# Complexity 26-1 Complexity Andrei Bulatov Interactive Proofs.

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Complexity 26-1 Complexity Andrei Bulatov Interactive Proofs

Complexity 26-2 Interactive Proofs ProverVerifier Has unlimited computational power Can perform polynomial time computations They can exchange messages Wants to convince Verifier in something Accepts or rejects after performing some computation

Complexity 26-3 Formal Model Prover and Verifier are represented by functions: Verifier (V) Input: an input string, w a random string, r message history Output: A message, that is a string, or accept or reject Prover (P) Input: an input string, w message history Output:A message, that is a string

Complexity 26-4 A Prover’s move is the message A Verifier’s move is the message We write if for some k and if for some k We assume that the length of messages, the length of r, and k are bounded with a polynomial p(|w|) Finally, we denote where r is a random string of length p(|w|)

Complexity 26-5 The Class IP Definition A language L belongs to IP if some polynomial time function V and arbitrary function P exist, such that for every function R and string w w  L implies Pr[V  P accepts w]  w  L implies Pr[V  R accepts w] 

Complexity 26-6 Examples Instance: Graphs G and H. Question: Are G and H non-isomorphic? Graph Non-Isomorphism Instance: A formula  in CNF and a number k Question: Is it true that  has exactly k satisfying assignments? #SAT Theorem #SAT  IP

Complexity 26-7 Proof Given a formula  and a number k. Let be the variables of  The first stage is “arithmetization” of  : We construct a polynomial such that

Complexity 26-8 By induction for X define the corresponding polynomial to be x for  X define the corresponding polynomial to be 1 – x for    with corresponding polynomials g and h, define the corresponding polynomial to be gh (the product) Arithmetization for    with corresponding polynomials g and h, define the corresponding polynomial to be 1 – (1 – g)(1 – h) for  with corresponding polynomial g, define the corresponding polynomial to be 1 – g

Complexity 26-9 Protocol Let be the result of arithmetization of  Then The protocol will go in n rounds On every round, we will construct from an equality an equality of the form

Complexity 26-10 The property we are trying to achieve is if and only if If this is possible, starting off with after n rounds we get an equality which is easy to verify The last equality is true if and only if the first one is true

If was true before the this round, then Complexity 26-11 Randomness Unfortunately, the condition above is impossible to achieve Instead, we use a probabilistic one is true after this round with probability 1 If was false before the this round, then is false after this round with probability 1– 

Complexity 26-12 Analysis Thus, after n rounds If we started off with a correct equality, then after n rounds we have a correct equality with probability 1 If we started off with an incorrect equality, then after n rounds the probability that we end up with a correct equality is the probability that the error was made in one of the n rounds, which is at most n . Thus, with probability at least 1 – n , we will have an incorrect equality We need to organize every round such that

Complexity 26-13 Round Having the equality Verifier sends to Prover the polynomial g Prover returns the polynomial or what he pretends this polynomial is Verifier checks if the degree of h is not higher than that of g Verifier checks if Verifier randomly chooses a number r, such that where m is the number of clauses in , and replace the original equality with

Complexity 26-14 Analysis If the original equality is true then Prover should behave honestly, that is return actual coefficients of h(x). Then - The degree of h is not higher than that of g - h(0) + h(1) = k - For any r,

Complexity 26-15 If the original equation is not true then consider two cases - if Prover is honest that is he returns the actual coefficients of h, then h(0) + h(1)  k - if Prover is dishonest that is he returns some polynomial p(x) such that the degree of p is not higher than that of g and p(0) + p(1) = k In this case p and h are different. Therefore their values can be equal for at most d = max{deg(p), deg(h)} values of x. The probability that for a randomly chosen is

Complexity 26-16 Note that the degree of f is at most mn. Therefore