# 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• 1998 $200,000 cracking machine (Deep Crack) brakes DES in 56 hours • 1999 time down to 22 hours • 2005 withdrawn as a standard • Changes • Double DES$C = E(k_2, E(k_1, P))$• Roughly equivalent to a 57 bit key.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.