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polyUnpackLeqEta

Unpack polynomial with coefficients in [Q-eta, Q+eta] from bytes. Output coefficients will not be normalized, but in [q-η, q+η].

ml_dsa.polyUnpackLeqEta

fn polyUnpackLeqEta(comptime eta: u8, buf: []const u8) Poly

File

lib/std/crypto/ml_dsa.zig:999

Code

fn polyUnpackLeqEta(comptime eta: u8, buf: []const u8) Poly {
    comptime {
        if (eta != 2 and eta != 4) {
            @compileError("eta must be 2 or 4");
        }
    }

    var p = Poly.zero;

    if (eta == 2) {
        // 3 bits per coefficient: unpack 8 coefficients from 3 bytes
        var j: usize = 0;
        var i: usize = 0;
        while (i < buf.len) : (i += 3) {
            p.cs[j] = Q + eta - (buf[i] & 7);
            p.cs[j + 1] = Q + eta - ((buf[i] >> 3) & 7);
            p.cs[j + 2] = Q + eta - ((buf[i] >> 6) | ((buf[i + 1] << 2) & 7));
            p.cs[j + 3] = Q + eta - ((buf[i + 1] >> 1) & 7);
            p.cs[j + 4] = Q + eta - ((buf[i + 1] >> 4) & 7);
            p.cs[j + 5] = Q + eta - ((buf[i + 1] >> 7) | ((buf[i + 2] << 1) & 7));
            p.cs[j + 6] = Q + eta - ((buf[i + 2] >> 2) & 7);
            p.cs[j + 7] = Q + eta - ((buf[i + 2] >> 5) & 7);
            j += 8;
        }
    } else { // eta == 4
        // 4 bits per coefficient: unpack 2 coefficients from 1 byte
        var j: usize = 0;
        for (0..buf.len) |i| {
            p.cs[j] = Q + eta - (buf[i] & 15);
            p.cs[j + 1] = Q + eta - (buf[i] >> 4);
            j += 2;
        }
    }

    return p;
}