add BFV newkey, encrypt, decrypt, and homomorphic addition impl

arithmetic/src/naive.rs

@@ -136,7 +136,7 @@ mod tests {
 
         let mut rng = rand::thread_rng();
         let uniform_distr = Uniform::new(0_f64, Q as f64);
-        let a = PR::<Q, N>::rand(&mut rng, uniform_distr)?;
+        let a = PR::<Q, N>::rand_f64(&mut rng, uniform_distr)?;
         // let a = PR::<Q, N>::new_from_u64(vec![36, 21, 9, 19]);
 
         // let a_padded_coeffs: [Zq<Q>; 2 * N] =
@@ -181,7 +181,7 @@ mod tests {
         let ntt = NTT::<Q, N>::new()?;
 
         let rng = rand::thread_rng();
-        let a = PR::<Q, { 2 * N }>::rand(rng, Uniform::new(0_f64, (Q - 1) as f64))?;
+        let a = PR::<Q, { 2 * N }>::rand_f64(rng, Uniform::new(0_f64, (Q - 1) as f64))?;
         let a = a.coeffs;
         dbg!(&a);
         let a_ntt = matrix_vec_product(&ntt.ntt, &a.to_vec())?;
@@ -189,6 +189,7 @@ mod tests {
         let a_intt = matrix_vec_product(&ntt.intt, &a_ntt)?;
         dbg!(&a_intt);
         assert_eq!(a_intt, a);
+        // TODO bench
 
         Ok(())
     }

arithmetic/src/ntt.rs

@@ -1,5 +1,6 @@
-//! Implementation of the NTT & iNTT, following the CT & GS algorighms, more
+//! Implementation of the NTT & iNTT, following the CT & GS algorighms, more details in
-//! details in https://github.com/arnaucube/math/blob/master/notes_ntt.pdf .
+//! https://eprint.iacr.org/2017/727.pdf, some notes at
+//! https://github.com/arnaucube/math/blob/master/notes_ntt.pdf .
 use crate::zq::Zq;
 
 #[derive(Debug)]
@@ -14,7 +15,8 @@ impl<const Q: u64, const N: usize> NTT<Q, N> {
 }
 
 impl<const Q: u64, const N: usize> NTT<Q, N> {
-    /// implements the Cooley-Tukey (CT) algorithm. Details at section 3.1 of
+    /// implements the Cooley-Tukey (CT) algorithm. Details at
+    /// https://eprint.iacr.org/2017/727.pdf, also some notes at section 3.1 of
     /// https://github.com/arnaucube/math/blob/master/notes_ntt.pdf
     pub fn ntt(a: [Zq<Q>; N]) -> [Zq<Q>; N] {
         let mut t = N / 2;
@@ -38,7 +40,8 @@ impl<const Q: u64, const N: usize> NTT<Q, N> {
         r
     }
 
-    /// implements the Gentleman-Sande (GS) algorithm. Details at section 3.2 of
+    /// implements the Cooley-Tukey (CT) algorithm. Details at
+    /// https://eprint.iacr.org/2017/727.pdf, also some notes at section 3.2 of
     /// https://github.com/arnaucube/math/blob/master/notes_ntt.pdf
     pub fn intt(a: [Zq<Q>; N]) -> [Zq<Q>; N] {
         let mut t = 1;

arithmetic/src/ring.rs

@@ -63,13 +63,20 @@ impl<const Q: u64, const N: usize> PR<Q, N> {
             evals: None,
         })
     }
-    pub fn rand(mut rng: impl Rng, dist: impl Distribution<f64>) -> Result<Self> {
+    pub fn rand_f64(mut rng: impl Rng, dist: impl Distribution<f64>) -> Result<Self> {
         let coeffs: [Zq<Q>; N] = array::from_fn(|_| Zq::from_f64(dist.sample(&mut rng)));
         Ok(Self {
             coeffs,
             evals: None,
         })
     }
+    pub fn rand_u64(mut rng: impl Rng, dist: impl Distribution<u64>) -> Result<Self> {
+        let coeffs: [Zq<Q>; N] = array::from_fn(|_| Zq::new(dist.sample(&mut rng)));
+        Ok(Self {
+            coeffs,
+            evals: None,
+        })
+    }
     // WIP. returns random v \in {0,1}. // TODO {-1, 0, 1}
     pub fn rand_bin(mut rng: impl Rng, dist: impl Distribution<bool>) -> Result<Self> {
         let coeffs: [Zq<Q>; N] = array::from_fn(|_| Zq::from_bool(dist.sample(&mut rng)));

bfv/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "bfv"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+anyhow = { workspace = true }
+rand = { workspace = true }
+rand_distr = { workspace = true }
+
+arithmetic = { path="../arithmetic" }

