# 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- 2005 withdrawn as a standard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.