Many-out-of-many proofs

Question

I need to prove that given vector of commitments of length N contains N-1 commitments to zero (and one to an arbitrary number). More formally, given vector: $$\textbf{a} = \begin{bmatrix} C(0, r_1) & C(0, r_2) & C(x, r_3) & ... & C(0, r_N) \end{bmatrix}$$ I want to prove that there is exactly one such commitment, that commits to x, rather than 0. Note that x is also private.
I've seen the opposite to what I need to prove in One-out-of-Many Proofs paper, but still can't come up with what I need.

Answer 1

Each of the $n$ commitments are of the form $C_i=a_iG+b_iH$, where $a_i$ is the value being committed to, and $b_i$ is the uniformly random blinding factor.

$G$ and $H$ are generator points, arbitrarily and fairly chosen so that $g$ such that $G=gH$ is unknowable (the EC discrete log assumption).

This means that if you treat $C_i$ as a public key on the generator $H$, the corresponding private key will be $a_ig+b_i$. Since $g$ is unknowable, the private key is only known if $a_i\overset{?}{=} 0$, i.e. the commitment is to the value zero.

Therefore, being able to provide a signature (such as a Schnorr signature) proving knowledge of the private key is equivalent to proving that the commitment is to the value zero. The same applies to knowledge of the private key of a sum of commitments.

First, calculate a challenge scalar $r_i$ for each commitment $C_i$. This can be done using an extendable-output function or HKDF, using the concatenation of all commitment EC-point bit strings as the initial keying material.

Calculate $S$ and $S'$ as follows:

$S=\sum\limits_{i=0}^{n-1}{C_i}$

$S'=\sum\limits_{i=0}^{n-1}{r_iC_i}$

Now, create $n$ public keys, as follows:

$P_i=S'-r_iS$

If the commitment to the value $x$ is at index $\pi$, then it will only be possible for $P_{\pi}$ to be a commitment to zero if all other commitments are commitments to zero (because multiplying zero by a random number is still zero). The private key will be $\sum\limits_{i=0}^{n-1}{r_ib_i}-r_{\pi}b_{\pi}$

If we provided a signature on the generator $H$ proving that $P_{\pi}$ is a commitment to zero, we would disclose $\pi$. To avoid disclosing the index $\pi$, we simply generate a ring signature instead.

Essentially, this approach is creating a ring, where each member of the ring contains the sum of $n-1$ commitments. Each ring position excludes a different commitment. The ring can only be signed if in at least one ring position, every commitment involved in the sum is a commitment to zero.