1use crate::{
 2    ciphertext::Ciphertext,
 3    elem::{Elem, ElemCommon},
 4    keys::SecretKey,
 5    parameters::Parameters,
 6    plaintext::Plaintext,
 7};
 8use base2k::{Module, SvpPPol, SvpPPolOps, VecZnx, VecZnxBigOps, VecZnxDft, VecZnxDftOps};
 9use std::cmp::min;
10
11pub struct Decryptor {
12    sk: SvpPPol,
13}
14
15impl Decryptor {
16    pub fn new(params: &Parameters, sk: &SecretKey) -> Self {
17        let mut sk_svp_ppol: SvpPPol = params.module().new_svp_ppol();
18        sk.prepare(params.module(), &mut sk_svp_ppol);
19        Self { sk: sk_svp_ppol }
20    }
21}
22
23pub fn decrypt_rlwe_tmp_byte(module: &Module, limbs: usize) -> usize {
24    module.bytes_of_vec_znx_dft(limbs) + module.vec_znx_big_normalize_tmp_bytes()
25}
26
27impl Parameters {
28    pub fn decrypt_rlwe_tmp_byte(&self, log_q: usize) -> usize {
29        decrypt_rlwe_tmp_byte(
30            self.module(),
31            (log_q + self.log_base2k() - 1) / self.log_base2k(),
32        )
33    }
34
35    pub fn decrypt_rlwe(&self, res: &mut Plaintext, ct: &Ciphertext<VecZnx>, sk: &SvpPPol, tmp_bytes: &mut [u8]) {
36        decrypt_rlwe(self.module(), &mut res.0, &ct.0, sk, tmp_bytes)
37    }
38}
39
40pub fn decrypt_rlwe(module: &Module, res: &mut Elem<VecZnx>, a: &Elem<VecZnx>, sk: &SvpPPol, tmp_bytes: &mut [u8]) {
41    let cols: usize = a.cols();
42
43    assert!(
44        tmp_bytes.len() >= decrypt_rlwe_tmp_byte(module, cols),
45        "invalid tmp_bytes: tmp_bytes.len()={} < decrypt_rlwe_tmp_byte={}",
46        tmp_bytes.len(),
47        decrypt_rlwe_tmp_byte(module, cols)
48    );
49
50    let (tmp_bytes_vec_znx_dft, tmp_bytes_normalize) = tmp_bytes.split_at_mut(module.bytes_of_vec_znx_dft(cols));
51
52    let mut res_dft: VecZnxDft = VecZnxDft::from_bytes_borrow(module, cols, tmp_bytes_vec_znx_dft);
53    let mut res_big: base2k::VecZnxBig = res_dft.as_vec_znx_big();
54
55    // res_dft <- DFT(ct[1]) * DFT(sk)
56    module.svp_apply_dft(&mut res_dft, sk, a.at(1));
57    // res_big <- ct[1] x sk
58    module.vec_znx_idft_tmp_a(&mut res_big, &mut res_dft);
59    // res_big <- ct[1] x sk + ct[0]
60    module.vec_znx_big_add_small_inplace(&mut res_big, a.at(0));
61    // res <- normalize(ct[1] x sk + ct[0])
62    module.vec_znx_big_normalize(a.log_base2k(), res.at_mut(0), &res_big, tmp_bytes_normalize);
63
64    res.log_base2k = a.log_base2k();
65    res.log_q = min(res.log_q(), a.log_q());
66    res.log_scale = a.log_scale();
67}