ziskos 1.1.0-alpha

Guest runtime and entrypoint for programs targeting the ZisK zkVM
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//! Operations in the base field Fp of the BN254 curve

use crate::{
    syscalls::{syscall_arith256_mod, SyscallArith256ModParams},
    zisklib::{eq, fcall_bn254_fp_inv, is_one, lt},
};

use super::constants::{P, P_MINUS_ONE};

/// Addition in the base field of the BN254 curve
#[inline]
pub fn add_fp_bn254(
    x: &[u64; 4],
    y: &[u64; 4],
    #[cfg(feature = "hints")] hints: &mut Vec<u64>,
) -> [u64; 4] {
    // x·1 + y
    let mut params =
        SyscallArith256ModParams { a: x, b: &[1, 0, 0, 0], c: y, module: &P, d: &mut [0, 0, 0, 0] };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );
    *params.d
}

/// Negation in the base field of the BN254 curve
#[inline]
pub fn neg_fp_bn254(x: &[u64; 4], #[cfg(feature = "hints")] hints: &mut Vec<u64>) -> [u64; 4] {
    // x·(-1) + 0
    let mut params = SyscallArith256ModParams {
        a: x,
        b: &P_MINUS_ONE,
        c: &[0, 0, 0, 0],
        module: &P,
        d: &mut [0, 0, 0, 0],
    };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );
    *params.d
}

/// Multiplication in the base field of the BN254 curve
#[inline]
pub fn mul_fp_bn254(
    x: &[u64; 4],
    y: &[u64; 4],
    #[cfg(feature = "hints")] hints: &mut Vec<u64>,
) -> [u64; 4] {
    // x·y + 0
    let mut params =
        SyscallArith256ModParams { a: x, b: y, c: &[0, 0, 0, 0], module: &P, d: &mut [0, 0, 0, 0] };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );
    *params.d
}

/// Squaring in the base field of the BN254 curve
#[inline]
pub fn square_fp_bn254(x: &[u64; 4], #[cfg(feature = "hints")] hints: &mut Vec<u64>) -> [u64; 4] {
    // x·x + 0
    let mut params =
        SyscallArith256ModParams { a: x, b: x, c: &[0, 0, 0, 0], module: &P, d: &mut [0, 0, 0, 0] };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );
    *params.d
}

/// Inversion in the base field of the BN254 curve
#[inline]
pub fn inv_fp_bn254(x: &[u64; 4], #[cfg(feature = "hints")] hints: &mut Vec<u64>) -> [u64; 4] {
    // if x == 0, return 0
    if eq(x, &[0, 0, 0, 0]) {
        return [0, 0, 0, 0];
    }

    // if x != 0, return 1 / x

    // Remember that an element y ∈ Fp is the inverse of x ∈ Fp if and only if x·y = 1 in Fp
    // We will therefore hint the inverse y and check the product with x is 1
    let inv = fcall_bn254_fp_inv(
        x,
        #[cfg(feature = "hints")]
        hints,
    );

    // Check that the inverse is canonical
    assert!(lt(&inv, &P), "Inverse is not canonical");

    // x·y + 0
    let mut params = SyscallArith256ModParams {
        a: x,
        b: &inv,
        c: &[0, 0, 0, 0],
        module: &P,
        d: &mut [0, 0, 0, 0],
    };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );
    assert!(is_one(params.d), "Inverse verification failed");

    inv
}