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Transaction circuit

Explanation

The circuit transaction.circom is meant as a simple template that should prove a transaction with these signals:

Input signals:

  • balanceInSender
  • balanceInReceiver
  • amount

Output signals:

  • balanceOutSender
  • balanceOutReceiver

While the template looks fine to me, when proving it, an error occurs during the C Points calculations.

The input.json is a sample input that should work correctly.

Installation

Please follow the instructions from here https://docs.circom.io/getting-started/installation to install the circom compiler and snarkjs.

Build & prove

  1. Run ./build.sh to generate WASM code for witness generation
  2. Run ./gen_witness.sh to generate the witness from the input.json
  3. Run ./gen_proof.sh. The last command in the file is the one that creates the problem
  • snarkjs groth16 prove zkey/transaction_0001.zkey witness.wtns proof/proof.json proof/public.json

For an explanation on all of the commands you can visit the other parts of the circom getting started guide:

Error

You can find the whole log (which I created by going through source code and adding console.log) and error in error.log file.

Explanation

What I've gathered from looking into the code where the error happens is that some data called C Points from the zkey is an array that's empty when it shouldn't be.

Well, I don't know whether it should or should not be empty, but it seems like it's expected to be non-empty because when empty it causes a Division by zero error, so the sScalar below becomes NaN.

It also seems that the data array that's gotten from the witness file that should be used in the function bellow is also empty, which I'm also not sure if it's allowed to be.

The error only happens on those C Points, and not on any data (Points) before them. The whole output for the _multiExp function for all the other points is in the error.log file. This one is just for the C Points.

From _multiExp function:

logText: multiexp C (C Points)
buffBases: Uint8Array(0) []
buffScalars: Uint8Array(0) []
buffBases.byteLength: 0
buffScalars.byteLength: 0
nPoints: 0
sScalar: NaN
sGIn: 64
inType: affine