Black-Box Models of Computation in Cryptology by Tibor Jager

By Tibor Jager

Generic staff algorithms remedy computational difficulties outlined over algebraic teams with no exploiting houses of a specific illustration of workforce parts. this can be modeled through treating the crowd as a black-box. the truth that a computational challenge can't be solved by means of a pretty limited type of algorithms might be noticeable as aid in the direction of the conjecture that the matter can also be demanding within the classical Turing computer version. additionally, a decrease complexity certain for convinced algorithms is a worthy perception for the hunt for cryptanalytic algorithms.

Tibor Jager addresses a number of basic questions touching on algebraic black-box types of computation: Are the commonly used workforce version and its versions an affordable abstraction? What are the restrictions of those types? do we chill out those versions to convey them towards the reality?

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Since φ is a ring-isomorphism and P performs only ring operations, it holds that P(x) = φ (P(x) mod p1 , P(x) mod p2 ) = φ (0, 0) and P(x ) = φ (P(x ) mod p1 , P(x ) mod p2 ) = φ (1, 1). The crucial observation is now that for each pair (x, x ) ∈ Z2N , there exist c, d ∈ ZN such that c = φ (x mod p1 , x mod p2 ) and d = φ (x mod p1 , x mod p2 ). Evaluating P with c or d yields P(c) = φ (P(x ) mod p1 , P(x) mod p2 ) = φ (1, 0) or P(d) = φ (P(x) mod p1 , P(x ) mod p2 ) = φ (0, 1). We therefore have gcd(N, P(c)) = p2 and gcd(N, P(d)) = p1 .

Thus the success probability of any algorithm when interacting with the original oracle is bounded by 1/2 + ε = Pr[Succ0 (A )] = Pr[Succ1 (A )] ≤ Pr[Succ2 (A )] + Pr[F ] ≤1/2 + Pr[F ], which implies ε ≤ Pr[F ]. 4 The Factoring Algorithm Consider a factoring algorithm B which samples a random element x ∈ C and runs A as a subroutine by implementing the generic ring oracle for A . That is, it performs all computations queried by A to x ∈ ZN . $ In parallel, B applies all queried operations to y ∈ ZN , where y ← U [C ] is chosen uniformly random at the beginning of the game.

Observe that O2 simulates O1 perfectly, unless O2 replies with 0 on an equality test query where O1 would have returned 1 (the opposite case is impossible). Note that this happens only if (ai , bi , ci ) = (a j , b j , c j ) but Li (x1 ) ≡ L j (x1 ) mod N . 6 Analysis of the Generic DCR Problem 55 Since ci = c j implies Li (x1 ) ≡ L j (x1 ) mod N , it suffices to consider the case where ci = c j and (ai , bi ) = (a j , b j ). 2) where x1 is uniformly random and independent of the algorithm’s view.

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