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Dan Boneh Block ciphers Exhaustive Search Attacks Online Cryptography Course Dan Boneh

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Dan Boneh Exhaustive Search for block cipher key Goal: given a few input output pairs ( m i, c i = E(k, m i ) ) i=1,..,3 find key k. Lemma: Suppose DES is an ideal cipher ( 2 56 random invertible functions ) Then ∀ m, c there is at most one key k s.t. c = DES(k, m) Proof: with prob. ≥ 1 – 1/256 ≈ 99.5%

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Dan Boneh Exhaustive Search for block cipher key For two DES pairs ( m 1, c 1 =DES(k, m 1 ) ), ( m 2, c 2 =DES(k, m 2 ) ) unicity prob. ≈ 1 - 1/2 71 For AES-128: given two inp/out pairs, unicity prob. ≈ 1 - 1/2 128 ⇒ two input/output pairs are enough for exhaustive key search.

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Dan Boneh DES challenge msg = “The unknown messages is: XXXX … “ CT = c 1 c 2 c 3 c 4 Goal: find k ∈ {0,1} 56 s.t. DES(k, m i ) = c i for i=1,2,3 1997: Internet search -- 3 months 1998: EFF machine (deep crack) -- 3 days (250K $) 1999: combined search hours 2006: COPACOBANA (120 FPGAs) -- 7 days (10K $) ⇒ 56-bit ciphers should not be used !! (128-bit key ⇒ 2 72 days)

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Dan Boneh Strengthening DES against ex. search Method 1: Triple-DES Let E : K × M M be a block cipher Define 3E: K 3 × M M as For 3DES: key-size = 3×56 = 168 bits. 3×slower than DES. (simple attack in time ≈2 118 ) 3E ( (k 1,k 2,k 3 ), m ) =

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Dan Boneh Why not double DES? Define 2E ( (k 1,k 2 ), m ) = E ( k 1, E(k 2, m) ) Attack: M = (m 1,…, m 10 ), C = (c 1,…,c 10 ). step 1: build table. sort on 2 nd column key-len = 112 bits for DES m m E(k 2, ⋅ ) E(k 1, ⋅ ) c c k 0 = 00…00 k 1 = 00…01 k 2 = 00…10 ⋮ k N = 11…11 k 0 = 00…00 k 1 = 00…01 k 2 = 00…10 ⋮ k N = 11…11 E(k 0, M) E(k 1, M) E(k 2, M) ⋮ E(k N, M) E(k 0, M) E(k 1, M) E(k 2, M) ⋮ E(k N, M) 2 56 entries

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Dan Boneh Meet in the middle attack Attack: M = (m 1,…, m 10 ), C = (c 1,…,c 10 ) step 1: build table. Step 2: for all k ∈ {0,1} 56 do: test if D(k, C) is in 2 nd column. if so then E(k i,M) = D(k,C) ⇒ (k i,k) = (k 2,k 1 ) m m E(k 2, ⋅ ) E(k 1, ⋅ ) c c k 0 = 00…00 k 1 = 00…01 k 2 = 00…10 ⋮ k N = 11…11 k 0 = 00…00 k 1 = 00…01 k 2 = 00…10 ⋮ k N = 11…11 E(k 0, M) E(k 1, M) E(k 2, M) ⋮ E(k N, M) E(k 0, M) E(k 1, M) E(k 2, M) ⋮ E(k N, M)

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Dan Boneh Meet in the middle attack Time = 2 56 log(2 56 ) log(2 56 ) < 2 63 << 2 112, space ≈ 2 56 Same attack on 3DES: Time = 2 118, space ≈ 2 56 m m E(k 2, ⋅ ) E(k 1, ⋅ ) c c m m E(k 2, ⋅ ) E(k 1, ⋅ ) c c E(k 3, ⋅ )

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Dan Boneh Method 2: DESX E : K × {0,1} n {0,1} n a block cipher Define EX as EX ( (k 1,k 2,k 3 ), m ) = k 1 E(k 2, mk 3 ) For DESX: key-len = = 184 bits … but easy attack in time = (homework) Note: k 1 E(k 2, m) and E(k 2, mk 1 ) does nothing !!

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Dan Boneh End of Segment

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