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Combined Attacks — from Boomerangs to Sandwiches and Differential-Linear

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Combined Attacks — from Boomerangs to Sandwiches and Differential-Linear Introduction Boomerang Diff-Lin Summary Combined Attacks — from Boomerangs to Sandwiches and Differential-Linear Orr Dunkelman Department of Computer Science, University of Haifa June 5th, 2014 Orr Dunkelman Combined Attacks 1/ 36 Introduction Boomerang Diff-Lin Summary Outline 1 A Quick Introduction Differential Cryptanalysis Linear Cryptanalysis 2 The Boomerang Attack The Boomerang Attack The Amplified Boomerang Attack Independence Assumptions The Sandwich Attack 3 Differential-Linear Cryptanalysis The Basic Concept A Differential-Linear Attack on 8-Round DES Several Extensions to Differential-Linear Cryptanalysis 4 Summary Orr Dunkelman Combined Attacks 2/ 36 Introduction Boomerang Diff-Lin Summary Differential Linear Differential Cryptanalysis ◮ Considers the development of differences through the encryption process. ◮ The core of the attack: a differential characteristic (a prediction of the development of differences through the encryption process). ◮ Given a differential characteristic with probability p, the adversary asks for O(1/p) pairs of plaintexts (P, P∗ = P ⊕ ΩP ). ◮ The attack tries to locate “right pairs”, i.e., a pair whose corresponding ciphertexts satisfy C ∗ = C ⊕ ΩC . ◮ Information about the key can be learnt from the right pair. Orr Dunkelman Combined Attacks 3/ 36 Introduction Boomerang Diff-Lin Summary Differential Linear Differential Cryptanalysis (cont.) ◮ To attack more rounds of the cipher than in the differential characteristic: ◮ Guess subkey material in the additional rounds, ◮ Partially encrypt/decrypt the plaintext/ciphertext pairs, ◮ Count how many “right pairs” exist, ◮ The counter for the right subkey is expected to be the highest. ◮ In such attacks, we care less about “which pair is a right pair”, and more about how many such pairs exist. ◮ Hence, for this sort of attacks, we are only interested in the input and output differences. ◮ This set of (ΩP , ΩC ) and the associated probability is called a differential. Its probability is the sum of the probabilities of all differential characteristics that share ΩP and ΩC . Orr Dunkelman Combined Attacks 4/ 36 Introduction Boomerang Diff-Lin Summary Differential Linear Differential Characteristic of DES A three-round differential characteristic of DES with probability 1/16: ΩP =400800 00 04000000x A =40 08 00 00 a =04 00 00 00 p = 1 ′ x F ′ x 4 B b p ′ =0x F ′ =0x =1 C =40 08 00 00 c =04 00 00 00 p = 1 ′ x F ′ x 4 ΩT =40080000 0400 0000x Orr Dunkelman Combined Attacks 5/ 36 Introduction Boomerang Diff-Lin Summary Differential Linear Differential Characteristic of DES (cont.) A 3-round truncated differential characteristic of DES: ΩP =400000 00 00000000x A a p ′ =0 F ′ =0 =1 B W XY Z b p ′ = 00 0 0 x F ′ =40 00 00 00x =1 = P(V 0 00 00 00x ) C M c W XY Z p ′ =?? ?? ? ??x F ′ = 00 0 0 x =1 = P(0? ?? ?? 0?x ) ΩT =?? ?? M??? 00 W 0 XY 0Zx Orr Dunkelman Combined Attacks 6/ 36 Introduction Boomerang Diff-Lin Summary Differential Linear Linear Cryptanalysis ◮ Tries to approximate the cipher (or a reduced-round variant of it) as a linear equation: λP · P ⊕ λC · C = λK · K with probability 1/2+ ǫ. ◮ 2 Collect N = O(ǫ− ) known plaintext/ciphertext pairs. The majority are expected to satisfy λP · P ⊕ λC · C = λK · K (when ǫ> 0). ◮ To attack more rounds than in the linear approximation: ◮ Guess subkey material in the additional rounds, ◮ Partially encrypt/decrypt the plaintext/ciphertext pairs, ◮ Count how many times λP · P ⊕ λC · C = 0, ◮ The counter for the right subkey is expected to be more biased. Orr Dunkelman Combined Attacks 7/ 36 Introduction Boomerang Diff-Lin Summary Differential Linear Linear Cryptanalysis (cont.) ◮ The attack is actually a random process. ◮ Consider the following scenario: ◮ There are 2s possible subkeys. ◮ We want the right subkey to be among the 2a most biased ones. x ◮ Let Φ(x)= 1 e x2/2dx. √2π − −∞ ◮ A linear attackR with N = c/ǫ2 known plaintexts has a success probability of 1 a 1 Ps = Φ 2c − Φ− 1 − 2− − . To achieve a success probability of Ps , set 1 1 a 1 2 Φ− (Ps ) + Φ− (1 − 2− − ) 2 N = · ǫ− . 