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Ln Lou 2014.8 Modern Cyphers Slide 2 0. Security System Key Plain Text Cipher Text Encryption (K P C) Decryption (K C P) Slide 3 1. Symmetric Key Algorithm 1.1 Stream Cyphers 1.2 Block Cyphers Slide 4 1.1 Stream cyphers RC4 (WEP, SSL) Salsa20/12 Slide 5 One-time Pad len(m) == len(c) == len(k) D(k, m) = k m E(k, c) = k c Slide 6 One-time Pad k = 1001110001010111 m = 1100010100101101 c = 0101100101111010 m = 1100010100101101 Slide 7 1.1 Stream Cyphers Pseudorandom Generator (PRG) {0, 1} s {0, 1} n n >> s Initialization Vector (IV) E(k, m) = m PRG(k, IV) D(k, c) = c PRG(k, IV) Slide 8 1.2 Block Cyphers DES 3-DES AES Slide 9 1.2.1 Mode of Operation ECB CBC CFB CTR Slide 10 1.2.1.1 ECB Slide 11 Slide 12 1.2.1.1 EBC Slide 13 1.2.1.2 CBC C i = E k (P i C i-1 ), C 0 = IV Slide 14 1.2.1.2 CBC P i = D K (C i ) C i-1, C 0 = IV Slide 15 1.2.1.3 CFB C i = E K (C i-1 ) P i, C 0 = IV Slide 16 1.2.1.3 CFB P i = E K (C i-1 ) C i, C 0 = IV Slide 17 1.2.1.4 OFB Slide 18 Slide 19 1.2.2.1 Iterated Cyphers key k k 0, k 1, k 2, , k n round function R m 0 = m m i+1 = R(k i+1, m i ) c = m n Slide 20 1.2.2.2 SP Network Substitution Permutation Slide 21 1.2.2.3 Feistel Cypher Encryption: L i+1 = R i R i+1 = L i F(K i, R i ) Decryption: R i = L i+1 L i = R i+1 F(K i, L i+1 ) Slide 22 2. Asymmetric Key Algorithm Encrypt: public key Decrypt: private key Slide 23 2.1 RSA Choose n = pq Compute (n) = n-(p+q-1) public key e: 1 < e < (n) && gcd(e, (n)) == 1 private key d: d e -1 (mod (n)) Slide 24 2.1 RSA Encryption: c = m e mod n Decryption: m = c d mod n Slide 25 Reference Wikipedia http://class.coursera.org/crypto-preview