-Box (s_boxes array), get the 32 bits output
2.5 permute the previous result (permut_32 array)
2.6 operate a XOR between the result of step 2.5 and Li
The computed value is then Li+1. Use Li as initial value for Ri+1
Step 3
At the output of the 16th round, the values are L16 and R16
L16 is appended to R16, then a reverse permutation is done (reverse_perm array)
Step 2.2 : Generating subkeys for each round.
An arbitrary 56 bits key is chosen, expanded to 64 bits with odd parity control : for each block of 7 bits,
an eight bit is added so that the number of 1s in the 8 bit-block is odd.
The key is divided in two blocks of 28 bits each :
The left block L is calculated from the key with the pc_1_left array
The right block R is calculated from the key with the pc_1_right array
Each of these two blocks is then left shifted by some number of positions, depending on the round.
For the first round, the shift is 1 bit left, for the second round, the shift is 2 bits left, … See the
keyshift table to know the left shift offset.
The shifted R block is then appended to the shifted L block, the resulting 48 blocks is finally extracted
using the pc_2 array.
This produces the key for the current round.