broken bootstrapping implementation

src/bootstrapping.rs

@@ -76,18 +76,60 @@ pub fn generate_bootstrap_key(
 }
 
 
-// /// Evaluates the decryption circuit to refresh a ciphertext (lower the noise level)
-// pub fn bootstrap(
-//     ciphertext: &Integer,
-//     bk: &BootstrapKey,
-//     pk: &PublicKey,
-//     rho: u32,
-//     sk: &PrivateKey,
-// ) -> Integer {
-//     // TODO: actual implementation
-//     let m = decrypt_bit(ciphertext, &sk.p);
-//     encrypt_bit_asym(m, pk, rho)
-// }
+pub fn encrypt_and_evaluate(
+    c: &Integer,
+    bootstrap_key: &BootstrapKey,
+    precision_bits: u32,
+) -> Vec<Rational> {
+    let mut result = Vec::with_capacity(bootstrap_key.y.len());
+
+    let scale = Integer::from(1) << precision_bits;
+    let scale_rat = Rational::from(&scale);
+
+    for y_i in &bootstrap_key.y {
+        let prod = Rational::from(c) * y_i;
+        let scaled = prod.clone() * &scale_rat;
+
+        let floored = scaled.clone().floor();
+        let frac = &scaled - floored;
+
+        result.push(frac);
+    }
+
+    result
+}
+
+pub fn bootstrap(
+    c: &Integer,
+    z: &[Rational],
+    encrypted_sk_bits: &[Integer],
+    pk: &PublicKey,
+    precision_bits: u32,
+) -> Integer {
+    let int_scale = Integer::from(1) << precision_bits;
+    let scale_rat = Rational::from(&int_scale);
+    let half_rat  = Rational::from((1, 2));
+    let x0        = &pk.xs[0];
+
+    let mut acc = Integer::from(0);
+    for (z_i, enc_s_i) in z.iter().zip(encrypted_sk_bits.iter()) {
+        let rounded: Integer = (z_i.clone() * &scale_rat + &half_rat).floor_ref().into();
+
+        acc += &rounded * enc_s_i;
+    }
+
+    acc %= x0;
+
+    let inv_scale = int_scale
+        .invert(x0)
+        .expect("2^precision_bits and x0 must be coprime");
+    let adjusted_sum = (acc * inv_scale) % x0;
+
+    let mut diff = c.clone();
+    diff -= &adjusted_sum;
+    diff.modulo(x0)
+}
+
 
 #[cfg(test)]
 mod tests {
@@ -165,4 +207,65 @@ mod tests {
             bound
         );
     }
+
+    #[test]
+    fn test_bootstrapping_preserve_m() {
+        let eta = 1000; // secret key bit size
+        let rho = 30; // initial noise
+        let theta = 1000; // Hamming weight of hint support
+        let n_theta = 1000; // total hint size
+        let kappa = 1000; // precision for y_i
+
+        let (sk, pk) = generate_keys(rho, eta, rho, theta);
+        let bk = generate_bootstrap_key(&pk, &sk.p, kappa, theta, n_theta, rho);
+
+        for m in 0..=1 {
+            let mut c = encrypt_bit_asym(m, &pk, rho);
+            for _ in 0..=100 {
+
+                let encrypted_z = encrypt_and_evaluate(&c, &bk, kappa);
+                c = bootstrap(&c, &encrypted_z, &bk.enc_s, &pk, kappa);
+
+                let m_boot = decrypt_bit(&c, &sk.p);
+                assert_eq!(
+                    m, m_boot,
+                    "Bootstrap failed: expected {}, got {}",
+                    m, m_boot
+                );
+            }
+        }
+    }
+
+    #[test]
+    fn test_bootstrapping() {
+        let eta = 1000; // secret key bit size
+        let rho = 30; // initial noise
+        let theta = 1000; // Hamming weight of hint support
+        let n_theta = 1000; // total hint size
+        let kappa = 1000; // precision for y_i
+
+        let (sk, pk) = generate_keys(rho, eta, rho, theta);
+        let bk = generate_bootstrap_key(&pk, &sk.p, kappa, theta, n_theta, rho);
+
+        // let c0 = encrypt_bit_asym(0u8, &pk, rho);
+        let c1 = encrypt_bit_asym(1u8, &pk, rho);
+
+        for m in 0..=1 {
+            let mut c = encrypt_bit_asym(m, &pk, rho);
+            for _ in 0..=100 {
+                // c += c0.clone();
+                c *= c1.clone();
+
+                let encrypted_z = encrypt_and_evaluate(&c, &bk, kappa*2);
+                c = bootstrap(&c, &encrypted_z, &bk.enc_s, &pk, kappa*2);
+
+                let m_boot = decrypt_bit(&c, &sk.p);
+                assert_eq!(
+                    m, m_boot,
+                    "Bootstrap failed: expected {}, got {}",
+                    m, m_boot
+                );
+            }
+        }
+    }
 }