WIP: NTRU Prime

Cargo.toml

@@ -1,7 +1,7 @@
 [workspace]
 resolver = "2"
 members = [
-    "ml-kem",
+    "ml-kem", "ntru",
 ]
 
 [profile.bench]

ntru/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "ntru"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+hybrid-array = { path="../../hybrid-array", features = ["extra-sizes"] }
+rand_core = "0.6.4"
+sha2 = "0.10.8"
+
+[dev-dependencies]
+aes="0.8.4"
+hex = "0.4.3"
+itertools = "0.13.0"
+rayon="1.10.0"

ntru/src/algebra/f3.rs

@@ -0,0 +1,70 @@
+//! arithmetic mod 3
+
+use super::fq::Fq;
+use crate::const_time::i32_mod_u14;
+use core::ops::Deref;
+
+/// always represented as -1,0,1
+#[derive(Eq, PartialEq, Debug, Copy, Clone, Default)]
+pub struct Small(i8);
+
+impl Small {
+    pub const ZERO: Small = Small(0);
+    pub const ONE: Small = Small(1);
+    pub const MONE: Small = Small(-1);
+
+    pub(super) fn new_i32(n: i32) -> Self {
+        debug_assert!(n < 2);
+        debug_assert!(n > -2);
+        Small(n as i8)
+    }
+    #[must_use]
+    pub fn new_i8(n: i8) -> Self {
+        debug_assert!(n < 2);
+        debug_assert!(n > -2);
+        Small(n)
+    }
+
+    #[must_use]
+    pub const fn freeze(x: i16) -> Self {
+        Small((i32_mod_u14((x as i32) + 1, 3).wrapping_sub(1)) as i8)
+    }
+}
+
+/// the benefit is from outside, anyone can access the inner value as number,
+/// but no one can modify it without refreezing
+impl Deref for Small {
+    type Target = i8;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<Q> From<Fq<Q>> for Small {
+    fn from(value: Fq<Q>) -> Self {
+        Small::freeze(*value)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::Small;
+    fn naive_freeze(x: i16) -> i8 {
+        // returns values in the set [-2, 2]
+        let res = (x % 3) as i8;
+        if res > 1 {
+            return res - 3;
+        }
+        if res < -1 {
+            return res + 3;
+        }
+        res
+    }
+    #[test]
+    fn test_freeze() {
+        for i in i16::MIN..i16::MAX {
+            assert_eq!(*Small::freeze(i), naive_freeze(i));
+        }
+    }
+}