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montgomeryReduce

Montgomery reduction: for input x, returns y where y ≡ x*R^(-1) (mod q). This is a generic implementation parameterized by the modulus q, its inverse qInv, the Montgomery constant R, and the result bound.

For ML-DSA: R = 2^32, returns y < 2q For ML-KEM: R = 2^16, returns y in range (-q, q)

ml_kem.montgomeryReduce

fn montgomeryReduce(
    comptime InT: type,
    comptime OutT: type,
    comptime q: comptime_int,
    comptime qInv: comptime_int,
    comptime r_bits: comptime_int,
    x: InT,
) OutT

File

lib/std/crypto/ml_kem.zig:1888

Code

fn montgomeryReduce(
    comptime InT: type,
    comptime OutT: type,
    comptime q: comptime_int,
    comptime qInv: comptime_int,
    comptime r_bits: comptime_int,
    x: InT,
) OutT {
    const mask = (@as(InT, 1) << r_bits) - 1;
    const m_full = (x *% qInv) & mask;
    const m: OutT = @truncate(m_full);

    const yR = x -% @as(InT, m) * @as(InT, q);
    const y_shifted = @as(@Int(.unsigned, @typeInfo(InT).Int.bits), @bitCast(yR)) >> r_bits;
    return @bitCast(@as(@Int(.unsigned, @typeInfo(OutT).Int.bits), @truncate(y_shifted)));
}