ziskos 1.1.0-alpha

Guest runtime and entrypoint for programs targeting the ZisK zkVM
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//! Operations in the scalar field Fn of the secp256r1 curve

use crate::{
    syscalls::{syscall_arith256_mod, SyscallArith256ModParams},
    zisklib::{fcall_secp256r1_fn_inv, is_one, is_zero, lt},
};

use super::constants::{N, N_MINUS_ONE};

/// Reduces a 256-bit value modulo the secp256r1 curve order N
pub fn reduce_fn_secp256r1(
    x: &[u64; 4],
    #[cfg(feature = "hints")] hints: &mut Vec<u64>,
) -> [u64; 4] {
    if lt(x, &N) {
        return *x;
    }

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

    *params.d
}

/// Addition in the scalar field of the secp256r1 curve
pub fn add_fn_secp256r1(
    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: &N, d: &mut [0, 0, 0, 0] };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );

    *params.d
}

/// Negation in the scalar field of the secp256r1 curve
pub fn neg_fn_secp256r1(x: &[u64; 4], #[cfg(feature = "hints")] hints: &mut Vec<u64>) -> [u64; 4] {
    // x·(-1) + 0
    let mut params = SyscallArith256ModParams {
        a: x,
        b: &N_MINUS_ONE,
        c: &[0, 0, 0, 0],
        module: &N,
        d: &mut [0, 0, 0, 0],
    };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );

    *params.d
}

/// Subtraction in the scalar field of the secp256r1 curve
pub fn sub_fn_secp256r1(
    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: &N_MINUS_ONE, c: x, module: &N, d: &mut [0, 0, 0, 0] };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );

    *params.d
}

/// Multiplication in the scalar field of the secp256r1 curve
pub fn mul_fn_secp256r1(
    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: &N, d: &mut [0, 0, 0, 0] };
    syscall_arith256_mod(
        &mut params,
        #[cfg(feature = "hints")]
        hints,
    );

    *params.d
}

/// Inversion in the scalar field of the secp256r1 curve. Returns 0 if `x` is 0.
pub fn inv_fn_secp256r1(x: &[u64; 4], #[cfg(feature = "hints")] hints: &mut Vec<u64>) -> [u64; 4] {
    // x == 0 has no inverse; return 0 (callers must check).
    if is_zero(x) {
        return *x;
    }

    // Hint y = x⁻¹ and verify x·y ≡ 1 (mod n).
    let inv = fcall_secp256r1_fn_inv(
        x,
        #[cfg(feature = "hints")]
        hints,
    );

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

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

    inv
}