2 Orr Dunkelman Combined Attacks 8/ 36 Introduction Boomerang Diff-Lin Summary Differential Linear Linear Approximation of DES A three-round linear approximation of DES with bias 20 2 25 1/2+2 · ( 64 ) =1/2+ 128 : λT =21040080 00008000x 20 A′ =21 04 00 80 a′ =00 00 80 00 1/2 − x F x 64 = P(00 00 F 0 00x ) B =0 b =0 1/2+1/2 ′ F ′ C =21 04 00 80 c =00 00 80 00 1/2 − 20 ′ x F ′ x 64 = P(00 00 F 0 00x ) λC = λT =2104 0080 00008000x Orr Dunkelman Combined Attacks 9/ 36 Introduction Boomerang Diff-Lin Summary Differential Linear Some General Comments ◮ Finding good differential characteristics/linear approximation is a hard task. ◮ Some automatic tools exist (Matsui’s method), but it is better to study the algorithm. ◮ Sometimes, a better attack is obtained when using differentials (approximations) of lower probability (bias). ◮ Many optimizations for both attacks exist. Consider differential cryptanalysis: ◮ Structures of plaintexts, ◮ Discarding wrong pairs (early abort), ◮ Using multiple differentials, Orr Dunkelman Combined Attacks 10/ 36 Introduction Boomerang Diff-Lin Summary Boomerang Amp. Boom. Independence Sandwich The Boomerang Attack ◮ Introduced by [W99]. P1 P3 ◮ Targets ciphers with good short α α P P differentials, but bad long ones. 2 4 ◮ The core idea: Treat the cipher as a E0 γ cascade of two sub-ciphers. Where T1 T3 β β in the first sub-cipher a differential γ E0 T T α −→ β exists, and a differential 2 4 E γ −→1 δ exists for the second. ◮ E1 The process starts with a pair of C1 C3 plaintexts: P1, P2 = P1 ⊕ α. δ C C ◮ 2 4 After the first sub-cipher, δ T1 ⊕ T2 = β. ◮ ButOrr the Dunkelman encryptionCombined process Attacks 11/ 36 Introduction Boomerang Diff-Lin Summary Boomerang Amp. Boom. Independence Sandwich The Boomerang Attack — Some Details ◮ If the probability of the first differential is p, and of the second differential is q, the total probability of the boomerang quartet is Pr[α → β]2 · Pr[γ → δ]2 =(pq)2. ◮ Note that we use three out of the four differentials in the backward direction. ◮ For regular differentials, the probability is the same. ◮ However, for truncated differentials, the probability is not necessarily the same. Orr Dunkelman Combined Attacks 12/ 36 Introduction Boomerang Diff-Lin Summary Boomerang Amp. Boom. Independence Sandwich The Boomerang Attack — Some More Details ◮ A right boomerang quartet discloses information about the key. ◮ At the same time, the attack is an adaptive chosen plaintext and ciphertext attack. ◮ This prevents us from using many of the cryptanalytic techniques that were proposed over the years. ◮ To overcome this, we need to transform the attack into a chosen plaintext attack. Orr Dunkelman Combined Attacks 13/ 36 Introduction Boomerang Diff-Lin Summary Boomerang Amp. Boom. Independence Sandwich The Amplified Boomerang Attack ◮ Pi Pj Introduced by [KKS00]. 1 1 ◮ Similar idea to the boomerang α α Pi Pj attack, but in a chosen plaintext 2 2 scenario. ◮ E0 γ Again, assume the existence of two T i T j 1 1 E0 β β differentials: α −→ β for the first γ E T i T j sub-cipher and γ −→1 δ for the 2 2 second. ◮ E1 Take many pairs of plaintext with C i C j 1 1 i i i δ difference α: P1, P2 = P1 ⊕ α. C i C j ◮ 2 2 After the first sub-cipher, for some δ i i of them T1 ⊕ T2 = β. ◮ If weOrr haveDunkelman manyCombined pairs Attacks 14/ 36 ii i Introduction Boomerang Diff-Lin Summary Boomerang Amp. Boom. Independence Sandwich The Amplified Boomerang Attack — Some Details ◮ If the probability of the first differential is p, and of the second differential is q, the total probability of the amplified boomerang quartet is 2 2 n 2 n Pr[α → β] ·Pr[γ → δ] ·2− =(pq) ·2− . ◮ In other words, the probability is less than 2−n! Orr Dunkelman Combined Attacks 15/ 36 Introduction Boomerang Diff-Lin Summary Boomerang Amp. Boom. Independence Sandwich The Amplified Boomerang Attack — Some Details (cont.) ◮ If we take N pair with input difference α, we obtain about N2/2 quartets. ◮ Hence, we expect 2 2 n N /2 · (pq) · 2− right amplified boomerang quartets. ◮ Start with N = O(2n/2/pq) pairs. ◮ n/2 As long as (pq) > 2− , we can have enough data to run the attack. ◮ Which is the same condition as for the boomerang attack. Orr Dunkelman Combined Attacks 16/ 36 Introduction Boomerang Diff-Lin Summary Boomerang Amp. Boom. Independence Sandwich The Rectangle Attack — Three Improvements i i j j 1 If the quartet ((P1, P2), (P1, P2)) is not a right quartet, i i j j then maybe ((P1, P2), (P2, P1)) is a right one? i i j j 2 If T1 ⊕ T2 = β′, but so does T1 ⊕ T2 = β′, we can still get a right quartet. i j i j 3 If T1 ⊕ T1 = γ′, but so does T2 ⊕ T2 = γ′, we can still get a right quartet. Expected number of right quartets starting with N pairs: 2 n+1 2 N · 2− · (pq) 2 n 2 N · 2− · (pq) 2 n E0 2 2 N · 2− · Pr[α −→ β′] q ′ ! Xβ 2 n E0 2 E1 2 N · 2− · Pr[α −→ β′] · Pr[γ′ −→ δ] Orr Dunkelman Combined′ Attacks ! ′ ! 17/ 36 Xβ Xγ Introduction Boomerang Diff-Lin Summary Boomerang Amp. Boom. Independence Sandwich A Technical Problem... ◮ In the boomerang attack the quartet is fully known. ◮ In the amplified boomerang attack, one needs to find the quartets among all possible ones.
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