COMFIC lecture notes

Crypto 14th

Next week: Reading week

Fiestel Ciphers-

Example that uses: DES

how it works

Split plaintext in two

Right half, run through round function, round key from password, so would be round 1 for example

This will produce random looking output

XOR otherside with output of function

Left hand side encrypted with function of right hand text

You then swap round and do same

Repeat through series of rounds

Encrypt XOR other

Then other side

Rinse and repeat

Decryption

Same key in round function, calc function of right hand side, XOR again

So repeat last round but decrypting and XORing

Rinse and repeat

The Des

64 bit block size

Keylength is 64 bits, 56 without parity really

16 rounds

Initial and final permutation that once disclosed, no security

S-boxes design criteria kept secret

Putting in key gives same sequence of keys