6-Encryption

Data Encryption Standard algorithm written in C++

 -hhhhhhhhhhhhhhhhh:             +hhhhhhhhhhhhhhhhhhhhhhh/          shhhhhhhhhhhhhs +hhhh.      
 :NNNNNNNNNNNNNNNNNo--.          oNNNNNNNNNNNNNNNNNNNNNNN+       .--dNNNNNNNNNNNNNd-yNNNN-      
 `--/NNNNh-------/NNNNy          .--sNNNNo----------+NNNN+       mNNNm-----------dNNNNNNN-      
    .NNNNh       .yyyNmyyo          +NNNN+          .yyyy/       mNNNd           syydNNNN-      
    .NNNNh           NNNNh          +NNNN+    `....              mNNNd              oNNNN-      
    .NNNNh           NNNNh          +NNNN+    hmmmm`             mNNNd              .::::`      
    .NNNNh           NNNNh          +NNNNy++++mNNNN`             hddNm+++++++++-                
    .NNNNh           NNNNh          +NNNNNNNNNNNNNN`             ```hNNNNNNNNNNo``````          
    .NNNNh           NNNNh          +NNNNy++++mNNNN`                :+++++++++Nmdddddd`         
    .NNNNh           NNNNh          +NNNN+    ymmmm`                         `mmmmmmmN///`      
    .NNNNh           NNNNh          +NNNN+    `....                           ......sNNNN-      
    .NNNNh           NNNNh          +NNNN+                       yhhhy              oNNNN-      
    .NNNNh       `---NNNNh          +NNNN+          `----`       mNNNm---           oNNNN-      
    .NNNNh       -mmmNh--.          +NNNN+          -mmmm+       mNNNNmmm.          oNNNN-      
 .yyyNNNNmyyyyyyyhNdyy+          /yydNNNNdyyyyyyyyyyhNNNN+       mNNNNydNyyyyyyyyyyydNhyy.      
 :NNNNNNNNNNNNNNNNN/             sNNNNNNNNNNNNNNNNNNNNNNN+       mNNNm oNNNNNNNNNNNNNN`         
 `/////////////////.             -///////////////////////.       :///: .//////////////          

The Data Encryption Standard is a symmetric-key algorithm for the encryption of electronic data.

Sample

How DES works

• Message (string text)
• Key (string text)
• Convert message and key to binary (string binary)

Key generation (16 keys needed)

• Run the binary key through PC-1 table (From 64 to 56 bit)
• Split the resulting 56bit key into 2 parts C and D
• Keys should be shifted to left 1 or 2 bits 16 times to generate other keys. Number of left shifts is determined with a table.
• After getting 16 pairs of C and D sets, concatenate them
• Run the resulting keys through PC-2 // 16 keys for each round.

Encyription algorithm

• Split binary message to 64 bit chunks

• Run 64 bit data through IP (initial permutation)

• Divide message into to parts: L(left) and R(right)

• Run these formulas for 16 rounds L(n) = R(n-1); R(n) = L(n-1) + f(R(n-1), K(n)); // '+' is XOR operation

  • f function first expanpandes R from 32 bit to 48 bit (run through E selection table)
  • f function then apply XOR operation: (R(n-1)^K(n))
  • f function then runs the result (8 times 6 bits = 48 bit) through 8 S boxes. S box is type of table. First and last bits help to find row and middle 4 bits help to find column. Table outputs 4 bit data.
  • f function, finally, passes the 32 bit result through a Permutation Function

• After finding L(16) and R(16), order should be reversed and concatenated like this: R(16)L(16)

• Concatinated string should be passed through final permutation IP(-1) table

Decryption is simply the inverse of encryption, follwing the same steps as above, but reversing the order in which the subkeys are applied.

Tools

• Visual Studio 2017 Community version
• C++