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useHint

Uses a hint to reconstruct high bits from a perturbed value. Given:

rp: the perturbed value (r' = r - f)
hint: the hint bit from makeHint Returns the reconstructed high bits r1.

This implements useHint from FIPS 204.

ml_dsa.useHint

fn useHint(rp: u32, hint: u32, comptime gamma2: u32) u32

File

lib/std/crypto/ml_dsa.zig:883

Code

fn useHint(rp: u32, hint: u32, comptime gamma2: u32) u32 {
    const decomp = decompose(rp, gamma2);
    const rp0_plus_q = decomp.a0_plus_q;
    var rp1 = decomp.a1;

    if (hint == 0) {
        return rp1;
    }

    // Depending on gamma2, handle the adjustment differently
    if (gamma2 == 261888) {
        // ML-DSA-65 and ML-DSA-87: max r1 is 15
        if (rp0_plus_q > Q) {
            rp1 = (rp1 + 1) & 15;
        } else {
            rp1 = (rp1 -% 1) & 15;
        }
    } else if (gamma2 == 95232) {
        // ML-DSA-44: max r1 is 43
        if (rp0_plus_q > Q) {
            if (rp1 == 43) {
                rp1 = 0;
            } else {
                rp1 += 1;
            }
        } else {
            if (rp1 == 0) {
                rp1 = 43;
            } else {
                rp1 -= 1;
            }
        }
    } else {
        @compileError("unsupported gamma2 value");
    }

    return rp1;
}