DES supplement

Why not 2DES? Why not C = E(E(P, K), K) ? Trick question --- it’s still just 56 bit key Why not C = E(E(P, K1), K2) ? D(C, K2) = D(E(E(P, K1), K2), K2) = E(P, K1) Not 2112 brute-force effort! A semi-practical known plaintext attack! Modern block ciphers are widely used to provide encryption of quantities of information, and/or a cryptographic checksum to ensure the contents have not been altered. We continue to use block ciphers because they are comparatively fast, and because we know a fair amount about how to design them. 2

Choosen Plaintext attack Short Procedures : Pre-compute table of E(P, K1) for every possible key K1 (sort the table has 256 entries table and remember key) For each possible K2 compute D(C, K2), look up in the table until a match in table is found When match is found, have E(P, K1) = D(C, K2) Result gives us keys: C = E(E(P, K1), K2) Only takes twice as long to break double DES using brute force! 2 x 256 = 257 Modern block ciphers are widely used to provide encryption of quantities of information, and/or a cryptographic checksum to ensure the contents have not been altered. We continue to use block ciphers because they are comparatively fast, and because we know a fair amount about how to design them. 3

3DES (TDEA) : 3 keys C = E(D(E(P, K1), K2), K3) Key size= 256x256x256 = 2168 Meet in the middle attack 2112 effort –too much Modern block ciphers are widely used to provide encryption of quantities of information, and/or a cryptographic checksum to ensure the contents have not been altered. We continue to use block ciphers because they are comparatively fast, and because we know a fair amount about how to design them. 4

3DES (TDEA) : 2 keys C = E(D(E(P, K1), K2), K1) Key size= 256x256 = 2112 Known-plaintext attack with effort 256, but requires 256 ⟨m,c⟩ pairs! Modern block ciphers are widely used to provide encryption of quantities of information, and/or a cryptographic checksum to ensure the contents have not been altered. We continue to use block ciphers because they are comparatively fast, and because we know a fair amount about how to design them. 5