## Monday, December 8, 2014

### Bivariate Polynomials Modulo Composites and their Applications

So this week is AsiaCrypt 2014 in Kaosiung, Taiwan and as such this will be one of a series of blog posts appearing over the course of the next few days. Unusually for me I have not chosen a paper on theoretical leakage resilience or anything close to what I usually work on. I have chosen to blog about a talk from the second session "New Proposals" entitled "Bivariate Polynomials Modulo Composites and their Applications" by Dan Boneh and Henry Corrigan-Gibbs, with Henry giving the talk.

So before I start talking about the presentation I just want to recap on RSA and its related problem. Given (equal size) primes $p,q$ we define $N=p\cdot q$, then the RSA problem is given $N$ recover $p,q$ and we assume that this is hard. Certainly we believe that factoring integers is hard and if we can solve the RSA problem, and years of using RSA at a large scale says this does appear to (currently) be a hard problem.

Using the RSA problem we can define a class of problems defined by a function $f$ where given $f(x)\mod{n}$ the goal is to find $x$. If $f(x)=x^2$ we get a problem similar to quadratic resoprocity, while if $f(x)=x^e$ we get RSA and if $f$ is random such that $f(x)=0\mod{N}$ then this is equivalent to factoring $N$.

The question that arose in this presentation is what if the function $f$ is replaced with a bivariate polynomial instead, can we get security and does this have any use? The three notions of security that were looked into were one way-ness, second preimage resistance and collision resistance.

The trick used by the authors is to look at $f(x,y)=c\in\mathbb{Q}$ to understand $f(x,y)\mod{N}$. Since if the problem if easy to solve in $\mathbb{Q}$ then it is easy to solve modulo $N$ (loosely speaking you just convert the answers as you would expect). Currently this is the only method known for solving these but it does raise the question of is it the only method?

Oneway-ness: It can be shown that (in $\mathbb{Q}$) $f(x,y)$ can not be one way if it is of genus 0, it is currently unknown how are (in general) it is to invert for a higher genus.

Second preimage resistant: For $f$ to be second preimage resistant, $f(x,y)=c\in\mathbb{Q}$ must have no non-trivial rational mapping to itself. If there is such a mapping then this mapping can be used to easily break the second preimage resistant property.

Collision resistance: For $f$ to be collision resistant, having $f$ be injective will give this. This gives us that if $f(x,y)$ is not injective then $f(x,y)\mod{N}$ is not collision resistant but it is an open question if $f(x,y)$ being injective automatically implies that $f(x,y)\mod{N}$ is collision resistent.

Before we even consider collision resistant functions, we have to ask if there even exists low degree injective polynomials over the rationals. Well it turns out that this is an open question in Number Theory! On the bright side $f_z(x,y)=x^7+3\cdot y^7$ has been conjectured to be injective over the rationals for 15 years now (can this be proved in the generic ring model?). From this point forward the authors use the assumption $f_z$ is in fact collision resistant and then ask what constructions they can get from this.

A commitment scheme
Commit(m): Choose a random $r$ modulo $N$ and return $c=f_z(m,r)\mod{N}$
Open(c,m,r): Check $c=f(m,r)\mod{N}$
Hiding follows from the fact that $m$ is blinded by a random $r$, while blinding follows from the fact that breaking blinding breaks the assumption that $f_z$ is collision resistant.

They also describe how to make a Chameleon hash and have a really novel use in which using succinct zero knowledge you can prove that 3 Pedersen commitments open to values which are the commitment (and message) of value for one of their commitment scheme values. While zero knowledge "proof that a commitment is of a commitment triple" sounds like a strange use it is actually used in anonymous bitcoin.

To conclude they show that using bivariate polynomials with the RSA problem leads to some interesting schemes and that it is certainly worhty of further study.