Zeroize full signingkey (#73)

* Zeroize full signingkey

Includes Default and DefaultIsZeroes impl for VerificationKey.

Resolves #72

* derive Zeroize for SigningKey

Cargo.toml

@@ -14,14 +14,14 @@ resolver = "2"
 features = ["nightly"]
 
 [dependencies]
-hex = { version = "0.4", default-features = false, features = ["alloc"] }
-sha2 = { version = "0.10", default-features = false }
-rand_core = "0.6"
 # "digest" is exempt from SemVer, so we should always use a specific version
 curve25519-dalek = { version = "=4.0.0-pre.5", default-features = false, features = ["alloc", "digest"] }
-serde = { version = "1", optional = true, features = ["derive"] }
-zeroize = "1.5"
 hashbrown = "0.12.0"
+hex = { version = "0.4", default-features = false, features = ["alloc"] }
+rand_core = "0.6"
+serde = { version = "1", optional = true, features = ["derive"] }
+sha2 = { version = "0.10", default-features = false }
+zeroize = { version = "1.5", features = [ "zeroize_derive" ] }
 
 [dev-dependencies]
 rand = "0.8"

src/signing_key.rs

@@ -3,13 +3,14 @@ use core::convert::TryFrom;
 use curve25519_dalek::{constants, digest::Update, scalar::Scalar};
 use rand_core::{CryptoRng, RngCore};
 use sha2::{Digest, Sha512};
+use zeroize::Zeroize;
 
 use crate::{Error, Signature, VerificationKey, VerificationKeyBytes};
 
 /// An Ed25519 signing key.
 ///
 /// This is also called a secret key by other implementations.
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, Zeroize)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 #[cfg_attr(feature = "serde", serde(from = "SerdeHelper"))]
 #[cfg_attr(feature = "serde", serde(into = "SerdeHelper"))]
@@ -103,13 +104,6 @@ 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]);
 

src/verification_key.rs

@@ -7,6 +7,7 @@ use curve25519_dalek::{
     traits::IsIdentity,
 };
 use sha2::Sha512;
+use zeroize::DefaultIsZeroes;
 
 use crate::{Error, Signature};
 
@@ -112,6 +113,20 @@ impl AsRef<[u8]> for VerificationKey {
     }
 }
 
+impl Default for VerificationKey {
+    fn default() -> VerificationKey {
+        let identity: EdwardsPoint = Default::default();
+        let identity_bytes = identity.compress().to_bytes();
+
+        VerificationKey {
+            A_bytes: VerificationKeyBytes::from(identity_bytes),
+            minus_A: -identity,
+        }
+    }
+}
+
+impl DefaultIsZeroes for VerificationKey {}
+
 impl From<VerificationKey> for [u8; 32] {
     fn from(vk: VerificationKey) -> [u8; 32] {
         vk.A_bytes.0