Secret inputs in witness obtainable

[RichardGazdik1]

Hello, I am interested in gnark privacy and wanted to know if it is possible to obtain secret inputs from an already created witness file. For example, I have a simple circuit with 1 public and 1 secret input with basic addition:

type Circuit struct {
    A frontend.Variable `gnark:"secret"` // Secret input
    B frontend.Variable `gnark:"public"` // Public input
}

func (circuit *Circuit) Define(api frontend.API) error {
    api.AssertIsEqual(api.Add(circuit.A, circuit.B), 15)
    return nil
}

I did all the needed steps to create the witness:

var circuit Circuit

r1cs, err := frontend.Compile(ecc.BN254.ScalarField(), r1cs.NewBuilder, &circuit)

pk, vk, err := groth16.Setup(r1cs)

assignment := &Circuit{
    A: 10, // Secret input
    B: 5,  // Public input
}

witness, err := frontend.NewWitness(assignment, ecc.BN254.ScalarField())

My question now is: Is it possible to obtain the value(s) of secret inputs from this witness file, or is it impossible to do so?

[ThomasPiellard]

Hi @RichardGazdik1 , if you want to have the full witness, you need to query the underlying vector that the witness stores, and cast it to the correct type:
v := witness.Vector().(fr.Vector)

(include "github.com/consensys/gnark-crypto/ecc/bn254/fr" in your example).

The entries in the vector correspond to the variables in the witness, in the same order than in the structure representing the circuit, with the public witness first, like this: [public || secret ] (for instance, the secret part contains the secret witness in the same order of appearance than in the structure defining the circuit)