k256: impl Drop for ecdsa::SigningKey

Adds a `Drop` handler which calls `zeroize()` on the `NonZeroScalar`
which the `SigningKey` newtype wraps.

Author: tarcieri

k256/src/ecdsa/sign.rs

@@ -19,6 +19,7 @@ use elliptic_curve::{
     ops::{Invert, Reduce},
     rand_core::{CryptoRng, RngCore},
     subtle::{Choice, ConstantTimeEq},
+    zeroize::Zeroize,
 };
 
 #[cfg(any(feature = "keccak256", feature = "sha256"))]
@@ -274,6 +275,12 @@ impl From<&NonZeroScalar> for SigningKey {
     }
 }
 
+impl Drop for SigningKey {
+    fn drop(&mut self) {
+        self.inner.zeroize();
+    }
+}
+
 #[cfg(feature = "pkcs8")]
 #[cfg_attr(docsrs, doc(cfg(feature = "pkcs8")))]
 impl FromPrivateKey for SigningKey {