## Cryptology ePrint Archive: Report 2012/609

A New Approach to Discrete Logarithm Problem with Auxiliary Inputs

Taechan Kim and Jung Hee Cheon

Abstract: Embedding an element of a finite field into auxiliary groups such as elliptic curve groups or extension fields of finite fields has been useful tool for analysis of cryptographic problems such as establishing the equivalence between the discrete logarithm problem and Diffie-Hellman problem or solving the discrete logarithm problem with auxiliary inputs (DLPwAI). Actually, Cheon's algorithm solving the DLPwAI can be regarded as a quantitative version of den Boer and Maurer. Recently, Kim showed in his dissertation that the generalization of Cheon's algorithm using embedding technique including Satoh's \cite{Sat09} is no faster than Pollard's rho algorithm when $d\nmid (p\pm 1)$.

In this paper, we propose a new approach to solve DLPwAI concentrating on the behavior of function mapping between the finite fields rather than using an embedding to auxiliary groups. This result shows the relation between the complexity of the algorithm and the number of absolutely irreducible factors of the substitution polynomials, hence enlightens the research on the substitution polynomials.

More precisely, with a polynomial $f(x)$ of degree $d$ over $\mathbf{F}_p$, the proposed algorithm shows the complexity $O\left(\sqrt{p^2/R}\log^2d\log p\right)$ group operations to recover $\alpha$ with $g, g^{\alpha}, \dots, g^{\alpha^{d}}$, where $R$ denotes the number of pairs $(x, y)\in\mathbf{F}_p\times \mathbf{F}_p$ such that $f(x)-f(y)=0$. As an example using the Dickson polynomial, we reveal $\alpha$ in $O(p^{1/3}\log^2{d}\log{p})$ group operations when $d|(p+1)$. Note that Cheon's algorithm requires $g, g^{\alpha}, \dots, g^{\alpha^{d}}, \dots, g^{\alpha^{2d}}$ as an instance for the same problem.

Category / Keywords: foundations / discrete logarithm problem, Cheon's algorithm, fast multipoint evaluation, Dickson polynomial, substitution polynomial, point counting, absolutely irreducible polynomial