Symmetric Block Ciphers

Triple DES

First some stats to think about:

Single DES uses a 56 bit key. So the number of keys is 2^56.

The number of bits in a block is 64 so any block can be mapped to one of 2^64 different blocks.

It seems reasonable then that DES using key K1 has a range of X and using key K2 has a range of Y where X and Y are different and the symmetric difference is non-empty latex2($\left( X \bigtriangleup Y \neq \emptyset \right)$). This is actually true so that latex2($E_{K1}(E_{K2}(P) \neq E_{K3}(P)$)

Using two keys doesn't do much good either though because we can use a meet in the middle attack. While DES requires effort on the order of 2^54 Double DES requires only effort on the order of 2^56. This type of attack works on any block cipher. So we go to 3DES.

Cost of attack on 3DES which uses two keys latex2($E_{K1}(D_{K2}(E_{K1})))$) is latex2($2^{112}$).

Blowfish

RC5

Characteristics of Advanced Symmetric Block Ciphers

RC4 Stream Cipher

Review Questions