From 8fedb433aeb8665f79172b123a8a766755b71386 Mon Sep 17 00:00:00 2001
From: Sam Hadow <sam.hadow@inbox.lv>
Date: Sun, 18 May 2025 17:26:05 +0200
Subject: [PATCH] bootstrapping, TODO: actual circuit evaluation

---
 src/bootstrapping.rs | 78 ++++++++++++++++++++++++++++++++++++++++++++
 src/main.rs          |  1 +
 2 files changed, 79 insertions(+)
 create mode 100644 src/bootstrapping.rs

diff --git a/src/bootstrapping.rs b/src/bootstrapping.rs
new file mode 100644
index 0000000..5a14da0
--- /dev/null
+++ b/src/bootstrapping.rs
@@ -0,0 +1,78 @@
+use crate::dghv::decrypt_bit;
+use crate::dghv_asym::{encrypt_bit_asym, PrivateKey, PublicKey};
+use rug::Integer;
+
+/// Bootstrap key, encrypted bits of the private key
+pub struct BootstrapKey {
+    pub encrypted_bits: Vec<Integer>,
+}
+
+/// Generates bootstrap key
+/// bootstrap key is a Vec with each bit of the private key encrypted with the public key
+pub fn generate_bootstrap_key(pk: &PublicKey, sk: &Integer, rho: u32) -> BootstrapKey {
+    let eta = sk.significant_bits();
+    let bits = get_bits(sk, eta);
+    let encrypted_bits: Vec<Integer> = bits
+        .into_iter()
+        .map(|bit| encrypt_bit_asym(bit, pk, rho))
+        .collect();
+
+    BootstrapKey { encrypted_bits }
+}
+
+/// 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)
+}
+
+// extract bits from an Integer
+fn get_bits(n: &Integer, num_bits: u32) -> Vec<u8> {
+    (0..num_bits).map(|i| n.get_bit(i) as u8).collect()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::dghv_asym::generate_keys;
+
+    #[test]
+    fn test_bootstrap_key_generation() {
+        let eta = 1024;
+        let gamma = 10000;
+        let rho = 64;
+        let theta = 10;
+        let (sk, pk) = generate_keys(gamma, eta, rho, theta);
+        let bk = generate_bootstrap_key(&pk, &sk.p, rho);
+
+        assert_eq!(bk.encrypted_bits.len(), eta as usize);
+        for bit in bk.encrypted_bits {
+            assert!(bit >= Integer::from(0));
+            assert!(bit < pk.xs[0]);
+        }
+    }
+
+    #[test]
+    fn test_bootstrapping() {
+        let eta = 1024;
+        let gamma = 10000;
+        let rho = 64;
+        let theta = 10;
+        let (sk, pk) = generate_keys(gamma, eta, rho, theta);
+        let bk = generate_bootstrap_key(&pk, &sk.p, rho);
+
+        let m = 1;
+        let c = encrypt_bit_asym(m, &pk, rho);
+        let bootstrapped_c = bootstrap(&c, &bk, &pk, rho, &sk);
+        let decrypted_m = decrypt_bit(&bootstrapped_c, &sk.p);
+
+        assert_eq!(decrypted_m, m);
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 1cc597c..bee6522 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,3 +1,4 @@
+mod bootstrapping;
 mod dghv;
 mod dghv_asym;
 mod utils;