ziskos 1.1.0-alpha

Guest runtime and entrypoint for programs targeting the ZisK zkVM
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//! Finite field Fr operations for BLS12-381

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

use super::constants::{R, R_MINUS_ONE};

/// Reduces a 256-bit value modulo the BLS12-381 scalar field order R
pub fn reduce_fr_bls12_381(
    x: &[u64; 4],
    #[cfg(feature = "hints")] hints: &mut Vec<u64>,
) -> [u64; 4] {
    if lt(x, &R) {
        return *x;
    }

    // x·1 + 0
    let mut params = SyscallArith256ModParams {
        a: x,
        b: &[1, 0, 0, 0],
        c: &[0, 0, 0, 0],
        module: &R,
        d: &mut [0, 0, 0, 0],
    };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );

    *params.d
}

/// Addition in the scalar field of the BLS12-381 curve
#[inline]
pub fn add_fr_bls12_381(
    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: &R, d: &mut [0, 0, 0, 0] };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );
    *params.d
}

/// Doubling in the scalar field of the BLS12-381 curve
#[inline]
pub fn dbl_fr_bls12_381(x: &[u64; 4], #[cfg(feature = "hints")] hints: &mut Vec<u64>) -> [u64; 4] {
    // 2·x + 0 or x·1 + x
    let mut params = SyscallArith256ModParams {
        a: x,
        b: &[2, 0, 0, 0],
        c: &[0, 0, 0, 0],
        module: &R,
        d: &mut [0, 0, 0, 0],
    };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );
    *params.d
}

/// Subtraction in the scalar field of the BLS12-381 curve
#[inline]
pub fn sub_fr_bls12_381(
    x: &[u64; 4],
    y: &[u64; 4],
    #[cfg(feature = "hints")] hints: &mut Vec<u64>,
) -> [u64; 4] {
    // y·(-1) + x
    let mut params =
        SyscallArith256ModParams { a: y, b: &R_MINUS_ONE, c: x, module: &R, d: &mut [0, 0, 0, 0] };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );
    *params.d
}

/// Negation in the scalar field of the BLS12-381 curve
#[inline]
pub fn neg_fr_bls12_381(x: &[u64; 4], #[cfg(feature = "hints")] hints: &mut Vec<u64>) -> [u64; 4] {
    // x·(-1) + 0
    let mut params = SyscallArith256ModParams {
        a: x,
        b: &R_MINUS_ONE,
        c: &[0, 0, 0, 0],
        module: &R,
        d: &mut [0, 0, 0, 0],
    };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );
    *params.d
}

/// Multiplication in the scalar field of the BLS12-381 curve
#[inline]
pub fn mul_fr_bls12_381(
    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: &R, d: &mut [0, 0, 0, 0] };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );
    *params.d
}

/// Squaring in the scalar field of the BLS12-381 curve
#[inline]
pub fn square_fr_bls12_381(
    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: &R, d: &mut [0, 0, 0, 0] };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );
    *params.d
}

/// Convert 32-byte big-endian scalar to [u64; 4] little-endian
pub fn scalar_bytes_be_to_u64_le_bls12_381(bytes: &[u8; 32]) -> [u64; 4] {
    let mut result = [0u64; 4];
    for i in 0..4 {
        for j in 0..8 {
            result[3 - i] |= (bytes[i * 8 + j] as u64) << (8 * (7 - j));
        }
    }

    result
}