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In this paper by Kate et al, a constant size polynomial commitment scheme is described.

The commitment scheme assumes a public reference string:

\begin{align*} \Big\{ \{ g^{ \tau^i }, g^{ \alpha \tau^i } \}_{i=-d}^{d} \}, \ \ \{ h^{ \tau^i }, h^{ \alpha \tau^i } \}_{i=-d}^{d}, \ \ e(g, h^{\alpha} ) \Big\} \end{align*}

where $\alpha, \tau \in \mathbb{F}$ are two secret values and $d$ is a large integer. The commitment goes as follows:

  • Commit: let $f \in \mathbb{F}[x]$ with degree at most $d$. The commitment is a group element $F$ defined by \begin{align*} F := g^{\alpha f(\tau) }. \end{align*}

  • To open the commitment to a certain value $z$, compute the proof $W$ which is another field element, defined by: \begin{align*} W := g^{ \frac{ f(\tau) -f(z) }{ \tau-z } } \end{align*}

  • To verify a correct opening given $(F, z, (v, W) )$: \begin{align*} e(W, h^{\alpha \tau} ) \cdot e(g^v \cdot W^{-z}, h^{\alpha} ) == e(F, h^{\alpha } ) \end{align*}

We note that the the commitment to the polynomial $f$ is constant size, as well as the proof of correct opening for a given value $z$.

My question is,

Given the above reference string, or a similar reference string of roughly the same size (linear in $d$), is there a known constant size commitment scheme to a bivariate polynomial?

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I think the main issue for the bivariate polynomials is the coefficients of the polynomial $f(x,y)$. Since $\tau$ is a secret value then to calculate $g^{\alpha f(\tau)}$ we can use the public parameters. As you know, the users can obtain the coefficients based on a standard basis and then solve a multiplication. $$g^{\alpha f(\tau)}=\prod_{i=0}^{n}{(g^{ \alpha \tau^i })}^{f_i}$$ For a univariate polynomial, the coefficients of $f_i$ are constant while if we consider bivariate polynomials then it is necessary to evaluate a second polynomial on the $\tau$. Precisely, the standard basis $(1,X,X^2,...,X^d)$ forms the univariate polynomials and the coefficients for a bivariate polynomial are changing by the second variable.

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