Cryptography

Cryptography

1. One Time Pad

   • C = K XOR M
   • Perfect secrecy
   • Random key

2. Stream cipher

   • Replace random key with pseudorandom
   • C = G(K) XOR M
   • Attacks
     • Two time pad
       • c1 XOR c2 = m1 XOR m2
     • Integrity attack
       • c = (m XOR k) c' = (c XOR p) m' = (c' XOR k)
   • Examples
     • RC4
       • 802.11b WEP
       • HTTPS
     • CSS
       • DVD (2 LFSR)
         • seed = 5 bytes
       • GSM (3 LFSR)
       • Bluetooth (4 LFSR)
     • eStream: Salsa 20
       • seed (k) + nonce (r)
       • Salsa = H(k, (r,0)) || H(k, (r,1)) ||...
       • H = h^10 (64 bits block) XOR (64 bits block)

3. Block cipher

   • 3DES
     • k = 168 bits
     • X = 64 bits
     • E(k_3, D(k_2, E(k_1, m)))
   • AES
     • k = 128 bits
     • X = 128 bits
     • Subs-Perm networks
       • 4x4 input
         • XOR k_0
           • ByteSub
           • ShiftRow
           • MixColumn
           • XOR k_1 --//--
       • Code size/performance tradeoff
   • Contains
     • Key expansion
     • Round function
   • DES
     • 16 rnd Feistel network
       • R_i = f_i(R_(i-1)) XOR L_i
       • L_i=R_(i-1)
   • Attacks
     • Side channel attacks
     • Fault attacks
     • Linear & differential attacks
     • Quantum attacks