Demystifying Array Inputs in zk-SNARKs: A Guide to ark-circom
Zero-knowledge proofs (ZKPs) are revolutionizing the way we interact with data. They enable us to prove knowledge of information without revealing the information itself. Libraries like ark-circom provide a user-friendly approach to building ZKP circuits, but handling arrays of input signals can pose a challenge.
This article will guide you through the process of effectively using ark-circom to manage and manipulate arrays within your ZKP circuits.
The Challenge of Array Inputs
Imagine you want to prove you know the sum of an array of numbers without disclosing the numbers themselves. With traditional cryptographic techniques, you'd have to reveal the individual numbers or use complex multi-party computations. ZKP solutions like ark-circom offer a more elegant way. However, you'll need to understand how to represent and manipulate arrays within your circuits.
Understanding the Basics
Let's start with a simple example: proving the sum of an array of three numbers. Here's a basic ark-circom code snippet:
pragma circom 2.0.0;
template SumCircuit(n) {
signal input in[n];
signal output out;
out <== 0;
for (var i = 0; i < n; i++) {
out <== out + in[i];
}
}
component main = SumCircuit(3);
This code defines a circuit that takes three input signals (in[0], in[1], in[2]) and calculates their sum, outputting it as out.
Explanation:
pragma circom 2.0.0;: This line indicates the version of the Circom language used.template SumCircuit(n): We define a template calledSumCircuitwith a parametern, representing the size of the array.signal input in[n];: This declares an input arrayinof sizen.signal output out;: This declares an output signalout.out <== 0;: We initializeoutto 0.for (var i = 0; i < n; i++) { ... }: This loop iterates over the array elements.out <== out + in[i];: Within the loop, we accumulate the sum of the elements intoout.component main = SumCircuit(3);: This instantiates theSumCircuittemplate with an array size of 3, making it the main circuit.
Beyond the Basics: Advanced Array Manipulation
Now, let's explore some advanced techniques for handling arrays in ark-circom:
1. Conditional Operations:
You can use conditional statements (if and else) to selectively manipulate array elements based on certain conditions:
pragma circom 2.0.0;
template FilterArray(n) {
signal input in[n];
signal output out[n];
for (var i = 0; i < n; i++) {
if (in[i] > 5) {
out[i] <== in[i];
} else {
out[i] <== 0;
}
}
}
This example demonstrates how to filter an array and set elements to 0 if they are less than or equal to 5.
2. Multi-Dimensional Arrays:
While ark-circom doesn't directly support multi-dimensional arrays, you can simulate them using a single-dimensional array and indexing:
pragma circom 2.0.0;
template MatrixMultiply(n) {
signal input a[n*n];
signal input b[n*n];
signal output c[n*n];
for (var i = 0; i < n; i++) {
for (var j = 0; j < n; j++) {
c[i*n + j] <== 0;
for (var k = 0; k < n; k++) {
c[i*n + j] <== c[i*n + j] + a[i*n + k] * b[k*n + j];
}
}
}
}
Here, we perform matrix multiplication on two n x n matrices represented as single-dimensional arrays. We use index calculations to access individual elements based on row and column positions.
Resources and Further Exploration
To delve deeper into advanced array manipulation, refer to the official ark-circom documentation: https://arkworks.rs/
Conclusion
Working with arrays in ark-circom enables you to construct powerful ZKP circuits for diverse applications. By understanding the fundamental concepts and employing advanced techniques like conditional operations and multi-dimensional array simulations, you can effectively leverage the power of ark-circom to prove statements about complex data structures, all while preserving privacy.
