Add Zeroize impl for SigningKey (#34)

* Add Zeroize impl for SigningKey

Cargo.toml

@@ -20,6 +20,7 @@ rand_core = "0.5"
 thiserror = "1"
 curve25519-dalek = "3"
 serde = { version = "1", optional = true, features = ["derive"] }
+zeroize = "1.1"
 
 [dev-dependencies]
 rand = "0.7"

src/signing_key.rs

@@ -106,6 +106,13 @@ impl From<[u8; 32]> for SigningKey {
     }
 }
 
+impl zeroize::Zeroize for SigningKey {
+    fn zeroize(&mut self) {
+        self.seed.zeroize();
+        self.s.zeroize()
+    }
+}
+
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 struct SerdeHelper([u8; 32]);