Jaap van Ginkel Security of Systems and Networks
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1 Jaap van Ginkel Security of Systems and Networks November 4, 2013 Part 4 Modern Crypto
2 Block Ciphers
3 (Iterated) Block Cipher Plaintext and ciphertext consist of fixed-sized blocks Ciphertext obtained from plaintext by iterating a round function Input to round function consists of key and output of previous round Usually implemented in software
4 Block Cipher Notation P = plaintext block C = ciphertext block Encrypt P with key K to get ciphertext C Decrypt C with key K to get plaintext P C = E(P, K) P = D(C, K) Note: P = D(E(P, K), K) and C = E(D(C, K), K) But P D(E(P, K1), K2) and C E(D(C, K1), K2) when K1 K2
5 DES DEA is algorithm 64 bits key with parity Effectively 56 bits Theoretically and practically considered cracked
6 Data Encryption Standard DES developed in 1970 s Based on IBM s Lucifer cipher DES was U.S. government standard NSB DES development was controversial NSA secretly involved Design process was secret Key length reduced from 128 to 56 bits Subtle changes to Lucifer algorithm
7 DES Numerology DES is a Feistel cipher with 64 bit block length 56 bit key length 16 rounds 48 bits of key used each round (subkey) Each round is simple (for a block cipher) Security depends heavily on S-boxes Each S-boxes maps 6 bits to 4 bits
8 Feistel Cipher: Encryption Feistel cipher is a type of block cipher, not a specific block cipher Split plaintext block into left and right halves: P = (L0,R0) For each round i = 1,2,...,n, compute Li= Ri 1 Ri= Li 1 F(Ri 1,Ki) where F is round function and Ki is subkey Ciphertext: C = (Ln,Rn) Part 1 Cryptography 8
9 Feistel Cipher: Decryption Start with ciphertext C = (Ln,Rn) For each round i = n,n 1,,1, compute Ri 1 = Li Li 1 = Ri F(Ri 1,Ki) where F is round function and Ki is subkey Plaintext: P = (L0,R0) Formula works for any function F But only secure for certain functions F Part 1 Cryptography 9
10 DES Overall Feistel Structure
11 DES Feistel function (F-Function)
12
13 DES Expansion Permutation Input 32 bits Output 48 bits
14 DES S-box 8 substitution boxes or S-boxes Each S-box maps 6 bits to 4 bits S-box number 1 input bits (0,5) input bits (1,2,3,4)
15 DES Last Word (Almost) An initial permutation before round 1 Halves are swapped after last round A final permutation (inverse of initial perm) applied to (R16,L16) None of this serves security purpose
16 Security of DES Security depends heavily on S-boxes Everything else in DES is linear Thirty+ years of intense analysis has revealed no back door Attacks, essentially exhaustive key search Inescapable conclusions Designers of DES knew what they were doing Designers of DES were way ahead of their time
17 Deep Crack
18 Triple DES Today, 56 bit DES key is too small Exhaustive key search is feasible But DES is everywhere, so what to do? Triple DES or 3DES (112 bit key) C = E(D(E(P,K1),K2),K1) P = D(E(D(C,K1),K2),K1) Why Encrypt-Decrypt-Encrypt with 2 keys? Backward compatible: E(D(E(P,K),K),K) = E(P,K) And 112 bits is enough
19 3DES 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)? A (semi-practical) known plaintext attack Pre-compute table of E(P,K1) for every possible key K1 (resulting table has 256 entries) Then for each possible K2 compute D(C,K2) 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)
20 Triple DES 3 times? In a smart way Key length between 80 en 112 bits EEE EDE with K1, K2, K3, often K1 equals K3.
21 ECB M1 M2 M3 M4 C1 C2 C3 C4
22 ECB effect
23 XKCD Encryptic (1286)
24 Cipher Block Chaining
25 CBC decryption
26 Cipher feedback (CFB)
27 Output Feedback Mode (OFB)
28 Use with error correcting codes
29
30 Counter (CTR) Mode Also known as Segmented Integer Counter (SIC) mode Random Access possible properties OFB
31 AES Competition NIST MARS RC-6 Rijndael Twofish Serpent
32 Rijndael Winner AES Joan Daemen en Vincent Rijmen
33
34
35
36 Chocolate Key Encryption Courtesy Prof. Ezra Brown of VA Tech.
37 Diffie-Hellman Key Agreement Method RFC 2631
38 Public Key Cryptography Asymmetric encryption Expensive/Slow Diffie Hellmann RSA PGP
39 El Gamal Dr. Taher Elgamal طاهر الجمل Egyptian American cryptographer
40 Diffie Hellman Merkle
41 Public Key Encryption
42
43
44 William Stanley Jevons William Stanley Jevons (September 1, August 13, 1882), English economist and logician,
45 Non Secret Encryption James Ellis Clifford Cocks Secret research at GCHQ
46 RSA Ron Rivest, Adi Shamir en Len Adleman
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