bfv/src/lib.rs

@@ -0,0 +1,152 @@
+//! Implementation of BFV https://eprint.iacr.org/2012/144.pdf
+#![allow(non_snake_case)]
+#![allow(non_upper_case_globals)]
+#![allow(non_camel_case_types)]
+#![allow(clippy::upper_case_acronyms)]
+#![allow(dead_code)] // TMP
+
+use anyhow::{anyhow, Result};
+use rand::Rng;
+use rand_distr::{Normal, Uniform};
+use std::ops;
+
+use arithmetic::PR;
+
+// error deviation for the Gaussian(Normal) distribution
+// sigma=3.2 from: https://eprint.iacr.org/2022/162.pdf page 5
+const ERR_SIGMA: f64 = 3.2;
+
+#[derive(Clone, Debug)]
+pub struct SecretKey<const Q: u64, const N: usize>(PR<Q, N>);
+
+#[derive(Clone, Debug)]
+pub struct PublicKey<const Q: u64, const N: usize>(PR<Q, N>, PR<Q, N>);
+
+// RLWE ciphertext
+#[derive(Clone, Debug)]
+pub struct RLWE<const Q: u64, const N: usize>(PR<Q, N>, PR<Q, N>);
+
+impl<const Q: u64, const N: usize> RLWE<Q, N> {
+    fn add(lhs: Self, rhs: Self) -> Self {
+        RLWE::<Q, N>(lhs.0 + rhs.0, lhs.1 + rhs.1)
+    }
+    fn mul(lhs: Self, rhs: Self) -> Self {
+        todo!()
+    }
+}
+
+impl<const Q: u64, const N: usize> ops::Add<RLWE<Q, N>> for RLWE<Q, N> {
+    type Output = Self;
+    fn add(self, rhs: Self) -> Self {
+        Self::add(self, rhs)
+    }
+}
+
+pub struct BFV<const Q: u64, const N: usize, const T: u64> {}
+
+impl<const Q: u64, const N: usize, const T: u64> BFV<Q, N, T> {
+    const DELTA: u64 = Q / T;
+
+    /// generate a new key pair (privK, pubK)
+    pub fn new_key(mut rng: impl Rng) -> Result<(SecretKey<Q, N>, PublicKey<Q, N>)> {
+        // WIP: review probabilities
+
+        // let Xi_key = Uniform::new(-1_f64, 1_f64);
+        let Xi_key = Uniform::new(0_u64, 2_u64);
+        // let Xi_key = Uniform::new(0_u64, 2_u64);
+        // use rand::distributions::Bernoulli;
+        // let Xi_key = Bernoulli::new(0.5)?;
+        let Xi_err = Normal::new(0_f64, ERR_SIGMA)?;
+
+        // secret key
+        let s = PR::<Q, N>::rand_u64(&mut rng, Xi_key)?;
+
+        // pk = (-a * s + e, a)
+        let a = PR::<Q, N>::rand_u64(&mut rng, Uniform::new(0_u64, Q))?;
+        let e = PR::<Q, N>::rand_f64(&mut rng, Xi_err)?;
+        let pk: PublicKey<Q, N> = PublicKey((&(-a) * &s) + e, a.clone());
+        Ok((SecretKey(s), pk))
+    }
+
+    pub fn encrypt(mut rng: impl Rng, pk: &PublicKey<Q, N>, m: &PR<T, N>) -> Result<RLWE<Q, N>> {
+        let Xi_key = Uniform::new(-1_f64, 1_f64);
+        let Xi_err = Normal::new(0_f64, ERR_SIGMA)?;
+
+        let u = PR::<Q, N>::rand_f64(&mut rng, Xi_key)?;
+        let e_1 = PR::<Q, N>::rand_f64(&mut rng, Xi_err)?;
+        let e_2 = PR::<Q, N>::rand_f64(&mut rng, Xi_err)?;
+
+        // migrate m's coeffs to the bigger modulus Q (from T)
+        let m = PR::<Q, N>::from_vec_u64(m.coeffs().iter().map(|m_i| m_i.0).collect());
+        let c0 = &pk.0 * &u + e_1 + m * Self::DELTA;
+        let c1 = &pk.1 * &u + e_2;
+        Ok(RLWE::<Q, N>(c0, c1))
+    }
+
+    pub fn decrypt(sk: &SecretKey<Q, N>, c: &RLWE<Q, N>) -> PR<T, N> {
+        let cs = c.0 + c.1 * sk.0; // done in mod q
+        let r: Vec<u64> = cs
+            .coeffs()
+            .iter()
+            .map(|e| ((T as f64 * e.0 as f64) / Q as f64).round() as u64)
+            .collect();
+        PR::<T, N>::from_vec_u64(r)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use anyhow::Result;
+    use rand::distributions::Uniform;
+
+    use super::*;
+
+    #[test]
+    fn test_encrypt_decrypt() -> Result<()> {
+        const Q: u64 = 2u64.pow(16) + 1;
+        const N: usize = 32;
+        const T: u64 = 4; // plaintext modulus
+        type S = BFV<Q, N, T>;
+
+        let mut rng = rand::thread_rng();
+
+        let (sk, pk) = S::new_key(&mut rng)?;
+
+        let msg_dist = Uniform::new(0_u64, T);
+        let m = PR::<T, N>::rand_u64(&mut rng, msg_dist)?;
+
+        let c = S::encrypt(rng, &pk, &m)?;
+        let m_recovered = S::decrypt(&sk, &c);
+
+        assert_eq!(m, m_recovered);
+
+        Ok(())
+    }
+
+    #[test]
+    fn test_addition() -> Result<()> {
+        const Q: u64 = 2u64.pow(16) + 1;
+        const N: usize = 32;
+        const T: u64 = 4; // plaintext modulus
+        type S = BFV<Q, N, T>;
+
+        let mut rng = rand::thread_rng();
+
+        let (sk, pk) = S::new_key(&mut rng)?;
+
+        let msg_dist = Uniform::new(0_u64, T);
+        let m1 = PR::<T, N>::rand_u64(&mut rng, msg_dist)?;
+        let m2 = PR::<T, N>::rand_u64(&mut rng, msg_dist)?;
+
+        let c1 = S::encrypt(&mut rng, &pk, &m1)?;
+        let c2 = S::encrypt(&mut rng, &pk, &m2)?;
+
+        let c3 = c1 + c2;
+
+        let m3_recovered = S::decrypt(&sk, &c3);
+
+        assert_eq!(m1 + m2, m3_recovered);
+
+        Ok(())
+    }
+}