Simple implementation of mimc, light-tested

[AlexandreBelling]

Add an implementation of MiMC on the bn254 scalar field in rust.

MIMC5 is a hash function suited for BN256's scalar field.

It has modulus p = 21888242871839275222246405745257275088548364400416034343698204186575808495617

The MIMC paper (https://eprint.iacr.org/2016/492.pdf) states that monomial x^d is a permutation in Fp if gcd(d,p) == 1. Exponent for MIMC is 5, which satisfies the above condition.

fn mimc5_permutation(constants, x, k):
    h := x;
    for i = 0..constants.len() {
        h := h + k + constants[i]
        h := h^5;
    }
    h = h + k;
    return h;

Interface

pub fn mimc_hash<T : AsRef<[u8]>>(bytes: T) -> Vec<u8>;

The implementation targets full consistency with the present implementation:

Consistency testing with gnark's implementation

gnark and the present implementation are tested to give the same result for the strings.

• 0x0000000000000000000000000000000000000000000000000000000000000000
• 0x0000000000000000000000000000000000000000000000000000000000000001
• 0x0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000000000003000000000000000000000000000000000000000000000000000000000000000400000000000000000000000000000000000000000000000000000000000000050000000000000000000000000000000000000000000000000000000000000006000000000000000000000000000000000000000000000000000000000000000700000000000000000000000000000000000000000000000000000000000000080000000000000000000000000000000000000000000000000000000000000009000000000000000000000000000000000000000000000000000000000000000a000000000000000000000000000000000000000000000000000000000000000b000000000000000000000000000000000000000000000000000000000000000c000000000000000000000000000000000000000000000000000000000000000d000000000000000000000000000000000000000000000000000000000000000e000000000000000000000000000000000000000000000000000000000000000f

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