Understanding Diffie-Hellman Problems in Cryptography
Exploring Diffie-Hellman assumptions and problems including Computational Diffie-Hellman (CDH) and Decisional Diffie-Hellman (DDH). Discusses the difficulty of solving the DDH problem compared to CDH and discrete logarithm assumptions. Covers examples and implications of these cryptographic challenges.
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Diffie-Hellman Problems Slides by Prof. Jonathan Katz. Lightly edited by me.
Diffie-Hellman problems Fix cyclic group G and generator g Define DHg(h1, h2) = DHg(gx, gy) = gxy
Diffie-Hellman assumptions Computational Diffie-Hellman (CDH) problem: Given g, h1, h2, compute DHg(h1, h2) Decisional Diffie-Hellman (DDH) problem: Given g, h1, h2, distinguish DHg(h1, h2) from a uniform element of G
Example In *11 <2> = {1, 2, 4, 8, 5, 10, 9, 7, 3, 6} So DH2(7, 5) = ? In *3092091139 What is DH2(1656755742, 938640663)? Is 1994993011 the answer, or is it just a random element of *3092091139 ?
DDH problem Let G be a group-generation algorithm On input 1n, outputs a cyclic group G, its order q (with q =n), and a generator g The DDH problem is hard relative to G if for all PPT algorithms A: | Pr[A(G, q, g, gx, gy, gxy)=1] Pr[A(G, q, g, gx, gy, gz)=1] | (n)
Relating the Diffie-Hellman problems Relative to G: If the discrete-logarithm problem is easy, so is the CDH problem If the CDH problem is easy, so is the DDH problem I.e., the DDH assumption is stronger than the CDH assumption I.e., the CDH assumption is stronger than the dlog assumption
