ntru: Add core streamlined NTRU implementation

Signed-off-by: Ahmed <>

Author: Ahmed

ntru/src/core/mod.rs

@@ -0,0 +1 @@
+pub mod streamlined;

ntru/src/core/streamlined.rs

@@ -0,0 +1,51 @@
+use hybrid_array::typenum::Unsigned;
+use rand_core::CryptoRngCore;
+
+use crate::{
+    algebra::{f3::Small, r3::R3, rq::Rq},
+    params::NtruCommon,
+};
+
+///  (h,f,ginv) = `key_gen()`
+pub fn key_gen<Params: NtruCommon>(
+    rng: &mut impl CryptoRngCore,
+) -> (Rq<Params>, R3<Params>, R3<Params>) {
+    let mut g = R3::default();
+    let mut ginv = R3::default();
+    loop {
+        g.small_random_buffer(rng);
+        if g.recip_buffer(&mut ginv) == 0 {
+            break;
+        };
+    }
+    let f = R3::short_random(rng);
+    let (finv, status) = Rq::recip3(&f);
+    debug_assert_eq!(status, 0);
+    let h = finv.mult_r3(&g);
+    (h, f, ginv)
+}
+
+pub fn encrypt<Params: NtruCommon>(r: &R3<Params>, h: &Rq<Params>) -> Rq<Params> {
+    let hr = h.mult_r3(r);
+    hr.round()
+}
+
+pub fn decrypt<Params: NtruCommon>(
+    c: &Rq<Params>,
+    f: &R3<Params>,
+    ginv: &R3<Params>,
+) -> R3<Params> {
+    let cf = c.mult_r3(f);
+    let cf3 = cf.mult3();
+    let e: R3<Params> = cf3.into();
+    let ev = e.mult(ginv);
+    let mask = ev.weight_w_mask() as i8;
+    let mut r = R3::<Params>::default();
+    for i in 0..Params::W as usize {
+        r.0[i] = Small::new_i8(((*ev.0[i] ^ 1) & !mask) ^ 1);
+    }
+    for i in Params::W as usize..Params::P::USIZE {
+        r.0[i] = Small::new_i8(*ev.0[i] & !mask);
+    }
+    r
+}

ntru/src/lib.rs

@@ -17,8 +17,9 @@
     clippy::similar_names,
 )]
 
+mod algebra;
 pub mod const_time;
-pub mod algebra;
+pub mod core;
 pub mod params;
 
 use hybrid_array::sizes::{U1013, U1277, U653, U761, U857, U953};