pedersen commitment combination证明

1. Zexe中的pedersen commitment

Zexe中基于的是pairing based曲线。
Zexe中的pedersen commitment 为Pedersen CRH，当做$Com(a_1\ast m_1,a_1\ast r_1)=a_1\ast Com(m_1,r_1)$时，
得特别注意WINDOW_SIZE的设置（最大可设置为与 randomness_generator一样，为域内最大位数：MODULUS_BITS）。（因为>=8的数值，因为input[u8]数组的bit位长度为8*N（N为整数）。）【PedersenCommitment::commit是逐bit进行commit求和。】
考虑到缓存情况等原因，应结合实际情况设置WINDOW_SIZE和NUM_WINDOWS。
Parameters.generators的组成为：

$\left(\begin{array}{ccccc}{G}_1& 2G_1& 4G_1& ...& 2^{WINDOW\_SIZE-1}{G}_1\\ G_2& 2G_2& 4G_2& ...& 2^{WINDOW\_SIZE-1}{G}_2\\ ...& ...& ...& ...& ...\\ G_{NUM\_WINDOWS}& 2G_{NUM\_WINDOWS}& 4G_{NUM\_WINDOWS}& ...& 2^{WINDOW\_SIZE-1}{G}_{NUM\_WINDOWS}\end{array}\right)$

fn setup(rng: &mut R) -> Result {
        let time = start_timer!(|| format!(
            "PedersenCOMM::Setup: {} {}-bit windows; {{0,1}}^{{{}}} -> G",
            W::NUM_WINDOWS,
            W::WINDOW_SIZE,
            W::NUM_WINDOWS * W::WINDOW_SIZE
        ));
        let num_powers = ::Params::MODULUS_BITS as usize;
        let randomness_generator = PedersenCRH::<_, W>::generator_powers(num_powers, rng);
        let generators = PedersenCRH::<_, W>::create_generators(rng);
        end_timer!(time);

        Ok(Self::Parameters {
            randomness_generator,
            generators,
        })
    }

impl PedersenCRH {
    pub fn create_generators(rng: &mut R) -> Vec> {
        let mut generators_powers = Vec::new();
        for _ in 0..W::NUM_WINDOWS {
            generators_powers.push(Self::generator_powers(W::WINDOW_SIZE, rng));
        }
        generators_powers
    }

    pub fn generator_powers(num_powers: usize, rng: &mut R) -> Vec {
        let mut cur_gen_powers = Vec::with_capacity(num_powers);
        let mut base = G::rand(rng);
        for _ in 0..num_powers {
            cur_gen_powers.push(base);
            base.double_in_place();
        }
        cur_gen_powers
    }
}

以下代码可证明Com(m_1,r_1) + Com(m_2,r_2)=Com(m_1+m_2, r_1+r_2)

    #[test]
    fn commitment_combination_test() {

        #[derive(Clone, PartialEq, Eq, Hash)]
        pub(super) struct Window;

        impl PedersenWindow for Window {
            const WINDOW_SIZE: usize = 8; // Should be no less than 8 , for `bytes_to_bits` in `commit`->`evalute` func will be always be 8*N (N as an integer bigger than 0)
            const NUM_WINDOWS: usize = 8;
        }

        let input_1 = [51u8, 5];

        let rng = &mut thread_rng();

        let randomness = PedersenRandomness(Fr::rand(rng));

        let parameters = PedersenCommitment::::setup(rng).unwrap();
        let primitive_result =
            PedersenCommitment::::commit(¶meters, &input_1, &randomness).unwrap();
        println!("primitive_result:{:?}, primitive_result + &JubJub::zero(): {:?}", primitive_result, primitive_result + &JubJub::zero());
        assert_eq!(primitive_result, primitive_result + &JubJub::zero());
        assert_eq!(parameters.generators[0][1], parameters.generators[0][0] * &Fr::from(2 as u64));
        //assert_eq!(primitive_result, parameters.generators[0][0] * &Fr::from(2 as u64));

        let input_2 = [5u8, 11];
        let randomness_2 = PedersenRandomness(Fr::rand(rng));
        let primitive_result_2 =
            PedersenCommitment::::commit(¶meters, &input_2, &randomness_2).unwrap();

        let a_1 = 4u8;
        let a_2 = 2u8;
        //let randomness_3 = PedersenRandomness(randomness.0 + &randomness_2.0);
        let randomness_3 = PedersenRandomness(randomness.0 * &Fr::from(a_1 as u64) + &(randomness_2.0 * &Fr::from(a_2 as u64)));
        //let input_3 = [214u8, 42]; //a_1*i1+a_2*i2=1*5+2*7=19
        let input_3: Vec= input_1.iter().zip(input_2.iter()).map(|(i1,i2)| a_1*i1+a_2*i2).collect();
        let primitive_result_3 =
            PedersenCommitment::::commit(¶meters, &input_3, &randomness_3).unwrap();
        println!("primitive_result:{:?}, a1:{:?}, primitive_result_2:{:?}, a_2:{:?}", primitive_result, a_1, primitive_result_2, a_2);
        //assert_eq!(primitive_result * &Fr::from(a_1 as u64) + &(primitive_result_2 * &Fr::from(a_2 as u64)), primitive_result_3);
        let primitive_expected = (primitive_result * &Fr::from(a_1 as u64) + &(primitive_result_2 * &Fr::from(a_2 as u64))).into_affine();
        let primitive_direct = primitive_result_3.into_affine();
        assert_eq!(primitive_direct.x, primitive_expected.x);
        assert_eq!(primitive_direct.y, primitive_expected.y);
    }

2. Qesa中的pedersen commitment

Qesa中基于的是curve25519。
任意取值均可通过
$Com(a_1\ast m_1,a_1\ast r_1)+Com(a_2\ast m_2,a_2\ast r_2)=Com(a_1\ast m_1+a_2\ast m_2,a_1\ast r_1+a_2\ast r_2)$

#[test]
fn test_commitment_combination() {
    let n = 1;
    let mut rng = rand::thread_rng();

    let G: Vec = (0..n).map(|_| RistrettoPoint::random(&mut rng)).collect();
    let G0: RistrettoPoint = RistrettoPoint::random(&mut rng);

    let w_1 = Scalar::random(&mut rng);
    let w_2 = Scalar::random(&mut rng);
    let r_1 = Scalar::random(&mut rng);
    let r_2 = Scalar::random(&mut rng);
    let c_1 = pedersen_commit(&G0, &G.clone(), &vec![w_1.clone()], &r_1);
    let c_2 = pedersen_commit(&G0, &G.clone(), &vec![w_2.clone()], &r_2);
    let a_1 = &Scalar::random(&mut rng);
    let a_2 = &Scalar::random(&mut rng);
    let w_combined = &w_1 * a_1 + &w_2 * a_2;
    let r_combined = &r_1 * a_1 + &r_2 * a_2;
    let c_combined = pedersen_commit(&G0, &G.clone(), &vec![w_combined.clone()], &r_combined);
    assert_eq!(a_1 * &c_1 + a_2*&c_2, c_combined);
}