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6-Encryption

Data Encryption Standard algorithm written in C++

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 `/////////////////.             -///////////////////////.       :///: .//////////////

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

Sample

Screen from program

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++