Apply input validation to serde as well

src/dp.rs

@@ -108,7 +108,7 @@ impl ZCdpBudget {
 impl DifferentialPrivacyBudget for ZCdpBudget {}
 
 /// Pure differential privacy budget. (ε-DP or (ε, 0)-DP)
-#[derive(Clone, Debug, Eq, PartialEq, Serialize, Deserialize, Ord, PartialOrd)]
+#[derive(Clone, Debug, Eq, PartialEq, Serialize, Ord, PartialOrd)]
 pub struct PureDpBudget {
     epsilon: Ratio<BigUint>,
 }
@@ -125,6 +125,31 @@ impl PureDpBudget {
 
 impl DifferentialPrivacyBudget for PureDpBudget {}
 
+/// This module encapsulates a deserialization helper struct. It is needed so we can wrap its
+/// derived `Deserialize` implementation in a customized `Deserialize` implementation, which makes
+/// use of the budget's constructor to enforce input validation invariants.
+mod budget_serde {
+    use num_bigint::BigUint;
+    use num_rational::Ratio;
+    use serde::{de, Deserialize};
+
+    #[derive(Deserialize)]
+    pub struct PureDpBudget {
+        epsilon: Ratio<BigUint>,
+    }
+
+    impl<'de> Deserialize<'de> for super::PureDpBudget {
+        fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+        where
+            D: serde::Deserializer<'de>,
+        {
+            let helper = PureDpBudget::deserialize(deserializer)?;
+            super::PureDpBudget::new(super::Rational(helper.epsilon))
+                .map_err(|_| de::Error::custom("epsilon cannot be zero"))
+        }
+    }
+}
+
 /// Strategy to make aggregate results differentially private, e.g. by adding noise from a specific
 /// type of distribution instantiated with a given DP budget.
 pub trait DifferentialPrivacyStrategy {
@@ -149,3 +174,17 @@ pub trait DifferentialPrivacyStrategy {
 }
 
 pub mod distributions;
+
+#[cfg(test)]
+mod tests {
+    use serde_json::json;
+
+    use super::PureDpBudget;
+
+    #[test]
+    fn budget_deserialization() {
+        serde_json::from_value::<PureDpBudget>(json!({"epsilon": [[1], [1]]})).unwrap();
+        serde_json::from_value::<PureDpBudget>(json!({"epsilon": [[0], [1]]})).unwrap_err();
+        serde_json::from_value::<PureDpBudget>(json!({"epsilon": [[1], [0]]})).unwrap_err();
+    }
+}