Generalized Hash Functions based on Multivariate Ideal Lattices

Lyubashevsky & Micciancio (2006) built collision resistant hash functions based on ideal lattices (in the univariate case) that in turn paved the way for the construction of other cryptographic primitives. Recently, in (Francis & Dukkipati, 2014), univariate ideal lattices have been extended to a multivariate case and its connections to Gr\"obner bases have been studied. In this paper, we show the existence of collision resistant generalized hash functions based on multivariate ideal lattices. We show that using Gr\"obner basis techniques an analogous theory can be developed for the multivariate case, opening up an area of possibilities for cryptographic primitives based on ideal lattices. For the construction of hash functions, we define an expansion factor that checks coefficient growth and determine the expansion factor for specific multivariate ideal lattices. We define a worst case problem, shortest polynomial problem w.r.t. an ideal in $\mathbb{Z}[x_1, ..., x_n]$, and prove the hardness of the problem by using certain well known problems in algebraic function fields.