classic_mceliece_rust/
synd.rs

1//! Syndrome computation
2
3use crate::gf::{gf_add, gf_inv, gf_mul, Gf};
4use crate::params::{SYS_N, SYS_T};
5use crate::root::eval;
6
7/// Given Goppa polynomial `f`, support `l`, and received word `r`
8/// compute `out`, the syndrome of length 2t
9pub(crate) fn synd(
10    out: &mut [Gf; SYS_T * 2],
11    f: &[Gf; SYS_T + 1],
12    l: &[Gf; SYS_N],
13    r: &[u8; SYS_N / 8],
14) {
15    out[0..2 * SYS_T].fill(0);
16
17    for i in 0..SYS_N {
18        let c: Gf = (r[i / 8] >> (i % 8)) as u16 & 1;
19        let e: Gf = eval(f, l[i]);
20        let mut e_inv: Gf = gf_inv(gf_mul(e, e));
21
22        for itr_out in out.iter_mut() {
23            *itr_out = gf_add(*itr_out, gf_mul(e_inv, c));
24            e_inv = gf_mul(e_inv, l[i]);
25        }
26    }
27}