DES

• The Data Encryption Standard
  • Developed by the IBM for the Government.
• This is a symmetric block cipher.
• Based on theory developed by Claude Shannon
  • Transposition
  • substitution
• The algorithm encrypts a block of code.
  • The block size is 64 bits.
  • The key size is 64 bits, but 8 of these are used for checksums
  • So the actual key size is 56 bits.
• It is something along the lines of
  • Takes a key and permutes the bits.
  • It then shifts the bits of the key.
  • It combines these with 1/2 of the block using a function (XOR)
  • It then Swaps the left and right side of the block.
  • Then repeats these steps 15 more times.
• It is based on standard "computer type" math and can be done quickly on a normal cpu.
  • But specialized devices have been constructed to break it.
• This was influenced by the NSA
  • Originally it was suspected that NAS made it easier to break.
  • Later it was discovered (much later) that they had strengthened it.
• Wikipedia has a nice timeline
  • 1977, published
  • 1998 $200,000 cracking machine (Deep Crack) brakes DES in 56 hours
  • 1999 time down to 22 hours
  • 2005 withdrawn as a standard
• Changes
  • Double DES $C = E(k_2, E(k_1, P))$
    • Roughly equivalent to a 57 bit key.
    • Doubles time to crack.
  • Triple DES $C = E(k_3,E(k_2, E(k_1, P)))$
    • Equivalent to 112 bit key.
    • This is far beyond normal computer capabilities.
  • Two Key Triple DES $C = E(k_1,D(k_2, E(k_1, P)))$
    • Equivalent to an 80 bit key.