/DES

DES implementation in C from Understanding Cryptography Course.

Primary LanguageC

Data Encryption Standard

This is my version of DES implemented from https://crypto-textbook.com/download/Understanding_Cryptography_Chptr_3---DES.pdf This version should stress readability first and foremost, with optimizations coming at a later time.

Delta Swap

Delta swap is an efficient bit permutation method available here https://reflectionsonsecurity.wordpress.com/2014/05/11/efficient-bit-permutation-using-delta-swaps/ . This implementation used on the website ultimately shifts the bit according to the delta in the left direction, so I have listed the original and the corrected version (using the delta defined to be positive in my code). Masks used are explained here:

  • Swap 1:
    Original:
    • 0x0055005500550055
    • 00000000 10101010 00000000 10101010 00000000 10101010 00000000 10101010 Used:
    • 0xAA00AA00AA00AA00
    • 10101010 00000000 10101010 00000000 10101010 00000000 10101010 00000000 selects every other bit skipping a row (it then swaps with a delta of 9, which for an 8x8 matrix for IP is 1 row down +1 right.)
  • Swap 2:
    Original:
    • 0x0000333300003333
    • 00000000 00000000 00110011 00110011 00000000 00000000 00110011 00110011
      Used:
    • 0xCCCC0000CCCC0000
    • 11001100 11001100 00000000 00000000 11001100 11001100 00000000 00000000
      selects alternate 4x4 groups of squares and swaps with the square in the quadrant with a delta of 18.
  • Swap 3: Original:
    • 0x000000000F0F0F0F
    • 00000000 00000000 00000000 00000000 00001111 00001111 00001111 00001111
      Used:
    • 0xF0F0F0F000000000
    • 11110000 11110000 11110000 11110000 00000000 00000000 00000000 00000000 selects upper left quadrant (with delta 36 (1/2 + 1/8 of total length), swaps it with the bottom right.
  • Swap 4: Original:
    • 0x00000000FF00FF00
    • 00000000 00000000 00000000 00000000 11111111 00000000 11111111 00000000
      Used:
    • 0x00FF00FF00000000
    • 00000000 11111111 00000000 11111111 00000000 00000000 00000000 00000000 selects 2nd and 5th rows, with delta 24 swaps them.
  • Swap 5: Original:
    • 0x000000FF000000FF
    • 00000000 00000000 00000000 11111111 00000000 00000000 00000000 11111111 Used:
    • 0xFF000000FF000000
    • 11111111 00000000 00000000 00000000 11111111 00000000 00000000 00000000 selects 1st and 5th rows, with delta 24 swaps them.
      This utilizes 30 bit operations total. Alternatives include bfly tables.

Debugging

Useful for debugging: http://styere.xyz/JS-DES.html just edit form to not be readonly.

TODO:

  • Utilize structures of P, E, PC-1, and PC-2 tables for more efficient bit ops.