math/notes_nova.tex

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\title{Notes on Nova}
\author{arnaucube}
\date{February 2023}

\begin{document}

\maketitle

\begin{abstract}
	Notes taken while reading Nova \cite{cryptoeprint:2021/370} paper.

	Usually while reading papers I take handwritten notes, this document contains some of them re-written to $LaTeX$.

	The notes are not complete, don't include all the steps neither all the proofs.
\end{abstract}

\tableofcontents

\section{Folding Scheme for Committed Relaxed R1CS}

\subsection{R1CS modification}

Want: merge 2 instances of R1CS with the same matrices into a single one. Each instance has $z_i = (W_i,~ x_i)$ (public witness, private values resp.).

\paragraph{traditional R1CS}
Merged instance with $z=z_1 + r z_2$, for rand $r$. But, since R1CS is not linear $\longrightarrow$ can not apply.

eg.
\begin{align*}
	Az \circ Bz &= A(z_1 + r z_2) \circ B (z_1 + r z_2)\\
	&= A z_1 \circ B z_1 + r(A z_1 \circ B z_2 + A z_2 \circ B z_1) + r^2 (A z_2 \circ B z_2)\\
	&\neq Cz
\end{align*}

$\longrightarrow$ introduce error vector $E \in \mathbb{F}^m$, which absorbs the cross-temrs generated by folding.

$\longrightarrow$ introduce scalar $u$, which absorbs an extra factor of $r$ in $C z_1 + r^2 C z_2$ and in $z=(W, x, 1+r\cdot 1)$.

\paragraph{Relaxed R1CS}
\begin{align*}
	&u=u_1+r u_2\\
	&E=E_1 + r (A z_1 \circ B z_2 + A z_2 \circ B z_1 - u_1 C z_2 - u_2 C z_1) + r^2 E_2\\
	&Az \circ Bz = uCz + E,~~ with~ z=(W,~x,~u)
\end{align*}
where R1CS set $E=0,~u=1$.

\begin{align*}
	Az \circ Bz &= A z_1 \circ B z_1 + r(A z_1 \circ B z_2 + A z_2 \circ B z_1) + r^2 (A z_2 \circ B z_2)\\
		    &= (u_1 C z_1 + E_1) + r (A z_1 \circ B z_2 + A z_2 \circ B z_1) + r^2 (u_2 C z_2 + E_2)\\
		    &= u_1 C z_1 + \underbrace{E_1 + r(A z_1 \circ B z_2 + A z_2 \circ B z_1) + r^2 E_2}_\text{E} + r^1 u_2 C z_2\\
		    &= u_1 C z_1 + r^2 u_2 C z_2 + E\\
		    &= (u_1 + r u_2) \cdot C \cdot (z_1 + r z_2) + E\\
		    &= uCz + E
\end{align*}

For R1CS matrices $(A,~B,~C)$, the folded witness $W$ is a satisfying witness for the folded instance $(E,~u,~x)$.



\vspace{20px}
Problem: not non-trivial, and not zero-knowledge. Solution: use polynomial commitment with hiding, binding, succintness and additively homomorphic properties.

\paragraph{Committed Relaxed R1CS}
Instance for a Committed Relaxed R1CS\\
$(\overline{E}, u, \overline{W}, x)$, satisfyied by a witness $(E, r_E, W, r_W)$ such that
\begin{align*}
	&\overline{E} = Com(E, r_E)\\
	&\overline{W} = Com(E, r_W)\\
	&Az \circ Bz = uCz+E,~~ where~z=(W, x, u)
\end{align*}


\subsection{Folding protocol}

V and P take two \emph{committed relaxed R1CS} instances
\begin{align*}
	\varphi_1&=(\overline{E}_1, u_1, \overline{W}_1, x_1)\\
	\varphi_2&=(\overline{E}_2, u_2, \overline{W}_2, x_2)
\end{align*}

P additionally takes witnesses to both instances
\begin{align*}
	(E_1, r_{E_1}, W_1, r_{W_1})\\
	(E_2, r_{E_2}, W_2, r_{W_2})
\end{align*}

Let $Z_1 = (W_1, x_1, u_1)$ and $Z_2 = (W_2, x_2, u_2)$.

% \paragraph{Protocol}
\begin{enumerate}
	\item P send $\overline{T} = Com(T, r_T)$,\\
		where $T=A z_1 \circ B z_1 + A z_2 \circ B z_2 - u_1 C z_2 - u_2 C z_2$\\
		and rand $r_T \in \mathbb{F}$
	\item V sample random challenge $r \in \mathbb{F}$
	\item V, P output the folded instance $\varphi = (\overline{E}, u, \overline{W}, x)$
		\begin{align*}
			&\overline{E}=\overline{E}_1 + r \overline{T} + r^2 \overline{E}_2\\
			&u = u_1 + r u_2\\
			&\overline{W} = \overline{W}_1 + r \overline{W}_2\\
			&x = x_1 + r x_2
		\end{align*}
	\item P outputs the folded witness $(E, r_E, W, r_W)$
		\begin{align*}
			&E = E_1 + r T + r^2 E_2\\
			&r_E = r_{E_1} + r \cdot r_T + r^2 r_{E_2}\\
			&W=W_1 + r W_2\\
			&r_W = r_{W_1} + r \cdot r_{W_2}
		\end{align*}
\end{enumerate}

P uses a zkSNARK showing that knows the valid witness $(E, r_E, W, r_W)$ for the committed relaxed R1CS without revealing its value.
Then, via Fiat-Shamir transform we achieve non-interactivity.

\section{IVC proofs}
\textbf{WIP}


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