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makeHint

Creates a hint bit to help recover high bits after a small perturbation. Given:

z0: the modified low bits (r0 - f mod Q) where f is small
r1: the original high bits Returns 1 if a hint is needed, 0 otherwise.

This implements makeHint from FIPS 204. The hint helps recover r1 from r' = r - f without knowing f explicitly.

ml_dsa.makeHint

fn makeHint(z0: u32, r1: u32, comptime gamma2: u32) u32

File

lib/std/crypto/ml_dsa.zig:859

Code

fn makeHint(z0: u32, r1: u32, comptime gamma2: u32) u32 {
    // If -alpha/2 < r0 - f <= alpha/2, then r1*alpha + r0 - f is a valid
    // decomposition of r' with the restrictions of decompose() and so r'1 = r1.
    // So the hint should be 0. This is covered by the first two inequalities.
    // There is one other case: if r0 - f = -alpha/2, then r1*alpha + r0 - f is
    // also a valid decomposition if r1 = 0. In the other cases a one is carried
    // and the hint should be 1.

    const cond1 = @intFromBool(z0 <= gamma2);
    const cond2 = @intFromBool(z0 > Q - gamma2);
    const eq_gamma2 = @intFromBool(z0 == Q - gamma2);
    const r1_is_zero = @intFromBool(r1 == 0);
    const cond3 = eq_gamma2 & r1_is_zero;

    return 1 - (cond1 | cond2 | cond3);
}