From 7735fbe85cc7cc77551565411c5b3108b0210be9 Mon Sep 17 00:00:00 2001
From: "Mayeul@Zama" <mayeul.debellabre@zama.ai>
Date: Thu, 8 Dec 2022 10:37:42 +0100
Subject: [PATCH] add rust crate to expose security weights

---
 .gitignore                               |  2 +
 rust-security-curves/Cargo.toml          |  8 +++
 rust-security-curves/src/lib.rs          | 67 ++++++++++++++++++++++++
 rust-security-curves/verified_curves.txt |  9 ++++
 4 files changed, 86 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 rust-security-curves/Cargo.toml
 create mode 100644 rust-security-curves/src/lib.rs
 create mode 100644 rust-security-curves/verified_curves.txt

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 000000000..ba51dfb7f
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+rust-security-curves/target/
+rust-security-curves/Cargo.lock
diff --git a/rust-security-curves/Cargo.toml b/rust-security-curves/Cargo.toml
new file mode 100644
index 000000000..facecdebe
--- /dev/null
+++ b/rust-security-curves/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "rust-security-curves"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/rust-security-curves/src/lib.rs b/rust-security-curves/src/lib.rs
new file mode 100644
index 000000000..c59091925
--- /dev/null
+++ b/rust-security-curves/src/lib.rs
@@ -0,0 +1,67 @@
+const SECURITY_WEIGHTS_ARRAY: [(f64, f64, u64, &str, u64); 9] = include!("../verified_curves.txt");
+
+#[derive(Clone, Copy)]
+pub struct SecurityWeights {
+    slope: f64,
+    bias: f64,
+    minimal_lwe_dimension: u64,
+}
+
+impl SecurityWeights {
+    pub fn secure_log2_std(&self, lwe_dimension: u64, ciphertext_modulus_log: f64) -> f64 {
+        // ensure to have a minimal on std deviation covering the 2 lowest bits on modular scale
+        let epsilon_log2_std_modular = 2.0;
+        let epsilon_log2_std = epsilon_log2_std_modular - (ciphertext_modulus_log);
+        // ensure the requested lwe_dimension is bigger than the minimal lwe dimension
+        if self.minimal_lwe_dimension <= lwe_dimension {
+            f64::max(
+                self.slope * lwe_dimension as f64 + self.bias,
+                epsilon_log2_std,
+            )
+        } else {
+            ciphertext_modulus_log
+        }
+    }
+}
+
+pub fn supported_security_levels() -> impl std::iter::Iterator<Item = u64> {
+    SECURITY_WEIGHTS_ARRAY
+        .iter()
+        .filter(|(_, _, _, status, _)| *status == "PASS")
+        .map(|(_, _, security_level, _, _)| *security_level)
+}
+
+pub fn security_weight(security_level: u64) -> Option<SecurityWeights> {
+    let index = SECURITY_WEIGHTS_ARRAY
+        .binary_search_by_key(&security_level, |(_, _, security_level, _, _)| {
+            *security_level
+        })
+        .ok()?;
+
+    let (slope, bias, _security_level, status, minimal_lwe_dimension) =
+        SECURITY_WEIGHTS_ARRAY[index];
+
+    if status == "PASS" {
+        Some(SecurityWeights {
+            slope,
+            bias,
+            minimal_lwe_dimension,
+        })
+    } else {
+        None
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        let weight = security_weight(128).unwrap();
+
+        let secure_log_2_std = weight.secure_log2_std(512, 64.);
+
+        assert!((-12.0..-10.0).contains(&secure_log_2_std));
+    }
+}
diff --git a/rust-security-curves/verified_curves.txt b/rust-security-curves/verified_curves.txt
new file mode 100644
index 000000000..f1ed1aed1
--- /dev/null
+++ b/rust-security-curves/verified_curves.txt
@@ -0,0 +1,9 @@
+[(-0.04042633119364589, 1.6609788641436722, 80, "PASS", 450),
+ (-0.03414780360867051, 2.017310258660345, 96, "PASS", 450),
+ (-0.029670137081135885, 2.162463714083856, 112, "PASS", 450),
+ (-0.02640502876522622, 2.4826422691043177, 128, "PASS", 450),
+ (-0.023821437305989134, 2.7177789440636673, 144, "PASS", 450),
+ (-0.02174358218716036, 2.938810548493322, 160, "PASS", 498),
+ (-0.019904056582117684, 2.8161252801542247, 176, "PASS", 551),
+ (-0.018610403247590085, 3.2996236848399008, 192, "PASS", 606),
+ (-0.014606812351714953, 3.8493629234693003, 256, "PASS", 826)]